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. 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 leaves exposed to freezing. Whole saplings of Liriodendron tulipifera were exposed to freezing and thawing trajectories designed to mimic natural spring freezes. We monitored the formation of freeze-thaw xylem embolism and damage to photosynthetic tissues to reveal a predictable progression of ice formation across the leaf surface that is strongly influenced by leaf vein architecture, notably the presence or absence of bundle sheath extensions. Our data also 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 geographic distributions, and show that bundle sheath extensions may stall or even prevent freezing spread.