A glasshouse light blocking film impacts Capsicum mesophyll ionic homeostasis and stomatal dynamics through altering light conditions

The use of light-blocking film (LBF) is a promising strategy to reduce energy consumption in high-tech glasshouses. However, it also reduces specific light spectra which affect the physiological responses of plants. The LBF reduces 8-25% of canopy-level photosynthetically active radiation (PAR) while targeting a reduction in biologically irrelevant heat-generating light. Here, we investigated the mesophyll and stomatal responses of a Capsicum annum genotype grown in a high-tech glasshouse facility under the LBF. Our results demonstrated that LBF significantly increased the rate of stomatal closure to stimulus (exogenously applied ABA). The capsicum crops grown under LBF also exhibited significantly faster stomatal response to changes in light intensities (e.g. low to high PAR), rather than to light spectral differences (e.g. blue light that induces stomatal opening), which are potentially due to upregulated expression of photoreceptors and light harvesting genes [Phototropin 1 (PHOT1), phytochrome A (PHYA) and Ribulose-1,5-bisphosphate carboxylase/oxygenase small subunit (RBCS)] in guard cells. Moreover, capsicum leaves under LBF also exhibited faster electron physiological responses to light intensity in mesophyll rather than to red light spectrum, which determines electron transfer in mesophyll for photosynthesis. However, leaf mesophyll in LBF showed enhanced K+ and Cl− efflux, Ca2+ influx, and reduced capability in proton pumping than those under control conditions, suggesting impaired mesophyll cell ion homeostasis in LBF. We propose that the LBF significantly affected stomatal responses to the light, which is partially linked with its modified mesophyll ionic status required for optimal photosynthesis in glasshouse capsicum plants.