Biophysical and physiological characteristics of nopalitos (Opuntia spp., Cactaceae) as influenced by domestication

A gradient of domestication based on morphological characteristics has been observed in species of Opuntia genus; but, a biophysical and physiological gradient have been not described. This study aimed to quantify biophysical and physiological characteristics in nopalitos (edible young cladodes of flat-stemmed spiny cacti) of 15 variants of five Opuntia species in a domestication gradient, including wild (O. streptacantha Lem.), semi-wild (O. hyptiacantha F.A.C. Weber), semi-domesticated (O. megacantha Salm-Dyck), more domesticated (O. albicarpa Scheinvar), and that recognized with the highest degree of domestication [O. ficus-indica (L.) Miller]. We hypothesized that biophysical and physiological characteristics of nopalitos are modified throughout the domestication process and that such changes will be evident when variants with different degree of domestication are grown under drought conditions. Tissue firmness, water potential (ψW), osmotic potential (ψπ), content of total soluble solids (TSS), content of malic acid, and membranes permeability were evaluated in nopalitos harvested from plants after 60 days without watering. A randomized design, with six replications and a plant as experimental unit by treatment, was used. The data were analysed by ANOVA, multiple comparisons of means by Tukey test (P ≤ 0.05) among variants and species, and multivariate analysis of principal components. Firmness (from 4.6 ± 0.082 to 6.8 ± 0.053 kgf/cm²), TSS (from 4.86 ± 0.105 to 6.33 ± 0.142 °Brix), membrane permeability (from 26 ± 0.712 to 31 ± 0.474 (µS cm⁻¹)/g⁻¹) and malic acid content (from 0.61 ± 0.008 to 0.82 ± 0.014 g 100 g⁻¹ wet matter) increased during domestication, and simultaneously ψW and ψS decreased (from −0.85 ± 0.034 to −1.1 ± 0.030 MPa and −1.01 ± 0.011 to −1.15 ± 0.011 MPa). The firmness, SST and ψS are the main characteristics that contributed to separate the Opuntia species on the gradient of domestication. Opuntia species on a domestication gradient based on morphometric characteristics of plants (cladodes, nopalitos and cactus pear fruits) correlate to biophysical and physiological characteristics of nopalitos.