Plant growth, the filling of carbon sinks, is much more than just acquiring photoassimilates. It means constructing new tissue, new cells, cell walls, proteins... The growth processes are very sensitive to low temperature and also require resources other than carbon compounds. Growth is very slow below 6 °C and is always zero at 0 °C, a temperature at which leaf photosynthesis already reaches substantial rates.

Try to find out what commonly limits alpine plant growth (outside the tropics) to a greater extent than lowland plant growth. A hint: Look for constraints which do not affect the instantaneous carbon balance, but still restrict growth, i.e. the formation of new plant structures (2 answers are correct). Please disregard exceptional situations and focus on common alpine life conditions as described in previous lessons.

Growth could theoretically continue for 24 hours and 365 days. If it does not, why?

The basic constraints for the formation of new tissue are time constraints induced by low temperature: In the 24 hour cycle, night hours are too cold for growth processes (often too close to 0 °C), over the year thermal seasonality restricts growth to the growing season, which can be as short as 6 weeks in the nival zone.

While photosynthesis (daylight hours) is hardly constrained by low temperature the structural investment of carbon is restricted to the warm daytime period. At low altitude plants can produce new cells day and night. At high altitude, they can do so during warm spells only. However, the cellular processes involved in growth are cold-adapted. At low temperatures alpine plants grow faster than lowland plants. At high temperatures lowland plants grow faster.

Remember: The growth of alpine plants is in large part restricted by the length of the growing season, and within the growing season, by too low temperatures during night and during bad weather situations: There is no shortage in photoassimilates.

Fig. 1 shows in situ measurements of growth in Poa alpina at 3000 m elevation in the Ötztal Alps (Austria). Linear variable displacement transducers (LVDT) are solenoids with a mobile core tied on one side to a leaf tip and bearing a counter weight on the other side.

• As the basal meristem produces new tissue, the leaf gets longer and the iron solenoid core moves up (pulled by the counter weight). The electrical signal produced by the change in electromagnetic induction permits a resolution of 1 μm (1/1000 mm) of leaf expansion growth.

• Note, this permits continuous 24 h records of leaf expansion, a surrogate for leaf growth.

2 - Using linear variable displacement transducers

29 August 2011
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