Photosynthesis and Respiration by Healthy and Beet Yellows Virus‐Infected Sugar Beets ( Beta vulgaris L.) 1

Beet Yellows Virus infection substantially reduced lightsaturated net photosynthesis (Pn) of sugar beets ( Beta vulgaris L.) grown outdoors under intense radiation in a warm environment. Smaller Pn reductions were observed with plants grown at moderate irradiances in a growth chamber at both moderate and warm daytime temperatures. Light‐saturated Pn rates for healthy plants were similar in all environments. Photorespiration estimated from low O 2 enhancement of Pn was lower in infected plants. Respiration per unit leaf area estimated by CO 2 evolution in the dark was slightly greater with infected leaves of outside plants but not with those in the growth chamber. Model simulations of crop assimilation rate predicted that the reduced Pn could cause the substantial reductions in crop growth rate that are often exhibited by virus‐infected plants. The model also predicted that the reductions in leaf area that result from infection may have little effect upon growth rate, providing that infection occurs after the early seedling stage.