CONCEPTS OF PLANT BIOTIC STRESS - SOME INSIGHTS INTO THE STRESS PHYSIOLOGY OF VIRUS-INFECTED PLANTS, FROM THE PERSPECTIVE OF PHOTOSYNTHESIS

The consequences of biotic stress have been poorly understood, partly because its application is difficult to control and partly because its physiological consequences are highly variable. Many plant viruses ar e recognised on the basis of leaf symptoms that depend on localised ch anges to chloroplast structure and function. This paper reviews recent progress in understanding early interactions between plant viruses an d the photosynthetic apparatus, using chlorophyll fluorescence analysi s of novel, defined algal-virus systems and using high resolution imag ing of chlorophyll fluorescence and other photosynthetic processes in higher plant systems. We then consider the consequences of viral effec ts on photosynthetic functioning for whole plants and populations with an emphasis on the potential interactions with other environmental fa ctors. Early responses indicated by increase in both non-photochemical quenching of fluorescence and increased reduction state of the primar y electron transport acceptor Q(A) suggest that, not surprisingly, bat h photoprotective and photoinhibitory processes contribute to the acce lerated local demise of the photosynthetic apparatus and symptom devel opment. In other cases, localised accumulations of carbohydrate and so urce-sink imbalance following infection may inhibit gene expression, l eading to altered levels of chloroplast protein complexes and enzymes of photosynthetic metabolism coincident with symptom development. Rece nt experiments suggest that much of the variability in plant responses to biotic stress may result from interactions with other environmenta l factors, such as light intensity and nutrition. Experiments suggest that virus infections may have greater effects on fitness and competit ive ability in low N, high light environments than in shaded, high nut rient conditions. Some ecological implications of these observations a re discussed.