Photosynthetic and storage limitations to yield in Sorghum bicolor (L. Moench)

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Abstract
Under favourable growing conditions, the source-sink relationships in grain yield of sorghum were analysed in terms of capacities for net assimilation, of head storage, and of the transport system to move assimilates between source and sink. In four commercial hybrids (Dekalb E57, Pacific Goldfinger, Texas 610SR, and Texas 626) grown at three population densities (20.2, 40.4 and 80.8 plants m-1), the assimilates supply was varied by increasing or decreasing the radiation available per plant (by thinning or shading), the potential gram storage capacity was decreased by spikelet removal, and the transport system was reduced by incision of the culm, all manipulations being performed at anthesis. Decreasing the number of grains increased the size of those remaining in all cultivars at all population densities The degree of Increase was greatest for T626 and T610 and least for E57 Thinning increased the grain size In all cultivars, but only sufficiently to cause a significant increase In gram yield In T610 and T626 Reduction in the assimilate supply by shading decreased the gram size and yield in all cultivars. Reduction In the transport system had no significant effect on gram yield. These results showed that there was surplus capacity for storage and transport In all cultivars In T626, T6L0, and Goldfinger, all post-anthesis assimilate was stored as gram and grams could grow larger The yield was therefore completely source-limited In E57, however, not all port-anthesis assimilate was stored as gram, and these grams showed little capacity to grow larger, which suggests that the yield was partially limited by both source and sink.