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Effects of Irradiance during Growth on Adaptive Photosynthetic Characteristics of Velvetleaf and Cotton ¹

Southern Weed Science Laboratory, Agricultural Research Service, United States Department of Agriculture, Stoneville, Mississippi 38776

We grew velvetleaf (Abutilon theophrasti Medic.) and cotton (Gossypium hirsutum L. var. Stoneville 213) at three irradiances and determined the photosynthetic responses of single leaves to a range of six irradiances from 90 to 2000 µeinsteins m^–2sec^–1. In air containing 21% O₂, velvetleaf and cotton grown at 750 µeinsteins m^–2sec^–1 had maximum photosynthetic rates of 18.4 and 21.9 mg of CO₂ dm^–2hr^–1, respectively. Maximum rates for leaves grown at 320 and 90 µeinsteins m^–2sec^–1 were 15.3 and 10.3 mg of CO₂ dm^–2hr^–1 in velvetleaf and 12 and 6.7 mg of CO₂ dm^–2hr^–1 in cotton, respectively. In 1 O₂, maximum photosynthetic rates were 1.5 to 2.3 times the rates in air containing 21% O₂, and plants grown at medium and high irradiance did not differ in rate. In both species, stomatal conductance was not significantly affected by growth irradiance. The differences in maximum photosynthetic rates were associated with differences in mesophyll conductance. Mesophyll conductance increased with growth irradiance and correlated positively with mesophyll thickness or volume per unit leaf area, chlorophyll content per unit area, and photosynthetic unit density per unit area. Thus, quantitative changes in the photosynthetic apparatus help account for photosynthetic adaptation to irradiance in both species. Net assimilation rates calculated for whole plants by mathematical growth analysis were closely correlated with single-leaf photosynthetic rates.