The Drought Response of Physiological and Structural Traits in Loblolly Pine (P. taeda L.) Clones with a Focus on Mesophyll Conductance to CO_(2)
Wilson, Elizabeth Susan
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Climate change will likely affect the productivity of forests through changes in precipitation and moisture availability. An important measure of a plant?s ability to assimilate carbon in photosynthesis with limited water loss, water use efficiency (WUE), is assessed by carbon stable isotopes using the Farquhar model. However, recent work has shown that mesophyll conductance to CO_(2) (g_(m)) is affected by environmental conditions, and the simplified model does not take into account this variability. Variation in this parameter could decrease the effectiveness of the stable isotope tool. A coupled gas exchange and carbon isotope system was developed and tested on well-watered loblolly pine seedlings. A cavity ring-down spectroscopy (CRDS) laser connected to a LI-6400XT gas exchange system allowed simultaneous measurements of photosynthesis and instantaneous carbon isotope discrimination. The standard deviation of two minute averaged delta over ten minute intervals ranged from 0.31? to 0.51?. Mesophyll conductance measured by this system on well-watered loblolly pine seedlings ranged from -0.74 to 0.99 mol m^(-2) s^(-1) and averaged 0.07 ? 0.04 mol m^(-2) s^(-1). A greenhouse study was conducted of the drought effects on g_(m) and other physical and biochemical traits in three clones of loblolly pine (P. taeda L.). Stomatal and hydraulic sensitivity to drought was assessed in the clones. Mesophyll conductance was estimated using several methods: gas exchange curve fitting, the variable J method of chlorophyll fluorescence, carbon isotope discrimination from leaf soluble carbohydrates, and instantaneous carbon isotope discrimination using a cavity ring-down spectroscopy laser. The three clones exhibited plasticity in stomatal conductivity and hydraulic conductivity in response to drought. The fastest growing clone also had the highest hydraulic conductivity and lowest WUE in well-watered conditions. There were no significant clonal or drought effects on g_(m), suggesting that the simplified Farquhar model is adequate for the use of stable isotopes as a proxy for WUE.