Ação de herbicidas sobre a atividade fotossintética de plantas com metabolismo C3 e C4

Abstract

The objective of this research wasto studythe physiological parametersrelated to photosynthesisusingchlorophyll a fluorescenceand gas exchangeon leaf discsfrom a C3and a C4plantsubmitted toPSII-inhibiting herbicides.Both transientand modulatedchlorophyll a fluorescence were studied. The herbicides used affect the synthesis ofcarotenoidsand the synthesis ofchlorophyll. The experimentwas conducted in a green-house in UFPel laboratory in the town of Capão do Leão/RS. We evaluatedleaf discsof corn and soybeanobtained frommature leaves ofplantsin vegetative stage. The factorial arrangement 3x4 was used, where the first factor was the type of herbicide (bentazon, clomazone and fomesafen) and the other factor was the concentration of the herbicide (0μM, 100μM, 250μM and 500μM). There were twodistinctexperiments, with 15and 5 replicatesper treatment. In the first experiment (n=15), the effects of herbicides onthetransient chlorophylla fluorescencewas measuredusing aportable fluorometer (HandyPEA, Hansatech). In the second experiment (n=5), we evaluated the effect of herbicides on modulated chlorophyll a fluorescence (relaxation kinetics and light curves).Gas exchange was studied using the parameters determined by the portable infra-red CO2 analyzer (model IRGA LI 6400XT, LI-COR). The intensityof chlorophyll fluorescenceand the other parameters were most strongly affected by the herbicidebentazon.The herbicidesclomazoneand fomesafencaused relatively smallchanges inthe transient fluorescencedata, whereas the modulatedfluorescence parametersand gas exchangewere more sensitiveto clomazoneand fomesafen action. Thedesempenho indexes and PIABSPIABStotal were the transient fluorescence parametersmost sensitive tothe actionof the three herbicides. For themodulatedfluorescence, FS, FM , ETR and ϕPSII were the most sensitive parametersactionof bentazon, clomazone and fomesafen. Cornleaves were lesssusceptible to the three herbicidesthansoybeanleaves. Such difference might suggest that C4 plants have the ability to better tolerateherbicides than species withonly the C3metabolism.