SummaryThe expansion of individual leaves was examined in 19 genotypes of potato (Sofunum tuberosumL.) grown either with irrigation or droughted from the time of plant emergence. Drought reduced the thermal time from emergence to leaf appearance in 10 genotypes but had no significant effect in the other genotypes. In all the genotypes, final size of leaves was reduced by drought, but the magnitude of the effect differed significantly (P<0.001) with genotype. In the droughted treatment, the final size of leaves was correlated (P<0.001) with the maximum rate of leaf expansion suggesting that reductions in final size of leaves were the result of reduced expansion rate rather than of effects on the duration of expansion.Both the constant and the slope of the relation between leaf expansion rate and soil moisture deficit differed between genotypes. Osmotic adjustment was limited, maximally 0.16 MPa, and did not correlate with the ability to maintain leaf expansion with increasing soil moisture deficit.