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Environmental and Stress Physiology

Triazine Resistance without Reduced Vigor in Phalaris paradoxa

Department of Agricultural Botany, Faculty of Agriculture, Hebrew University of Jerusalem, Rehovot 76100, Israel, Department of Field and Vegetable Crops, Faculty of Agriculture, Hebrew University of Jerusalem, Rehovot 76100, Israel

A triazine-resistant (R) biotype of Phalaris paradoxa L. (hood canarygrass) was superior to a triazine-susceptible (S) biotype in seed-germinability and seedling emergence. It was equal or superior to the S-biotype in growth under noncompetitive conditions. Rates of CO₂ uptake by R-plants were similar to those of S-plants, except at very low photon flux densities, where S-plants exhibited higher rates of CO₂ uptake. Fluorescence induction curves of chloroplasts isolated from R-plants indicated an alteration in photosystem II. Analysis of the light dependence of electron transport shows a reduction in quantum yield (Q_y) in R- compared to S-chloroplasts. The same analysis, however, shows for R-chloroplasts an increase in the light-saturated electron transport rate (V_max). The increase in V_max compensates for the reduction of Q_y over a wide range of photon flux densities, which may explain the similarity between R- and S-biotypes in photosynthetic potential and growth.