Showing papers in "Journal of Experimental Botany in 1995"

Changes in plant response to NaCl during development of rice (Oryza sativa L.) varieties differing in salinity resistance

Abstract: The effects of NaCl stress on growth and development of rice (Oryza sativa L.) were studied and compared in varieties of various origins. During the vegetative stage, tall indica landraces (Nona Bokra, Buhra Rata, Panwell, and Pokkali) appeared to be resistant throughout while in japonica varieties (I Kong Pao (IKP) and Tainung 67) and elite breeding lines (IR 4630, in 2153 and in IR 31785), resistance fluctuated. Panwell, which was the only indica variety evaluated during the reproductive stage, also expressed salt resistance during booting, heading and grain maturation while varieties with the greatest variability in salt stress response during the vegetative phase (IR 4630, in 31785 and IKP) also showed the greatest variability during reproductive development. Thus, varietal levels of resistance to salinity at different growth stages are not necessarily interdependent characteristics. Variability in salt resistance of different genotypes during the vegetative and reproductive phases of development was not correlated to their mean level of relative resistance. There was an ontogenic evolution of salt resistance and the young seedling stage appeared to be the most sensitive to NaCl during vegetative growth. Nevertheless, short- and middle term effects of stress have to be distinguished for each genotype since some varieties showed better growth during the second week of stress than during the first, whilst others showed an opposite trend. Moreover, even at specific stages of development, plant responses to NaCl were quite variable according to the criterion used to quantify salinity resistance. The identification of genotypes resistant to NaCl at specific developmental stages is essential to improve the understanding of the effects of salt stress upon phenology and to elaborate further breeding programmes.

Perspectives on photoinhibition and photorespiration in the field: quintessential inefficiencies of the light and dark reactions of photosynthesis?

Abstract: Taking the long-held view that photoinhibition embraces several processes leading to a reduction in the efficiency of light utilization in photosynthesis, and that photorespiration embraces several processes associated with 02 uptake in the light, photoinhibition and photorespiration now can be considered as inevit able, but essential inefficiencies of photosynthesis which help preserve photosynthetic competence in bright light. Photorespiratory 02 uptake via Rubisco, and 02 uptake via the Mehler reaction, both promote non-assimilatory electron transport, and stimulate photon utilization during C02-Iimited photosynthesis in bright light. Although fluorescence studies show that the proportion of total photon use via oxygenase photorespiration in air may decline to only about 10% in full sunlight, mass spectrometer studies show that 02 uptake in Mehler reaction photorespiration in C3 and CAM plants can still account for up to 50% of electron flow in saturating C02 and light. The Mehler ascorbate peroxidase reaction has an additional role in sustaining membrane energization which promotes dynamic photoinhibition and photon protection (rapidly reversible decrease in PSII efficiency involving dissipa tion of the energy of excess photons in the antennae). Net C02 and 02 exchange studies evidently underesti mate the extent of total electron transport, and hence overestimate the extent of photon excess in bright light, leading to overestimates of the role of energy dependent photon dissipation through dynamic photo inhibition. Nevertheless, in C3 plants in air all of these processes help to mitigate chronic photoinhibition and photon damage (slowly reversible decrease in PSII effi ciency involving loss of reaction centre function). The possibility remains that residual electron transport to 02 from intermediates in the vicinity of PSII may also lead to reactive 02 species that potentiate this photon

Dynamic measurements of root hydraulic conductance using a high-pressure flowmeter in the laboratory and field

Abstract: A new high-pressure flowmeter (HPFM) is described which is capable of rapid water-flow measurements. The HPFM permits dynamic determination of hydraulic conductance of roots, K r , and can be used in the laboratory or field. The base of a root is connected to the HPFM and water is perfused into the root system opposite to the normal direction of flow during transpiration. The perfusion pressure is changed at a constant rate of 3-7 kPa s −1 while measuring the flow into the root every 2-4 s. The slope of the plot of flow versus applied pressure is K r . This paper describes the HPFM, presents the theory of dynamic flow measurements, discusses sources of error, presents evidence that dynamic measurements of K r in Ficus maclellandi (and six other tropical species from Panama) yield the correct result, and demonstrates the use of the method under field conditions in Panama on Cecropia obtusifolia and Palicourea guianensis

Identification of QTL for drought responses in maize and their use in testing causal relationships between traits

