Relative phosphorus utilization efficiency, growth response, and phosphorus uptake kinetics of Brassica cultivars under a phosphorus stress environment

Plants grown in highly weathered or highly alkaline calcareous soils often experience phosphorus (P) stress but never a P-free environment. Thus, applications of mineral P fertilizers are often required to achieve maximum yield, but recovery of applied P fertilizers is notoriously low. Phosphorus deprivation elicits a complex array of morphological, physiological, and biochemical adaptations among plant species and genotypes to enhance P acquisition and utilization efficiency. Ten Brassica cultivars were grown hydroponically to investigate their relative efficiency to utilize deficiently (20-mu M) and adequately (200-mu M) supplied P, using Johnson's modified solution. Cultivars differed significantly (P < 0.001) in biomass accumulation. Orthophosphate concentration and uptake in shoot and root, absolute and relative growth rate, and P-utilization efficiency (PUE) were also significantly different among various Brassica cultivars. Root-shoot ratio and specific absorption rate were substantially increased in plants subjected to low P supply. Shoot and root dry-matter yield as well as total biomass production correlated significantly (P < 0.01) with their total P uptake and PUE. Cultivars, which were efficient in P utilization, were also efficient accumulators of biomass under adequate as well as deficient levels of P supply. As part of the study, kinetic parameters of P uptake were evaluated for six contrasting Brassica cultivars in PUE, grown in nutrient solution. The kinetic parameters related to P influx were maximal transport rate (Vmax), the Michaelis-Menten constant (Km), and the external concentration when net uptake is zero (Cmin). Lower Km and Cmin values were indicative of P-uptake ability of the cultivars, evidencing their adaptability to P-stress conditions. In another experiment, six cultivars were exposed to no P nutrition for 27 days after initial feeding on optimum nutrition for 14 days. All the cultivars retranslocated P from aboveground parts to their roots during growth in P-free conditions, the magnitude of which was variable in different cultivars. Phosphorus concentration at 41 days after transplanting was higher in developing leaves than developed leaves. Translocation of absorbed P from metabolically inactive sites to active sites in plants growing under P-stress conditions may have helped the tolerant cultivars to establish a better rooting system, which provided basis for tolerance against P-deficiency stress and increased PUE.