QUANTITATIVE GENETICS OF GROWTH AND DEVELOPMENT IN POPULUS - III - PHENOTYPIC PLASTICITY OF CROWN STRUCTURE AND FUNCTION

The whole-tree leaf area of a poplar derives from three different shoo t types, current terminal, sylleptics and proleptics. The response of these architectural components to two sharply contrasting environments was examined in two replicated plantations containing a cloned inters pecific hybrid pedigree of Populus trichocarpa and P. deltoides that i ncluded the original parents, two F-1 parents, and 375 F-2 genotypes. In the warmer, sunnier, and better-watered environment of interior Boa rdman, Oregon, 2-year-old trees produced more, larger, and wider leave s, as well as more and longer branches, than in the cooler and more cl oudy coastal conditions of Clatskanie, Oregon. Sylleptic branches were phenotypically more plastic than the other elements of the crown. Mos t trees produced significantly more and longer sylleptics in the near- optimal growth environment in Boardman than in the suboptimal environm ent of Clatskanie. In both environments, all crown traits displayed si gnificant genotype effects in the F-2 family, but for most traits, bro ad-sense heritabilities were statistically larger in Clatskanie (0.44- 0.78) than Boardman (0.28-0.70). For all traits except for the number of sylleptics, the values of across-environment genetic correlations w ere significantly larger than zero but less than one, indicating that nonparallel responses of genotypes to environment had led to significa nt genotype x environment interactions, although some genetic basis wa s shared between the two environments. A nonsignificant genetic correl ation of sylleptic number expressed in the two different environments suggests that the phenotypic plasticity of this trait is under strong genetic control. All architectural traits, except for branch and canop y traits of sylleptics, were correlated with growth more strongly in C latskanie than Boardman. In both environments, contrary to observation s in an earlier study, proleptic traits were better predictors of stem height and basal area growth than sylleptic traits.