Abstract: A difficulty in identifying traits that help crop plants maintain their yield under droughted conditions is distinguishing between those traits that contribute to yield stability under drought and traits that do not affect yield. With the development of molecular markers for many crops it is now possible to identify major quantitative trait loci (QTL) regulating specific drought responses. By comparing the coincidence of such QTL for specific traits it is possible to test much more precisely than before whether a particular constitutive or adaptive response to drought stress is likely to be of significance in improving drought resistance. We have used this approach to identify QTL for ABA content and other traits likely to be important in determining drought response in maize. Eighty-four RFLP markers were mapped in an F 2 population of 81 plants from a cross between parents, Polj17 (drought resistant) and F-2 (drought sensitive), that differ markedly in many constitutive and adaptive responses to drought stress. In a soil glasshouse experiment, from which water was withheld for 3 weeks after anthesis, flowering time, stomatal conductance, tissue ABA contents, leaf water relations parameters and fluorescence characteristics, root pulling force, and nodal root number were measured. The minimum number and location of genes having major effects on the traits were determined and possible causal relationships amongst them tested. Comparing the coincidence of QTL for ABA content and stomatal conductance showed that xylem ABA content was more likely to have had a regulatory effect on the stomatal conductance of those plants than the whole leaf ABA content. However, both xylem and leaf ABA contents were significantly associated with root characteristics, suggesting that the rooting behaviour (either constitutive or adaptive) was important in regulating stress responses, particularly in determining xylem ABA contents. We also found that F m (a measure of the activity of photosynthetic reaction centres) was positively associated with chlorophyll concentration per unit area. Different methods for comparing QTL are presented and discussed.

Summer survival of leaves in a soft-leaved shrub (Phlomis fruticosa L., Labiatae) under Mediterranean field conditions: avoidance of photoinhibitory damage through decreased chlorophyll contents

Abstract: Photosynthetic pigments and relative water content of young leaves of P. fruticosa decreased considerably with the onset of the summer dry period and stabilized at low values for the last two summer months, while leaf growth was arrested. Corresponding decreases in photochemical efficiency of photosystem II, as judged by chlorophyll fluorescence measurements of predarkened leaves, were, however, negligible. Following the first autumn heavy rains, growth was restored and photosynthetic pigments and relative water content increased to the pre-drought values. The results indicate that the reduction of chlorophylls does not result from severe photoinhibitory damage but, instead, it may be an adaptive response against the adverse conditions of the Mediterranean summer. Some photosynthetic and photoprotective characteristics of P. fruticosa leaves at two stages of their development, i.e. at the severely dehydrated state with arrested growth during late summer and after their revival following the first heavy autumn rains were compared. Apart from the chlorophyll loss, the photon yield of O 2 evolution and the photosynthetic capacity at saturated CO 2 were considerably suppressed during the summer, indicating that the extremely low net photosynthetic rates observed in the field were the combined result of stomatal and mesophyll limitations. Epoxidation state was low at midday during the summer, indicating an active, photodissipative xanthophyll cycle. Although zeaxanthin content did not increase at midday after the rains, the potential of the cycle was maintained in the revived leaves, as judged by the high concentrations of the cycle components. After the rains, the activities of the anti-oxidant enzymes (superoxide dismutase, ascorbate peroxidase) remained relatively unchanged on a chlorophyll basis, but increased when expressed on a leaf surface area or protein basis. It may be concluded that P. fruticosa leaves avoid severe photoinhibitory and oxidative damage during the long, warm, dry and sunny Mediterranean summer by reducing light harvesting and electron flow capacity, whilst maintaining an adequate photoprotective ability. The preservation of a remarkable photodissipative and anti-oxidative potential after the rains may be related to the low predictability of precipitation even during the rainy winter.

Aluminium/silicon interactions in higher plants

Abstract: Aluminium and silicon are usually abundant in soil mineral matter, but their availability for plant uptake is limited by low solubility and, in the case of AI, high soil pH causes precipitation of the element in insoluble forms. Al toxicity is a major problem in naturally occurring acid soils and in soils affected by acidic precipitation. Al has no known role in higher plants, and is generally known as a toxic element, whereas Si is generally regarded as a beneficial element. Recently, it has been suggested that Al toxicity can be ameliorated by Si in a variety of animal systems. In this review the evidence that amelioration of Al toxicity by Si can also occur in plants is assessed. At present such amelioration has been shown in sorghum, barley, teosinte, and soybean, but not in rice, wheat, cotton, and pea. Plant species vary considerably in the amounts of Al and Si that they transport into their tissues, and it seems that very high Si accumulation and very high Al accumulation are mutually exclusive. The mechanisms considered for amelioration are: solution effects; codeposition of Al and Si within the plant; effects in the cytoplasm and on enzyme activity; and indirect effects

