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

Abstract

Seasonal changes in water relations and net photosynthesis were measured over a year in current and 1-year-old foliage of Abiesamabilis (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 22 June, highest rates of stomatal conductance and net photosynthesis did not occur until September and October. One-year-old foliage had the highest rates of net photosynthesis (12.9 mg CO2•dm−2•h−1) and stomatal conductance (3.1 mm•s−1) during the summer. Net photosynthesis decreased with needle age, but foliage as old as 7 years had rates of net photosynthesis as high as 5.0 mg CO2•dm−2•h−1. There was no evidence of photosynthetic adjustment to seasonal change in temperature. The optimum temperature for photosynthesis remained at 15 ± 1.5 °C throughout the year. No water stress was observed during the summer. Xylem water potential never decreased below −1.65 MPa 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 carbon gain. These results also suggested that a strategy for competitive success by Abiesamabilis in this cold, stressful environment is minimum dependence on the carbon gain of any individual age-class of foliage. Instead trees rely on the combined photosynthetic capacity of many years of foliage.