Differential Light Responses of Photosynthesis by Triazine-resistant and Triazine-susceptible Senecio vulgaris Biotypes

Abstract

Studies were conducted to determine a physiological basis for competitive differences between S. vulgaris L. biotypes which are either resistant or susceptible to triazine herbicides. Net C fixation of intact leaves of mature plants was higher at all light intensities in the susceptible biotype than in the resistant biotype. Quantum yields measured under identical conditions for each biotype were 20% lower in the resistant than in the susceptible biotype. O2 evolution in continuous light measured in stroma-free chloroplasts was higher at all light intensities in the susceptible biotype than in the resistant biotype. O2 evolution in response to flashing light was measured in stroma-free chloroplasts of both biotypes. The steady-state yield per flash of resistant chloroplasts was less than 20% that of susceptible chloroplasts. Susceptible chloroplasts displayed oscillations in O2 yield per flash typically observed in normal chloroplasts; the pattern of oscillations in resistant chloroplasts was noticeably damped. Modification of the herbicide binding site which confers s-triazine resistance may affect the oxidizing side of photosystem II, making photochemical electron transport much less efficient. This alteration resulted in a lowered capacity for net C fixation and lower quantum yields in whole plants of the resistant type.