STOMATAL RESPONSES TO VARIABLE SUNLIGHT IN BUR OAK (QUERCUS-MACROCARPA MICHX) LEAVES WITH DIFFERENT PHOTOSYNTHETIC CAPACITIES

Interspecific comparisons of stomatal response to variable light have generally supported the hypothesis that stomatal adjustments are more rapid in plants with high photosynthetic capacity. We compared stomatal responses to variable light in bur oak (Quercus macrocarpa) leaves that had low photosynthetic capacity and high apparent anthocyanins levels (''red leaves'') with responses in green leaves. When incident sunlight was experimentally varied, stomatal conductance (g(s)) in red leaves was no less responsive than in green leaves. In both leaf types, g(s) declined at the same rate (18.9 mmol m(-1) s(-1) min(-1)) upon shading and increased at the same rate (13.4 mmol m(-2) s(-1) min(-1)) when reilluminated. The only difference in stomatal response in red leaves was an overall reduction in and compared with green leaves. In shade, net photosynthetic rates (A(net)) in red leaves were very low (ca. 2.3 mu mol m(-2) s(-1)) but still nearly 70% of full sun rates (3.2 mu mol m(-2) s(-1)). In contrast, A(net) in green leaves reduced by nearly 50% in shade (7.2 mu mol m(-2) s(-1)) relative to fun sun rates(l4.1 mu mol m(-2) s(-1)). Internal CO2 concentrations (C-i) were always ca. 50 mu L L(-1) higher in red than in green leaves. Upon reillumination, A(net) in red leaves immediately returned to preshade levels, while green leaves recovered more slowly, perhaps because of stomatal limitation, as indicated by reduced C-i. We concluded that, although rapid stomatal responses are often correlated with photosynthetic capacity among species, within Quercus macrocarpa, stomatal responses to rapidly varying Light are not related to leaf photosynthetic capacity.