Pleiotropy in Triazine-Resistant Brassica napus: Leaf and Environmental Influences on Photosynthetic Regulation

Abstract

Since the first discovery of s-triazine-resistance (R ) in higher plants, 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 plant productivity in many reports. W hat is less clear in the literature is whether this change in D -1 structure and electron transport function 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 of R is complex, involves more than one aspect of photosynthesis, and can be mitigated by other processes in the system. Electron transport limitations are only one possible regulatory point in the photosynthetic pathway leading from light-harvesting and the photolysis of water, through ribulose bisphosphate carboxylase/oxygenase, to sucrose biosynthesis and utilization. Herein we discuss this complex issue, arguing that D -1 function can’t be evaluated in isolation from the leaf, the organism, and possibly from the community response. Carbon assimilation in R and the susceptible wild type (S) is a function of several interacting factors. These include the pleiotropic effects resulting from the psbA mutation (the dynamic reorganization of the R chloroplast) interacting with other regulatory components of photosynthesis, microenvironmental conditions, and time (including ontogeny and time of day). Previous work in our laboratory indicated a consistant, differential, pattern of Chi a fluorescence, carbon assimilation, leaf temperature, and 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. Dekker and Sharkey have shown that the primary limitation to photosynthesis changes with changes in leaf temperature, and that electron transport limitations in R may be significant only at higher temperatures. The recognized R plants interact with the environment in a different way than does S. Under environmental conditions highly favorable to plant growth, S often has an advantage over R. Under certain less favorable conditions to plant growth, stressful conditions, R can be at an advantage over S. These conditions may have been cool (or hot), low light conditions interacting with other biochemical and diurnal plant factors early and late in the photoperiod, as well as more complex physiological 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 numerous S individuals in a population of a species. Under certain conditions R might have exploited a photosynthetic niche under-utilized by S.