LOW TEMPERATURE-ENHANCED INHIBITION OF PHOTOSYNTHESIS BY OXIDES OF NITROGEN IN LETTUCE (LACTUCA-SATIVA L)

The response of photosynthetic gas exchange to oxides of nitrogen (NO(x)) was studied in leaves of lettuce (Lactuca sativa L.) at different temperatures. Exposure to high concentrations (e.g. 1.3-mu-mol NO(x) mol-1), similar to those often found in commercial glasshouses, caused a rapid inhibition of the net assimilation of CO2. This appeared to be by a direct effect on photosynthesis rather than by a change in the stomatal conductance. In ambient CO2 (345-mu-mol mol-1), the percentage inhibition at 10 and 5-degrees-C was approximately 3 x and 5 x, respectively, that measured at 20-degrees-C. This effect of temperature also occurred when measured in CO2 enriched air (1050-mu-mol mol-1), which would normally accompany NO(x) in a glasshouse. The extent of photosynthetic inhibition caused by NO(x) was, however, always less in high than in low CO2. The results suggest that when burning fuel to raise the CO2 concentration and heat the glasshouse air, growers should avoid generating high concentrations of NO(x) in conditions of low temperature.