Acclimation responses of mature Abies amabilis sun foliage to shading

This paper addresses two main questions. First, can evergreen foliage that has been structurally determined as sun foliage acclimate physiologically when it is shaded? Second, is this acclimation independent of the foliage ageing process and source-sink relations? To investigate these questions, a shading and debudding experiment was established using paired branches on opengrown Abies amabilis trees. For each tree, one branch was either shaded, debudded, or both, from before budbreak until the end of summer, while the other branch functioned as a control. Foliage samples were measured both prior to and during treatment for photosynthesis at light saturation (A _max), dark respiration, nitrogen content, chlorophyll content, chlorophyll-to-nitrogen ratio and chlorophyll a:b ratio. All age classes of foliage responded similarly during the treatment, although pre-treatment values differed between age classes. Within 1 month after the treatment began, A _max was lower in shaded foliage and remained lower throughout the treatment period. For debudded branches, A _max was lower than the controls only during active shoot elongation. At the end of the treatments in September, A _max in shade-treated sun foliage matched the rates in the true shade-formed foliage, but nitrogen remained significantly higher. By 1.5 months after treatment, chlorophyll content in shaded foliage was higher than in controls, and the chlorophyll a:b ratio was lower for the shaded foliage. On debudded branches, chlorophyll content and chlorophyll a:b ratio were similar to the values in control samples. Shading lowered the rate of nitrogen accumulation within a branch, while removing debudding decreased the amount of sequestered N that was exported from the older foliage to supply new growth. By September, chlorophyll content in shade-treated foliage was higher than that in the control sun foliage or in true shade foliage. The chlorophyll increase as a result of shading was unexpected. However, the chlorophyll-to-nitrogen ratio was identical for the shade-treated sun foliage and the true shade foliage while being significantly lower than the control sun foliage. It appears that acclimation to shading in mature foliage involves a reallocation of nitrogen within the leaf into thylakoid proteins. A redistribution of resources (nitrogen) among leaves is secondary and appears to function on a slower time scale than reallocation within the leaf. Thus, A. amabilis foliage that is structurally determined as sun foliage can acclimate to shade within a few months; this process is most likely independent of ageing and is only slightly affected by source-sink relations within a branch.