Leaf area modulates the chlorophyll fluorescence of Leymus chinensis in response to different drought scenarios

The photosynthetic response of plants to drought has been widely explored, primarily through indoor cultivation or short-term physiological monitoring. However, studies linking the photosynthesis of forage with plant traits and production under various drought conditions, especially in the context of global precipitation changes, are limited. We conducted a four-year field experiment involving different precipitation treatments: ambient precipitation, intense drought (ID, precipitation exclusion during June), chronic drought (CD, reducing half precipitation amount from June to August), and reducing half precipitation frequency from June to August (RF, precipitation redistribution without changing precipitation amount). Our results showed that ID and CD significantly decreased the actual maximum photochemical quantum yield of PSII (ΦPSII) and maximum photochemical quantum yield (F_v/F_m), indicating a decline in photosynthetic capacity in Leymus chinensis. Meanwhile, the increase in regulatory energy dissipation quantum yield (Φ(NPQ)) highlighted enhanced photoprotection. Additionally, the CD increased the non-regulatory energy dissipation quantum yield (Φ(NO)), indicating that the photoprotection mechanism was insufficient to dissipate excess excitation energy, leading to photodamage at the reaction center. In contrast, under the RF scenario, plants effectively managed excess excitation energy by increasing Φ(NPQ), which prevented damage and maintained stable ΦPSII and F_v/F_m levels. Through regulating leaf area, drought increased Φ(NO) and decreased F_v/F_m. Although this strategy mitigated further photosynthetic damage, it also reduced photosynthetic efficiency and productivity of L. chinensis. This study represents the first exploration of patterns and mechanisms of plant photosynthetic processes in response to diverse drought scenarios. It underscores the crucial role of key plant traits, i.e. leaf area, in regulating photosynthetic responses amid changing precipitation patterns, and provides valuable information for grassland management and continuous forage supply.