PHYSIOLOGICAL DETERMINANTS OF GENOTYPIC DIFFERENCES IN CARBON-ISOTOPEDISCRIMINATION IN POTATO GROWN IN WELL-WATERED CONDITIONS

Carbon isotope discrimination (Delta), leaf conductance (g(s)), photos ynthetic capacity, and plant growth were measured in well-watered, gla sshouse-grown potato plants of clones from a cross made between diploi d Solanum tuberosum and Solanum vernei. Clones showed significant diff erences (P < 0.001) in g(s), Delta, stomatal density, root growth, and total dry matter production. Carbon isotope discrimination of genotyp es was positively correlated (P < 0.001) with g(s). There was no corre lation between g(s) and stomatal density indicating that differences i n g(s) reflected differences in stomatal aperture. Differences in root ing characteristics or in root/shoot ratio did not contribute to diffe rences in g(s) or Delta. Genotypic differences in photosynthetic capac ity were not statistically significant, and there was no correlation b etween Delta and photosynthetic capacity. Total dry matter production and Delta were positively correlated (P < 0.001) when differences in t he time of plant emergence were included in the regression model. It i s concluded that differences in Delta among potato genotypes is largel y determined by g(s), but confounding of g(s) and photosynthetic capac ity reduces genotypic variation in Delta compared with that in g(s). T otal dry matter production is largely determined by processes other th an carbon assimilation rate per unit area in individual leaves. Effect ive use of Delta as a character for selection in plant breeding depend s on elucidating the effects that differences in stomatal characterist ics have on crop production both in well-watered and in water-limited crops.