Modulation of antioxidant defenses in Conocarpus lancifolius under variable abiotic stress

Conocarpus lancifolius Engl. (Combretaceae) is a non-native ornamental shrub with a high tolerance to semi-arid conditions, grown in the State of Kuwait. Although the plant has a remarkable growth rate under extreme conditions of drought and temperature, biochemical defense-related mechanisms remain unexplored. The objective of this study was to gain an understanding of the physiology of C. lancifolius under abiotic stress conditions. The plant was exposed to variable drought, salinity and temperature stress conditions and in addition to antioxidant defense compounds, photosynthetic and electron transport rates were measured. Ascorbic acid, phenols and flavonoids were quantified as defense compounds in C. lancifolius. The photosynthetic and electron transport rates decreased under drought and high salinity, but increased with increasing temperature. Phenolic, flavonoid and ascorbic acid contents were positively correlated with electron transport and photosynthetic rates in experiments at higher temperature. Plant phenols increased significantly (p ≤ 0.05) from 5.37 mg g ^-1 in controls to 8.75 mg g ^-1 in drought-stressed plants. This increase paralleled with an increase in flavonoid content. A consistent increase in phenols/flavonoids was also observed with increasing temperature, salinity and polyethylene glycol (PEG stress). The tolerance of C. lancifolius to extreme temperature stress showed a high correlation with the synthesis of ascorbic acid and flavonoids, indicating their biochemical role in protecting plant cells from damaging reactive oxygen species produced under stress. Ascorbic acid significantly increased (p ≤ 0.05) from 426 μg g ^-1 in response to temperature stress at 10 °C to 477 μg g ^-1 in plants grown at 40 °C. Meanwhile, flavonoid content increased significantly from 3.38 to 5.07 μg g ^-1 DW in response to higher temperature and drought. A decrease in the concentration of ascorbic acid and photosynthetic and electron transport rates, however, was observed in PEG-induced drought, with water deficit and salt-stressed plants. The decrease in ascorbic acid was accompanied with an increase in the total phenols and flavonoids.