Chlorophyll a fluorescence kinetics uncover differential photosystem II response and salinity tolerance mechanisms in pearl millet (Pennisetum glaucum L.)

Salinity stress is a major abiotic constraint that impairs plant growth and photosynthetic efficiency. This study aimed to screen pearl millet (Pennisetum glaucum L.) varieties for salinity tolerance and to elucidate the underlying physiological and photobiological mechanisms. Eighteen varieties were evaluated under control and 200 mM NaCl conditions. Based on growth and biomass retention, three varieties (YBS-93, YBS-94, YDR-8-1) were identified as salt-tolerant and three (YBS-98, YCMP-19, YCMP-34) as salt-sensitive. Tolerant varieties exhibited superior ion homeostasis, maintaining lower leaf Na⁺ and higher K⁺/Na⁺ ratios, and experienced less oxidative stress, as indicated by lower hydrogen peroxide accumulation. While salt-sensitive varieties showed a marked increase in antioxidant enzymes likely as a compensation for severe oxidative damage, tolerant varieties maintained stable enzymatic activity. In-depth chlorophyll a fluorescence (OJIP) analysis revealed that salinity stress severely impaired the photosynthetic electron transport chain in sensitive varieties, causing to damage at both the donor (oxygen-evolving complex) and acceptor (plastoquinone pool) sides of Photosystem II (PSII). This was evidenced by the positive K- and L-bands, increased Vⱼ, and a decline in the Performance Index (PIABS) and maximum quantum yield (Fv/Fm). In contrast, tolerant varieties maintained PSII structural integrity and functional stability. The findings demonstrate that an integrative analysis of growth, ion homeostasis, and chlorophyll fluorescence parameters, particularly JIP-test indices, provides a robust non-destructive framework for screening salinity tolerance in pearl millet. 1. Chlorophyll-a fluorescence was employed as a sensitive tool to assess PSII performance under salinity stress in pearl millet. 2. Eighteen varieties were screened, identifying YBS-93, YBS-94, and YDR-8-1 as salt-tolerant, and YCMP-19, YCMP-34, and YBS-98 as salt-sensitive. 3. Salinity stress (200 mM NaCl) significantly increased H₂O₂ levels, particularly in sensitive varieties, indicating oxidative stress. 4. Antioxidant enzymes were upregulated under stress, contributing to partial ROS detoxification. 5. OJIP transients and JIP-test parameters (ABS/RC, TR₀/RC, ET₀/RC, DI₀/RC) were disrupted, especially in YCMP-34, reflecting PSII damage. 6. Positive K and L bands, reduced Fv/Fm, and lower φPo, φEo, and Ψo confirmed impaired photosynthetic efficiency in sensitive genotypes.