Regulation of non-photochemical quenching by PsbS and zeaxanthin under variable light and drought

Plants are often exposed to rapidly changing irradiance and intermittent periods of reduced water availability. Efficient use of light for photosynthesis depends on the plant's ability to respond flexibly to dynamic environmental constraints. Our aim was to assess the impact of light quantity and dynamics on the induction of photoprotective mechanisms in wild-type A. thaliana and mutants impaired in NPQ mechanisms. Wild-type (wt) plants and mutants impaired in key NPQ components, npq1 and npq4, were grown under optimal watering (OW) or drought stress (DS) and exposed to constant low (LL), medium (ML), and high light (HL), or to variable light (VL). The results showed that both PsbS and zeaxanthin (Zx) were essential for full induction of energy-dependent quenching (qE) under HL and VL. Notably, qE in wt under ML, which had the same average photon flux density as VL, was lower than qE under VL, indicating that temporal light dynamics, and not only average irradiance, critically affected qE induction. Furthermore, both PsbS and Zx were important for limiting the accumulation of sustained, slowly relaxing quenching under DS and HL. In turn, alternating HL and LL phases during VL restricted the build-up of this residual quenching to levels closer to those observed under ML. Under combined VL and DS, wt plants accumulated Zx to levels comparable to those observed under constant HL. This response was markedly reduced in the npq4 mutant lacking PsbS. In contrast, under constant light conditions, Zx accumulation in npq4 was similar to that in wt, indicating that the contribution of PsbS to Zx accumulation becomes particularly important under dynamic light combined with DS. These findings demonstrate that effective photoprotection and acclimation to combined drought and variable light depend on the coordinated action of PsbS and Zx.