Aluminum (Al) toxicity is an important growth‐limiting factor for plants in many acid soils. The problem is not always economically correctable with conventional liming practices. But plant species and genotypes within species differ widely in tolerance to excess Al, and some of these differences are genetically controlled. Hence, an alternative or supplemental approach to the problem is to select or breed plant genotypes having greater tolerance to Al. A plant genetic approach has great potential for solving difficult soil fertility problems such as Al toxicity in acid subsoils. An important part of this approach is the determination of plant genetic, physiological and biochemical mechanisms by which plants tolerate mineral stress. Better understanding of stress tolerance mechanisms could lead to the development of more tolerant plants and more effective liming and fertilization practices for plants already in use. The objective of this presentation is to discuss our state of knowledge concernin...

Plant adaptation to acid, aluminum‐toxic soils

The influence of varied supply of phosphorus (10 and 250 mmol P m -3 ), potassium (50 and 2010 mmol K m -3 ) and magnesium (20 and 1000 mmol Mg m -3 ) on the partitioning of dry matter and carbohydrates (reducing sugars, sucrose and starch) between shoots and roots was studied in bean (Phaseolus vulgaris) plants grown in nutrient solution over a 12 d period. Shoot and root growth were quite differently affected by low supply of P, K, and Mg. The shoot/root dry weight ratios were 4.9 in the control (sufficient plants), 1.8 in P-deficient, 6.9 in K-deficient and 10.2 in Mg-deficient plants. In primary (source) leaves, but not in trifoliate leaves, concentrations of reducing sugars, sucrose and starch were also differently affected by low nutrient supply

Partitioning of shoot and root dry matter and carbohydrates in bean plants suffering from phosphorus, potassium and magnesium deficiency

Magnesium (Mg) deficiency exerts a major influence on the partitioning of dry matter and carbohydrates between shoots and roots. One of the very early reactions of plants to Mg deficiency stress is the marked increase in the shoot-to-root dry weight ratio, which is associated with a massive accumulation of carbohydrates in source leaves, especially of sucrose and starch. These higher concentrations of carbohydrates in Mg-deficient leaves together with the accompanying increase in shoot-to-root dry weight ratio are indicative of a severe impairment in phloem export of photoassimilates from source leaves. Studies with common bean and sugar beet plants have shown that Mg plays a fundamental role in phloem loading of sucrose. At a very early stage of Mg deficiency, phloem export of sucrose is severely impaired, an effect that occurs before any noticeable changes in shoot growth, Chl concentration or photosynthetic activity. These findings suggest that accumulation of carbohydrates in Mg-deficient leaves is caused directly by Mg deficiency stress and not as a consequence of reduced sink activity. The role of Mg in the phloem-loading process seems to be specific; resupplying Mg for 12 or 24 h to Mg-deficient plants resulted in a very rapid recovery of sucrose export. It appears that the massive accumulation of carbohydrates and related impairment in photosynthetic CO2 fixation in Mg-deficient leaves cause an over-reduction in the photosynthetic electron transport chain that potentiates the generation of highly reactive O2 species (ROS). Plants respond to Mg deficiency stress by marked increases in antioxidative capacity of leaves, especially under high light intensity, suggesting that ROS generation is stimulated by Mg deficiency in chloroplasts. Accordingly, it has been found that Mg-deficient plants are very susceptible to high light intensity. Exposure of Mg-deficient plants to high light intensity rapidly induced leaf chlorosis and necrosis, an outcome that was effectively delayed by partial shading of the leaf blade, although the Mg concentrations in different parts of the leaf blade were unaffected by shading. The results indicate that photooxidative damage contributes to development of leaf chlorosis under Mg deficiency, suggesting that plants under high-light conditions have a higher physiological requirement for Mg. Maintenance of a high Mg nutritional status of plants is, thus, essential in the avoidance of ROS generation, which occurs at the expense of inhibited phloem export of sugars and impairment of CO2 fixation, particularly under high-light conditions.

