Dynamics of fruit growth and oil quality of sunflower (Helianthus annuus L.) exposed to brief intervals of high temperature during grain filling

Brief periods of high temperature have been shown to reduce grain weight and quality in temperate cereals but the nature of sunflower grain growth and oil quality responses to exposure to high temperature are unknown. We examined these issues by exposing the capitulae of plants of cv. HA89 growing at 25 degreesC to temperatures of ca. 35, 37 and 40 degreesC for seven consecutive days in each of the three different intervals during grain filling. Final grain weight was reduced (-40%, on average) by temperatures >35 degreesC during early grain development due to a reduction of the duration of grain growth. Oil grain percentage was also reduced (-30%) by temperatures >35 degreesC, largely through an increase in the pericarp:embryo ratio, although with extreme temperatures a reduction of embryo oil percentage was also observed. Final oil composition proved to be sensitive to the timing of heat stress. The fatty acid pattern was strongly modified during exposure to heat stress but final oil composition was only altered by exposure to stress in the period from 19 to 26 days after anthesis (daa). Early heat stress allowed partial recovery of oleate desaturase (ODS) activity in the post-stress period, but this recovery did not occur when the ending of exposure to high temperature took place close to the ending of the oil accumulation process. We conclude that the period from 12 to 19 daa showed the greatest sensitivity to heat stress regarding embryo and grain weight responses, whereas the period of greatest sensitivity for oil quality was from 19 to 26 daa. A clear step-change in response to temperature was found, indicating a threshold for grain growth responses at 35 degreesC. Brief periods of heat stress produced sunflower grain with a lower weight, a greater percentage of pericarp, a lower oil content and an altered fatty acid composition, and hence, of lower commercial quality. (C) 2003 Elsevier Science B.V. All rights reserved.