Senior Lecturer Arne Sellin, University of Tartu
Dr. Patrizia Trifilò, University of Messina, Italy
Water plays an essential role in the life of plants. Since leaves continually lose water by transpiration, they need a constant water supply from the soil. Leaf hydraulic conductance (KL) is an important physiological parameter expressing the plants' inner water transport efficiency. Since KL is positively related to stomatal conductance and photosynthetic capacity, it influences significantly plant productivity, growth rate and fitness. The objective of the thesis was to investigate the variability of leaf hydraulic conductance both spatially and temporally within a forest canopy in relation to environmental factors. The study revealed that KL is significantly affected by light intensity, spectral composition and duration of exposure. KL varies considerably with position within the crown due to vertical environmental gradients. Under increased atmospheric humidity, leaf hydraulic conductance decreases as a result of declined transpirational flux. Also disruption of phloem transport by means of phloem girdling causes a decline in KL. In addition, KL is diurnally variable, primarily driven by air temperature and relative humidity in natural forest stands. Trees are characterised by hydraulic segmentation: leaves constitute a major part (>30%) of the total liquid-phase resistance of a tree and are also hydraulically more constrained and functionally more vulnerable than branches. To summarize, leaf hydraulic conductance is a highly dynamic parameter, enabling plants under changing environmental conditions to optimize their water use and achieve a positive water balance. In light of global climate change, understanding the principles of plant hydraulic transport and its regulatory mechanisms gives us the opportunity to predict the responses of plant water relations and contribute to elaboration of forest management measures to improve plant growth and alleviate possible adverse consequences on trees performance under changing conditions.