PEANUT LEAF PHOTOSYNTHETIC ACTIVITY IN COOL FIELD ENVIRONMENTS

Controlled environment studies have shown that peanut (Arachis hypogae a L.) leaf CO2 exchange rates (CER) are sensitive to night temperature s <20 degrees C and that genotypes vary in their ability to acclimate after exposure to continuous cool nights. Our objectives were to inves tigate the relationship between minimum air temperature (Tmin) and lea flet CER for peanut genotypes grown in the held and to determine the e xtent of differences between Tmin and minimum canopy temperatures unde r existing climatic conditions. Eight peanut genotypes were grown in t he held at Delhi, Ontario, in 1991 and six genotypes common to the 199 1 study were sown in 1992. Leaflet CER was recorded on a number of occ asions in both seasons, and canopy temperatures were recorded under bo th clear and overcast conditions at night during 1992. Variation in le aflet CER between sampling dates was large in 1992 (e.g., 1.5-36.0 mu mol m(-2) s(-1) for Early Bunch) but small in 1991. Effects of Tmin co uld account for 73 to 95% of the CER variation in 1992, depending on g enotype. Genotypes differed significantly in CER response to Tmin in 1 992, with relative tolerance to low Tmin of OAC Ruby > OAC Garroy > al l other genotypes. The cultivar OAC Ruby showed a reduction in CER of approximate to 25% when Tmin fell from 20 to 5 degrees C. In contrast, Early Bunch, Chico, VA910212, and OAC Tango did not differ from each other and were relatively sensitive to low Tmin, with CER reduced by a pproximately 55% for the same reduction in Tmin. After Tmin of 20 degr ees C, however, CER of sensitive genotypes was 17% higher than that of OAC Ruby. Canopy minimum temperatures at Delhi were 1 to 3 degrees C cooler than Tmin, primarily due to radiative cooling. Microclimate dif ferences between environments will affect the magnitude of differences between Tmin and Tmin (leaf) and therefore affect the utility of Tmin -CER relationships developed here.