Maximum Heat Tolerance of Alpine Plants

In many ecosystems high temperature is a crucial abiotic stress factor for plant recruitment and survival and will become even more important over the following decades. This is also true for high mountain regions, as many alpine species have evolved heat cumulating growth forms, like cushions and rosettes, which may - against the background of global warming - increasingly act as fatal heat traps causing heat stress and lethal damage. While mean global surface temperature increase was +0.60.2 C during the 20th century, in alpine habitats the temperature increase was found to be more than double that. Although plants may adjust their heat tolerance within short time-spans (e.g. up to 2.2 K h-1 in Saxifraga paniculata) to some extent, it is evident that on clear and calm summer days high solar radiation can cause substantial overheating of plants. Possible consequences are disturbances of the photosynthetic processes as well as lethal damage and may have a great impact on plant survival, plant distribution ranges and the entire ecosystem. Knowledge of the species-specific maximum heat tolerance of alpine plants is insufficient because appropriate measurement systems are still missing. Additionally, much of the available data is not derived from in situ measurements and does not allow for the study of recuperation and repair greatly reducing its ecological relevance. As many questions concerning the dynamics of heat tolerance are still unsolved (e.g. the influence of solar radiation on the dynamics of heat tolerance) and there is no existing measurement system which would allow maximum heat tolerance to be determined in situ, a novel computer-controlled and field-suitable heat tolerance test instrument (HTTS.12) will be designed and constructed. Maximum heat tolerance, the influence of solar radiation on heat tolerance, the maximum heat limits of PS II, responses of the photosynthetic gas-exchange and the pool sizes of all components of the xantophyll cycle and ROS de-toxifying substances to heat stress including after effects will be measured for a wide range of representative alpine and subnival plant species. Furthermore, extended microscopy studies are planned to describe the impact on ultrastructure (in terms of adaptation and disruption) of heat stress, the status of heat tolerance and photosynthetic functions. Two altitudinally different study sites in the Tuxer Alps (1995 m a.s.l) and in the Zillertaler Alps (2660 m a.s.l.) will be chosen to cover the alpine as well as the subnival ecotone and a wide range of significant plant species. On both sites extended micrometeorological data will be recorded to document the actual frequency and extent of heat stress on the related plant species. While heat stress in high mountain plants is normally closely linked to high solar radiation, in tropical rainforests plants experience heat as convection heat at low solar radiation and high relative humidity of the surrounding air. Comparative studies on selected tropical species are expected to enhance our comprehension of functional and structural adaptations. For this reason complementary measurements will be conducted in the tropical Terai in Nepal. This research project will greatly increase our understanding of the capacity of alpine and subnival plants to survive in their natural habitats, which will increasingly be affected by global warming. It will close existing gaps in our knowledge about the effects of heat stress and photosynthetic function on the cellular ultrastructure and vice versa and help to make predictions on the future destiny of high mountain plants and ecosystems more reliable.

Many alpine plant species have evolved prostrate growth forms such as cushions and rosettes, which allow for effective decoupling plant body temperature from the cool ambient. Against the background of global warming which particularly affects high mountain regions of the northern hemisphere (Austria: + 1.4 C is expected until 2050) this can be fatal. For future risk assessment it is therefore necessary to gain knowledge on the heat hardening capacity of alpine plants and on their specific defense mechanisms against heat stress. This particularly with regard to the emergence of radical oxygen species (ROS) and ROS-detoxifying pathways as well as adaptations at the level of cellular ultrastructure such as chloroplasts. A prerequisite for such work is equipment for applying controlled heat stress and for determining heat tolerance under natural environmental conditions. Such equipment was not available until now. Therefore, we designed a novel Heat Tolerance Testing System (HTTS) which allows for determining heat tolerance in situ at the presence of natural solar irradiation and without the necessity of detaching plants parts. We could demonstrate that heat tolerance of photosynthesis and of the entire leaf tissue in most cases is significantly higher by several degrees when determined with the HTTS as compared to standard laboratory based test assays. However, the underlying mechanisms are not yet fully understood. Still, the higher heat tolerance in the field as such is good news. The less positive news is that based on long-term recording, leaf temperature of certain plants even under present conditions may occasionally surpass the level upon which restrictions of photosynthesis and also initial leaf damage may occur. We could demonstrate that even short-term and sub-lethal spells may restrict photosynthetic assimilation for several days, but also that natural sunlight during heat stress may have a significant protective effect on photosynthetic functions and leaf tissue. Leaf pigments such as xanthophylls and antioxidants like ascorbate and glutathione were shown to play an important role in antioxidative defense under the prevailing stress conditions. Long-term in situ heat hardening of selected plant species induced maximum heat tolerance in a highly ecologically significant manner. From these data first risk assessment of suffering from heat under current and future conditions will be possible. In summary, the project significantly contributed to our understanding on the methodological, (eco-) physiological and ultrastructural level. From the development of the HTTS not only eco-physiological research will benefit but also plant breeding and cultivar testing in regard to providing more heat tolerant crops for the in future changed climate.