Studies of grain production in Sorghum bicolor (L. Moench). IV.* Some effects of increasing and decreasing photosynthesis at different stages of the plant's development on the storage capacity of the inflorescence

Abstract

Sorghum plants (cv. RS610) grown in field stands at two population densities were manipulated to increase the supply of assimilates (by removing neighbouring plants) at one of three developmental stages—10-15 days after floral initiation, 1 week prior to three-quarters anthesis, and 1 week after three-quarters anthesis. Post-initiation exposure increased the number of grains per inflorescence 1.8-fold and 3.5-fold in medium and high density populations respectively, but had relatively less effect on grain size. Higher grain number resulted largely from more grains per secondary branch in the lower part of the inflorescence. Neither of the post-heading exposure treatments influenced grain number, but the higher supply of assimilates resulted in larger grains at both densities. Differences at one density only between yield characteristics of plants exposed at the two times provide evidence of inter-plant competition for assimilates to the extent that the potential size of the grain may be affected. Shading (10% light transmission) of plants grown in a glasshouse, whether for 1 week at anthesis or during grain filling, reduced grain yield at maturity by the same amount as the immediate reduction at the end of the shading period. The experiment was unable to demonstrate changes in the potential size of grains resulting from the loss of assimilates at anthesis. There was substantial compensation for the loss by translocation from other plant parts. *Part III, Aust. J. Agric. Res., 26: 11 (1975).