Peter J. Randall

Aluminum Tolerance in Wheat (Triticum aestivum L.) (II. Aluminum-Stimulated Excretion of Malic Acid from Root Apices).

We investigated the role of organic acids in conferring Al tolerance in near-isogenic wheat (Triticum aestivum L.) lines differing in Al tolerance at the Al tolerance locus (Alt1). Addition of Al to nutrient solutions stimulated excretion of malic and succinic acids from roots of wheat seedlings, and Al-tolerant genotypes excreted 5- to 10-fold more malic acid than Al-sensitive genotypes. Malic acid excretion was detectable after 15 min of exposure to 200 [mu]M Al, and the amount excreted increased linearly over 24 h. The amount of malic acid excreted was dependent on the external Al concentration, and excretion was stimulated by as little as 10 [mu]M Al. Malic acid added to nutrient solutions was able to protect Al-sensitive seedlings from normally phytotoxic Al concentrations. Root apices (terminal 3–5 mm of root) were the primary source of the malic acid excreted. Root apices of Al-tolerant and Al-sensitive seedlings contained similar amounts of malic acid before and after a 2-h exposure to 200 [mu]M Al. During this treatment, Al-tolerant seedlings excreted about four times the total amount of malic acid initially present within root apices, indicating that continual synthesis of malic acid was occurring. Malic acid excretion was specifically stimulated by Al, and neither La, Fe, nor the absence of Pi was able to elicit this response. There was a consistent correlation of Al tolerance with high rates of malic acid excretion stimulated by Al in a population of seedlings segregating for Al tolerance. These data are consistent with the hypothesis that the Alt1 locus in wheat encodes an Al tolerance mechanism based on Al-stimulated excretion of malic acid.

Characterisation of Al-stimulated efflux of malate from the apices of Al-tolerant wheat roots

Aluminium (Al) stimulates the efflux of malate from the apices of wheat (Triticum aestivum L.) roots (Delhaize et al. 1993, Plant Physiol. 103, 695–702). The response was five to tenfold higher in Al-tolerant seedlings than Al-sensitive seedlings and the capacity for Al-stimulated malate efflux was found to co-segregate with Al tolerance in a pair of near-isogenic wheat lines differing in Al-tolerance at a single dominant locus. We have investigated this response further using excised root apices. Half-maximal efflux of malate from apices of Al-tolerant seedlings was measured with 30 μM Al in 0.2 mM CaCl2, pH 4.2, while saturating rates of 2.0 nmol·apex−1·h−1 occurred with concentrations above 100 μM Al. The stimulation of malate efflux by Al was accompanied by an increase in K+ efflux which appeared to account for electroneutrality. The greater stimulation of malate efflux from Al-tolerant apices compared to Al-sensitive apices could not be explained by differences in the activities of phosphoenolpyruvate carboxylase or NAD-malate dehydrogenase. Several other polyvalent cations, including gallium, indium and the tridecamer Al13, failed to elicit malate efflux. Aluminium-stimulated efflux of malate was correlated with the measured concentration of total monomeric Al present, and with the predicted concentrations of Al3+ and AlOH2+ ions in the solution. Several antagonists of anion channels inhibited Al-stimulated efflux of malate with the following order of effectiveness: niflumic acid≈NPPB>IAA-94≈A-9-C>ethacrynic acid. Lanthanum, chlorate, perchlorate, zinc and α-cyano-4-hydroxycinnamic acid inhibited malate release by less than 30% at 100 μM while 4,4′-diisothiocyanatostilbene-2,2′-disulphonate (DIDS) had no effect. These results suggest that the Al3+ cation stimulates malate efflux via anion channels in apical cells of Al-tolerant wheat roots.

Characterization of a Phosphate-Accumulator Mutant of Arabidopsis thaliana

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.

