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UBC Theses and Dissertations

Physiological basis of variation in mesophyll conductance of black cottonwood (Populus trichocarpa Torr. & Gray) Momayyezi, Mina


There is dramatic provenance level variation in tree species with geographically wide ranges. For example, in Populus trichocarpa Torr. & Gray, net photosynthesis (An) and stomatal conductance (gs) both increase with latitude of origin. This thesis reports a parallel cline in mesophyll conductance (gm) and explores its physiological basis. In addition to anatomical constraints, variation in gm should depend on chloroplast positioning, transmembrane CO₂ diffusion through aquaporins (AQPs), and biochemical facilitation of the CO₂↔HCO₃− equilibrium by carbonic anhydrase (CA), but evidence for the former has been lacking. I found that gm increases with latitude across 12 genotypes, as measured by chlorophyll fluorescence, and confirmed this pattern by the isotope discrimination method in six representative genotypes. Northern genotypes had greater CA activity. An inhibitor of CA, acetazolamide, reduced CA activity, gm, gs, chloroplast CO₂ concentration and An at normal CO₂ (400 µmol mol-¹), the latter being reversible at saturating CO₂. The relationship between CA activity and gm was similar whether the variation was inherent or inhibitor-induced. I then explored the role of chloroplast positioning in relation to gm, driven by the ratio of blue (BL) to red light supplied to leaves. Repositioning was manifested by a reversible decrease in chlorophyll content index (CCI), while actual chlorophyll content remained unchanged. Although gm was found to decrease as BL increased, and more so in northern genotypes, cytochalasin D, an inhibitor of chloroplast motility, blocked the effect of BL on CCI but not gm, suggesting that BL can mediate gm independently of repositioning. High BL reduced CA activity, consistent with a possible reduction in protein-facilitated diffusion, which might also involve AQPs. I found that the AQP inhibitor mercuric chloride reduces gm more in northern genotypes than in southern genotypes, both absolutely and proportionally, but also reduces CA activity. Although greater gm in high-latitude genotypes likely reflects contributions from several components of the liquid-phase diffusion pathway, this thesis draws particular attention to a major role for CA. Because gm is an equal or greater limitation on photosynthesis than gs, these findings may help direct crop improvement efforts to promote resource use efficiencies and yield.

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