Leaf shrinkage causes a decline of leaf hydraulic conductance during mild drought so proctecting xylem from embolism

Plant productivity is closely dependent on CO2 uptake through stomata and carbon fixation through photosynthesis. Both processes critically depend on plant and leaf water status. Reduced water supply to the mesophyll can induce stomatal closure thus affecting photosynthesis rates. Leaves represent the terminal part of the soil-plant-atmosphere continuum, and the water potential drop between the leaf and the atmosphere drives water loss due to evaporative processes and ultimately generates the driving force for water movement through the plant. Thus, the efficiency of the liquid water transport through the leaf (i.e. leaf hydraulic conductance, KL) plays a key role for whole plant hydraulics and, ultimately, productivity. Recently it has been suggested that, during mild dehydration, the decline of leaf hydraulic conductance triggering partail stomatal closure is mainly caused by the decline of the conductance of the mesophyll pathways outside the xylem (Kox ), and this drop could preserve the leaf from xylem water cavitation. However, no direct measurement has been performed to test the hypothesis. We studied in this respect two evergreen (i.e. Aleurites moluccana L. (Willd) and Magnolia grandiflora L.) and two deciduous (i.e. Quercus rubra L. and Vitis labrusca L.) species. We specifically measured the decline of leaf hydraulic conductance and of its components as well as leaf shrinkage in response to dehydration. Leaf hydraulic resistance (RL=1/KL) values were correlated with leaf thickness during dehydration in M. grandiflora and Q. rubra but not in A. moluccana and V. labrusca. The slope of leaf thickness shrinkage at values of leaf water potential around -0.5 MPa was linearly correlated to values of xylem water potential inducing 80% loss of hydraulic conductance and to the modulus of elasticity (emax ) across the four species. Moreover, leaf density was related with different leaf shrinkage traits (i.e. leaf thickness at turgor loss point, the ratio of leaf thickness values and leaf water potential values up to turgor loss point). Our findings suggest new interesting physiological roles for Kox in modulating the leaf xylem resistance to cavitation and plant resistance to drough