Altiudinal transect in frost resistance of growing leaves

Research project P 14524 Alitudinal transect of frost resistance in growing leaves Gilbert NEUNER 08.05.2000 Freezing stress is one of the most important environmental constraints limiting plant distribution. Frost damage can be a regional contributor to the high altitude treeline formation. The occurrence of spring frost damage to unfolding leaves has been suggested to increase as one effect of climatic warming. Know-ledge of frost resistance of leaves during the growing period and of particular frost susceptible stages during bud burst can be considered significant as it is the most susceptible ontogenetic stage that determines the persistence of a species in a given habitat. Complete data sets on frost resistance of growing leaves of wild plants are, however, scarce and often lack the knowledge of the most susceptible stage. Critical temperature thresholds for frost damage in growing leaves of European plant species available by now range between -1.5 and 12.5C. These data, however, can hardly be compared and used in models as different testing procedures, testing times, frost resistance definitions, the neglection of the ability to short-term frost hardening and dehardening and the determination of frost resistance on detached leaves are factors of uncertainty. With these data field frost survival may not reliably be predicted. By developing a new equipment aim to determine critical temperature thresholds for field frost survival and their variability that may serve as reliable basis for future model predictions. By selecting a broad range of plant species we want to take the opportunity of the location of Innsbruck where different plant life and growth forms can be studied in close vicinity along an altitudinal transect (600 m - 2600 in) in their natural habitat within and at their natural distribution boundary. By this the same species can be studied in various environments and in different developmental stages at the same time and for a longer period due to gradual bud burst at different altitudes. Alpine habitats are stress-dominated as abiotic factors overrule biotic ones. Alpine plant species, therefore, will respond particularly sensitive to climatic changes (bioindicators of climate changes). By testing a broad range of plant species- general traits can be assessed that are important for field frost survival of leaves during the growing period and for model predictions concerning the future distribution of plants.

Freezing stress is one of the most important constraints limiting plant distribution. Plant cells are not always similarly susceptible to frost damage. The most frost susceptible time is during flushing of buds and during summer. Warmer winter periods due to climate change can cause an earlier bud break that increases the risk of damage by late frosts. The results of the investigation show a high variability of frost resistance within a single species during flushing of buds and in summer of up to 11C. Expanding leaves (between 0 and 60% of full expansion) were most frost susceptible. These findings relativise earlier climate change modells and make a prognosis of late frost damage and hence the potential climate change induced alteration of distribution boundaries of plants very difficult. Until now, the frost resistance of leaves during flushing of buds and in summer was considered to be rather stable. Depending on the species frosts between -1.5 and -12.5C were thought to be critical to frost survival. Variability of frost resistance was assessed on a broad variety of plant species and growth forms, including low land species and high alpine plants extending to the uppermost boundaries of higher plant life. In tree species frost resistance increased in tendency with an increasing upper distribution boundary. Although tree line trees were more frost resistant than trees from low altitude sites, their new leaves get frost damaged in regular perennial intervals during late spring frosts. This may locally be a reason for the formation of the timberline. Within the plants growing at highest altitudes some very frost resistant species but also some quite frost susceptible species were found. The frost susceptible species compensate by a fast restitution capacity through below ground organs that usually remain undamaged. By the development of a new instrumentation for the determination of frost resistance and ice formation in plants at their natural growing sites completely new insights were possible. Despite earlier suggestions during summer ice formation within the leaf tissue outside of the cells was recorded at species-specific different freezing temperatures (-0.6 - -3.5C). Frost damage did not develope at this stage. However, this extracellular ice successively withdraws water from the cells and ice cristalls cause a mechanical deformation of cells. During further lowering of temperaure in most species a second ice formation within cells was recorded that commenced at the same time as frost damage. Only very frost resistant alpine plants and cushions did not show a second freezing event. In these specialists frost damage must hence be due to freeze dehydration, as usually observed in other species during winter. By this these species are physiological model organisms that can show how frost susceptible crop plants could survive frosts during the active growing period without frost damage.