Differential Photosynthetic Responses of Green and Purple Basil to Drought Stress and Recovery: The Protective Role of Anthocyanins

TL;DR

Purple basil is more drought-tolerant than green basil due to higher anthocyanin levels, which protect photosynthesis and reduce oxidative stress.

Contribution

Demonstrates the protective role of anthocyanins in enhancing drought tolerance and recovery in basil.

Findings

Purple basil showed less photosynthetic damage under drought stress compared to green basil.

Higher anthocyanin levels in purple basil correlate with reduced oxidative stress and better recovery after re-watering.

Purple basil's photosynthetic functions recover almost completely after drought, unlike green basil.

Abstract

Drought is a major environmental threat to agriculture. This study examined the role of anthocyanins in plant drought tolerance by comparing two basil varieties differing in leaf anthocyanin content: green basil (Ocimum basilicum L. Italiano Classico) and purple basil (Ocimum basilicum L. Dark Opal). The impact of the PEG-induced drought stress was assessed by monitoring changes in chlorophyll a fluorescence parameters (JIP and PAM), leaf pigment content, anthocyanin and total phenolic levels, oxidative stress markers (malondialdehyde, hydrogen peroxide and membrane integrity), as well as radical-scavenging capacity (DPPH assay). Drought stress led to a modification on both the donor (Wk) and acceptor (Vj) sides of PSII, which influences QA reoxidation and amounts of the closed reaction centers (1-qP). These changes inhibited photosystem II photochemistry, the rate of the electron transport (ETR), and the rate of the photosynthesis (RFd) and decreased performance indices (PIABS, PItotal), as well as the photosystem I photochemistry. The drought-induced changes were associated with an increase in the dissipated energy per reaction center (DI0/RC). The results show that photosynthetic functions in purple basil were less affected under drought stress compared to green basil. The reason for better tolerance of purple basil is associated with elevated anthocyanin levels, which correlate with enhanced antioxidant capacity, reduced hydrogen peroxide accumulation, lower membrane lipid peroxidation, improved relative water content and membrane stability. In addition, rapid cyclic electron flow around photosystem I and a higher carotenoid to chlorophyll ratio contribute to drought tolerance in purple basil. After re-watering, purple basil recovers its photosynthetic function almost completely, unlike green basil, which shows further suppression. The increase in the anthocyanin content and radical-scavenging capacity, as well as the smaller oxidative damage under drought stress, are the main reasons for the better recovery in purple basil. Overall, the findings highlight that higher anthocyanin accumulation in purple basil confers greater drought tolerance and recovery capacity by stabilizing photosynthetic processes and reducing oxidative stress.

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