YIELD, YIELD COMPONENTS AND SOURCE SINK RELATIONSHIPS IN WATER-STRESSED SUNFLOWER

Assimilate fluxes to/from various organs (J) of sunflower (Helianthus annuus L.) were estimated in order to examine the following hypotheses: (1) plants with small storage capacity in stems as a result of short stature are able to maintain greater J(head) with consequently larger floret survival than taller plants when exposed to water stress before anthesis; and (2) plants with long stems (i.e. greater capacity to store assimilates) are able to maintain a greater J(seed) than short-stature plants when exposed to water stress after anthesis. Assimilate fluxes were estimated using a budget accounting for labile carbohydrates, structural biomass and synthesis respiration, and maintenance respiration. Glasshouse-grown plants of three cultivars of different stature were compared under four water regimes. The reduction in floret number caused by water stress before anthesis was not related with J(head). This, together with a greater concentration of labile carbohydrate in the heads of stressed plants relative to the well-watered controls, indicated that the survival of florets of water-stressed plants was probably not limited by assimilate availability. Seed filling depended strongly on stem assimilates as indicated by a significant negative association between J(seed) and J(stem) in the seed-filling period. The apparent contribution of stem assimilates to seed yield was cultivar dependent but was not strictly associated with plant height. The implications for selection of cultivars with improved mobilisation of stem assimilate to seeds are discussed.