Carbon Production of Sunflower Cultivars in Field and Controlled Environments. I. Photosynthesis and Transpiration of Leaves, Stems and Heads

Commercial cultivars of sunflower were grown either with adequate water in glasshouses, or in the field using stored moisture or with supplementary irrigation. Diurnal measurements of photosynthesis, transpiration, respiration and water use efficiency were made as leaves expanded and aged: several leaf positions of each cultivar and treatment were examined throughout the season. Responses to quantum flux density were also determined. Comparable treatments in the field and glasshouse gave similar results and any differences in gas exchange per unit leaf area among cultivars were very small. All leaves, regardless of position on the plant had the same age-determined pattern of gas exchange per unit leaf area. Rates peaked some 10-12 days after leaves were 5 cm2 and had fallen to 50% of these values 50 days later: the decline was slightly faster in field canopies. Instantaneous rates of photosynthesis were occasionally reduced in plants growing on stored moisture when leaf water potential fell below 1.0 MPa, but on a diurnal scale these reductions were small. Water use efficiency declined with leaf age though under saturating light the decline was only 13% in 60 days: efficiency was markedly reduced at quantum flux densities below 800 �E m-2 s-1. The contribution of heads and stems to photosynthesis and transpiration throughout grain growth is discussed. It is concluded that the water use efficiency of sunflower in the short term is similar to that of other C3 species in spite of the high rates of gas exchange of sunflower. On a diurnal basis, its characteristic of maintaining open stomata under conditions of high evaporative demand results in poor water economy. Sunflower appears to be set to maximize carbon fixation per unit leaf area almost regardless of conditions.