Photosynthesis and Water Relations in Brazilian Sugarcane

The expansion of sugarcane ( Saccharum spp.) in Brazil has been boosted by the growing demand for bio-fuels all over the world and requires the occupation of new agricultural areas, including regions with unfavorable agro-climatic conditions. This observational study makes an analysis of the relationship between stomatal conductance, gas exchange and photosynthesis in sugarcane fields subject to different ambient conditions and growth phases. It was noted that the stomatal conductance had a much higher control on photosynthesis than on transpiration. Even under high stomatal conductance, photosynthesis maintained a high correlation to it. Sugarcane had an instantaneous water use efficiency of approximately 4.4 µmol of fixed CO 2 for each mmol of H 2 0 expended in transpiration. Lower leaf water potential, besides leading to an internal reduction of CO 2 , also reduced the instantaneous carboxylation efficiency, indicating that the photosynthesis apparatus was also affected. The variety RB92579, seems to have good water absorption system, because even under low leaf water potential, it maintained a good stomatal conductance, with a higher transpiration and photosynthesis than the others varieties. These results suggest that this variety has a tolerance mechanism to endure rather than avoid drought.