Showing papers on "Plant physiology published in 1988"

Some aspects of aluminum toxicity in plants

Abstract: Aluminum toxicity is a major factor in limiting growth in plants in most strongly acid soils. Toxic effects on plant growth have been attributed to several physiological and biochemical pathways, although the precise mechanism is still not fully understood. In general, root elongation is hampered through reduced mitotic activity induced by Al, with subsequent increase in susceptibility to drought. The initial site of uptake is usually the root cap and the mucilaginous secretion covering the epidermal cells. Al ions bind very specifically to the mucilage by exchange adsorption on the polyuronic acid, complexing with the pectic substances and by the formation of polyhydroxy forms, increasing the number of Al atoms per positive charge. Toxicity has been suggested to be initiated at the sites of mucopolysaccharide synthesis. Al is absorbed on all Ca-binding sites on the cell surface. In the intact tissues, most of the Al is bound to the pectic substances of the cell wall and a part to the nucleic acids and cell membrane. Al is also reported to enter the plant by moving into meristematic cells via the cortex, bypassing the endodermal barrier. Being a polyvalent cation, it follows principally the apoplasmic pathway of transport through cortical cells, but may also enter the stele through the plasmalemma. Ultrastructural studies have shown the maximum accumulation to be in the epidermal and cortical cells. The interaction of Al with different systems follows different pathways. The plasma membrane at the outer boundary of the root cell is a potential target and its physical properties can be altered by Al through interaction with membrane-bound ATPase, lipids, carbohydrates and proteins. The Golgi apparatus has been suggested as the primary site of action, followed by damage to the plasmalemma. Aluminum interferes with the uptake, transport and use of several essential elements, including Cu, Zn, Ca, Mg, Mn, K, P and Fe. Excess of Al reduces the uptake of certain elements and increases that of others, the patterns being dependent on the element, the plant part and species involved. A major factor is the pH concentration. At an acid pH, below 5.5, the antagonism between Ca and Al is probably the most important factor affecting Ca uptake by plants. The molecular mechanism of tolerance of Al is as yet not clear. Tolerant plants reduce the absorption by the root or detoxify Al after absorption. Al tolerant plants may be grouped into those with higher Al concentrations in tops and those with less. In the latter, more Al is entrapped in roots. Uptake of Al may be reduced by binding to cell wall or to membrane lipid. Tolerance may be different in different species and seems to be controlled by one or more genes. Absorption of Al in non-metabolic conditions is affected only slightly by temperature. Anaerobic conditions, like the presence of nitrogen and metabolic inhibitors, damage the endodermal membrane barrier, increasing the uptake and enhancing injurious effects. Aluminum also causes morphological damage to plant parts. It affects photosynthesis by lowering chlorophyll content and reducing electron flow. Reduced respiratory activity might be due to reduced metabolic energy requirement. Protein synthesis is decreased probably due to effect on ribosome distribution at endoplasmic reticulum. Aluminum is known to bind to DNA and nuclei. However, its penetrance to DNA of mitotically active centers is slow. On accumulating in roots, it initially inhibits mitotic activity, possibly through affecting the integrated control function of the root meristem. Aluminum toxicity in acid soil is of special importance due to the destruction of components of forest ecosystems under specific conditions. It reduces biomass yield and tree growth and represses litter-degrading microflora. Further information is required on the factors affecting membrane permeability, distribution and accumulation of Al in different plant parts and different species. Al tolerance may be studied with relation to the presence of different ligands, nitrogen metabolism (nitrate reductase and protein accumulation), nitrogen tolerance in relation to pH change and metal ion activities, the role of Ca and P and interference with water relations and litter degradation.

Effect of light intensity on root growth, mycorrhizal infection and phosphate uptake in onion (Allium cepa L.)

Abstract: It is well established that the growth responses of plants to mycorrhizal infection are influenced by the amount of phosphorus (P) supplied in the soil. In addition previous work has shown that at low light intensity (LI) plants have lower rates of root growth and smaller responses to mycorrhizal infection than plants grown at high LI (Bethlenfalvay and Pacovsky, 1983; Daft and El-Giahmi, 1978; Hayman, 1974; Tester et al., 1985). The decrease in growth enhancement of roots at low LI’s results from a higher investment by the plant in its leaves (Bethlenfalvay and Pacovsky, 1983). This adjustment in source-sink relationships follows decreases in photosynthesis with decreasing LI. However, in mycorrhizal plants there is a further confounding factor: the increased demand on the host plant for carbohydrates used by the fungus.

Carbon dioxide exchange rates in soybeans during podfilling as a function of potassium supply

Abstract: Photosynthetic and dark respiration rates of single leaflets and whole plant canopies were measured during podfilling of soybean plants that were grown under low and high K regimes. Dark respiration rates of detached seed from these plants were also determined during the latter part of seed development. The study was carried out to test the hypothesis that low oil/protein ratios of seed from K-deficient plants resulted from the reduction in carbon availability within the plant, as a result of lower carbon assimilation rates and higher rates of respiratory carbon loss. Photosynthetic rates of upper canopy leaflets during early podfilling were depressed under K deficiency but this effect did not occur with whole plant canopies. In fact, towards the latter part of the podfilling period canopy photosynthetic rates were higher in K-deficient plants as nitrogen was exported earlier from the leaves in high-K plants, resulting in earlier leaf senescence in these plants. The level of K supply had no consistent effect on dark respiration rates of single leaflets or plant canopies, and had no effect on CO2 evolution rates from detached, developing seed. The findings do not substantiate the hypothesis that reduced photosynthetic efficiency or enhanced respiratory carbon loss are responsible for lower oil/protein ratios in seed from K-deficient soybean plants.