Impact of combined drought and light stress on the structure and function of photosynthetic apparatus in Pisum sativum

This study demonstrates the impact of combined (drought and high light) stress on thylakoid organization in Pea, Pisum sativum. The combined stress significantly reduced gas exchange parameters, indicating compromised photosynthetic activity. Chlorophyll a fluorescence analysis confirmed a decrease in the photochemical efficiency of photosystem (PS)II. This was accompanied by alterations in thylakoid macro-organization, specifically a reduction in PSII-light-harvesting complex (LHC)II supercomplexes and PSII dimers, coupled with an increase in LHCII monomers. This pattern indicates a redistribution of LHCII from tightly assembled PSII supercomplexes into monomeric forms, reflecting a stress-induced disassembly of the antenna system. In response to the combined stress, the plants exhibited photoprotective mechanisms, including increased carotenoid content, accompanied by decreased chlorophyll content. Additionally, elevated reactive oxygen species were observed, likely as a consequence of the stress combination, which contributed to the thylakoid membrane disorganization and a subsequent decline in membrane protein content. The plants also activated protective mechanisms such as increased non-photochemical quenching and elevated PSBS (PS II subunit S) protein levels to mitigate photoinhibition. Furthermore, the thylakoid stacks displayed a looser arrangement under combined stress, potentially due to the observed changes in thylakoid supercomplexes. Both PSI and PSII were equally affected, showing a reduced abundance of proteins under combined stress. Simultaneously, the abundance of antioxidant proteins increased, reflecting the plant's attempt to counteract the oxidative stress.