Survival strategies of Centrosema molle and C. macrocarpum in response to drought

Abstract The response of a genotype each of C. molle and C. macrocarpum to drought (low soil moisture availability) was studied in a seasonally dry tropical environment throughout 3 consecutive years. Changes in soil water content, leaf water relations and gas exchange, chlorophyll fluorescence, forage yield and leaf area index (LAI) were compared in well-watered and draughted plots. Soil water depletion during the study occurred mostly at 0-20 cm depth. Minimum values of leaf relative water content, water potential (Pi) and net photosynthesis in unwatered plants were within the ranges: 68 (C. molle) to 70% (C. macrocarpum); -1.6 (C. molle) to -0.9 MPa (C. macrocarpum); and 8 (C. molle) to 10 (imol/m2/s (C. macrocarpum), respectively. Leaf movements helped to avoid excessive solar radiation incidence, yet efficiency of chloroplast Photosystem II in stressed leaves of C. molle was negatively affected. Above-ground biomass and LAI were reduced only in C. macrocarpum (45-50% reduction) as a result of moisture stress. Leaves of both species behaved as isohydric, though larger declines in Pi in C. molle may suggest a less effective control of water loss; this promoted more leaf senescence. Drought survival in these species depends on a combination of avoidance and tolerance strategies; the relative importance of both mechanisms depends on species and the duration and intensity of water deficit. Further studies with a higher number of accessions/ecotypes of each species are suggested in order to corroborate our findings.