Photosynthetic responses in developing and year-old Douglas-fir needles during new shoot development

Summary A transient decline in photosynthetic rate and several correlates of photosynthetic function in year-old shade needles coincided with shoot elongation in 15 fullsib 8-year-old Douglas-fir Pseudotsuga menziesii (Mirb.) Franco] saplings. In year-old needles and current year needles collected from May to November from branchlets with a single terminal bud and at least 4 needle age classes, chlorophyll (chl) content, photosynthetic rate and non-photochemical quenching of chl fluorescence declined during the period of flushing of the new shoots and recovered as shoot elongation slowed. Developing shade needles did not achieve the same oxygen evolution rate per unit area as the year-old needles, but did develop a higher quantum yield (estimated from chl a fluorescence). In short, in shade branchlets shoot development occurred at a cost of photosynthetic function in year-old needles. In year-old sun needles collected from the upper portions of the same trees, total protein concentration increased prior to, and decreased during, flushing. The concentration of ribulose 1,5-bisphosphate carboxylase-oxygenase (Rubisco) rose and decreased more than chlorophyll-binding proteins. In general, protein concentration in needles reflected age class rather than sun or shade environment. A specific decline in Rubisco in year-old sun needles during the period of new shoot elongation strengthens the hypothesis that degradation of this photosynthetic protein contributes to development of the new shoot.