Showing papers in "Journal of Experimental Botany in 1973"

Nutrient Supply and the Growth of the Seminal Root System in Barley I. THE EFFECT OF NITRATE CONCENTRATION ON THE GROWTH OF AXES AND LATERALS

Abstract: A method was devised by which different zones along a single seminal axis of an intact plant could be exposed for extended periods to contrasting concentrations of nitrate (either 0-01 or 1-0 mM) in continuous flow, the supply of all other nutrients being favourable throughout. The concentration of nitrate was found to exert a direct and strictly localized effect upon the growth of lateral roots which, depending upon the supply of assimilates from the shoot, resulted in marked modifications to the form of the root system. Zones receiving 1-0 mM nitrate showed an increase in the number and extension rate of both firstand second-order laterals, associated with a preferential accumulation of dry matter, compared with zones in 0-01 mM nitrate. The average number of laterals (both first and second order) per cm of parent root was 4-4 in the presence of 1-0 mM nitrate and 2-2 in 0-01 mM. The average ex tension rates of first-order laterals were 0-61 and 0-26 cm d"1 and second-order laterals, 0-10 and 0-05 cm d-1 for nitrate concentrations of 1-0 and 0-01 mM respectively. The precise numbers and extension rates of laterals in any one zone were affected, however, by the rate of growth of laterals in other parts of the root system. In contrast, the extension rates of axes were little affected by the concentration of nitrate to which their apical meristems were exposed and approached 2-0 cm d_1 provided the plants were not nitrogen-starved. The significance of these results to the physiology of root growth and soil-plant relations is dis cussed.

The Carbon Balance of a Legume and the Functional Economy of its Root Nodules

Abstract: Budgets for carbon and nitrogen m shoot, root, and nodules of garden pea (Pisum sativum L.) are drawn up for a 9-d interval in the life cycle, from data on nitrogen fixation, carbon accumu lation in dry matter, respiratory output of plant organs, and organic solute exchange between shoot and nodulated root. Of the carbon gained photosynthetically by the shoot from the atmosphere 26 per cent is incorporated directly into its dry matter, 32 per cent translocated to the nodules, and 42 per cent to the supporting root. Of the nodules' share, 5 per cent is consumed in growth, 12 per cent in respiration, and 15 per cent returned to the shoot via the xylem, as amino compounds generated in nitrogen fixation. Growth and respiration of the root utilize, respectively, 7 and 35 per cent. The respiratory efficiency of a nodulated root in terms of nitrogen fixation (5-9 mg Cpermg N2-N fixed) is found to be very similar to that of an uninoculated root assimilating nitrate (6-2 mg C per mg N03-N reduced). The nodules require in growth, respiration, and export 4-1 mg C (ss 10-3 mg carbohydrate) for each mg N which they fix. The nodules consume 3 ml 02 for every 1 ml N2 utilized in fixation. In exporting a milligram of fixed nitrogen the nodules require at least 0-35 ml of water. Almost half of this requirement might be met by mass flow into the nodules via the phloem.

The Role of Endogenous Abscisic Acid in the Response of Plants to Stress

Abstract: When a continuous stream of warm air (38 °C) was directed on to the leaves of dwarf bean seedlings they wilted and then gradually regained turgor. This process of adaptation was accompanied by an increasing abscisic acid (ABA) level in the leaves and an increase in leaf resistance (RL). It is suggested that the leaf-water deficit induced by the warm-air treatment caused the increase in ABA level and that the latter was responsible for stimulating stomatal closure, enabling the plants to regain full turgor. A similar type of adaptation, brought about by an increased level of ABA in the leaves, is believed to occur in tomato, dwarf bean, and wheat plants when they are flooded. Predictably, in rice, a species adapted to a flooded environment, seedlings showed no increase in ABA level as a result of flooding. It is proposed that adaptation may involve the formation of an equilibrium between ABA and its conjugate form (i.e. the glucose ester). The ABA-conjugate was observed to disperse slowly from leaves recovering from a water deficit and therefore it may act as a metabolic 'back-stop', enabling the 'free' ABA level to remain high for a period even when the leaves have regained turgor. Abscisic acid appears to be responsible for alleviating the effects of water stress in plants, making it possible for plants to pass through periods of stress with little harm.

Genotypic Differences in the In Vitro Formation of Embryoids from Rice Pollen

Abstract: Twenty varieties of rice were assessed for the ability of their pollen to give rise to ombryoids and haploid plantlets in anther culture. There were wide differences in response between the varieties, a primitive cultivar from Assam being the most responsive. Only anthers containing uninucleate microspores showed differentiation of pollen embryoids. It appeared that during the initial stages of development the embryoids were attached to the wall of the anthors by a suspensor-like attachment. Genotypic differences, besides several other factors, have a definite role in shifting the normal course of development of the pollen.

