The response of photosynthetic carbon assimilation and chlorophyll fluorescence quenching to changes in intercellular CO2 partial pressure (C1), 02 partial pressure, and leaf temperature (15-350C) in triazine-resistant and -susceptible biotypes of Brassica napus were examined to determine the effects of the changes in the resistant biotype on the overall process of photosynthesis in intact leaves. for optimal photosynthetic efficiency consistent with the altered D-1 binding properties (19). The rate of electron transfer in PSII from the primary to the secondary electron-accepting plastoquinone is less in resistant biotype than in the susceptible (2). Hart and Stemler (I 1) have suggested that resistant Brassica napus is more susceptible to photoinhibition, which reduces the rate of electron transport.Decreased electron transport through PSII in resistant biotypes could result in lower levels of light-harvesting Chl protein phosphorylation (1). This could lead to the pleiotropic effects observed, including increased thylakoid grana stacking (28) and an increase in fatty acid unsaturation in grana lamellae (20). It has been suggested that these changes in lipid composition in chloroplast membranes in resistant biotypes may confer enhanced resistance to low temperature stress and greater fluidity at low temperatures (20).Several studies have compared carbon assimilation in resistant and susceptible variants ofa species. Resistant biotypes of several species, including S. vulgaris, had lower carbon assimilation and quantum yields than susceptible biotypes (16). van Oorschot and van Leeuwen (26) found carbon assimilation rates in resistant and susceptible biotypes were comparable in Chenopodium album, Polygonum lapathifolium, Poa annua, Solanum nigrum, and Stellaria media. Dekker and Burmester (5, 6) have shown that the carbon assimilation rate in resistant and susceptible B. napus differs at different times of the day: carbon assimilation in resistant biotypes is often greater than in susceptible biotypes early and late in the photoperiod, whereas susceptible is usually greater than resistant at midday.The effect of temperature on resistant and susceptible biotypes is unclear. Several studies have indicated resistant biotypes of several species are more heat sensitive than their susceptible counterparts (8,12
