CANOPY PHOTOSYNTHESIS OF CO2-ENRICHED LETTUCE (LACTUCA-SATIVA L) - RESPONSE TO SHORT-TERM CHANGES IN CO2, TEMPERATURE AND OXIDES OF NITROGEN

The canopy net photosynthesis (P-n) of lettuce (Lactuca sativa L. cv. 'Ambassador') was analyzed under controlled conditions simulating the winter glasshouse atmosphere. Prior to measurements the plants were gr own in CO2- enriched air of 1000 mu mol mol(-1), at a photosynthetic p hoton flux density (PPFD) of 280 mu mol m(2) s(-1) (400-700 nm) and a day/night air temperature of 16/13 degrees C. Short-term changes in CO 2 concentration significantly changed the initial gradient of the phot osynthetic response to incident PPFD. Maximum photosynthetic efficienc y of the crop increased from 0.041 mol CO2 mol photons(-1) (equivalent to 8.2 mu g CO2 J(-1) and 9.4% on an energy basis) at 350 mu mol mol( -1) to 0.055 mol CO2 photons(-1) (10.9 mu g CO2 J(-1) and 12.7% on an energy basis) at 1000 mu mol mol(-1). Transfer from low to high CO2 al so lowered the light compensation point, but did not affect dark respi ration. The large response of P-n to transient changes in CO2 indicate d that the lettuce canopy did not acclimate to growth in 1000 mu mol C O2 mol(-1), in contrast with the effect of growth in high CO2 on P-n i n single mature leaves reported earlier. A reduction in air temperatur e from 16 to 6 degrees C at a concentration of 1000 mu mol CO2 mol(-1) halved the rate of dark respiration and reduced the light compensatio n point, but had no direct effect on the maximum efficiency with which the crop utilized light. Subsequently, at low light (below 200 mu mol m(-2) s(-1)) P-n was greater at 6 than 16 degrees C. Between a PPFD o f 250 and 300 mu mol m(-2) s(-1) canopy P-n was similar at all tempera tures. Addition of 2.0 mu mol mol(-1) nitric oxide to an atmosphere of 1000 mu mol CO2 mol(-1) caused a rapid and reversible reduction of ca nopy P-n which was greater at the lowest temperatures. The average inh ibition was 6.6% at 16 degrees C and 28.8% at 6 degrees C; this was no t explained by differences in the rate of pollutant uptake, which was less in the cooler conditions. The results are discussed in relation t o development of optimal growing conditions for production of glasshou se lettuce at low light and low temperature during winter in the UK.