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 shoot 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 interspecific hybrid pedigree of Populus trichocarpa and P. deltoides that included the original parents, two F₁ parents, and 375 F₂ genotypes. In the warmer, sunnier, and better-watered environment of interior Boardman, Oregon, 2-year-old trees produced more, larger, and wider leaves, as well as more and longer branches, than in the cooler and more cloudy coastal conditions of Clatskanie, Oregon. Sylleptic branches were phenotypically more plastic than the other elements of the crown. Most trees produced significantly more and longer sylleptics in the near-optimal growth environment in Boardman than in the suboptimal environment of Clatskanie. In both environments, all crown traits displayed significant genotype effects in the F₂ family, but for most traits, broad-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 were significantly larger than zero but less than one, indicating that nonparallel responses of genotypes to environment had led to significant genotype x environment interactions, although some genetic basis was shared between the two environments. A nonsignificant genetic correlation 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 canopy traits of sylleptics, were correlated with growth more strongly in Clatskanie than Boardman. In both environments, contrary to observations in an earlier study, proleptic traits were better predictors of stem height and basal area growth than sylleptic traits.