Little is known about the physiological processes involved in gas exchange of guava (Psidium guajava L.) leaves. In this research, the rates of CO2 net assimilation (A), respiration, and transpiration (E), and the stomatal conductance (gs), were measured in two stages of leaf development (mature and immature leaves) and in two levels of soil moisture (irrigated and drought stressed plants), on 3.5 years old guava trees planted in Iguala, Guerrero, México. The photosynthetic parameters in response to the intercellular concentration of CO2 (Ci) and to the photosynthetic active radiation (PAR) were also determined. Measurements were done with two portable photosynthesis apparatus, one of them equipped with devises for regulating PAR and Ci, keeping constant other environmental variables. It was found that immature leaves showed a lower A but higher respiration rate than mature leaves, and that the induced drought stress diminished gs by 50 % and A by 55 %, while E was completely inhibited. The photosynthetic parameters of guava leaves corresponded to a C3 plant since in response to Ci, A became saturated at 540 μmol·mol-1 and the compensation point occurred at 69.2 μmol·mol-1. In response to PAR, leaves reached photosaturation at 700 μmol photon·m-2·s-1 and the compensation point at 70 mmol photon·m-2·s-1.

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