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 (ΦPSII), photosynthetic capacity (maximum electron transfer rate ETRmax, maximum photosynthetic rate Anmax, maximum carboxylation rate Vcmax25) 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 ΦPSII - PAR relationship in the wheat leaves(a C3 plant) was more sensitive to water stress than was that in the maize leaves (a C4 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 (gsw), 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 ETRmax in wheat than in maize. Anmax showed a weaker relationship with SWC in both wheat and maize, whereas Vcmax25 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.