Family variation in photosynthesis of 22-year-old black spruce: a test of two models of physiological response to water stress

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Abstract
Gas exchange and water potential were measured in 22-year-old black spruce (Piceamariana (Mill.) BSP) trees from four full-sib families on two sites (one drier and one wetter) at the Petawawa National Forestry Institute, Ontario. Based on an observed genotype × environment interaction and earlier work with seedlings, a hypothesis was formed that at high soil moisture availability, no genetic differences in net photosynthesis (Pn) would exist and as soil moisture decreases, genetic differences in Pn would increase. From results of initial research with mature trees we formed an alternative hypothesis that genetic differences in Pn are constantly maintained under an array of soil moisture conditions. The two models were rigorously tested over a range of soil moisture conditions using two physiological measurement crews who switched sites throughout the day. Second-year foliage Pn of mature black spruce was more affected by nonstomatal limitations than by stomatal limitations. Progeny of one female had 12.5% and 7.4% higher Pn than progeny of another female on the dry and wet site, respectively. Genetic variation in Pn was consistent over a range of soil water potential. Thus, the first hypothesis was rejected in favor of the alternative hypothesis. Genetic variation in Pn appeared to be due to differential response to vapor pressure deficit. Suggestions as to how to reconcile the observed genotype × environment interaction in growth with the genetic differences in Pn are discussed.
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