A novel system for in situ determination of heat tolerance of plants: first results on alpine dwarf shrubs.

Othmar Buchner,Ines Bauer,Matthias Karadar,Gilbert Neuner

doi:10.1186/1746-4811-9-7

Abstract

BackgroundHeat stress and heat damage to plants gain globally increasing importance for crop production and plant survival in endangered habitats. Therefore the knowledge of heat tolerance of plants is of great interest. As many heat tolerance measurement procedures require detachment of plants and protocols expose samples to various heat temperatures in darkness, the ecological relevance of such results may be doubted. To overcome these constraints we designed a novel field compatible Heat Tolerance Testing System (HTTS) that opens the opportunity to induce controlled heat stress on plants in situ under full natural solar irradiation. Subsequently, heat tolerance can be evaluated by a variety of standard viability assays like the electrolyte leakage test, chlorophyll fluorescence measurements and visual assessment methods. Furthermore, recuperation can be studied under natural environmental conditions which is impossible when detached plant material is used. First results obtained on three alpine dwarf - shrubs are presented.ResultsWhen heat tolerance of Vaccinium gaultherioides Bigelow was tested with the HTTS in situ, the visual assessment of leaves showed 50% heat injury (LT50) at 48.3°C, while on detached leaves where heat exposure took place in small heat chambers this already happened at 45.8°C. Natural solar irradiation being applied during heat exposure in the HTTS had significantly protective effects: In Loiseleuria procumbens L. (Desv.), if heat exposure (in situ) took place in darkness, leaf heat tolerance was 50.6°C. In contrast, when heat exposure was conducted under full natural solar irradiation heat tolerance was increased to 53.1°C. In Rhododendron ferrugineum L. heat tolerance of leaves was 42.5°C if the exposure took place ex situ and in darkness, while it was significantly increased to 45.8°C when this happened in situ under natural solar irradiation.ConclusionsThe results obtained with the HTTS tested in the field indicate a mitigating effect of natural solar irradiation during heat exposure. Commonly used laboratory based measurement procedures expose samples in darkness and seem to underestimate leaf heat tolerance. Avoidance of detachment by the use of the HTTS allows studying heat tolerance and recuperation processes in the presence of interacting external abiotic, biotic and genetic factors under field conditions. The investigation of combined effects of heat exposure under full solar irradiation, of recuperation and repair processes but also of possible damage amplification into the results with the HTTS appears to be particularly useful as it allows determining heat tolerance of plants with a considerably high ecological significance.

Highlights

Heat stress and heat damage to plants gain globally increasing importance for crop production and plant survival in endangered habitats
While at moderate relative humidity (Rh) leaf temperatures were substantially lower than the set - point temperature, heat exposure of leaves at high Rh (95 - 100%) in any case led to a significant approximation of leaf temperatures to the target temperature with a simultaneous reduction of temperature scattering in terms of minima and maxima
We presented a novel system for determining heat tolerance of plants in situ without the necessity of detachment of plant organs

Summary

Introduction

Heat stress and heat damage to plants gain globally increasing importance for crop production and plant survival in endangered habitats. The main reason for heat as an important stress factor in alpine sites are growth forms like rosettes and cushions that plants may have evolved to favor a decoupling from ambient cold air temperature [2] This improvement of thermal conditions may in turn cause critical overheating as the heat trapping nature can get fatal in situations with high solar irradiation input, combined to restricted transpiration and calm winds causing situations where the thermal high temperature thresholds can get exceeded. As high altitudes are not excepted from climate change [3] and high mountain vegetation is vulnerable to it [4] especially prostrate species and plants over bare and quickly desiccating soils like many pioneer plants and their seedlings may be affected by heat stress [5,6,7] This if the projected increase in global mean temperature [8] should come true. Food production especially in hot and arid regions due to increased temperature has already become an agricultural problem in many areas of the world [9]

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