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Disturbance of cytoplasmic Ca 2+ homeostasis, leading to breakdown in Ca 2+ -mediated signal transduction processes, has been suggested as a primary mechanism of aluminium (Al) rhizotoxicity in plants. To test this hypothesis, Al-related changes in cytoplasmic free activity of Ca 2+ ions ([Ca 2+ ] c ) in root apical cells of near-isogenic wheat (Triticum aestivum L.) lines differing in Al tolerance at a single locus were examined by visualising the Ca 2+ -sensitive probe Fluo-3 with confocal laser scanning microscopy. Exposure of roots to 50 µM AlCl 3 (pH 4.2) led to an increase in [Ca 2+ ] c of root apical cells in both Al-sensitive (ES8) and Al-tolerant (ET8) wheat lines. An increase in [Ca 2+ ] c was greater in ES8 than in ET8; after 1-h treatment with 50 µM AlCl 3 an increase in [Ca 2+ ] c was 48 and 27 % in ET8 and ES8, respectively. An increase in [Ca 2+ ] c of ES8 roots, but not ET8 roots, was observed upon treatment with 2.6 µM AlCl 3 (pH 4.5). Al-related increases in [Ca 2+ ] c were correlated with inhibition of root growth. The Al-induced increase in the [Ca 2+ ] c was reversible upon removing AlCl 3 . These findings provide direct evidence to support the hypothesis that Al interactions with cytoplasmic Ca 2+ are involved in the Al toxicity syndrome in plants.

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A novel approach to the sustainable management of potassium (K) resources in agro-ecosystems is through better exploitation of genetic differences in the K efficiency of crop plants. Potassium efficiency is a measure of genotypic tolerance to soils with low potassium availability and can be quantified as the K efficiency ratio (the ratio of growth at deficient and adequate K supply). This study investigated the magnitude of variation in K efficiency among wheat (Triticum aestivum L.) genotypes grown in a glasshouse and in the field. Genotypes differed significantly in response to low soil K availability in terms of shoot biomass during the vegetative growth phase and grain yield at maturity under glasshouse (144 genotypes) and field (89 genotypes) conditions. K-efficient and K-inefficient genotypes were identified. The main factor determining K efficiency for grain yield was the capacity of genotypes to maintain a high harvest index (grain yield/total shoot weight) at deficient K supply. Genotypes that had reduced harvest index under deficient K supply were K-inefficient. Capacity to tolerate low concentrations of K in shoot tissue where K supply was deficient was also important in determining K efficiency for grain yield. Potassium-efficient genotypes have the potential to enhance the productivity and sustainability of cereal cropping systems.

The effect of colonization with the arbuscular mycorrhizal (AM) fungus Glomus mosseae (Nicol.& Gerd.) Gerdemann & Trappe on the growth and physiology of NaCl-stressed maize plants ( Zea mays L. cv. Yedan 13) was examined in the greenhouse. Maize plants were grown in sand with 0 or 100 mM NaCl and at two phosphorus (P) (0.05 and 0.1 mM) levels for 34 days, following 34 days of non-saline pre-treatment. Mycorrhizal plants maintained higher root and shoot dry weights. Concentrations of chlorophyll, P and soluble sugars were higher than in non-mycorrhizal plants under given NaCl and P levels. Sodium concentration in roots or shoots was similar in mycorrhizal and non-mycorrhizal plants. Mycorrhizal plants had higher electrolyte concentrations in roots and lower electrolyte leakage from roots than non-mycorrhizal plants under given NaCl and P levels. Although plants in the low P plus AM fungus treatment and those with high P minus AM fungus had similar P concentrations, the mycorrhizal plants still had higher dry weights, soluble sugars and electrolyte concentrations in roots. Similar relationships were observed regardless of the presence or absence of salt stress. Higher soluble sugars and electrolyte concentrations in mycorrhizal plants suggested a higher osmoregulating capacity of these plants. Alleviation of salt stress of a host plant by AM colonization appears not to be a specific effect. Furthermore, higher requirement for carbohydrates by AM fungi induces higher soluble sugar accumulation in host root tissues, which is independent of improvement in plant P status and enhances resistance to salt-induced osmotic stress in the mycorrhizal plant.

Aluminum (Al) toxicity is a key factor limiting plant growth and crop production on acid soils. Increasing the plant Al-detoxification capacity and/or breeding Al-resistant cultivars are a cost-effective strategy to support crop growth on acidic soils. The plasma membrane H+-ATPase plays a central role in all plant physiological processes. Changes in the activity of the plasma membrane H+-ATPase through regulating the expression and phosphorylation of this enzyme are also involved in many plant responses to Al toxicity. The plasma membrane H+-ATPase mediated H+ influx may be associated with the maintenance of cytosolic pH and the plasma membrane gradients as well as Al-induced citrate efflux mediated by a H+-ATPase-coupled MATE co-transport system. In particular, modulating the activity of plasma membrane H+-ATPase through application of its activators (e.g., magnesium or IAA) or using transgenics has effectively enhanced plant resistance to Al stress in several species. In this review, we critically assess the available knowledge on the role of the plasma membrane H+-ATPase in plant responses to Al stress, incorporating physiological and molecular aspects.