Water movement through Quercus rubra I. Leaf water potential and conductance during polycyclic growth

Abstract

ABSTRACTTwo experiments examined simultaneous changes in leaf area (AL), root length (Lr), stomatal conductance (gs), leaf water potential (ΨL), transpiration and hydraulic plant conductance per unit leaf area (G) during the first three shoot cycles of northern red oak (Quercus rubra L.) grown under favourable and controlled conditions. Each shoot cycle consisted of bud swell, stem elongation, leaf expansion and rest; roots grew almost continuously. The gs of all leaves decreased substantially while leaves of the newest flush were expanding and increased modestly when seedling leaf area remained constant. Overall, gs decreased. The ΨL of mature leaves decreased during leaf expansion and increased by an equivalent amount during intervening periods. Possible explanations for the paired changes in gs and ΨL are considered. Changes in G closely paralleled those of canopy gs. These parallel changes during polycyclic seedling growth should act to keep seedling ΨL relatively constant as plant size increases and thereby help prevent ΨL from dropping to levels that would cause runaway embolism.