Exogenous Applications of Spermidine Improve Drought Tolerance in Seedlings of the Ornamental Grass Hordeum jubatum in Northeast China

Abstract

Hordeum jubatum L. is a potential ornamental grass species. Spermidine (Spd) plays a regulatory role in plant stress. This study measured seedling growth, photosynthesis, chlorophyll fluorescence, osmotic regulatory substance contents, and antioxidant enzyme activities in H. jubatum seedlings pretreated with Spd (0–1.5 mM at 0.5 mM intervals) in drought treatments simulating natural water loss. The results indicated that the water content, photosynthetic parameters maximal quantum yield of PSII photochemistry (Fv/Fm), actual quantum yield of photochemical energy conversion in PSII (ϕPSII), and photochemical quenching coefficient (qp) values of H. jubatum shoots decreased significantly with increasing drought intensity. Simultaneously, the contents of malondialdehyde (MDA) and the nonphotochemical quenching coefficient (NPQ) value increased. Spd improved growth and photosynthesis under drought stress. Spd also increased osmotic regulatory substance contents and antioxidant enzyme activities. These results suggest that the drought stress inhibited the growth of H. jubatum and damaged the photosynthetic systems, triggering a range of protective responses. Exogenous Spd mitigated the damage by promoting a variety of responses and adaptive mechanisms, such as adjusting stomatal conductance, promoting photosynthetic capacity, accelerating the synthesis of osmoregulatory molecules, and activating antioxidant enzyme systems. Additionally, 1.5 mM Spd-treated H. jubatum had the best drought tolerance. This study will help to develop an understanding of the effects of exogenous Spd on improving drought resistance and provide a strategy for the H. jubatum landscape effect to be achieved under water-limited conditions.