The role of sucrose synthase in the response of soybean nodules to drought

Abstract: Experiments were carried out to investigate the effects of drought stress on enzymatic activities related to carbon and nitrogen metabolism in soybean nodules. Gradual drought stress, imposed by withholding water/ nutrients, resulted in declines in the water potential of leaves and nodules consistent with a significant decline in N 2 fixation. However, the amounts of nitrogenase components 1 and 2 were virtually unaffected by drought stress. Similarly, no significant changes could be detected in aspartate aminotransferase, phosphoenolpyruvate carboxylase, glutamine synthetase or alkaline invertase activities throughout the experiment. In contrast, sucrose synthase (SS), one of the enzymes involved in sucrose metabolism in legume modules, declined dramatically in activity and in content within a few days of withholding water. Coincident with this decline in SS activity were significant increases in the nodule contents of sucrose, total free amino acids and ureides. The amounts of proline, however, did not increase until some days later. It is suggested that SS may play a key role in the regulation of nodule carbon metabolism and, therefore, of nitrogen fixation under drought stress conditions.

Diurnal regulation of NO3− uptake in soybean plants I. Changes in NO3− influx, efflux, and N utilization in the plant during the day/night cycle

Abstract: The effect of light on NO 3 - utilization was investigated in non-nodulated soybean (Glycine max L. Merr., cv. Kingsoy) plants during a 14/10 h light/dark period at a constant temperature of 26°C. A 30-50% decrease of net NO 3 - uptake rate was observed 2-6 h after the lights were turned off. This was specifically due to an inhibition of NO 3 - influx as measured by 15 N incorporation during 5 min. The absolute values of NO 3 - efflux depended on whether the labelling protocol involved manipulation of the plants or not, but were not affected by illumination of the shoots. Darkness had an even more marked effect in lowering the reduction of 15 NO 3 - in both roots and shoots, as well as xylem transport of 15 NO 3 - and reduced 15 N. Concurrently with this slowing down of transport and metabolic processes, accumulations of NO 3 - and Asn were significantly stimulated in roots during the dark period. These data are discussed in view of the hypothesis that darkness adversely affects NO 3 - uptake through specific feedback control, in response to alterations in the later steps of N utilization which are more directly dependent on light.

Activated oxygen production and detoxification in wheat plants subjected to a water deficit programme

Abstract: Wheat plants (Triticum durum L. cv. Ofanto) were grown in a controlled environment. In one set, control plants were regularly watered ; the other set of plants was subjected to two water deficit periods obtained by withholding water and rewatering to field capacity at the end of the first period. After both periods of stress, water potential (Ψ w ), pressure potential (Ψ p ) and relative water content (RWC) decreased ; osmotic potential (Ψ π ) decreased by 0.3 MPa only after the second period of stress. In both treatments no osmo-regulation mechanism occurred, however, an elastic adjustment took place and turgor was maintained at positive levels. Following the first treatment the good functionality of the ascorbate/glutathione cycle allowed the plants to maintain hydrogen peroxide to the control level despite a greater capacity of the thylakoid membranes to leak electrons towards oxygen ; moreover, the ascorbate (AsA)/dehydroascorbate (DHA) ratio was unchanged while reduced glutathione (GSH)/oxidized glutathione (GSSG) increased in comparison with the control. Following the second period of stress, the decreased enzyme defence activities of the glutathione reductase (GR), dehydroascorbate reductase (DHAR) and ascorbate peroxidase (AsAP) together with a minor glutathione content might be a consequence of a reduced rate of activated oxygen production.

Water channels in plants: do basic concepts of water transport change?

