Chapters authored
The Shade Avoidance Syndrome Under the Sugarcane Crop
Part of the book: Crop Plant
Acid Phosphatase Kinetics as a Physiological Tool for Assessing Crop Adaptability to Phosphorus Deficiency
Acid phosphatases (APase) exuded from the roots is important in mobilizing organic phosphate in the soil . Enzyme kinetics can provide reliable physiological markers to detect the potential for superior plant performance under low P . Kinetic constants for the secreted APase could be used as an early physiological indicator for P stress tolerance in legumes, Desmodium tortuosum , Phaseolus vulgaris,Vigna unguiculata and Crotalaria juncea were grown from seed in +P and -P nutrient solutions and plants were harvested during the early vegetative phase in order to collect the root exudates in vivo and for dry biomass, leaves soluble Pi, and total P in the dry biomass. Root surface Na-soluble APase was extracted from +P and -P grown plants by incubating three intact plants in beakers with their roots immersed in a 0.1 M NaCl solution. Secreted APase was obtained with the roots of three plants individually immersed in a dialysis tube (12 kD) containing NaCl 100 mM and then transferred to a recipient containing 3L of the same solution. Kinetic constants Km and Vmax were determined using a range substrate (p-NPP)concentration (S). Activity (v) was expressed as µmoles PNP/h per g root fresh (FWr) or dry weight DWr. Graphical representations were used for the determination of the Km and Vmax: Linewaver-Burk double reciprocal plot 1/v vs. 1/S plot; Hanes-Wolf plot S/v vs. S and Woolf-Augustinsson-Hofstee plot v vs. v/S. The first visual indication of P deficiency was a reduction in leaf area and dry biomass and a higher soluble Pi in the leaves of +P plants. Activity was higher in -P plants at the beginning of the growth period and the proper timing for the onset of the P-stress was apparently crucial for the induction of APase. For Phaseolus vulgaris Km values apparently indicate the lack of phosphate starvation-inducible APase and a higher Vmax in -P plants; however, with the combination of a high Km with a high Vmax plant behaviour could be improved under P deficiency. In Vigna unguiculata the low Vmax in -P plants may be compensated for by its lower Km. Crotalaria juncea showed considerably greater kinetic diversity, but Km was lower in -P plants. The practical implications of Km and Vmax are explained in terms of the potential for P-liberation under limiting Pi ; to be efficient an increase in Pi uptake is likely to occur if the APase released has a low Km (in the neighborhood of the soil P concentration) and a high Vmax as found for Desmodium, Phaseoulus and Vigna. The Km provided a means of comparing the enzyme from high or low-P plants indicating that Km is a reliable physiological tool for assessing plant adaptability to P-deficiency and it is suggested that Km, Vmax with total leaf area and relative growth rate (RGR).
Part of the book: Plants for the Future