Photosynthetic response of bottle gourd [Lagenaria siceraria (Molina) Standl.] to drought stress: Relationship between cucurbitacins accumulation and drought tolerance

Cucurbitacins are a group of secondary metabolites produced by cucurbits including bottle gourd [Lagenaria siceraria (Molina) Standl.]. The role of cucurbitacins in plant responses to drought stress is not well-determined. The objective of this study was to investigate the response of bottle gourd to drought stress and to establish the relationship between cucurbitacins accumulation and drought tolerance. Leaf gas exchange, chlorophyll fluorescence and level of cucurbitacins production were investigated on 12 bottle-gourd landraces grown under drought-stressed (DS) and non-stressed (NS) conditions. Results showed that stomatal conductance (gs), transpiration (T) rate, net CO2 assimilation rate (A), the ratio of CO2 assimilation rate and intercellular CO2 concentration (Ci/Ca), intrinsic and instantaneous water-use efficiencies declined in landraces that were subjected to drought stress. Intercellular CO2 concentration, the ratio of intercellular and atmospheric CO2 concentration increased significantly in response to drought stress. The maximum quantum efficiency of photosystem II photochemistry (Fv'/Fm'), the effective quantum efficiency of photosystem II photochemistry, photochemical quenching (qP) and non-photochemical quenching (qN) were not affected by drought stress; whereas, electron transport rate, electron transport to oxygen molecules (ETR/A) and alternative electron sink (AES) declined. Cucurbitacin E and I were detected under NS and DS conditions in some of the tested bottle gourd landraces. Significant and positive correlations were observed between cucurbitacin I content with ETR/A and AES. The current study identified drought tolerant bottle gourd landraces namely: BG-48, BG-58, BG-70, BG-78 and BG-79 based of high values for gs, T, A, Fv'/Fm' and qN under DS condition. These selections may be useful for drought tolerance breeding in bottle gourd or related cucurbits. There is a need for a further investigation whether cucurbitacin I accumulation may be involved in the regulation of the physiological processes evaluated in the present study.