Abstract: A review and re-examination of literature data shows that highly selective water channels (aquaporins) have marked effects on the overall transport properties of the plasma membrane of plant cells. The application of the channel blocker HgCl 2 (50 μM) or of high external concentrations of permeating solutes reduced the water permeability (hydraulic conductivity, Lp) of Chara internodes down to 25% of the control. In treated cells, reflection coefficients (σ s ) of permeating low molecular weight organic test solutes (alcohols, amides, ketones) were markedly reduced as well, but solute permeability (permeability coefficient, P s ) remained constant. A similar relation between Lp and σ s was found with untreated cells of isolated epidermis of Tradescantia virginiana. The results can not be interpreted in terms of conventional membrane models (pore or solubility membrane) ; for instance, the classical frictional pore model (Dainty and Ginzburg, 1963) fails to explain low σ s of rapidly permeating solutes. The results fit into a model which treats the membrane as a composite structure with proteinaceous arrays (containing water channels) in parallel with lipid arrays ('composite transport model'). Test solutes predominantly pass across the lipid array which was not affected by treatments. Water, however, largely uses the water channel array which was affected. When using heavy water (HDO) as an osmotic solute, the transport pattern changed as predicted by the model. As indicated by low channel reflection coefficients of test solutes, water channels did not completely exclude small uncharged molecules and do show some permeability for the test solutes used. Low σ s values of water channels are interpreted by a single-file mechanism of water and solute flow. Absolute values of transport coefficients (Lp, P s , σ s ) represent mixed values to which the different membrane arrays contribute according to concepts available from irreversible thermodynamics. The patchy structure of the cell membrane results in a circulation flow of water in the membrane. The fact that water channels can be triggered by factors such as heavy metals and high concentration suggests that water transport can be regulated by opening or closing water channels. The results have consequences for our basic understanding of osmosis and water transport in plants.

Timing of the phases in adventitious root formation in apple microcuttings

Abstract: As with other regeneration processes, adventitious root formation may be divided into three phases, namely dedifferentiation, induction and differentiation. We assumed that the appropriate hormonal conditions for rooting, in particular a high level of auxin and a low level of cytokinin, are required only during the induction phase. Hence, the effect of 24 h pulses with indolebutyric acid (IBA) or benzylaminopurine (BAP) should be maximal during this phase. On this assumption, the timing of the three phases was determined in microcuttings of Malus 'Jork 9'. The promotion of rooting was largest for 24 h IBA-pulses given 24-48 h or 48-72 h after the onset of culture. During culture of shoots on IBA-containing medium 24 h BAP-pulses were given. Inhibition of rooting was maximal for the BAP-pulses given between 24 and 96 h. An analysis of the kinetics of root emergence also indicated that added IBA acted from 24 h onwards: emergence of roots from the tissue occurred simultaneously for the IBA pulses at 0-24h and 24-48 h, but lagged 1 d behind for the 48-72 h pulse and 2 d for the 72-96 h pulse. We concluded that dedifferentiation occurred from 0 to 24 h, induction from 24 to 72 or 96 h, and after that differentiation. Histological observations showed that after 24 h, cells with swollen nuclei and dense cytoplasm had appeared in the regions of the stem where the roots were formed. The first cell divisions were observed after 48 h. After 96 h, meristemoids of c. 30 cells had been formed. After the BAP-pulses at 24-48 h or 48-72 h, these meristematic cells formed callus.

Expansion of pea leaves subjected to short water deficit: cell number and cell size are sensitive to stress at different periods of leaf development

Abstract: We have followed the expansion of individual pea leaves from initiation to maximum area, over two markedly different periods. During the first one (2/3 of total leaf development time), cell production occurred while cell and leaf expansions were slow. Rapid expansion (95% of total) occurred for a second period lasting 1/3 of total development time, whereas cell division was virtually completed. Water deficits of 15 d were applied during either slow or rapid expansion, and characterized by measurements of soil water potential, stomatal conductance, leaf water potential and xylem [ABA]. Plants which experienced water deficit during the slow expansion period had markedly reduced expansion during the second period (i.e. 1 or 2 weeks after cessation of deficit), while all variables characterizing water status were returned to the level of the control. This 'after effect' was accounted for by a reduced cell number per leaf, while individual cell area was not affected. In contrast, water deficit occurring during rapid leaf expansion immediately reduced leaf expansion via cell area, without affecting cell number per leaf. These experiments indicate a role, in the response to water deficits, for events occurring very early in the development of pea leaves, while leaf expansion is too slow to be measured with macroscopic methods. This role would be accounted for by cell production during the first 2/3 of leaf development while cell expansion would account for changes in the area of leaves experiencing a later stress. These results suggest that long-term temporal analysis may be essential in the study of dicot leaf expansion compared to monocot leaves where temporal analysis can be inferred from spatial analysis.

