Photosynthesis in Sitka Spruce (Picea Sitchensis (Bong.) Carr.): X. Acclimation to Quantum Flux Density Within and Between Trees

Abstract

The hypothesis was tested that acclimation to shade by foliage within young trees of Sitka spruce does not have the same effects on shoot and leaf morphology and photosynthesis as acclimation to photon flux density by leaves on different plants exposed in entirety. The photosynthetic capacity of low light phenotypes was higher than that of high light phenotypes, whereas the photosynthetic capacity of sun foliage was higher than that of shade foliage in the same plant, whether expressed on a leaf area or dry weight basis. Shoots on the low light phenotypes were longer, had more lateral buds, more needles per shoot, longer but narrower needles and a larger specific leaf area than shoots on the high light phenotypes, but about the same needle area per shoot. Shade shoots had a much smaller leaf area than sun shoots on the same plant as a result of fewer, shorter and narrower needles. The differences between phenotypes may result from a homeostatic response to photosynthate levels caused by the much higher rates of photosynthesis by the high light phenotypes than by the low light phenotypes. Within the trees of a particular phenotype, acclimation to the local light environment (both amount and quality) apparently leads to the most economical morphology and photosynthetic capacity. In studying acclimation to shade it is important to distinguish between the acclimation of entire plants and of foliage within plants.