Comparison of single and combined salt and cold stress effects and their challenges for hyperspectral measurements of different Capsicum species

This study investigates the capability of leaf reflectance measurements to identify stress responses under combined stress treatment. Crops are subjected to various environmental stresses, mostly occurring in combination. Research on combined stresses is important, but most studies focus on single stresses. We analyzed physiological responses of two Capsicum species to single cold and salt stresses, which differ from the responses to combinations of these stresses. Combined stress caused growth to decrease more than individual stresses. Single cold stress significantly reduced photosynthetic pigments in both species. However, single salt stress increased pigments in C. annuum. Under combined stress, photosynthetic pigments were decreased to a lower extent compared to single cold stress. An increase in leaf reflectance around 550 nm and a significant shift in the red-edge peak of the first derivative corresponded with chlorophyll content. The effects of single cold and combined stress were similar, differing only in magnitude. Only C. chinense showed a response in leaf reflectance to salt stress. Spectral vegetation indices distinguished single cold from single salt stress, whereas the effects of single cold and the combined stress were similar, indicating a dominating effect of cold stress. The photochemical reflectance index (PRI), however, distinguished between all three treatments. This research confirms that the responses to combined stresses are unique and different from responses to individual stresses. A strong effect of one stress can mask another. This can lead to misinterpretation when combined stresses occur. The use of hyperspectral signals for quantification of responses to combined stresses must be carefully evaluated and established for further research to assist breeding of climate-resilient crops performing well under multi-stress events.