Nitrogen-limited growth of lettuce is associated with lower stomatal conductance

C assimilation (A) has been shown to limit the growth of young Lactuca sativa (lettuce) plants following an interruption in their external N supply. Further data from these plants were used to test two hypotheses: that N-limited growth of lettuce is associated with lower stomatal conductance (g(s)); and that reductions in g(s) result from adjustments to stomatal frequency or distribution. The photosynthetic characteristics, nitrate and organic N-concentrations, as well as epidermal and stomatal distributions, were determined in leaves of hydroponically grown lettuce plants supplied continuously with N or with N removed for up to 14 d. Although N-limited plants had lower maximum rates of A, comparisons at equivalent values of g(s) showed that A was not directly limited by organic-N but by g(s) Reductions in g(s) under N-limiting conditions did not associate with adjustments to stomatal frequency or distribution. Associations between plant N and A could arise either through stomata responding directly to signals induced by N deprivation or to increased CO2 partial pressure at the sites of carboxylation.