Low temperature growth limits of alpine plants and winter crops

Temperature is a major driver of plant growth in cold ecosystems such as boreal, arctic and alpine environments and of winter crops in temperate regions during winter and springtime. Plant growth and productivity in those settings are presumably constrained by low temperature through slow tissue formation (sink activity) rather than photosynthesis (source activity). The exact temperature thresholds and the growth dynamics under critically cold conditions are needed to explain the success of wild plants and winter crops under critically low temperatures. The overall hypothesis of this doctoral thesis is that all cold adapted plants, regardless of whether they are alpine species or winter crop varieties are limited by similar temperature related limitations and are constrained by similar thermal thresholds. This hypothesis was tested by three field experiments exploring the thermal limits of leaf growth in monocot and dicot winter crops and for root growth in alpine plants. Both, leaves and roots are expected to be affected by similar temperature related limitations at the cell and tissue level. To elucidate the thermal limits for growth of leaves and roots, accurate, non-destructive and particularly continuous measurements with short measurement intervals were used on the basis of modern image sequence analysis methodologies.
Leaves in the studied winter cereals and rapeseed plants reached zero growth rates both at 0 °C air temperature. Absolute minimum temperatures for root length increment in cold-adapted alpine species were found between 0.8 and 1.4 °C. These zero points of growth are associated with characteristic changes in the apical tissues' structure. Meristematic root cells were inhibited to expand beyond a certain temperature limit and led to a shortened root elongation zone and the root tissue was less lignified. These findings suggest that cell differentiation and xylem lignification are decisive processes that feed back on cell production and prevent any further extension of root tips. Similar physiological limitations might also apply to leaf growth but this remains to be proven by histological examinations of cold grown leaf tissues.
It is concluded that the low temperature limit for leaf growth in cold adapted plants, the so-called base temperature, is at 0 °C and is similar for both, monocot and dicot plants. However, differences were observed with respect to the growth dynamics with rising temperature and the influences of other environmental parameters. While the leaf growth dynamics in monocots are strictly temperature controlled, dicot leaf growth at low temperatures shows a transition from pronounced environmental regulation below 4 °C and a superposition of environmental and internal, circadian-clock-dependent regulation. These findings support the initial hypothesis that the limitation of plant growth by low temperature is similar in all cold adapted plants. Thus, the adaptation to low temperature is a uniform evolutionary mechanism, responsible for the success of arctic alpine plants or crops in winter. The detected genotype specific responses of cereal leaf growth/elongation to temperature appear to be promising for breeding for cold tolerant winter crops that have the potential to accumulate more biomass during winter. Such varieties could be harvested earlier in the season to avoid summer droughts and thus increase yield stability in the context of climate change.
This thesis presents developmental and cellular explanations of plant growth in the cold. The results are both useful to explain and define the growing season and the distribution limits of arctic-alpine plants, and to assess the performance of cold tolerant winter crops for plant breeding. The methods refined during this thesis enable in situ measurements under natural field conditions as well as in crop fields, and they will permit advances in our understanding of other growth related processes under field conditions and thus present a promising tool for scientist and breeders.