Leaf litter ash alkalinity and neutralisation of soil acidity

Soil acidification is a major factor limiting the sustainability of agricultural production systems throughout the world. Liming may not always be economically possible and therefore alternative methods or complementary methods of amelioration are required. Leaf litter collected from several tree species was examined for ash alkalinity (as an estimate of organic anion content) and ability to ameliorate an acid soil. Ash alkalinity measured by titration of the ash and excess cation values obtained by calculation as the difference between cation and anion content were correlated. Values obtained by the latter method ranged from 247 cmolc kg-1 for Melia azedarach (white cedar) to 36 cmolc kg-1 for Eucalyptus globoidea (white stringybark). There was a significant linear correlation between ash alkalinity and the Ca concentration in the litter. When added to an acid soil (pH 4.04 measured in 0.01 M calcium chloride) and incubated for 8 weeks, leaf litter raised the pH. Species differed markedly with Melia azedarach having the greatest effect. The increase in pH was proportional to the quantity of ash alkalinity (organic anions) added, expressed as calcium carbonate equivalents. Aluminium levels on the exchange complex were lowered by treatment with leaf litter through direct precipitation of a solid phase and again Melia azedarach litter was most effective. There was also indirect evidence of organo-Al complexes affecting the concentration of monomeric Al in soil treated with litter from Liquidambar styraciflua (liquidambar), Quercus robur (English oak) and Pinus radiata (radiata pine).

The response of dryland canola to nitrogen fertilizer: partitioning and mobilization of dry matter and nitrogen, and nitrogen effects on yield components

Abstract Canola (Brassica napus) was grown under dryland conditions in field experiments at Greenethorpe (1988) and Canowindra (1989) in the cereal belt of New South Wales to determine (1) the response of the crop to nitrogen (N) fertilizer when grown late in a cropping sequence; (2) the seasonal course of dry-matter production and N accumulation; (3) the distribution of dry matter and N among plant parts, including shed leaves; and (4) the apparent mobilization of dry matter and N from stems and leaves to seeds. At both sites, maximum dry-matter production and seed yields occurred at 75 kg applied N ha−1. Seed yields increased from 2.3 to 3.5 t ha−1 at Greenethorpe, and from 0.85 to 2.5 t ha−1 at Canowindra. Topdressing with a single application of N at the 5–6 leaf rosette stage, flower buds visible or the start of flowering resulted in 70–90% of the seed yields obtained when the equivalent amount of N was applied pre-sowing. At maximum seed yield, canola accumulated 165 kg N at Greenethorpe and 110 kg N ha−1 at Canowindra. Averaged over both seasons and all N treatments, 52% of the N content of the mature plants accumulated before flowering and 50% of the dry-matter content of the mature plants accumulated during flowering. Maximum dry-matter and N contents for leaves occurred at the start of flowering, and for the stem at the end of flowering. Averaged over all N treatments at Greenethorpe, about 20% of the dry matter and 60–65% of the N was apparently mobilized from the stem and leaves, after flowering. The combined mobilization from the stem + leaves could have contributed to about 17% of the dry matter and 55% of the N accumulated by seeds. Amounts of dry matter and N lost in shed leaves ranged from 1-1.75 t and 10–30 kg ha−1, and N removal in seed ranged from 63–112 and 27–96 kg ha−1 at Greenethorpe and Canowindra, respectively. N concentrations in whole shoots and vegetative organs declined during the season, irrespective of the rate of N fertilizer applied. N fertilizer increased pod number per plant but had little effect on seed number per pod or 1000 seed weights. Seed oil concentrations were unaffected by the N rate at which maximum seed yield was obtained. N fertilizer rate had no effect on dry-matter harvest indices (mean both sites 30%) which, when expressed on the basis of the biosynthetic costs for straw and seed production, were comparable to those reported for wheat (37%). N harvest indices (mean both sites 76%) were reduced only at the highest N rates at Greenethorpe. Indices of N fertilizer use-efficiency generally decreased with increasing N fertilizer rate, and were similar to values reported for wheat when differences in the biosynthetic cost of grain production were taken into account.

Mineral Nutrition of Linseed and Fiber Flax

Publisher Summary This chapter describes the mineral nutrition of linseed and fiber flax. Flax is a temperate winter-growing annual ranging in height from 20 cm to over 100 cm depending on the cultivar. Seedling establishment is slow at the low temperatures experienced in winter in the Australian wheat belt, and the small seedling competes poorly with weeds. In Australia, linseed competes with temperate cereals, especially wheat, for a place in rotations based on subterranean clover pasture, and it also competes with rapeseed as a winter oilseed crop. Most of the reported research on the nutrition of linseed and fiber flax concerns N and P and, to a lesser extent, K. There is also a considerable, but often restricted, range of information on other nutrients. It is found that flax seeds are also very sensitive to P fertilizer, and yields are generally reduced by the application of more than 5 kg/ha of water-soluble orthophosphate P fertilizer placed with seeds, as it suppresses germination. It is observed that application of P fertilizer to linseed may increase seed oil concentration but usually it does not alter the iodine value of the oil.

cDNA and protein sequence of a major pea seed albumin (PA 2 : Mr≈26 000).

