In the last decades, the number of occurrences of extreme fluctuations in temperatures has been increasing. The extent to which plants are able to acclimate to short term temperature changes needs further investigation.

The objective of this experiment was to test whether fluctuating temperatures affect plant growth and to what extent plants are able to acclimate to short term changes in temperature.

For this purpose, poplar saplings (Populus deltoides x nigra) were grown in a growth chamber under constant PAR (300mmolm^-2s^-1) and RH (50%) but under three different temperature regimes: control (C), medium fluctuations (M) and extreme fluctuations (E). In the C treatment, the day/night temperature was constant throughout the experiment. In the other two treatments, temperature increased for five days, by 5ºC (M) or 10ºC (E) over the control and then decreased for five days by the same amount. The cycles of high and low temperature were continuously repeated until the end of the experiment. As a result, plants were subjected to equal heat sums but to different temperature amplitudes.

Sapling height and diameter, leaf area and biomass as well as the response of photosynthesis (A) and respiration (R) to changes in temperature were measured to evaluate the effect of the treatments on growth and carbon balance.

Results/Conclusions

Our results show that even though the plants experienced equal heat sums, the temperature regimes affected growth, A and R differently depending on the extent of the temperature changes imposed. Daily photosynthetic carbon assimilation was not different when temperature increased or decreased 5ºC (M) but was greatly depressed at a temperature change of 10ºC (E), compared to the control. Similarly the R:A ratio was found higher in the E treatment during the high temperature cycle, suggesting a negative impact of high temperature on the carbon balance. The increase in temperature resulted in an increase in stomatal conductance and plant transpiration when temperature increased.

In spite of the depression observed in daily photosynthetic carbon assimilation under extreme high and low temperatures, biomass accumulated during the experiment was higher in E and M treatment than in C, probably as a consequence of increased leaf biomass and area.

In conclusion, our results underline the importance of assessing the effects of temperature fluctuations as well as mean temperatures on growth and physiological functions.