Abstract
Forage cactus is a CAM-optional plant according to its water status. The objective of this study was to characterize the net photosynthesis rate (Pn), transpiration rate (E), stomatal conductance (C), internal CO 2 concentration(CO 2int. ), water use efficiency (WUE) and instantaneous carboxylation efficiency ( CEi ) of forage cactus varieties of the genus Opuntia and Nopalea grown in the Brazilian semiarid region under screen conditions and periodic irrigation. The varieties IPA-200016 ( O. stricta ), IPA-100003 ( O. ficus indica ), IPA-200008 ( O. atropes ), IPA-200149 ( O. larreri ) and IPA-100004 ( N. cochenillifera ) were grown under screen conditions, with 50% light reduction, and irrigated every seven days using 1/3 of total weekly evapotranspiration. Measurements were made in all cladodes, on two-year-old plants, at 6:00 a.m., 12:00 p.m., 6:00 p.m. and 0:00 a.m. with the aid of CIDBio-Science Ultra-Light CI-340®. All variables analyzed vary between genotypes and times. The varieties had the same pattern for Pn , E , C , CO 2int. , WUE and CEi . Gas exchange takes place at day time (optional-CAM). Assimilation of CO 2 predominates at night (CAM). The loss of H 2 O is most evident during the day (C3). Stomatal conductance is identical to those reported for C3 and C4 plants. The IPA-200149 variety stands out in relatin to WUE and CEi in relatin to the others.
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