Variation in phosphorus uptake efficiency by genotypes of cowpea (Vigna unguiculata) due to differences in root and root hair length and induced rhizosphere processes

The lengths of roots and root hairs and the extent of root-induced processes affect phosphorus (P) uptake efficiency by plants. To assess the influence of variation in the lengths of roots and root hairs and rhizosphere processes on the efficiency of soil phosphorus (P) uptake, a pot experiment with a low-P soil and eight selected genotypes of cowpea (Vigna unguiculata (L) WALP) was conducted. Root length, root diameter and root hair length were measured to estimate the soil volume exploited by roots and root hairs. The total soil P was considered as a pool of Olsen-P, extractable with 0.5 M NaHCO3 at pH 8.5, and a pool of non-Olsen-P. Model calculations were made to estimate P uptake originated from Olsen-P in the root hair zone and the Olsen-P moving by diffusion into the root hair cylinder and non-Olsen-P uptake. The mean uptake rate of P and the mean rate of non-Olsen-P depletion were also estimated. The genotypes differed significantly in lengths of roots and root hairs, and in P uptake, P uptake rates and growth. From 6 to 85% of total P uptake in the soil volume exploited by roots and root hairs was absorbed from the pool of non-Olsen-P. This indicates a considerable activity of root-induced rhizosphere processes. Hence the large differences show that traits for more P uptake-efficient plants exist in the tested cowpea genotypes. This opens the possibility to breed for more P uptake-efficient varieties as a way to bring more sparingly soluble soil P into cycling in crop production and obtain capitalisation of soil P reserves.