The estimation of pollen viability in rice

Abstract: Methods for testing pollen viability in rice were evaluated by comparing staining with aniline blue in lactophenol, two tetrazolium salts and fluorescein diacetate with germination in an in vitro culture medium. Staining with thiazolyl blue (MTT) was nicely correlated with germination. Under non-saline conditions, rice pollen was only viable for a short period of time, with an approximately 50% loss of viability within 20 min of shedding. When plants were salinized at either panicle initiation or the booting stage, pollen viability was reduced.

Reversible closing of water channels in Chara internodes provides evidence for a composite transport model of the plasma membrane

Abstract: Treatment of internodal cells of Chara corallina with the water channel blocker HgCl 2 caused a decrease of the hydraulic conductivity of the membrane (Lp) by a factor of three to four. In the presence of (practically) non-permeating solutes such as sugars or salts, the osmotic responses were similar to those found in controls, i.e. reflection coefficients ( s ) were close to unity. However, when treating the internodes with osmotic solutions of rapidly permeating lipophilic substances such as low molecular weight alcohols or acetone, the pirture of biphasic pressure relaxations due to the exchange of both water and solutes changed considerably. In the presence of HgCl 2 , reflection coefficients were substantially reduced and even became negative for some solutes (anomalous osmosis). Different from reflection coefficients, permeability coefficients (P s ) remained constant upon treatment. When using heavy water (HDO) as a small hydrophilic solute which should cross the membrane largely via water channels, results were different: the reflection coefficient of HDO increased and permeability decreased. Treating the cells with 5 mM 2-mercaptoethanol to remove the mercury from transport proteins (water channels) reversed changes in Lp and s . The results disagree with conventional membrane models, which imply a homogeneous membrane. They are readily explained by a composite model of the membrane in which proteinaceous arrays with specific water channels are arranged in parallel with lipid arrays. The latter account for most of the permeability of the small organic test solutes used and are not affected by the channel blocker. From the data, detailed information is obtained about transport properties of the arrays (water channels).

Responses of young trees to wind and shading: effects on root architecture

Abstract: Two wind tunnels were designed to detect influences of wind on the development of the root systems of young trees : Picea sitchensis and Larix decidua were grown in the first and L. decidua only in the second. In the second experiment, the tap root of each L. decidua seedling was removed in order to mimic the formation of a shallow root-plate. Responses of shoot growth to wind stress were small, although uneven irradiance levels resulted in asymmetric growth of both shoots and roots ; the distribution of root biomass around the tree was related spatially to that of shoot growth. In both experiments there was an increase in the number of large windward and leeward roots in both species. In the first experiment, the sum of the cross-sectional area (ΣCSA) of lateral root bases was greater on the windward side of the tree in both species, whereas in the second experiment, it was greatest on the leeward side. Windward woody lateral roots of P. sitchensis had a higher incidence of branching than leeward roots. Woody tips of windward roots were significantly longer with a larger diameter than on leeward roots. It appears that wind action stimulates diameter growth and alters morphogenesis of those roots most important for anchorage. As a tree sways in the wind, leeward and especially windward lateral roots are placed under the most stress. Larger roots, or a greater branching density in these areas will help counteract wind stresses on the tree.

The regulation of Rubisco by Rubisco activase

Abstract: The activity of Rubisco depends on the conversion of the inactive form (E) to the active form (ECM); the binding of the inhibitors CA1P and RuBP to ECM and E, respectively; and the catalytic formation of inhibitory sugar bisphosphates from the enediol intermediate that precedes carboxylation/oxygenation. The regulatory protein, Rubisco activase, modulates the activity of Rubisco by kinetically increasing the dissociation rate of these sugar bisphosphates from their respective enzyme forms in a process that requires ATP hydrolysis. The activity of Rubisco activase is determined by the ATP:ADP ratio. The activity also appears to be influenced by thylakoid membranes in a light-dependent manner, but only a few details of this effect are currently understood. A species dependence in the interaction between heterologous Rubisco and Rubisco activase proteins has been observed. Whether the regulation of Rubisco is a constraint to crop productivity is debatable and will only be decided by detailed study of plants transformed with appropriately modified Rubisco activase and/or Rubisco proteins.

