GAS EXCHANGE RESPONSES OF GRAPEVINE TO MILDEW INFECTIONS

The relationships between pathogens and their hosts can be very complex and their final effect can result in high economical losses due to the costs of crop protection and/or to decreases of yield in qualitative or quantitative terms. A correct approach to the study of these relationships must consider the real effect of disease on plant activity: the responses of crop to pathogen should be investigated on the basis of the involved processes, such as photosynthesis, translocation, growth, development, etc. (1). Among the physiological processes, gas exchanges are some of the most sensible to different conditions of stress, and their measurements are frequently used to analyse and describe the responses of crop (2). Diseases, such as grapevine (Vitis vinifera) mildews, are known to induce many alterations at physiological level (3, 4), but the knowledge of the real impact of disease on the plant is still very limited (5). Many mechanisms have been proposed to explain the modification of photosynthesis. Generally they are classified in physical and cellular or metabolic factors. The first mainly consist in alteration of CO2 diffusion, while the second includes several processes, such as destruction of photosynthetic machinery, increase of respiration, carbohydrate accumulation, inhibition of electron transport. The identification of the precise causes of gas exchange alteration can be very difficult, also because in field studies environmental factors can introduce larger variability and even additional stresses (6). Moreover, alteration of photosynthetic performances, that are typically localised in the area of disease attack, are also reported for leaf tissue outside the typical lesions. Several studies describe this effect for many patho-systems as the result of the presence of virtual lesions determining physiological alteration without corresponding visible symptoms (7). This paper presented the results obtained during 1996 and 1997, when the responses of photosynthesis (A), stomatal conductance (GS) and other physiological aspects were studied on grapevine leaves during downy (Plasmopara viticola) and powdery (Uncinula necatrix) mildew attacks which naturally developed under field conditions. Healthy and diseased leaves were measured under controlled conditions to outline the responses of plant to different level of infection. A tentative estimation was done using a simple modelling approach to relate photosynthetic rate and disease severity. Moreover daily and seasonal trends of gas exchange were measured under field conditions on healthy and diseased leaves, to investigate physiological alterations.