Change in photosynthesis and water relations with age and season in Abiesamabilis

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Abstract
Seasonal changes in water relations and net photosynthesis were measured over a year in current and 1-yr-old foliage of A. amabilis (Dougl.) Forbes, a subalpine conifer. Responses were compared with maximum rates achieved in older foliage. Current-year foliage developed slowly during the growing season. Although growth began on June 22, highest rates of stomatal conductance and net photosynthesis did not occur until Sept. and Oct. One-year-old foliage had the highest rates of net photosynthesis (12.9 mg CO2 .cntdot. dm-2 .cntdot. h-1) and stomatal conductance (3.1 mm .cntdot. s-1) during the summer. Net photosynthesis decreased with needle age, but foliage as old as 7 years had rates of of net photosynthesis as high as 5.0 mg CO2 .cntdot. dm-2 .cntdot. h-1. There was no evidence of photosynthetic adjustment to seasonal change in temperature. The optimum temperature for photosynthesis remained at 15 .+-. 1.5.degree. C throughout the year. No water stress was observed during the summer. Xylem water potential never decreased below -1.65 MPa [megapascals] and was always well above the turgor loss point. The lack of any apparent water stress, combined with photosynthetic characteristics, indicated that summer was the most important season for C gain. These results also suggested that a strategy for competitive success by A. amabilis in this cold, stressful environment is minimum dependence on the C gain of any individual age-class of foliage. Instead trees rely on the combined photosynthetic capacity of many years of foliage.