Photosynthesis and Carbon Partitioning / Source-sink Relationships

Photosynthesis is one of the principle biochemical processes underpinning plant growth and development. Because of its basic nature, it is intimately involved with reproductive growth and determining crop yields. Photosynthesis of a crop canopy can be broken down into three components: 1) leaf area development, 2) photosynthetic rate per leaf area, and 3) partitioning assimilates between vegetative and reproductive growth, or source-to-sink relationships (Krieg, 1983). The leaf surface area intercepts the solar radiation and allows for the photosynthetic conversion of that radiant energy into chemical energy. This production of chemical energy and the subsequent use of that chemical energy use to fix CO2 into photosynthetic carbon assimilates constitutes the source side of yield development. The fruiting buds, flowers, and fruit development constitute the reproductive sink side of the yield equation, although other vegetative growing points can operate as secondary sinks. This interplay between the vegetative source and the reproductive sink can influence crop photosynthesis because the capacity for carbon assimilation can be somewhat regulated by the utilization of those photoassimilates in many crops (Krieg, 1983). The scope of this review chapter is to examine the intimate but complex relationship cotton photosynthesis (source) has with flowering and yield development (sink). The complexity occurs because major interactions change as the boll load increases with day length, temperature, and water availability decreasing as the season progress.