Stronger cooling effects of transpiration and morphology of the plants from a hot dry habitat than from a hot wet habitat

Hua Lin, Yajun Chen, Houlei Zhang, Peili Fu, Zexin Fan

 Leaf temperature exerts an important impact on the physiological processes of leaves. It is predominately determined by ambient temperature, but can also be regulated by leaf morphology and transpiration. When plants are exposed to hot conditions, they can reduce the amount of radiation absorbed through reflection and movement, and can dissipate excessive heat via radiation emission, heat convection, and transpiration. However, the relative contribution of each of these processes differs greatly in different habitats or between different plant groups. For plants in arid habitats, morphological adjustments are commonly believed to be the main means by which plants have evolved to achieve heat dissipation, while the contribution of transpiration to leaf cooling is often ignored. However, some arid habitat species use another strategy: they rapidly fix carbon in the short periods when water is available, during which they may have high transpiration. In this case, transpiration cooling is beneficial for achieving high rates of photosynthesis.

We investigated 38 canopy species of seedlings in a greenhouse, including 18 dominant species from a hot wet habitat (HW) and 20 dominant species from a hot dry habitat (HD). We found that the species from HD always had lower leaf temperatures than the species from HW under the same conditions. Both transpiration capacity and cooling effect of leaf morphology were stronger for the plants from HD. Transpiration-related structures (e.g. a denser vein network and large stomatal area) were also more developed in plants from HD than plants from HW.

Our results confirmed that transpiration and leaf morphology provided double insurance for avoiding overheating, particularly for plants from HD. We emphasize that transpiration is a more effective way to cool leaves than morphology when water is sufficient, so that the plants from HD can achieve high photosynthesis whilst cool leaves efficiently during sporadic rainfall. Present research separates thermal effects of morphology and transpiration under natural environment. Our results provide further insight into the relationship between morphology and transpiration for the regulation of leaf temperature, the co-evolution of gas exchange (CO2 and H2O exchange) and thermal regulation of leaves.

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Image provided by authors.

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