Ice and air: Visualisation of freeze-thaw embolism and freezing spread in young L. tulipifera leaves

Spring freezing is an unforgiving stress for young leaves, often leading to death, with consequences for tree productivity and survival. With an increasingly unpredictable climate leading to more spring freezing events, it is important the we understand how freezing damages young leaf tissue. While both the plant water transport system and living tissues are vulnerable to freezing, we do not know whether damage to one or both of these systems causes death in young leaves exposed to freezing and thawing. Whole saplings of Liriodendron tulipifera were exposed to freezing and thawing trajectories designed to mimic natural spring freezes. We visualised freezing damage to the water transport system (xylem embolism) and living tissues (mesophyll freezing, decline in chlorophyll fluorescence). We 1.) provide the first visualisation of freeze-thaw embolism in leaves and compare this to drought-embolism, 2.) reveal a predictable progression of ice formation within the mesophyll which is strongly influenced by leaf vein architecture, notably the presence or absence of bundle-sheath extensions, and 3.) show that freeze-thaw embolism occurs only in the largest vein orders where mean vessel diameter exceeds 30µm. With evidence of both freeze-thaw embolism and damage to photosynthetic tissue, we conclude that this dual-mode lethality may be common among other wide-vesseled angiosperm-leaves, potentially playing a role in limiting tree distributions, and show that bundle-sheath extensions may stall or even prevent freezing spread.