Genetic effects on transpiration, canopy conductance, stomatal sensitivity to vapour pressure deficit, and cavitation resistance in loblolly pine

Physiological uniformity and genetic effects on canopy-level gas-exchange and hydraulic function could impact loblolly pine (Pinus taeda L.) plantation sustainability and ecosystem dynamics under projected changes in climate. Over a 1-year period, we examined genetic effects on mean and maximum mid-day canopy conductance (Gs, Gsmax) and transpiration (E, max-E) within a juvenile loblolly pine plantation composed of 'genotypes' (e.g. different genetic entries) from each of the three different genetic groups (clones, full-sibs, open-pollinated). We also compared reference canopy conductance (Gs-ref or Gs at a vapour pressure deficit (D) = 1 kPa), maximum E (Emax) in response to D, stomatal sensitivity to D, specific hydraulic conductivity (ks), and cavitation resistance among genotypes. Based on genetic and physiological principles, we hypothesized that (1) within genotypes, physiological uniformity will increase as inherent genetic diversity decreases and (2) genotypes with greater ks and higher canopy-level gas-exchange rates will be more sensitive to increases in D, and more susceptible to loss of ks. In our results, high- and low-genetic diversity genotypes showed no differences in E and Gs uniformity over time. However, E and max-E were significantly different among genotypes, and genotypes showed significant seasonal variability in Gs and Gsmax. Additionally, there were significant differences in Emax, Gs-ref, Gs sensitivity to D, and the pressure at which 50% loss of ks occurs (P50) among individual genotypes. We found no relationship between mean hydraulic conductivity parameters and overall Gs-ref or Gs sensitivity. However, the genotype full embolism point (P88) and loss of ks rate (LCrate) both showed a significant positive relationship with genotype Gs-ref during the spring, indicating that genotypes with higher Gs were less resistant to cavitation. Overall, genetic effects on canopy-level gas-exchange and cavitation resistance were significant, implying that physiological differences among genotypes might affect stand water use, carbon gain, drought tolerance, and hydrologic processes. Contrary to our expectations, uniformity in physiological process rates did not increase as inherent genetic diversity decreased, suggesting that clonal genotypes exhibit high physiological plasticity under plantation conditions. Lastly, our results imply that genotypes with higher spring-time gas-exchange rates may be more susceptible to catastrophic loss of ks. With changes in climate expected to continue, physiological differences among genotypes may affect loblolly pine plantation carbon and water cycling.