REGULATION OF PHOTOSYNTHESIS IN TRIAZINE-RESISTANT AND TRIAZINE-SUSCEPTIBLE BRASSICA-NAPUS

The response of photosynthetic carbon assimilation and chlorophyll flu orescence quenching to changes in intercellular CO2 partial pressure ( C(i)), O2 partial pressure, and leaf temperature (15-35-degrees-C) in triazine-resistant and -susceptible biotypes of Brassica napus were ex amined to determine the effects of the changes in the resistant biotyp e on the overall process of photosynthesis in intact leaves. Three cat egories of photosynthetic regulation were observed. The first category of photosynthetic response, ribulose-1,5-bisphosphate carboxylase/oxy genase (Rubisco)-limited photosynthesis, was observed at 15, 25, and 3 5-degrees-C leaf temperatures with low C(i). When the carbon assimilat ion rate was Rubisco-limited, there was little difference between the resistant and susceptible biotypes, and Rubisco activity parameters we re similar between the two biotypes. A second category, called feedbac k-limited photosynthesis, was evident at 15 and 25-degrees-C above 300 microbars C(i). The third category, photosynthetic electron transport -limited photosynthesis, was evident at 25 and 35-degrees-C at moderat e to high CO2. At low temperature, when the response curves of carbon assimilation to C(i) indicated little or no electron transport limitat ion, the carbon assimilation rate was similar in the resistant and sus ceptible biotypes. With increasing temperature, more electron transpor t-limited carbon assimilation was observed, and a greater difference b etween resistant and susceptible biotypes was observed. These observat ions reveal the increasing importance of photosynthetic electron trans port in controlling the overall rate of photosynthesis in the resistan t biotype as temperature increases. Photochemical quenching of chlorop hyll fluorescence (q(p)) in the resistant biotype never exceeded 60%, and triazine resistance effects were more evident when the susceptible biotype had greater than 60% q(p), but not when it had less than 60% q(p).