Intermittent versus continuous drought: chlorophyll a fluorescence reveals photosystem resilience in tomato

Recurrent drought threatens Mediterranean tomato yields, yet how the time-scale of drought shapes photochemical tolerance remains unclear.

We evaluated six genotypes, three commercial cultivars ('Sintonía', 'Marejada', 'Valenciano') and three Mediterranean landraces ('82', '264', '260', under greenhouse conditions. Plants received either two short pulses (WS1) or a single prolonged drought (WS2). We tracked stem water potential (Ψ_stem) and computed a stress integral (SI), and measured gas exchange, leaf chlorophyll, and chlorophyll a fluorescence (OJIP test) across key time points and after rewatering.

The first WS1 pulse transiently increased performance index (PI_ABS) and electron‑transport efficiencies (Ψ_E0, ϕ_E0) by 20-40 % in four cultivars. Photosynthesis declined by -70 to -80 % but recovered within three days of irrigation. 'Sintonía' showed early increases in dissipation (ϕ_D0) and fluxes (ABS/CS₀, DI₀/CS₀), while maximum quantum yield (ϕ_P0) remained unchanged across genotype. Sustained WS2, however, reduced PI_ABS and ϕ_P0 by -18 to -50 %, increased ϕ_D0, ABS/CS₀ and DI₀/CS₀ by 30-60 % in all except '260', whose OJIP profile remained stable. Photosynthesis dropped near zero but recovered in five genotypes; '264' recovered only 50 %, showing irreversible damage. Chlorophyll content stayed constant, so shifts were pigment‑independent.

Findings support a three-stage resilience model: (i) reversible photoprotective adjustment to short severe drought; (ii) cumulative photochemical damage under sustained deficit; and (iii) genotype-dependent recovery.

Combining temporal stress integrals with OJIP screening provides a robust pipeline for breeding Mediterranean tomatoes adapted to future drought, advancing knowledge of drought resilience mechanisms.