Responses of succulents to drought: Comparative analysis of four Sedum (Crassulaceae) species

The increasing frequency and intensity of drought periods is a serious threat for agriculture, prompting research to select and develop crop species and cultivars with enhanced water stress tolerance. Drought responses were studied in four ornamental Sedum species under controlled greenhouse conditions, by withholding watering of the plants for four weeks. Determination of growth parameters (stem length, fresh weight) allowed establishing the relative degree of tolerance of the selected species as S. spurium > S. ochroleucum > S. sediforme > S. album. The levels of photosynthetic pigments (chlorophylls a and b and total carotenoids), oxidative stress [using malondialdehyde (MDA) as a marker], non-enzymatic antioxidants (total phenolic compounds and total flavonoids) and putative osmolytes [proline (Pro) and total soluble sugars] were measured in leaves of control and stressed plants, to correlate drought tolerance with the activation of specific response mechanisms. The results obtained indicate that a higher tolerance to water deficit in Sedum is associated with: a) relatively lower stress-induced degradation of chlorophylls and carotenoids, especially of the latter (which did not decrease in water-stressed plants of S. spurium, the most tolerant species, whereas it was reduced to about 40% of the control in S. album, the most sensitive); b) no increase in MDA levels, reflecting the lack of drought-induced oxidative stress; and c) higher Pro contents in the non-stressed controls of the taxa most resistant to drought, which could be the basis of constitutive mechanisms of tolerance. However, Pro contribution to drought tolerance in Sedum must be based on an 'osmoprotectant' role, as its concentrations, below 16 μmol g−1 DW in all cases, are too low to have any significant osmotic effect. The identification of these biochemical markers of drought tolerance should help to develop rapid and efficient screening procedures to select Sedum taxa with enhanced tolerance when comparing different species within the genus, or different cultivars within a given species.