SummaryPea albumin 2 (PA2:Mr≈26000) is a major component of the albumin fraction derived from aqueous salt extracts of pea seed. Sodium dodecylsulfate-polyacrylamide gel electrophoresis and chromatography on DEAE-Sephacel resolve PA2 into two closely related components (PA2a and PA2b). A cDNA clone coding for one of these components has been sequenced and the deduced amino acid sequence compared with partial, chemically-determined sequences for cyanogen bromide peptides from both PA2 components. Complete amino acid sequences were obtained for the C-terminal peptides. The PA2 molecule of 230 amino acids contains four imperfect repeat sequences each of approximately 57 amino acids in length.The combined sequence data, together with a comparison of PA2-related polypeptides produced in vitro and in vivo, indicate that PA2 is synthesized without a signal sequence and does not undergo significant post-translational modification. Although both forms of PA2 contain Asn-X-Thr consensus sequences, neither form is glycosylated. Accumulation of PA2 contributes approximately 11% of the sulfur-amino acids in pea seeds (cysteine plus methionine equals 2.6 residues percent). Suppression of levels of PA2 polypeptides and their mRNAs in developing seeds of sulfur-deficient plants is less marked than that for legumin, in spite of the lower content of sulfur-amino acids in legumin.

Root Exudates in Phosphorus Acquisition by Plants

This chapter discusses the processes in the rhizosphere that are determined by exudates from roots and that in turn affect the availability of phosphorus to plants. These include control of rhizosphere pH, exudation of organic acids and root phosphatases. Possibilities for manipulating these processes in order to improve phosphorus acquisition in agricultural plants are discussed in the context of the phosphorus cycle in agriculture.

Sulphur nutrition of sunflower (Helianthus annuus) as affected by nitrogen supply: Effects on vegetative growth, the development of yield components, and seed yield and quality

Abstract The effects of sulphur and nitrogen nutrition on the growth and yield components of a hybrid confectionery sunflower (cv. Kernel) were investigated in a glasshouse experiment using controlled S and N supplies that ranged from deficient to more than adequate, and in which the S supplies of some plants were changed at defined stages of growth. S deficiency delayed floret initiation and anthesis but not seed maturity; N deficiency delayed all developmental phases including seed maturity. S and N deficiencies reduced plant height and leaf area. N deficiency caused a reduction in leaf number, but S deficiency did not. Both S and N deficiencies reduced yield by reducing the number of seeds per plant and by decreasing single seed weight. Oil concentration in seeds was the same for all levels of S supplied, but it was reduced by increased N supply. Changing the S supply at the end of floret initiation and at anthesis indicated that floret number and therefore seed number per plant were mainly established by the S suplly before the end of floret initiation. Single seed weight responded positively to increased S supply after floret initiation but only if the N supply was adequate; there was no response to increased S supply after anthesis. Concentrations of S and N in plant organs increased with increasing S and Nsupplies, but seeds were much less responsive than vegetative organs. Plants grown on high N but low S had the highest concentrations of N in their tissues, but the highest S concentrations did not occur in high S low N plants. Cysteine and methionine decreased by 30% in seeds of S-deficient but N-sufficient plants, whereas arginine increased by 34% compared with nutrient-adequate plants. We suggest that an adequate supply of S to young sunflower plants is required, particularly up to floret initiation, to obtain large floret numbers and maximum leaf area. An adequate supply of S is also required between the end of floret initiation and anthesis to prevent floret abortion and allow development of large seeds. Sulphur stress in sunflower during seed filling results in kernels with low levels of essential S-containing amino acids.

Aluminium tolerance in wheat: Analysis of polypeptides in the root apices of tolerant and sensitive genotypes

Proteins in root apices of wheat (Triticum aestivum L.) genotypes differing in Al tolerance were analyzed by two-dimensional gel electrophoresis. In vivo experiments using radiolabelled amino acids showed that at an Al concentration which would ultimately kill a sensitive cultivar (Egret) but not a tolerant cultivar (Carazinho) the synthesis of a range of polypeptides was induced in both cultivars. Although most of the constitutive and Al-induced polypeptides were common to both cultivars, some were found to be cultivar specific. Al tolerance in Carazinho is consistent with the presence of a major, dominant gene controlling the tolerance and it was hypothesized that a unique polypeptide encoded by this gene was present in Carazinho root apices. However, in progeny derived from Egret crossed to Carazinho and segregating for Al tolerance it was shown that none of the Carazinho specific polypeptides co-segregated with the Al tolerant phenotype.