Coupled soil water stress and environmental effects on changing photosynthetic traits in wheat and maize.

Soil water deficits limit the photosynthetic productivity of crops, resulting in reduced yields. However, the effects of soil water stress on leaf gas exchange parameters, actual quantum efficiency (Φ PSII), photosynthetic capacity (maximum electron transfer rate ETRmax, maximum photosynthetic rate An max , maximum carboxylation rate Vc max25) and environmental parameters, such as photosynthetic active radiation (PAR), vapour pressure deficit (VPD), remains unclear. Especially, the dynamic responses of crops with different canopy structure types and phenological stages to water stress needs to be further clarified. In this study, we conducted a field experiment using wheat and maize under complete rainfall isolation to study the effects of coupled water stress and environmental factors on leaf gas exchange processes and photosynthetic capacity. Our results showed that the Φ PSII - PAR relationship in the wheat leaves(a C 3 plant) was more sensitive to water stress than was that in the maize leaves (a C 4 plant) and significantly differed with phenological stage. The coupling of water stress with VPD had a more pronounced effect on the gas exchange parameters (net photosynthetic rate (An), stomatal conductance (g sw), transpiration efficiency (TE), intrinsic water use efficiency (WUEi)) than coupling with PAR, especially for wheat, where the coupling effect of soil water content (SWC) with VPD was more pronounced as the degree of water stress increased. The SWC status did not significantly alter the wheat An -PAR relationship, in contrast to that in maize, wherein a strong effect of SWC on the An -PAR relationship was observed. Water stress had a more pronounced limiting effect on ETR max in wheat than in maize. An max showed a weaker relationship with SWC in both wheat and maize, whereas Vc max25 exhibited a stronger relationship with SWC. Additionally, the physiological response process should take into account the differences between phenological periods. Our study can be used as a reference for precise field irrigation. ● The response mechanism of the Φ PSII -PAR relationship to soil water stress was established. ● The mechanism of coupling soil water stress and environmental parameters on leaf gas exchange was clarified. ● Contribution of soil moisture and photosynthetic pigment content to leaf photosynthetic capacity. ● The effect of potential phenological stages on physiological characteristics under soil water stress. [ABSTRACT FROM AUTHOR]