Carbon Isotope Discrimination in Coffee Genotypes Grown under Limited Water Supply

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Abstract
Photosynthetic gas exchange, plant-water relations characteristics, and stable carbon isotope discrimination (.DELTA.) were evaluated for five Coffea arabica L. genotypes growing under two soil moisture regimes in the field. The .DELTA. of leaf tissue was strongly correlated (r = -0.95) with inherent water use efficiency (ratio of assimilation to stomal conductance; A/g). The variation in inherent water use efficiency (WUE) among genotypes was 30% for plants irrigated weekly. The higher WUE exhibited by some of these plants resulted from reduced g rather than increased photosyntehtic capacity at a given g. Withholding irrigation for 1 month caused .DELTA. to decline substantially in expanding leaf tissue of all genotypes. A strong correlation (r = 0.92) was found between .DELTA. and plant hydraulic efficiency estimated as the ratio of g to the diurnal range in leaf water potential (.PSI.L). The .DELTA. values for plants irrigated weekly adequately predicted drought-induced changes in .DELTA. (r = 0.99) and midday .PSI.L (r = 0.95). The results indicated that .DELTA. might be used to evaluate several aspects of plant performance and response to specific environmental conditions, once suitable background physiological data have been gathered.