# The Effects of Cold Stress on Photosynthesis in Hibiscus Plants

## TL;DR

This study explores how cold stress affects photosynthesis in Hibiscus plants and how different parts of the plant respond to chilling.

## Contribution

The study reveals how root temperature modifies cold stress effects and the role of cyclic electron flow and chlororespiration in protecting photosystems.

## Key findings

- Cold stress reduces PSII efficiency and electron transport capacity in Hibiscus plants.
- Cyclic electron flow helps protect PSII when the whole plant is chilled, but not when only the stem is chilled.
- Chlororespiratory enzymes increase when only the stem is chilled, indicating their role in stress response.

## Abstract

The present work studies the effects of cold on photosynthesis, as well as the involvement in the chilling stress of chlororespiratory enzymes and ferredoxin-mediated cyclic electron flow, in illuminated plants of Hibiscus rosa-sinensis. Plants were sensitive to cold stress, as indicated by a reduction in the photochemistry efficiency of PSII and in the capacity for electron transport. However, the susceptibility of leaves to cold may be modified by root temperature. When the stem, but not roots, was chilled, the quantum yield of PSII and the relative electron transport rates were much lower than when the whole plant, root and stem, was chilled at 10°C. Additionally, when the whole plant was cooled, both the activity of electron donation by NADPH and ferredoxin to plastoquinone and the amount of PGR5 polypeptide, an essential component of the cyclic electron flow around PSI, increased, suggesting that in these conditions cyclic electron flow helps protect photosystems. However, when the stem, but not the root, was cooled cyclic electron flow did not increase and PSII was damaged as a result of insufficient dissipation of the excess light energy. In contrast, the chlororespiratory enzymes (NDH complex and PTOX) remained similar to control when the whole plant was cooled, but increased when only the stem was cooled, suggesting the involvement of chlororespiration in the response to chilling stress when other pathways, such as cyclic electron flow around PSI, are insufficient to protect PSII.