Photosynthetic induction responses of two Leymus chinensis ecotypes under drought stress

Drought often alters both steady-state photosynthesis and photosynthetic induction, yet their joint responses within species remain incompletely described. Here, we examined photosynthetic and stomatal responses of two ecotypes (gray-green, GG; yellow-green, YG) of Leymus chinensis to a step increase in irradiance under contrasting soil moisture conditions. For both ecotypes, experimentally imposed drought (three soil water content levels: well-watered 60-75%, moderate drought ~ 35-50%, severe drought ~ 10-25% v/v; 60 d) reduced steady-state photosynthetic rate and prolonged photosynthetic induction. The GG ecotype showed faster stomatal responses and higher cumulative carbon gain during light induction than the YG ecotype. Specifically, GG exhibited a faster Rubisco activation rate and smaller but more numerous stomata, which were associated with faster photosynthetic induction under drought. This combination of traits may help maintain carbon gain during transient drought episodes, while involving trade-offs in water use. In contrast, the YG ecotype required more time to reach steady state and maintained lower steady-state photosynthetic rates under drought. Overall, our results quantify how declining soil moisture differentially affects induction and steady-state photosynthesis within a species and reveal ecotypic contrasts in stomatal and biochemical responses under drought, highlighting the role of photosynthetic induction and stomatal behaviour in ecotypic adaptation to drought.