Intraspecific variation in photosynthetic traits of Populus trichocarpa

Abstract

Three experiments were conducted to evaluate net photosynthesis in black cottonwood (Populus trichocarpa Torrey & Gray) from mesic and xeric regions of Washington. In 1986, six clones each from the lower Nisqually (mesic) and Yakima (xeric) river valleys and growing in a common garden were measured for their photosynthetic rates. On 2 clear days in summer, Yakima clones had significantly (p ≤ 0.10) higher rates (means: 32 and 25μmol CO2 m−2 s−1) than Nisqually clones (means: 25 and 22μmol CO2 m−2 s−1). The next year, cuttings from these clones were potted, grown separately for 2 months in a maritime (Puyallup, Wash.) and a continental climate (Wenatchee, Wash.), and then transferred to growth chambers in Seattle, Wash. Photosynthetic rates were determined at four light levels (200, 500, 800, and 1500 μmol m−2 s−1) and two temperatures (24 and 32 °C). Yakima plants had significantly (p ≤ 0.05) higher rates at high light (20μmol CO2 m−2 s−1) and also at moderate light in high temperature than Nisqually plants (16 μmol CO2 m−2 s−1 in high light). Significant acclimation (p ≤ 0.01) was also revealed: at the two moderate light levels in high temperatures the Wenatchee-grown plants, regardless of source, had higher rates than those grown at Puyallup. In 1991, eight clones from the lower elevation, xeric region of the Yakima and eight from the upper elevation, mesic region were selected in a Puyallup stoolbed, and their photosynthesis and water-use efficiency were measured on a clear summer day. Lower elevation plants had a higher photosynthetic rate than those from the upper elevations along the Yakima; values were similar to those of 1986. The pattern for water-use efficiency was reversed; values were 56 μmol CO2/mol H2O for the lower and 84 μmol CO2/mol H2O for the upper Yakima plants. These three experiments provide evidence for significant genetic variation in photosynthetic processes both between and within river valleys. Key words: Populus trichocarpa, black cottonwood, poplar, photosynthesis, water-use efficiency, adaptation.