Vein cavitation and stomatal behaviour of sunflower (Helianthus annuus) leaves under water limitation

The impact of leaf vein cavitation and embolism on stomatal response and leaf hydraulic conductance was studied in potted plants of sunflower subjected to water limitation. Plant dehydration was achieved either by cutting well-watered plants near their base and leaving them dehydrating in air or by depriving intact plants of irrigation. The vein cavitation threshold (Psi(CAV)) was estimated in terms of ultrasound acoustic emissions (UAE) from the leaf blade versus leaf water potential (Psi(L)). This was found to be the same (Psi(CAV) approximate to -0.6 MPa) for leaves of both cut and intact plants where stomata began to close in coincidence with starting vein cavitation. Vein embolism was detected by infiltrating leaves at different Psi(L) with 0.7 mM fluorescein and measuring the percentage fluorescent area as percentage of total leaf surface area. A distinct loss of vein functionality (up to 50%) was found to occur in leaves at progressively decreasing Psi(L), starting when leaves reached Psi(CAV). A linear positive relationship with high statistical significance was found to exist between g(L) and percentage leaf fluorescent area, thus indicating that stomata were sensitive to vein embolism. The hydraulic conductance (K-L) of the leaf was affected by leaf dehydration less than expected (K-L decreased by about 20% between near full turgor and Psi(L) = -1.3 MPa). When the extravascular leaf compartment was excluded either by killing cells by immersing leaves in 70% ethanol or by cutting the main leaf venous system through to allow flow to bypass it, K-L turned out to increase 5.5 times, thus suggesting that the high dominance of the hydraulic resistance of the extravascular leaf compartment over the total leaf resistance might buffer or mask possibly large local changes in K-L inducing stomatal closure.