Drought intensity–responsive changes in photosynthesis-related proteins, carbohydrate pools and forage quality in Kentucky bluegrass (Poa pratensis L.)

Abstract

Drought is a major factor affecting annual forage production and quality. The present study aimed to characterize drought stress–responsive changes in carbohydrate composition and forage quality–related parameters as being linked to the responses of physiological parameters during the regrowth of Kentucky bluegrass (Poa pratensis L.). Drought treatment gradually decreased leaf water potential (Ψ_w) to a minimum value of −2.94 MPa at day 46. Drought-responsive increases in H₂O₂ concentration and lipid peroxidation were significant from day 22 (Ψ_w = −1.38 MPa), accompanied by the decreased chlorophyll content. Drought stress repressed Rubisco large subunit and several thylakoid membrane protein complexes (PSI, PSII dimer, PSII monomer and cytochrome b6/f). Drought also increased glucose, fructose and sucrose concentrations from day 22. A distinct increase in monosaccharides and disaccharides coincided with a reduction of fructan, especially from day 36 onward, when the enzymatic activity of fructan exohydrolases (1-FEH and 6-FEH) became greatly enhanced. Significant changes in forage nutritional parameters also occurred from day 36 (Ψ_w = −2.74 MPa), as shown by increases in neutral detergent fiber, acid detergent fiber (ADF) and ADF-lignin with enhanced activity of polyphenol oxidase, as well as a decrease in crude protein content. These drought-responsive changes in nutritional parameters were strongly correlated with a decrease in in vitro dry matter digestibility (IVDMD). A correlation analysis revealed that drought-induced accumulation of H₂O₂ along with decreasing Ψ_w was closely associated with increases in fiber compounds and ADF-lignin leading to a decrease in IVDMD. Our results indicate that drought-responsive reductions in forage quality mainly occurred under severe drought stress (day 36 to day 46, Ψ_w ≤ −2.74), when H₂O₂ accumulation and lipid peroxidation were high, were characterized by an accompanied increase in ADF.