Source-sink relationships and grain weight at different positions within wheat spike.

The response in growth and final weight of grains from basal, middle and apical spikelets to increases in assimilate availability was studied by modifying source-sink relationships during the grain growth period in a wheat cultivar Lok-1. The source-sink relationship was altered in 1500 plants by removing spikelets in various positions i.e. either apical, middle or basal spikelets were pinched off on the day of anthesis. The final grain weight, grain growth rate and grain filling period (duration) was significantly affected by increasing the source capacity per grain by trimming i.e. removing some spikelets. The final grain weight was positively significantly correlated with grain growth rate while it was significantly but inversely correlated with grain filling duration. A clear effect of position of grain in spikelet and its (spikelet) position on the spike could be visualized. The results imply that there is a possibility that the availability of photosynthates is limiting and does not fully satisfy grain growth requirements. It is suggested that in this cultivar during post-anthesis period, grain yield is source limited.