THE EFFECT OF VAPOR-PRESSURE DEFICIT ON CARBON ISOTOPE DISCRIMINATION IN THE DESERT SHRUB LARREA-TRIDENTATA (CREOSOTE BUSH)

Larrea tridentata (creosote bush) seedlings were subjected to three regimens of atmospheric humidity in a growth chamber experiment. Relative humidity was varied to achieve daytime vapour pressure deficits (VPD) during growth of 2.9, 4.8 and 7.7 kPa. Photosynthetic gas exchange, carbon isotope composition and biomass production were measured after 8-10 weeks of treatment. Whereas stomatal conductance (g(s)) declined linearly with increasing ambient VPD, CO2 assimilation rate (A) was not measurably affected by changes in ambient VPD. This resulted in a decrease in intrinsic water use efficiency (ratio of CO2 assimilation to stomatal conductance; A/g(s)) with increasing VPD. Leaf carbon isotope discrimination (DELTA) was negatively correlated (r2 = 0.88) with A/g(s) ratios. Carbon isotope discrimination also correlated positively with ratios of internal (c(i)) to ambient (c(a)) CO2 levels determined by gas exchange measurements (c(i)/c(a)). The ratio of c(i) to c(a) was lower at higher VPD levels. Leaf biomass decreased with increasing ambient VPD and correlated positively with DELTA. Root to leaf biomass ratio increased at higher VPD levels and correlated negatively with DELTA.