Cyanogenesis in cassava (Manihot esculenta Crantz)

Abstract: Cassava is the most agronomically important of the cyanogenic crops. Linamarin, the predominant cyanogenic glycoside in cassava, can accumulate to concentrations as high as 500 mg kg -1 fresh weight in roots and to higher levels in leaves. Recently, the pathway of linamarin synthesis and the cellular site of linamarin storage have been determined. In addition, the cyanogenic enzymes, linamarase and hydroxynitrile lyase, have been characterized and their genes cloned. These results, as well as studies on the organ- and tissue-specific localization of linamarase and hydroxynitrile lyase, allow us to propose models for the regulation of cyanogenesis in cassava. There remain, however, many unanswered questions regarding the tissue-specific synthesis, transport, and accumulation of cyanogenic glycosides. The resolution of these questions will facilitate the development of food processing, biochemical and transgenic plant approaches to reducing the cyanogen content of cassava foods.

Molecular analysis of carbon partitioning in solanaceous species

Abstract: Due to their importance as crop species, solanaceous plants such as tobacco, potato and tomato have been studied in great detail with respect to carbohydrate partitioning using classical physiological methodology. During the last decade, molecular tools have enabled the isolation of many genes involved in carbohydrate metabolism, and the development of efficient transformation technology, especially for the Solanaceae, has allowed the creation of transgenic plants, that are altered by the activity of individual genes. The physiological analysis of these transgenic plants has contributed considerably to our current understanding of how assimilates are produced, mobilized, translocated, and allocated in higher plants. This review aims to cover the current knowledge in the area of molecular genetics of sucrose biosynthesis, transport and utilization in these species

Vulnerability of xylem to embolism in relation to leaf water potential and stomatal conductance in Fagus sylvatica f. purpurea and Populus balsamifera

Abstract: The vulnerability of xylem vessels to water stressinduced cavitation was studied by measuring hydraulic conductivity and ultrasound acoustic emissions [AEs) in Fagus sylvatica L. f. purpurea (Ait.) Schneid. and Populus balsamifera L.. The occurrence of xylem embolism in summer was investigated in relation to leaf water potential and stomatal conductance. Populus was extremely vulnerable to cavitation, losing functional vessels due to embolism at water potentials lower than —0.7 MPa. Fagus experienced embolism when water potential fell below —1.9 MPa. Midday water potentials often approached these threshold values. When evaporative demand increased rapidly on sunny days, water loss became limited by low stomatal conductance. Thus water potentials fell only slightly below the threshold values inducing cavitation. Despite the differences in vulnerability, both species tolerated a similar embolism rate of about 10% in the summer. There was no embolism reversal during prolonged periods of rain. AEs were predictive of loss in hydraulic conductivity, indicating that AEs were mainly confined to vessels. Finally, vessel length distribution, vessel diameter (tangential axis), vessel density, and vessel wall thickness had been determined for both species investigated. Populus had longer and wider vessels than Fagus, whereas vessel wall thickness was similar in both species.

The effect of nitrogen and growth regulators on stem and root characteristics associated with lodging in two cultivars of winter wheat

Abstract: The effects of nitrogen and plant growth regulators (stem shorteners) on root and shoot characteristics associated with lodging resistance were investigated in two winter wheat (Triticum aestivum L.) cultivars of contrasting lodging resistance: the susceptible Galahad and the resistant Hereward. The morphology and mechanical strength of the stems and anchorage systems grown at two levels of nitrogen and with or without growth regulators were measured and related to the incidence of lodging recorded in a field trial. In both cultivars high levels of nitrogen increased the height of the stem, thereby increasing the 'self-weight' moment transmitted into the ground and weakened both the stems and the anchorage coronal roots. As a result, the anchorage strength was also reduced, plants failing in the root system in simulated lodging tests. Growth regulators, in contrast, had little effect on the bending strength of the shoots and root systems, but reduced plant height so that the overturning moments generated by the weight of the shoot were less. There were also differences between cultivars: Galahad plants had weaker anchorage due to the smaller number and lower strength of the coronal roots. The morphological and mechanical measures were used to calculate a safety factor against both stem and root lodging. Five factors were found to influence the safety factors, these were: cultivar type, the type of lodging, the rate of nitrogen and growth regulator application, and time, being lowest in Galahad plants at high levels of nitrogen and without growth regulators and at grain filling when the ears were heaviest. This was consistent with the observed pattern of lodging: root lodging occurred at grain filling and only in Galahad which had been treated with high nitrogen rates, most strongly in plants without growth regulators.

