Factors affecting the relationship between carbon isotope discrimination and transpiration efficiency in blue oak (Quercus douglasii)

Abstract

In a greenhouse study, variation in plant and leaf transpiration efficiency(W, A/E)and carbon isotope discrimination (D) were assessed forQuercus douglasii Hook & Arn. (blue oak) from a‘wet’ (930 mm) and a ‘dry’ (500 mm) site. Plants weregrown at 75–100% (wet) and 50–75% (dry) of fieldcapacity. Family variation masked population level differences, and twocontrasting patterns emerged. The expected pattern was observed with drytreatment plants having lower leaf internal CO ₂concentration (c _i ) and D, andhigher W andA/E. For families with verylarge increases in plant size from dry to wet treatments, however, wettreatment plants had lower c _i andD, and higher W andA/E, reflecting a greaterinfluence of the maximum assimilation rate (A_max ) compared with stomatal conductance(g _s ) on c_i . In addition, large within-population variation inplant size appears to have affected both n (the vapor pressure deficit) andø _c (a measure of respiredcarbon). Lower n for large plants may be due to the higher rates of stomatalconductance and greater leaf cooling. Values of ø_c appear to have increased for plants below 2.5 g (drymatter, DM). Variation in these factors would directly affect the relationshipbetween W and D. This study illustrates a case where significant within-population variation can occur in the relative effect ofg _s and A_max on c _i .In addition this study indicates that ø_c may not be a constant for a species.