PHOTOSYNTHETIC RESPONSE TO CHILLING IN PEANUT

Recent evidence suggests peanut (Arachis hypogaea L.) may be sensitive to relatively mild night temperature in terms of both dry matter accu mulation and yield. The objective of this research was to quantify the effects of a range of night temperatures (9-20 degrees C) on leaf CO2 exchange rate (CER) of unhardened plants and to study the mechanisms involved in the contrasting response of peanut cultivars to differing night temperature. Four peanut cultivars (Ontario Agricultural College [OAC] Ruby, OAC Garroy, Chico, and Early Bunch) were grown in either controlled-environment cabinets or outdoors during August and Septembe r at Guelph, Ontario. Results of the outdoor study indicated a linear decline in CER as ambient night temperature fell from 20 to 13 degrees C for all cultivars except OAC Garroy, with OAC Ruby less sensitive t o low night temperature than Chico or Early Bunch. Limitations to CER after chilling at 10 degrees C in the dark were greater for Chico and Early Bunch (CER reduced by approximate to 35%) than for OAC Ruby and OAC Garroy (CER reduced by approximate to 15%). All cultivars showed t he ability to recover CER within 24 to 48 h of alleviation of chilling conditions. Cultivars grown indoors showed differing responses to one or two successive chilling nights (Chico was more affected than all o thers), but after four successive chilling nights, all cultivars showe d a similar linear response to night temperature (i.e., CER = 0.693 x Tmin + 11.8; r(2) = 0.90). An analysis of the relative contribution of stomatal and mesophyll constraints to CER in indoor studies (chilled at 9 degrees C) indicated significant limitations due to each componen t, with the relative importance dependent on cultivar and chilling per iod duration. Results suggest that night temperatures may be an import ant factor in determining relative cultivar performance in cool produc tion environments.