CONTRIBUTION OF PREANTHESIS ASSIMILATES TO GRAIN-FILLING IN IRRIGATED AND WATER-STRESSED SUNFLOWER CROPS .2. ESTIMATES FROM A CARBON BUDGET

Carbon budgets for the grain-filling period of irrigated and water-stressed sunflower crops were constructed using data from measurements of canopy carbon-dioxide exchange, root respiration, crop biomass and biomass composition. These budgets showed that carbon fixed during the pre-anthesis period contributed 56 and 79 g C m-2 ground surface, or about 15 and 27% of the total carbon (i.e. fixed C + respired C) uptake of the grain in irrigated and stressed crops, respectively. Carbon-budget estimates of pre-anthesis assimilate contributions to grain-filling were about 30% smaller than those obtained for the same crops using the much simpler labelled-carbon technique. Head and stem respiration rates, measured at weekly intervals, head temperature records, head biomass and biomass proximate composition data were used to define the carbon balance of the head and grain. Head synthesis respiration accounted for about one-half and one-third of total head respiration in irrigated and stressed crops, respectively. Estimated mean maintenance coefficients for the heads at 30°C were 15.0 and 18.7 mmolC molC-1 day-1 in irrigated and stressed crops, respectively. Daytime head temperature maxima exceeded ambient temperature maxima by about 10°C. This temperature regime may be crucial in determining the importance or the maintenance respiration component of total head carbon dioxide production. © 1990.