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

## TL;DR

This study explores how different pepper plants respond to combined cold and salt stress, and how these responses can be detected using leaf reflectance measurements.

## Contribution

The study demonstrates that combined stress responses in Capsicum species are unique and can be masked by dominant stress effects, requiring careful evaluation of hyperspectral signals.

## Key findings

- Combined stress caused greater growth reduction than single stresses in Capsicum species.
- Leaf reflectance around 550 nm and red-edge peak shifts correlated with chlorophyll content under stress.
- The photochemical reflectance index (PRI) distinguished all three stress treatments in Capsicum species.

## Abstract

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.

The online version contains supplementary material available at 10.1007/s00425-025-04865-0.