Chilling Injury in Cucumber Leaves

Abstract: When cucumber plants are chilled at 5 °C and 85 per cent, r.h. the leaves wilt rapidly and lose water. Chlorophyll is lost in the light, but not in the dark. If chilled leaves are placed in water, electrolytes leak out rapidly, the amount depending on the duration of chilling. There are marked reductions in the rates of respiration and photosynthesis; Q10 for respiration is 5 0 between 15 and 5 °C. Phospholipid levels decline, but more slowly than the rate of water loss. Leaves recover their initial fresh weight, rate of electrolyte leakage, and respiration and photosynthesis rates if the plants are returned to warm conditions within 1 to 2 d. When leaves are chilled at 100 per cent r.h. they do not lose water or phospholipids; electro lyte leakage is little more than in controls, but respiration and photosynthesis are reduced as at 85 per cent r.h. It is envisaged that the leaf cell membranes suffer a physical change at 5 °C which reduces the rates of respiration and photosynthesis, but allows the passage of water and electrolytes out of the cells; the water evaporates away but electrolytes become concentrated in the walls and can be leached out of the leaf. As water is lost from the leaf symptoms of dehydration appear. Phospholipid loss may be associated with enzyme action following the loss of compart mentation.

Frictional Potential Losses and Total Water Potential in Plants: a Re-evaluation

Abstract: A theoretical treatment is given of isothermal potential losses due to frictional resistances against water flow in plants. It becomes apparent that in the past two main difficulties have been widely ignored: the proper choice of dimensions for fluxes and resistances deserves consideration, and the original van den Honert concept seems to be especially prone to mis interpretation. A revised equation is presented for total water potential at a certain point in the plant, and some implications of this approach are outlined.

Membrane Lipids in Senescing Green Tissues2

Abstract: At a late stage in senescence cucumber cotyledons lose fresh weight rapidly ; at the same time there is an increase in apparent free space and large quantities of electrolyte leak out when discs of cotyledon tissue are floated on water It is concluded that tonoplast and plasma membrane become leaky at this time Phosphatidyl choline, the major phospholipid present, begins to disappear once the cotyledons reach maximum fresh weight ; by the time rapid water loss starts, 56 per cent has gone, and phosphatidyl ethanolamine and inositol start to disappear Only at maturity is there enough of these phospholipids to furnish more than two complete membranes around each cell ; it is suggested that the decline in phospholipid level at senescence destroys mem brane integrity and allows leakage The glyolipids begin to disappear at the same time as chlorophyll, 2 weeks before weight loss starts

Studies on the Growth in Culture of Plant Cells XV. UPTAKE AND UTILIZATION OF URIDINE DURING THE GROWTH OF ACER PSEUDOPLATANUS L. CELLS IN SUSPENSION CULTURE

Abstract: [2-14C]-uridine is rapidly taken up by sycamore cells in suspension culture. A proportion of the radioactivity enters RNA without measurable delay, whilst the remainder equilibrates with a large pool of phosphorylated compounds, the major radioactive component of which is 5'-UMP. Both the uracil and cytosine residues of RNA receive label from [14C]-uridine and, when the cells are supplied with high concentrations of uridine, these bases are derived almost exclusively from the nucleoside. [14C]-uridine is incorporated into RNA at all stages of the growth cycle of batch cultures; its continuing incorporation, when the total RNA content of the cells is rapidly decreasing, indicates a high rate of turnover of the total RNA. Long-term labelling experiments also indicate turnover of RNA during the phase of active cell division and suggest that a large proportion of the degradation products are not re-utilized for RNA synthesis. Sycamore cells degrade [2-14C]-uridine with release of 14C02. The proportion degraded increases from 25 per cent at an external uridine concentration of 10 '6 M to 75 per cent at 10~3 M. Despite this, nucleic acids are the only macromolecules that receive a significant amount of radioactivity from [2-14]C-uridine.

Seed Dormancy in Acer CHANGES IN ENDOGENOUS CYTOKININS, GIBBERELLINS AND GERMINATION INHIBITORS DURING THE BREAKING OF DORMANCY IN ACER SACCHARUM MARSH

Abstract: The present studies on changes in the levels of cytokinins, gibberellins, and germination inhibitors during stratification of Acer saccharum seeds suggest that the breaking of dormancy is a phasic process Stratification at 5 °C led to a marked increase in the butanol-soluble cytokinins after 20 d Further chilling resulted in a loss of cytokinin The majority of the detectable cytokinin activity co-chromatographed with zeatin Similarly, at day 40, acidic gibberellin-like substances increased and declined with further chilling A loss of acidic germination inhibitors was also detected during stratification The acidic fraction was shown to contain abscisic acid (ABA) by its behaviour on GLC and isomerization with ultra-violet light Determinations of endogenous levels of ABA by combined GLC and isotope dilution methods indicated a loss of 97-9 per cent ABA during stratification The data suggest that the breaking of seed dormancy at low temperature involves the initiation of an ordered sequence of events that include the changes in growth hormones necessary to determine the course of development for the removal of dormancy

A Study of the Relationship between Root and Shoot Metabolism

Abstract: The influence of light and dark on the respiration rate, sugar level, and rubidium uptake of the roots of young sunflower and barley plants has been investigated. The rate of root respiration was found to be closely correlated with the rate of carbohydrate transport from the shoots. This enabled the energy required for root growth to be calculated. A period of darkness caused a reduction in rubidium uptake and transport which appeared to be due to the reduced rate of root metabolism in the dark. The experiments of some earlier workers on the effect of transpira tion on rubidium uptake by young sunflower and barley plants were repeated. The results indicated that they were studying the metabolic effect of light and dark on salt uptake as well as the effect of transpiration.

