THE INFLUENCE OF VERTICILLIUM-DAHLIAE AND DROUGHT ON POTATO CROP GROWTH .1. EFFECTS ON GAS-EXCHANGE AND STOMATAL BEHAVIOR OF INDIVIDUAL LEAVES AND CROP CANOPIES

Potato (Solanum tuberosum L.) plants cv. Saturna were subjected to infection with Verticillium dahliae and drought stress. At the early stages of growth, stomatal conductance, transpiration and net photosynthesis were measured at light saturation (PAR>300 m-2) on individual leaves and with mobile field equipment with the aid of field enclosures. No significant changes in stomatal conductance and gas exchange characteristics occurred as a result of V. dahliae instomatal conductance, transpiration and and photosynthetic rates, especially on older leaves and on plants exposed to direct sunlight for a longer period of time. In combination with drought, V. dahliae only occasionally showed interaction; their effects being less than additive. High values of coefficients of variatoon necessitated a high number of measurements per treatment; the more so in the inoculated plants which shows that V. dahliae seems to affect certain leaves while not affecting others early in growth. Crop photosynthesis was less reduced by V. dahliae than individual leaf photosynthesis due to the levelling effect of integration over the whole canopy and possibly through a stimulation of the top leaves. The upper non-affected leaves are responsible for the bulk of photosynthetic crop activity. The results indicate that following an infection with V. dahliae photosynthesis is reduced early in growth as a result of drought stress in the leaves. © 1990 Koninklijke Nederlandse Planteziektenkundige Vereniging.