Soil-plant water relations, root distribution and biomass partitioning in Lupinus albus L. under drought conditions

Abstract: Lupinus albus L from different climatic origins responded to a 15 d period of water shortage during flowering by losing 50% of the total leaf canopy and gaining 55% in stem dry weight Water deficits also led to a significant increase in the fine root length density and a slight increase in the fine root dry weight The latter increase was especially pronounced in the deeper soil layers Some marginal differences among genotypes were observed in the responses Stomatal closure by midday was an early response to water deficit, giving rise to constant predawn leaf water potentials during the first week of water shortage in spite of a decrease of 60% in the available soil water No osmotic regulation or adjustments of the cell wall properties were observed in any of the lupin lines We explained the maintenance of seed production in water-stressed plants by their ability to accumulate assimilates in the shoot, which would be diverted to the pods during the seed filling stage

Genotype, explant and position effects on organogenesis and somatic embryogenesis in eggplant (Solanum melongena L.)

Abstract: The relative importance of genotype and explant, and their interactions for in vitro plant regeneration via both organogenesis and somatic embryogenesis in Solanum melongena (eggplant) has been studied. Hypocotyl, cotyledon and leaf explants of four commercially grown Indian cultivars, Pusa Purple Long, Long White Cluster, Pusa Kranti, and Pusa Purple Cluster were used in the study. A combination of benzyladenine (11.1 μM) and indoleacetic acid (2.9 μM) was found to be optimum for shoot regeneration. Naphthalene acetic acid induced embryogenesis in all the three explants; 32.2 μM was optimum for hypocotyl explants while 10.7 μM yielded maximum number of somatic embryos from cotyledon and leaf explants. Genotype, explant and genotype-explant interaction had highly significant effects on both organogenesis and somatic embryogenesis with genotype exerting maximum effect on both these processes. Pusa Purple Long was found to be the most responsive genotype for regeneration of both adventitious shoots and somatic embryos among the cultivars. Among the explants, hypocotyls yielded the maximum number of adventitious shoots followed by cotyledons and leaves. The embryogenic response of leaves and cotyledons was, however, significantly higher than that of hypocotyl explants. Significant differences for morphogenetic potential were also observed within a single explant (hypocotyl). There was a basipetal gradient for organogenesis (i.e. decrease in number of shoots from base to apex) while the terminal hypocotyl segments showed better embryogenic potential than the median segments

Development of white spruce (Picea glauca (Moench.) Voss) somatic embryos during culture with abscisic acid and osmoticum, and their tolerance to drying and frozen storage

Abstract: The limit of permeability of white spruce (Picea glauca [Moench.] Voss) somatic embryo cell walls to molecules was in the order of 30 A. Polyethylene glycols (PEGs) and dextrans of molecular weights greater than 1000 and 6000, respectively, produced a nonpermeating (non-plasmolysing) water stress which improved embryo development. Somatic embryos converted to plantlets at frequencies of 76-84% following slow drying and storage at − 20°C for 1 year, which was similar to the 77% recorded for control somatic embryos slowly dried then germinated without freezing or storage. Culture for 7-8 weeks with medium containing abscisic acid, 3% sucrose, and 7.5% PEG 4000 yielded somatic embryos with five times the embryo storage lipid content recorded for zygotic embryos. During culture with PEG the moisture content of the somatic embryos decreased from 96% for immature suspension-cultured somatic embryos, to 47% for mature embryos. Somatic embryos cultured for 7-8 weeks survived rapid drying to 5% moisture content, and converted to plantlets at frequencies of 60-70%, but no somatic embryos survived rapid drying when cultured for only 4 weeks; however, slow drying did induce desiccation tolerance in 3-week cultured somatic embryos. Abscisic acid was important to maintain embryos in a developmental state, but ABA alone did not induce desiccation tolerance. In order to induce desiccation tolerance a water stress treatment was required. Tolerance of rapid drying coincided with moisture contents below 55%, which occurred after 5 weeks of culture in the presence of PEG 4000 and abscisic acid

Effects of light and atmospheric carbon dioxide enrichment on photosynthesis and carbon partitioning in the leaves of tomato (Lycopersicon esculentum L.) plants over-expressing sucrose phosphate synthase