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Role of magnesium in carbon partitioning and alleviating photooxidative damage

We have characterized a novel mutation of Arabidopsis thaliana at a locus designated pho2. pho2 mutants accumulated up to 3-fold more total P in leaves, mostly as inorganic phosphate (Pi), than wild-type seedlings. In addition, we isolated a mutant (locus designated pho1-2, an allelle of pho1-1 described by Y. Poirier, S. Thoma, C. Somerville, J. Schiefelbein [1991] Plant Physiol 97: 1087-1093) with low Pi concentrations in leaves. When grown under high transpiration conditions, leaves of pho2 seedlings became severely P intoxicated, whereas shoots of pho1-2 mutants were P deficient and wild-type seedlings were normal. A pho1/pho2 double mutant resulting from a cross between the single mutants was identified in the F2 generation and shown to have a pho1 phenotype. Prior to the development of P toxicity symptoms, P was the only mineral nutrient whose concentration was greater in pho2 mutants than wild-type seedlings. Compared to wild-type, pho2 mutants had greater Pi concentrations in stems, siliques, and seeds, but roots of pho2 mutants had similar or lower Pi concentrations than either pho1 mutants or wild-type seedlings. We suggest that the pho2 mutation affects a function normally involved in regulating the concentration of Pi in shoots of Arabidopsis.

Characterization of a Phosphate-Accumulator Mutant of Arabidopsis thaliana

We investigated (1) the effect of constant and altered inorganic phosphate (P i ) supply (1-100 mmol m -3 ) on proteoid root production by white lupin (Lupinus albus L.); and (2) the variation in citrate efflux, enzyme activity and phosphate uptake along the proteoid root axis in solution culture. Proteoid root formation was greatest at P i solution concentrations of 1-10 mmol m -3 and was suppressed at 25 mmol m -3 P i and higher. Except at 1 mmol m -3 P i , the formation of proteoid roots did not affect plant dry matter yields or shoot to root dry matter ratios, indicating that proteoid roots can form under conditions of adequate P supply and not at the expense of dry matter production. Plants with over 50% of the root system as proteoid roots had tissue P concentrations considered adequate for maximum growth, providing additional evidence that proteoid roots can form on P-sufficient plants. There was an inverse relationship between the P i concentration in the youngest mature leaf and proteoid root formation. Citrate efflux and the activities of enzymes associated with citric acid synthesis (phosphoenolpyruvate carboxylase and malate dehydrogenase) varied along the proteoid root axis, being greatest in young proteoid rootlets of the 1-3 cm region from the root tip. Citrate release from the 0-1 and 5-9 cm regions of the proteoid root was only 7% (per unit root length) of that from the 1-3 cm segment. Electrical potential and 32 P i uptake measurements showed that P i uptake was more uniform along the proteoid root than citrate efflux.

Effect of phosphorus supply on the formation and function of proteoid roots of white lupin (Lupinus albus L.)

Effects of nitrogen nutrition on leaf expansion and photosynthesis of Trifolium subterraneum L. 1. Comparison between different levels of nitrogen supply.

ln an experiment with lucerne (Medicago sativa) and subterranean clover (Trifolium subterraneum), grown in solution cultures at two levels of phosphorus, each at five aluminium levels and pH 4, growth and aluminium transport to the leaves were comparable in both species up to the fourth aluminium level (40¦M All. The highest aluminium level (200 ¦M Al) caused a much greater growth depression in lucerne than in subterranean clover. In lucerne, aluminium concentrations in leaf dry matter peaked at the fourth aluminium level. Concentrations below this peak could indicate non-toxic as well as toxic supplies of aluminium. Leaf analysis, therefore, appears to be of little value in the detection of aluminium problems in lucerne. In subterranean clover leaves, aluminium concentrations increased to values in excess of 1 70 ppm of the dry matter at the highest aluminium level. In a second experiment, in which subterranean clover plants were grown in pots in two acid soils at two phosphorus x five aluminium levels, close, inverse, linear relations were found between relative yield (Al/no Al x 100) and leaf aluminium concentrations. These relations differed between soils, harvests and phosphorus levels, and would therefore be of little help in providing quantitative estimates of yield depressions in subterranean clover caused by aluminium. However, the lines all converged within the range of 100-1 50 ppm aluminium in leaf dry matter of plants in the no aluminium treatment, suggesting that this range represented a 'threshold' above which yield reductions due to aluminium occurred.

