Reproduction of mountain plants under temperature stress

During the growing season, mountain plants are exposed to large temperature fluctuations. On the one hand, with increasing altitude night frosts become more frequent; on the other hand, high irradiation on clear summer days can result in prostrate plants reaching temperatures up to 30 K above air temperature. Earlier investigations have shown that, under moderate temperature stress, leaves of mountain plants are sufficiently frost and heat resistant. Following sudden cold spells or during extended hot weather periods, however, the threshold values for frost and heat injury can be reached or even exceeded. The extent to which reproductive organs (flower buds, flowers, immature and mature fruits with seeds) are endangered by extreme temperatures is largely unknown. As the reproductive phase is generally a very vulnerable phase, it can be expected that reproductive organs are at a higher risk of becoming frost and heat damaged than vegetative organs. If flowering and seed formation frequently fails, there are far-reaching consequences for the maintenance and the colonization potential of a species. The present project investigates the temperature resistance of reproductive organs in different developmental phases in common mountain plants of the alpine and nival zone in the European Alps and compares this with the temperature resistance of vegetative above-ground organs at the same time. In field experiments, intact plants are exposed to different levels of temperature stress in small, temperature controlled chambers. Frost and heat effects are further analyzed in the laboratory using different methods. During freezing, ice nucleation temperatures are recorded and ice propagation in reproductive organs is followed. Targeted in situ temperature experiments on naturally growing plants should reveal the frost and heat resistance of reproductive tissues and in particular show the after effects of temperature extremes on further reproductive development and reproductive success. Comparative in situ investigations on plants growing in the northern part of the Chilean Andes,, where temperature extremes during the growing season are the rule, are planned. Frost and heat susceptibility of reproductive organs will be analysed against the background of possible temperature extremes at the natural sites at different times during the growing season. It is assumed that plant species with a short developmental period are at lower risk of injury from temperature extremes than species with an extended developmental period. Further, reproductive tissues of plant species reaching the upper distribution limit are expected to resist subzero temperatures better than they do in species from lower elevations, and vice versa, as plants from lower elevations are believed to be less sensitive to heat. Temperature resistance is an important key for the altitudinal range of a plant species and the potential to persist in a changing climate.

High-mountain plants are exposed to severe winter frost, summer frost during cold spells, and heat near the ground during clear weather in summer. The question arises how vulnerable flower buds, flowers and young fruits are, and to what extent reproduction of mountain plants is endangered by temperature extremes.We investigated heat and frost resistance in reproductive and vegetative shoots of common plant species from the subalpine, alpine and nival zone of the European Alps during different stages of reproductive development. With regard to frost, both, the direct impact and the after effects of frost on the reproductive success were assessed. Additionally, the extent of supercooling and the pattern of ice propagation were determined.During winter, the dormant flower buds remain undamaged down to -10C and show only marginal frost injuries down to 20/-30C. Flower buds are safe below the snow at temperatures between 0 to -5C but may be damaged during periods of severe frost at exposed sites. During the growing season, the reproductive shoots are particularly frost-susceptible during bolting, anthesis and early fruiting. The weakest links within the reproductive structures are the peduncle including flower stalks, the stigma and the style (first frost damage between -2 and -4C). Flowering shoots of most species do not tolerate ice formation in their tissues. However, due to supercooling, not all shoots of an individual freeze at the same time, but 2-9 K below the temperature causing frost damage. An exception is the nival plant species R. glacialis whose inflorescences freeze at -3C and are ice tolerant. By linking data of frost resistance with microclimate data from the natural habitats and calculating the frost damage risk we could show that low-statured species of the alpine zone are rarely endangered as long as they are protected by snow. The situation is different in the subnival and nival zone, where frost-susceptible reproductive shoots may become frost damaged even below the snow. Without snow protection, absolute temperature minima would cause severe frost damage to all nival species tested. Overall, reproductive shoots are markedly more frost-susceptible than vegetative shoots, but supercooling, phenological adjustment and snow protection effectively minimize the risk of frost damage. Reproductive shoots of high-mountain plants are remarkably heat resistant. In the most heat-susceptible species, first damage occurs at 44 to 45C. Stalked inflorescences or flowers, because of permanent air movement, reach maximum temperatures of 30-33C which is far below the damage threshold. In unstalked buds and flowers, however, we recorded up to 40C. It is conceivable that the critical temperature thresholds may be exceeded during hot and dry periods. This may become a problem for heat-susceptible species when temperatures rise in mountain areas as predicted.