Genotypic variability of pigeonpea in distribution of photosynthetic carbon at low phosphorus level

Pigeonpea (Cajanus cojan L. Millsp.) is an important grain protein crop which thrives in the P-deficient soils of tropical and subtropical Asia. The varietal differences in soil phosphorus uptake and partitioning of the crop are well documented, but the mechanism of carbon distribution between the Source and sink at low P supply is not understood. In the present Study, the effect of a low P Supply On the distribution of photosynthetic carbon of one high-yielding improved hybrid cultivar. ICPH 8 and two low-yielding non-hybrid cultivars, ICPL 87 and UPAS 120, were compared by feeding the plants (CO2)-C-13 at the vegetative and reproductive stages of development. A low P supply significantly decreased plant biomass, P uptake, average leaf area and photosynthetic activity of all genotypes the decrease was smallest in ICPH 8 and largest in UPAS 120. ICPH 8 retained more plant biomass at low P Supply due to higher P-absorption capacity rather than high P-utilization efficiency. The tolerance of ICPH 8 to low P is mostly related to its maintenance of a high leaf area and rate of photosynthesis. In contrast, P deficiency severely reduced leaf initiation of the sensitive cultivar UPAS 120, and reduced its photosynthesis. Under limiting P supply, partitioning of C-13 increased in favor of the roots in all cultivars, but the resistant cultivars ICPH 8 partitioned less 13C to roots, and invested more in shoot growth compared to the non-hybrid cultivars. similarly, partitioning of 13C to the raceme was greater in ICPL 87 than in the highly sensitive UPAS 120. It is concluded that P deficiency affects primarily sink activity by restricting cellulose biosynthesis in file meristematic sink cells. (C) 2003 Elsevier Ireland Ltd. All rights reserved.