Blue light differentially represses mesophyll conductance in high vs low latitude genotypes of Populus trichocarpa...

To explore what role chloroplast positioning might have in relation to latitudinal variation in mesophyll conductance (g_m) of Populus trichocarpa Torr. & Gray (black cottonwood), we examined photosynthetic response to different blue light treatments in six representative genotypes (three northern and three southern). The proportion of blue (B) to red light was varied from 0:100, 10:90, 20:80, 40:60, and 60:40 while keeping the total photosynthetic photon flux density constant. Mesophyll conductance was estimated by monitoring chlorophyll fluorescence in combination with gas exchange. Compared to the control (10% B), g_m was significantly lower with increasing blue light. Consistent with a change in chloroplast positioning, there was a simultaneous but reversible decrease in chlorophyll content index (CCI), as measured by foliar greenness, while the extracted, actual chlorophyll content (ACC) remained unchanged. Blue-light-induced decreases in g_m and CCI were greater in northern genotypes than in southern genotypes, both absolutely and proportionally, consistent with their inherently higher photosynthetic rate. Treatment of leaves with cytochalasin D, an inhibitor of actin-based chloroplast motility, reduced both CCI and ACC but had no effect on the CCI/ACC ratio and fully blocked any effect of blue light on CCI. Cytochalasin D reduced g_m by ∼56% under 10% B, but did not block the effect of 60% B on g_m, which was reduced a further 20%. These results suggest that the effect of high blue light on g_m is at least partially independent of chloroplast repositioning. High blue light reduced carbonic anhydrase activity by 20% (P<0.05), consistent with a possible reduction in protein-mediated facilitation of CO₂ diffusion.