Responses to bleaching herbicides by leaf chloroplasts of maize plants grown at different temperatures

The effects of growth temperature on chloroplast responses to norflurazon a nd amitrole, two herbicides inhibiting carotenogenesis, at phytoene desatur ation and lycopene cyclization, respectively, were studied in leaves of mai ze plants grown at 20 degreesC and 30 degreesC in light. At the lower tempe rature both chemicals caused severe photo-oxidative damage to chloroplasts. In organelles of norflurazon-treated leaves neither carotenoids nor chloro phylls were detectable and the thylakoid system was dismantled, In organell es of amitrole-treated leaves lycopene was accumulated, but small quantitie s of p-carotene and xanthophylls were also produced. Moreover, some chlorop hyll and a few inner membranes still persisted, although these latter were disarranged, lacking essential protein components and devoid of photosynthe tic function. The increase in plant growth temperature to 30 degreesC did n ot change the norflurazon effects on carotenoid synthesis and the photooxid ative damage suffered by chloroplasts. By contrast, in organelles of amitro le-treated leaves a large increase in photoprotective carotenoid biosynthes is occurred, with a consequent recovery of chlorophyll content, ultrastruct ural organization and thylakoid composition and functionality. This suggest s that thermo-modulated steps could exist in the carotenogenic pathway, bet ween the points inhibited by the two herbicides. Moreover it shows that, un like C-3 species, C-4 species, such as maize, can express a strong toleranc e to herbicides like amitrole, when supplied to plants growing at their opt imum temperature conditions.