The effects of water stress on variability in mesophyll conductance of loblolly pine (Pinus taeda L.) leaves
Climate models predict increased temperatures and changing precipitation regimes with a decrease in moisture availability to forests in the Southeastern US predicted by some models. Breeding for drought tolerance in Pinus taeda L. (loblolly pine) is important for continued productivity and potential mitigation of rising CO2 levels by carbon sequestration. Carbon stable isotope analysis has been used as a proxy for water use efficiency (WUE) in breeding and ecophysiology in a number of species. However, the most commonly used version of the Farquhar model does not include variability in mesophyll conductance to CO2 (gm). Recent studies have shown evidence that gm varies with several environmental conditions, suggesting limitations to the model as well as potential targets for selection in breeding for traits related to water use and carbon gain. We designed a full-factorial, randomized complete block greenhouse experiment to examine drought effects on the capacity of loblolly pine to vary gm. Five family clones were planted in pots with potting media and were subjected to different watering treatments—well-watered, moderate drought, and severe drought. gm was estimated from measurements of stomatal conductance, photosynthesis, and instantaneous carbon isotope discrimination (Δ13C) using a gas exchange system coupled to a cavity ring-down spectroscopy analyzer.
Two of the five planted clones had higher maximum photosynthesis and more negative bulk leaf δ13C values than the other three clones, consistent with genetically-based differences in leaf traits related to carbon gain and water loss through transpiration. Significant differences in estimated gm were also observed in relation to clonal varieties and drought treatments. Our work is broadly consistent with other findings related to variable gm and points to the need to modify how C isotope results are interpreted for P. taeda. We intend to extend our greenhouse findings to an assessment of carbon isotope discrimination in an existing field experiment with drought treatments. This work has advanced our understanding of the controls on gm and will now allow us to better assess the accuracy of our current carbon isotope models.