Apical Dominanace in Xanthium strumariumA DISCUSSION IN RELATION TO CURRENT HYPOTHESES OF CORRELATIVE INHIBITION

Abstract: The influence of the spectral distribution of illumination on the gibberellin, cytokinin, auxin, and abscisic acid levels and the correlation with the degree of branching in Xanthium strum arium is presented and discussed. Gibberellins do not appear to play a major role in apical dominance but may be important for bud extension following the initial release from domin ance. The cytokinin level was much higher in inhibited buds than in released buds. It is sug gested that the cytokinins present were probably not able to participate in bud growth because of an auxin-induced accumulation of abscisic acid in the buds themselves. The con centration of abscisic acid as measured by bioassay and gas-liquid chromatography was between 50 and 250 times that occurring in all other plants parts examined. This level fell dramatically following release from apical dominance by decapitation. The results are discussed in relation to current hypotheses of apical dominance.

Action Potentials in Lupinus angustifolius L. shoots

Abstract: The refractory periods for action potentials (AP) were investigated in Lupinus shoots by application of pairs of electrical stimuli (d.c.). The response (AP) for the second stimulus is characterized. The second response differs in amplitude, range, and propagation velocity from the first, depending on the strength of the stimuli and the time interval between them. The absolute refractory period determined on this basis lasts 12 min, and the relative one from 12 to about 90 min. The character of the refractory phenomenon is similar in the case of nerves, only the processes in Lupinus are 103 to 105 times slower. The possibility of transmission of excitation from cell to cell is discussed.

Glucose Oxidation during Shoot Initiation in Tobacco Callus Cultures

Abstract: Involvement of the Embden—Meyerhof-Parnas and the pentose phosphate pathways in glucose oxidation in tobacco callus was examined. Marked changes in the activities of gluco kinase, aldolase, glucose-6-phosphate dehydrogenase, and phosphogluconate dehydrogenase were observed during culture of tobacco callus under shoot-forming and non-shoot-forming conditions. Activities of these enzymes were higher in shoot-forming tissue than in non-shoot forming tissue. Furthermore, the activities of the pentose phosphate pathway enzymes showed greater differences than those of the Embden-Meyerhof-Parnas pathway. Confirmation of these findings was obtained by investigating the contributions of 14C from [14C-1]and [14C-6]-glucose to C02 released. The significance of these findings on glucose oxidation in relation to the shoot-initiation process are discussed.

Studies on the Growth in Culture of Plant Cells XVI. NITROGEN ASSIMILATION DURING NITROGEN-LIMITED GROWTH OF ACER PSEUDOPLATANUS L. CELLS IN CHEMOSTAT CULTURE

Abstract: Cultures of Acer pseudoplatanus L. cells have been established in a chemostat with nitrogen as the limiting nutrient factor. During prolonged growth at three different dilutions several enzymes concerned with the assimilation of nitrate and urea from the culture medium achieved steady states of activity. Neither the enzyme activities nor the amino-acid contents of the cells showed marked changes with change in dilution under the nitrogen-limiting conditions employed here. One of the steady states was perturbed by supplementing the culture medium with gluta mate as an additional nitrogen source. This caused a rapid and considerable enhancement of the alanine content of the cells, presumably resulting from transamination between pyruvate and the incoming glutamate. In the longer term, the transition caused an elevation of the levels of enzymes concerned with the diversification of nitrogen from glutamate (glutamate oxaloacetate and glutamate-pyruvate transaminases and y-glutamyl transferase), whilst enzymes concerned with glutamate formation (urease and, most probably, nitrate reductase) declined in activity.

Nucleic-Acid and Protein Contents of Embryogenic Tobacco Pollen

Abstract: Anthers of Nicotiana tabacum cv. White Burley containing microspores in mitosis were cultured for 12—14 d at 23 °C in order to induce mitosis of the vegetative cell (embryogenic grains). After this period the total DNA content of such grains estimated by gallocyanin-chrome alum photometry was double that of the microspores in mitosis, whereas the total RNA content was reduced by about one-third. Total protein estimated by naphthol yellow S photometry was approximately the same as at inoculation. In contrast, total RNA and total protein contents of non-embryogenic grains in the same anthers were at least four times greater than at inoculation. It is suggested that degradation of cytoplasmic information concerned in gametophytic differentiation takes place prior to mitosis of the vegetative cell.