title: Blue light differentially represses mesophyll conductance in high vs low latitude genotypes of Populus trichocarpa Torr. & Gray.
abstract: To explore what role chloroplast positioning might have in relation to latitudinal variation in mesophyll conductance (gm) 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), gmwas 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 gmand 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 gmby ∼56% under 10% B, but did not block the effect of 60% B on gm, which was reduced a further 20%. These results suggest that the effect of high blue light on gmis 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 CO2diffusion.
authors.affiliation: Department of Forest and Conservation Sciences, Faculty of Forestry, University of British Columbia, Forest Sciences Centre, 2424 Main Mall, Vancouver, BC V6T 1Z4, Canada.
authors.forename: Mina
authors.initials: M
authors.lastname: Momayyezi
authors.affiliation: Department of Forest and Conservation Sciences, Faculty of Forestry, University of British Columbia, Forest Sciences Centre, 2424 Main Mall, Vancouver, BC V6T 1Z4, Canada. Electronic address: rob.guy@ubc.ca.
authors.forename: Robert D
authors.initials: RD
authors.lastname: Guy
mesh_terms: D002734:Chlorophyll / Q000378:metabolism; D002736:Chloroplasts / Q000378:metabolism / Q000528:radiation effects; D005838:Genotype; D008027:Light; D058503:Mesophyll Cells / Q000378:metabolism / Q000528:radiation effects; D010788:Photosynthesis; D018515:Plant Leaves / Q000378:metabolism / Q000528:radiation effects; D018526:Plant Transpiration / Q000528:radiation effects; D032107:Populus / Q000378:metabolism* / Q000528:radiation effects*
chemical_list: D002734:Chlorophyll
keywords: Black cottonwood; Chloroplast repositioning; Internal conductance; Photosynthesis