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BELLOTI, M. Decarboxylation mechanism of the C4 photosynthesis and isotope discrimination for selection of Saccharumsppdrought tolerant genotypes.2012. 89 p. Dissertação (Mestrado) – Centro de Energia Nuclear na Agricultura, Universidade de São Paulo, Piracicaba, 2012. Drought stress in sugarcane (Saccharum spp.) is one of the most harmful environmental stresses causing yield losses. To develop drought tolerant cultivars, the selection of genotypes with improved adaptation to this adverse environmental condition are highly required for sugarcane breeding programs. The objective of this dissertation was to investigate drought responses mechanism present in sugarcane by evaluation of physiological measurements, carbon isotope discrimination (ΔC) and leakage of CO2 (φ) between two contrasting sugarcane cultivars to drought stress. Further, the effect of drough on C4 photosynthetic mechanism was analized to inquire the enzymatic pathway involved in the transport of the C4 acids from mesophyll cells to the bundle sheath cells. Through physiological experiments, two contrasting cultivars, „IACSP94 2094‟ (tolerant) and „IACSP97 7065‟ (sensitive) to water deficit were screened among 10 cultivars submmited to drought prone field conditions. The Δ 13 C and φ analysis showed significant variation between the two cultivars during drought growth condictions wherein „IACSP94-2094‟ cultivar displayed high ΔC and φ rates. A quantitative expression profile and enzymatic activity were analyzed considering the three alternative pathways of C4 decarboxylation, NADP-dependent malic enzyme (NADP:ME), NAD dependent malic enzyme (NAD:ME) or phosphoenolpyruvatecarboxykinase (PCK) during drought stress. Despite the NADP-ME C4 mechanism present in Sugarcane, the increased expression and activity levels of PCK decarboxylation pathway was shown in the tolerant cultivar („IACSP94 2094‟) during drought. However, the drought sensitive cultivar lacks the ability to triggers such a decarboxylation mechanism which suggest that this alternative mechanism might be associated with drought tolerance. The increased φ in PCKtype C4 plants was detected over the NADP:ME-type C4 plants which supports the presence of this alternative decarboxylation process. Altogether, the data presented here suggest that the PCK mechanism of decarboxylation is induced by drought stress and can be used to select drought tolerant genotypes.

Mecanismo de descarboxilação da fotossíntese C4 e discriminação isotópica para seleção de genótipos do complexo Saccharumspp. tolerantes à seca