Induction of new isoforms of superoxide dismutase and catalase enzymes in the flag leaf of wheat during monocarpic senescence.

Leaf senescence is a programmed cell death phenomenon and involves oxidative stress. Superoxide dismutase (SOD, EC 1.15.1.1) and catalase (CAT EC 1.11.1.6) activities were studied in the flag leaf of Triticum aestivum cv. Kundan at different stages of grain development. Both SOD and CAT activities showed a decline during monocarpic senescence. Three SOD isozymes were observed in the cytosol, of which one isozyme was observed in the chloroplasts as well. Mitochondria showed the presence of three low abundant SOD isoforms. Inhibitor studies revealed the cytosolic and chloroplastic isoforms to be Cu/Zn SODs. In mitochondria however, two isozymes were MnSOD and one of them appeared to be FeSOD. These isoforms present in the mitochondria increased in activity as senescence progressed. Three isoforms of CAT were observed in peroxisomes which responded differentially during monocarpic senescence. The changes in the kind and pattern of the antioxidant enzymes supported the ordered sequence of events during leaf senescence. This is the first report showing an increase in mitochondrial FeSOD activity during leaf senescence.

Lipid peroxidation is an early event in necrosis of wheat hybrid.

We previously reported enhanced superoxide anion generation in an F1 necrotic hybrid produced from normal parents (Khanna-Chopra et al., Biochem. Biophys. Res. Commun. (1998) 248, 712-715). Further investigation of the mechanism of necrosis shows the possibility of lipid peroxidation as an early event in the death of necrotic leaves. Lipid peroxidation resulting from the inability of free radical scavenging is often associated with cell death. In this study the accumulation of malondialdehyde, an end product of lipid peroxidation, was measured in hybrid leaves and those of the parents. Lipid peroxidation was higher in the hybrid leaves through out the leaf ontogeny. This was accompanied by increased membrane permeability. Cell viability measured by a TTC reduction test showed a significant correlation with conductivity. There was no apparent effect on photosynthetic pigments and maximum efficiency of PSII (Fv/Fm) until the appearance of necrotic lesions on the hybrid leaf. There seems to be a close relationship among lipid peroxidation, membrane permeability, and cell viability in the leaves undergoing necrosis. This suggests the possibility of a genetic mechanism whereby the scavenging of free radical is impaired, leading to enhanced lipid peroxidation and membrane permeability, resulting in necrosis and death of the hybrid leaves in wheat.

Drought induces many forms of cysteine proteases not observed during natural senescence.

Drought-induced senescence and natural senescence was characterised in the cowpea leaf, with a focus on cysteine proteases. Soluble protein content and ribulose 1,5-bisphosphate carboxylase (Rubisco) content declined as senescence progressed. Endopeptidase activity with Rubisco as a physiological substrate exhibited significant increase at acidic (pH 4.8) than at neutral (pH 7.0) during drought induced senescence and declined during recovery. Natural senescence was associated with a several-fold increase in the endopeptidase activity at both the pHs. Cysteine proteases were analyzed using western blot with polyclonal antibodies raised against papain. Several polypeptides of molecular weights 57, 52, and 43 kDA were recognized by the antibodies, the levels of which showed an increase under water deficit conditions, followed by a decrease during recovery. Three polypeptides of molecular weights 69, 60, and 48 kDa appeared only during the water stress conditions, whereas, during natural senescence, only a single 48 kDa polypeptide with maximum intensity at 9 days after flowering was observed. The results suggests the possibility of distinguishing drought-induced and natural senescence.

A genetic system involving superoxide causes F1 necrosis in wheat (T. aestivum L.).

A genetic system in wheat is described in which F1 produced by crossing a drought tolerant cultivar C306 and high yielding cultivar WL711 exhibits leaf necrosis leading to the death of the plant. The mechanism underlying hybrid necrosis is not yet known. The hybrid exhibited a higher level of superoxide anion compared to the healthy leaves of parents at similar developmental stages. This increase in superoxide generation preceded necrotic lesion formation and displayed a gradient from the leaf tip to base. The leaf tip where necrotic lesions make their first appearance exhibited a higher level of superoxide compared to the base. Superoxide anion thus appears to play a vital role in necrosis of leaves in F1 hybrid. This genetic system can be a model system for understanding cell death in higher plants.

A Simple Technique of Studying Water Deficit Effects on Nitrogen Fixation in Nodules without Influencing the Whole Plant.

Cowpea (Vigna unguiculata L. Walp cv C-152) plants were grown in a system in which watering was withheld from the soil zone containing nodules, while the plants were able to maintain normal water status. The system was developed in a pot by making two soil zones, an upper and a lower separated by a gravel column between these two zones. Plants extended their roots into the lower layer of soil and were able to absorb water. The dry matter accumulation, photosynthesis rate, and leaf area development of the plant were not affected when the upper soil zone was dried, but the water potential of the nodules was lower than in the nodules in fully irrigated pots. Nitrogenase activity in the nodules obtained from plants stressed in the upper zone only was lower than in nodules obtained from fully irrigated plants. The present technique is helpful in distinguishing the direct water stress effects on nitrogen fixation compared to those mediated via photosynthate availability.

Photosynthesis, photosynthetic enzymes and leaf area development in relation to hybrid vigour in Sorghum vulgare L.

Heterotic hybrids of sorghum produced more dry matter than their respective parents. Therefore, an analysis of leaf are development, rate of photosynthesis and activities of RuBP carboxylase and PEP carboxylase was made to determine whether the superior dry matter production in the hybrids could be attributed to any of these characteristics. Heterosis in leaf area was maintained at all stages in plant growth. Heterosis in photosynthesis was observed only during grain development in certain hybrids. At all other stages, the photosynthesis rate in hybrids were either intermediate or similar to one of the parents. No heterotic effect was observed in enzyme activity at any stage of growth. It is suggested that a multiplicative interaction between the heterotic leaf area and photosynthesis rate could possibly explain heterosis in dry matter production in heterotic hybrids.

RuBP carboxylase activity in wheat following ear emergence and evaluation of the role of ear.

Ribulose bisphosphate carboxylase activity and chlorophyll content were measured in the leaves and reproductive parts of two wheat cultivars after ear emergence. The chlorophyll content of the flag leaf was mostly higher than that of the awns and glumes. Awns had the highest chlorophyll content among reproductive parts. Light transmission to the lower leaves was higher in the dwarf cultivar Moti than in the medium tall cultivar Kalyansona. RuBP carboxylase activity in Kalyansona leaves was higher than in Moti leaves. In postanthesis stages there was no difference in RuBP carboxylase activity in the flag leaf and lower leaves between cultivars. Awns had the maximum activity of RuBP carboxylase followed by glumes and grains among ear parts. The relative capacity for photosynthesis in the ear parts was several times higher than in the flag leaf on a unit chlorophyll basis. It is suggested that in a crop canopy in the field, the spike(ear) may have a greater importance in grain development than has been previously estimated.