KERNEL MASS AND ASSIMILATE ACCUMULATION OF WHEAT - CULTIVAR RESPONSES TO 50-PERCENT SPIKELET REMOVAL AT ANTHESIS

Removal of spikelets at anthesis from primary culms of soft red winter wheat (Triticum aestivum L.) decreased kernel number and induced a range of compensatory growth responses for kernels on the primary culm. Cultivars with kernels on the primary culm that failed to increase in mass were classified as nonresponsive to 50% spikelet removal (SR), while those that increased were classified as responsive. With SR from only the primary culm, differences in assimilate synthesis and export from the primary culm to tillers with intact spikes might explain differential compensatory growth responses for kernels on the primary culm. Consequently, effects were compared for SR from only the primary culm and/or all culms on assimilate levels and kernel mass of two nonresponsive and four responsive cultivars. For nonresponsive cultivars, growth of kernels on the primary culm and tillers was unaffected by either SR treatments. For responsive cultivars, growth of kernels on the primary culm was significantly increased (9 to 23%) to a similar extent for both SR treatments, but growth of tiller kernels increased (10 to 26%) only when all spikes received SR treatment. Apparently, growth of primary-culm kernels after SR was unaffected by the reproductive-sink size of tillers. At maturity, the total water soluble carbohydrate content of stem tissues from both nonresponsive and responsive cultivars was increased by SR, but the higher levels of assimilate were associated with greater kernel growth of only responsive cultivars. Enhanced availability of assimilates and differential kernel responses caused by SR indicate kernel growth of nonresponsive cultivars was sink-limited, while kernel growth of responsive cultivars was initially source-limited and later sink-limited once maximum kernel growth was reached.