Transfer conductance in second growth Douglas-fir (Pseudotsuga menziesii (Mirb.)Franco) canopies

The internal conductance from intercellular spaces to the sites of carboxylation ( g i ) has only been measured in a few tree species and not in conifers, despite the fact it may impose a large limitation on photosynthesis. The present study provides the first estimates of g i for a coniferous species, and examines variation in g i with height and its relationships to anatomical, biochemical and physiological traits. Measurements were made on upper and lower canopy current-year needles of 50-year-old Douglas-fir ( Pseudotsuga menziesii (Mirb.) Franco). Needle thickness and specific leaf area decreased by 30% from the top to bottom of the canopy. These anatomical/morphological changes were accompanied by modest variation in allocation of N to chlorophyll and the chlorophyll a / b ratio. Allocation of N to Rubisco did not vary with height, but the ratio of Rubisco to chlorophyll did owing to the aforementioned changes in allocation to chlorophyll. The value of g i was estimated in one tree from concurrent measurements of carbon isotope discrimination and net photosynthesis. To examine the variation in g i among trees a second independent method based on day respiration and the difference between the chloroplastic and intercellular photocompensation points (photocompensation point method) was used. Estimates of g i obtained by the two methods agreed well with values varying between 0.14 and 0.20 mol m - 2 s - - 1 . It is estimated that g i limits photosynthesis by approximately 20% as compared to an approximately 30% stomatal limitation (under well-watered conditions). The value of g i scaled approximately with maximum rates of photosynthesis, which were significantly greater in upper canopy needles. Nevertheless, g i did not vary significantly with canopy height, owing to greater variability in g i than photosynthesis.

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