Photosynthetic acclimation of sweet sorghum under progressive water stress

• Energy dissipative mechanisms become ineffective as sorghum plants age.
• PSI was less sensitive to drought than donor side of PSII at young stages.
• Glucose and sucrose counteract the oxidant effect of drought stress in young plants.

This study addressed photosynthetic acclimation of sweet sorghum under progressive soil drying at different growth stages: (i) three fully expanded leaves (D1); (ii) growing point differentiation (D2); (iii) booting (D3); (iv) half-blooming (D4). Soil water potential (SWP), photosynthetic traits and soluble sugars accumulation were determined. Under progressive drought stress, D1 and D2 photosynthesized until 20 days after stress imposition (SWP ≈ −3.5 MPa), while, D3 and D4 were phothosynthetically inactive 12 days after irrigation suspension (SWP ≈ −1.5 MPa). Photoprotection mechanisms also changed with plant age: in mature plants, electron transport rate per cross section (ETo/CSm), and silent PSII reaction centers (RCs) drastically declined at SWP of −1.0 MPa, while remained unchanged in young plants until −1.5 MPa. As water stress increased (SWP < −1.8 MPa), the light energy absorption (ABS/CSm), the trap energy flux (TRo/CSm) and ETo/CSm dramatically decreased in mature plants (−70% compared to control). Soluble sugars significantly correlated with thermal energy dissipation (DIo/CSm) and density of active reaction centers (RCo/CSm) at young stages. The acceptor side of Photosystem I (PSI) was affected by plant age resulting in a lower efficiency than donor side of PSII.