Water Loss From Bagged Leaves During Storage: Why and When?

In ecophysiology leaves are frequently stored for hours after sampling before measuring their leaf water potential (Ψ_leaf). Here, we address a previously unidentified source of error, that metabolic heat generation can cause continuous water loss from leaves stored in impermeable bags, leading to a Ψ_leaf drop over time. We tested Ψ_leaf drop rates under various conditions: two bag materials, two species, initial Ψ_leaf above or below the turgor loss point (Ψ_tlp), and storage at 25°C versus 4°C. We partitioned leaf water loss due to condensation on the inner bag surface or permeation through the bag. We found that Ψ_leaf dropped by up to 0.39 MPa per hour, with 41%-89% of the water leaving the leaf condensed on the inner bag surface. Plastic bags showed higher Ψ_leaf drop rates than aluminium bags, and leaves above Ψ_tlp experienced greater drops. Storing leaves at 4°C reduced the Ψ_leaf drop rate by 60% compared to 25°C. Leaves were 0.2-0.3°C warmer than the bags, likely due to metabolic heating. Our energy balance model suggests that water loss is lower when storing leaves at cooler temperatures, using leaves with low stomatal conductance, deflated bags, and leaves with low Ψ_leaf.