Gas exchange in forage cactus cultivars of genera Opuntia and Nopalea (Cactaceae)

Resumo

Forage cactus species are cultivated in northeaster Brazil for feed, mainly during dry periods, as cacti are xerophytes well adapted to drought. Gas exchange studies have elucidated the physiological mechanisms underlying plant adaptation to ecosystems with low water availability. Here, we evaluated the variability between and within cactus forage genera Opuntia and Nopalea with respect to gas exchange behaviour. Twenty-one cultivars were used, 16 of which belonged to genus Opuntia, while six belonged to genus Nopalea, particularly, O. cochenillifera. Gas exchange was evaluated in the secondary cladode at 00:00 hours using a portable infrared gas analyser. Net CO₂ assimilation rate, stomatal conductance, CO₂ intercellular concentration, transpiration rate, and leaf temperature were evaluated. Additionally, instantaneous water use efficiency, intrinsic water use efficiency, and instantaneous carboxylation efficiency were calculated. Boxplots were used to assess value dispersion between the genera prior to submitting the data to multivariate analysis of principal components and cultivar-based grouping, and subsequently organizing the species studied in a dendrogram. Variability between the two genera and among cactus forage cultivars was detected for gas exchange. Cultivars were classified into three groups and two main components were identified. Cultivars 'Verdura' and 'Orelha de Elefante Mexicana’ showed higher photosynthetic performance.