Abstract: Photosynthetic carbon assimilation, carbon parti tioning and foliar carbon budgets were measured in the leaves of transformed tomato plants expressing a maize sucrose-phosphate synthase (SPS) gene in addi tion to the native enzyme, and in untransformed con trols. The maize SPS gene was expressed under control of either the promoter of the small subunit of ribulose 1,5-bisphosphate carboxylase (rbcS promoter; lines 2, 9 and 18) or the 35S promoter from cauliflower mosaic virus (CaMV promoter; line 13). The rate of sucrose synthesis was increased relative to that of starch and sucrose/starch ratios were higher through out the photoperiod in the leaves of all plants expressing high SPS activity. The leaf carbon budget over the day/night cycle in air at low irradiance (180//mol photon m~2 s1) was similar in all plants. Net photosynthesis measured in air and at elevated C02 (800-1500 //I r1) on whole plants grown in air at 400//mol m~2 s-1 irradiance was significantly increased in the high SPS expressors compared to the untransformed controls and was highest where SPS activity was greatest. At high C02 the stimulation of photosynthesis was more pronounced. We conclude that SPS activity is a major point of control of photo synthesis particularly under saturating light and C02.

Hydraulic architecture and xylem structure of the dimorphic root systems of South-West Australian species of Proteaceae

Abstract: Specific hydraulic conductivities (K s ) of trunks (stem), superficial lateral roots, and deeply penetrating sinker (tap) roots were examined in variously-aged tree or shrub species of Proteaceae (Banksia prionotes, Banksia ilicifolia and Adenanthos cygnorum) in native habitat on deep sand overlying water tables located at 1.8-3.8 m depth. K s values (on the basis of organ transectional area) for sinker roots (range from 30 to 780 x 10 -3 m 2 MPa -1 s -1 ) were consistently greater than in associated laterals (2 to 50 x 10 -3 m 2 MPa -1 s -1 ), and much greater again than in trunks (0.5 to 9 x 10 -3 m 2 MPa -1 s -1 ). Wood was essentially diffuse-porous and mean lengths of vessels in the trunk, laterals and sinker roots of B. prionotes were 4 cm, approximately 75 cm and greater than 1.5 m, respectively. A 5-20-fold increase in K s from top to base of a sinker was associated with progressive increases in mean radii of conducting elements, proportional areas of stele occupied by xylem conduits and percentage of organ fresh weight displaceable by mild vacuum extraction of water from vessel lumina. Resulting from these changes, conductance (K h ) of a sinker was relatively constant along its length, or even increased with depth, despite a 10-15-fold downward attenuation in transectional area over the 1.3-1.9 m length of root studied. Correlation plots of mean vessel radius and K s (lumen area basis) grouped data for laterals separately from that of sinkers and showed interspecific differences in K s values for similar range classes of lumen radius. Increases in conduit flow with increasing radius were less than predicted from the Hagen-Poiseuille equation.

Circumnutation and gravitropism cause root waving in Arabidopsis thaliana

Abstract: Arabidopsis thaliana roots grow in a wavy pattern on inclined agar plates. This waving behaviour has been interpreted as representing a gravitropism-dependent thigmotropic response. We argue instead that this root waving represents primarily a flattened spiral growth pattern resulting from circumnutation and gravitropism

The respiratory energy requirements involved in nocturnal carbohydrate export from starch-storing mature source leaves and their contribution to leaf dark respiration

Abstract: The present study explores the potential contribution of the energy requirements associated with nocturnal carbohydrate export to (1) the fraction of dark respira­ tion correlating with leaf nitrogen concentration and (2) the dark respiration of mature source leaves. To this end, we determined the nocturnal carbohydrate-export rates from leaves with an optimal nitrogen supply, and the correlation between the nitrogen concentration and the dark respiration of leaves. The specific energy costs of carbohydrate export from starch-storing source leaves were determined both experimentally and theor­ etically. The present estimate of the specific energy cost involved in carbohydrate export as obtained by linear regression (0. 70 mol C02 (mol sucrose) - 1 ), agrees well with both literature data obtained by different methods (0.4 7 to 1.26 mol C02 (mol sucrose) - 1 ) and the theoretically calculated range for starch-storing species (0.40 to 1.20 mol C02 (mol sucrose) - 1 ). The conversion of starch in the chloroplast to sucrose in the cytosol is a major energy-requiring process. Maximally 42 to '107'% of the slope of the relationship between respiration rate and organic nitrogen concentration of primary bean leaves, may be ascribed to the energy costs associated with nocturnal export of carbohyd­ rates. Total energy costs associated with export were derived from the product of the specific costs of carbo- hydrate export and the export rates, either measured on full-grown (primary) leaves of potato and bean or derived from the literature. These export costs account, on average, for 29% of the dark respiration rate in starch-storing species. We conclude that nocturnal carbohydrate export is a major energy-requiring pro­ cess in starch-storing species.