INTRASPECIFIC VARIATION IN PHOTOSYNTHETIC TRAITS OF POPULUS-TRICHOCARPA

Three experiments were conducted to evaluate net photosynthesis in bla ck cottonwood (Populus trichocarpa Torrey & Gray) from mesic and xeric regions of Washington. In 1986, six clones each from the lower Nisqua lly (mesic) and Yakima (xeric) river valleys and growing in a common g arden were measured for their photosynthetic rates. On 2 clear days in summer, Yakima clones had significantly (p less than or equal to 0.10 ) higher rates (means: 32 and 25 mu mol CO2 m(-2) s(-1)) than Nisquall y clones (means: 25 and 22 mu mol CO2 m(-2) s(-1)). The next year, cut tings from these clones were potted, grown separately for 2 months in a maritime (Puyallup, Wash.) and a continental climate (Wenatchee, Was h.), and then transferred to growth chambers in Seattle, Wash. Photosy nthetic rates were determined at four light levels (200, 500, 800, and 1500 mu mol m(-2) s(-1)) and two temperatures (24 and 32 degrees C). Yakima plants had significantly (p less than or equal to 0.05) higher rates at high light (20 mu mol CO2 m(-2) s(-1)) and also at moderate l ight in high temperature than Nisqually plants (16 mu mol CO2 m(-2) s( -1) in high light). Significant acclimation (p less than or equal to 0 .01) was also revealed: at the two moderate light levels in high tempe ratures the Wenatchee-grown plants, regardless of source, had higher r ates than those grown at Puyallup. In 1991, eight clones from the lowe r elevation, xeric region of the Yakima and eight from the upper eleva tion, mesic region were selected in a Puyallup stoolbed, and their pho tosynthesis and water-use efficiency were measured on a clear summer d ay. Lower elevation plants had a higher photosynthetic rate than those from the upper elevations along the Yakima; values were similar to th ose of 1986. The pattern for water-use efficiency was reversed; values were 56 mu mol CO2/mol H2O for the lower and 84 mu mol CO2/mol H2O fo r the upper Yakima plants. These three experiments provide evidence fo r significant genetic variation in photosynthetic processes both betwe en and within river valleys.