Relations between plant aluminium content and the growth of lucerne and subterranean clover: their usefulness in the detection of aluminium toxicities

A simple and rapid method is presented for the assessment of the phosphorus status of subterranean clover. The method rests on extracting fresh leaf tissue (400 mg) with five drops 10 N H2SO4 and measuring phosphorus in the filtered extract by a molybdenum blue colour method, visually or calorimetrically. No special skills or equipment are required and the method is therefore potentially suitable for use by advisers and farmers. Two standard blue colours would be sufficient for a visual separation between non-deficient, moderately deficient or severely deficient plants. The method was tested in early spring with leaf samples obtained from two field experiments (five phosphorus levels), a sand culture experiment, and a pot experiment with phosphorus deficient soil (five phosphorus levels in each). Close relations were obtained between relative yields (yields as a percentage of the maximum in each experiment) and extractable phosphorus (R2 = 0.93). The curve fitted to the relation had a clearly defined inflexion point, indicating a critical value of 150 ppm extractable phosphorus at 90% of the fitted asymptote for relative yield. The relation for total phosphorus and yields was not as close (R2=0.77) and the critical value not clearly defined. Extractable phosphorus was closely related to total phosphorus below the critical value for extractable phosphorus, but not above this value. Likely physiological are discussed.

Phosphorus status of subterranean clover: a rapid and simple leaf test

Effects of Nitrogen Nutrition on Leaf Expansion and Photosynthesis of Trifolium subterraneum L. II. Comparison between Nodulated Plants and Plants Supplied with Combined Nitrogen

Field experiments were carried out in south-eastern New South Wales to establish the relationships between three plant tests for phosphorus and for sulphur, and the yield responses of subterranean clover pastures to applied phosphorus and sulphur. Subterranean clover plants (CV. Mt. Barker), sampled early in the growing season of 1963 and on five occasions at approximately monthly intervals in 1964, were analysed for total and soluble phosphorus and for total and reducible sulphur. The third plant test involved a comparison of the leaf area responses measured seven days after transfer to appropriate nutrient solutions of clover plants sampled in the field plots at the beginning of each growing season. Correlation coefficients (R) varying from 0.546 to 0.908 were obtained for the curvilinear regression of relative yields on total phosphorus contents of the clover, but only under conditions of an adequate sulphur supply. The correlation coefficients for soluble phosphorus were generally lower and differed greatly between samplings. The correlation coefficients for the curvilinear regressions of yield responses on the total or reducible sulphur content of clover, under conditions of ample phosphorus supply, were never less than 0.606 at the end of the season, and for some of the earlier samplings were as high as 0.947. The correlation coefficients (r) between leaf area responses and yield responses to phosphorus were 0.576 and 0.716, and those for sulphur 0.710 and 0.692 in 1963 and 1964 respectively. In contrast to those based on leaf analysis, the relationships between leaf area responses and yield response for each one of the elements were not affected by the level of supply of the other element.

D Bouma

Papers

Effects of nitrogen nutrition on leaf expansion and photosynthesis of Trifolium subterraneum L. 1. Comparison between different levels of nitrogen supply.

Relations between plant aluminium content and the growth of lucerne and subterranean clover: their usefulness in the detection of aluminium toxicities

Phosphorus status of subterranean clover: a rapid and simple leaf test

Effects of Nitrogen Nutrition on Leaf Expansion and Photosynthesis of Trifolium subterraneum L. II. Comparison between Nodulated Plants and Plants Supplied with Combined Nitrogen

Assessment of the phosphorus and sulphur status of subterranean clover pastures. 3. Plant tests