Contrasting photosynthesis, photoinhibition and oxidative damage in honeysuckle (Lonicera japonica Thunb.) under iso-osmotic salt and drought stresses

Honeysuckle (Lonicera japonica Thunb.) is a traditional Chinese medicinal crop and belongs to the glycophyte with certain salt tolerance. This study aimed to deeply dissect its salt adaptability by contrasting photosynthesis, photoinhibition and oxidative damage under moderate and severe iso-osmotic salt (150 and 300 mM NaCl) and drought (19.3 % and 28 % PEG-6000) stresses with hydroponic protocol. Photosynthesis was more susceptible to drought stress than iso-osmotic salt stress in honeysuckle according to drought-induced greater decrease in photosynthetic rate. In contrast to salt-induced mild PSII and PSI photoinhibition, severe photosystem II (PSII) and photosystem I (PSI) photoinhibition arose upon iso-osmotic drought stress, indicated by greater decreased the maximal photochemical efficiency of PSII and PSI and remarkable loss of their reaction center proteins. However, PSII and PSI interaction hardly contributed to salt stability of photosynthetic apparatus because of salt-induced finite restriction on electron flow from PSII to PSI. Consistent with photosystems photoinhibition, leaf lipid peroxidation, H2O2 production and electrolyte leakage were elevated much greater by drought stress than iso-osmotic salt stress, confirming drought-induced severe oxidative stress in honeysuckle. Furthermore, the principal components analysis comprehensively showed higher salt adaptability in honeysuckle due to larger cluster separation upon drought stress than iso-osmotic salt stress. As an apparent reason, honeysuckle could prevent drought-induced tremendous leaf water loss upon iso-osmotic salt stress, and had a capacity to dispose accumulated Na+. Therefore, honeysuckle resembles halophytes in this respect and seems appropriate for planting in coastal saline land.