Abstract
Heat shock is considered an abiotic stress for plant growth, but the effects of heat shock on physiological responses of cucumber plant leaves with and without downy mildew disease are still not clear. In this study, cucumber seedlings were exposed to heat shock in greenhouses, and the responses of photosynthetic properties, carbohydrate metabolism, antioxidant enzyme activity, osmolytes, and disease severity index of leaves with or without the downy mildew disease were measured. Results showed that heat shock significantly decreased the net photosynthetic rate, actual photochemical efficiency, photochemical quenching coefficient, and starch content. Heat shock caused an increase in the stomatal conductance, transpiration rate, antioxidant enzyme activities, total soluble sugar content, sucrose content, soluble protein content and proline content for both healthy leaves and downy mildew infected leaves. These results demonstrate that heat shock activated the transpiration pathway to protect the photosystem from damage due to excess energy in cucumber leaves. Potential resistance mechanisms of plants exposed to heat stress may involve higher osmotic regulation capacity related to an increase of total accumulations of soluble sugar, proline and soluble protein, as well as higher antioxidant enzymes activity in stressed leaves. Heat shock reduced downy mildew disease severity index by more than 50%, and clearly alleviated downy mildew development in the greenhouses. These findings indicate that cucumber may have a complex physiological change to resist short-term heat shock, and suppress the development of the downy mildew disease.
Highlights
Heat shock is defined as the exposure of plants to a sudden and significant temperature increase for a short period of time [1]
We investigated the responses of physiological processes such as photosynthesis, chlorophyll fluorescence, carbohydrate metabolism, antioxidant enzyme activity and osmolytes to short-term heat shock in cucumber plants growing under normal greenhouse conditions as would occur in large-scale cucumber greenhouse production
These results indicate that when the cucumbers suffered from heat stress, the leaves opened their stomata to reduce leaf temperature through transpiration [39, 40]
Summary
Heat shock is defined as the exposure of plants to a sudden and significant temperature increase for a short period of time [1]. As global temperature increases combined with the effects of more frequent extreme weather events, heat shock could have significant influences on plant physiology and development. Plants are subject to physical stress in their environment [2]. The initial effects of high temperature on plants include growth reduction, water loss, a change in photosynthetic efficiency, and oxidative stress [3]. Longer periods of high temperature stress may result in wilting, necrosis, leaf pigmentation loss for herbaceous plants, as well as leaf elongation repression [4]. Heat stress is considered as a problem for agriculture throughout the world [5, 6], and it may lead to dramatic losses in crop yields [7]
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