PLEIOTROPY IN TRIAZINE-RESISTANT BRASSICA-NAPUS - LEAF AND ENVIRONMENTAL-INFLUENCES ON PHOTOSYNTHETIC REGULATION

Since the first discovery of s-triazine-resistance (R) in higher plant s, the altered D-1 protein product of the psbA gene has been regarded as less photosynthetically efficient in those R biotypes of a species. Decreases in electron transport function in the chloroplast have been believed to be the cause of decreased carbon assimilation rates and p lant productivity in many reports. What is less clear in the literatur e is whether this change in D-1 structure and electron transport funct ion directly modifies whole-leaf photosynthesis and plant productivity or only indirectly influences these functions. The dynamic nature of these responses have led several to conclude that the primary effect o f R is complex, involves more than one aspect of photosynthesis, and c an be mitigated by other processes in the system. Electron transport l imitations are only one possible regulatory point in the photosyntheti c pathway leading from light-harvesting and the photolysis of water, t hrough ribulose bisphosphate carboxylase/oxygenase, to sucrose biosynt hesis and utilization. Herein we discuss this complex issue, arguing t hat D-1 function can't be evaluated in isolation from the leaf, the or ganism, and possibly from the community response. Carbon assimilation in R and the susceptible wild type (S) is a function of several intera cting factors. These include the pleiotropic effects resulting from th e psbA mutation (the dynamic reorganization of the R chloroplast) inte racting with other regulatory components of photosynthesis, microenvir onmental conditions, and time (including ontogeny and time of day). Pr evious work in our laboratory indicated a consistant, differential, pa ttern of Chl a fluorescence, carbon assimilation, leaf temperature, an d stomatal function between S and R Brassica napus over the course of a diurnal light period and with ontogeny, i.e. R is a chronomutant. De kker and Sharkey have shown that the primary limitation to photosynthe sis changes with changes in leaf temperature, and that electron transp ort limitations in R may be significant only at higher temperatures. T he recognized R plants interact with the environment in a different wa y than does S. Under environmental conditions highly favorable to plan t growth, S often has an advantage over R. Under certain less favorabl e conditions to plant growth, stressful conditions, R can be at an adv antage over S. These conditions may have been cool (or hot), low light conditions interacting with other biochemical and diurnal plant facto rs early and late in the photoperiod, as well as more complex physiolo gical conditions late in the plant's development. It can be envisioned that there were environmental conditions in the absence of s-triazine -herbicides in which R had an adaptive advantage over the more numerou s S individuals in a population of a species. Under certain conditions R might have exploited a photosynthetic niche under-utilized by S.