Fresh out of the seed coat: first leaves of germinants are equally heat tolerant but less cold tolerant than later life stages

BACKGROUND AND AIMS: Plant photosystem thermal thresholds are regularly used as indicators for plant tolerance under extreme temperature scenarios. Photosystem tolerance can vary with species, leaf traits and environmental conditions. To date, most research into plant thermal tolerance has been conducted on established plants; likewise, leaf traits are routinely measured on fully expanded, adult leaves. To fully understand function and survival of plant communities, the first leaves of early life stages must be considered. METHODS: We sought to understand germinant photosystem thermal tolerance breadth through upper and lower critical temperature (Tcrit) thresholds, leaf mass per area (LMA) and leaf water content (LWC) for 22 species from three contrasting biomes - extreme desert and alpine, and benign coastal temperate. For seven of these species, we compared the relationship between these traits with those of older juvenile and adult life stages. KEY RESULTS: Biome of origin had no effect on germinant thermal thresholds. Thermal tolerance breadth (heat minus cold tolerance thresholds) varied significantly among germinant species, but not across the three life stages. However, although germinant heat tolerance within species was comparable to adult thermal tolerance, germinants were significantly less cold tolerant than adults. LMA was greater in older life stages than in germinants, LWC decreased across the life stages. LMA was greater in older life stages than in germinants and LWC decreased as LMA increased. CONCLUSIONS: In the first known study examining photosystem thermal tolerance breadth of newly emerged leaves of germinants, we found germinant PSII to be as heat tolerant as later life stages, but less cold tolerant, associated with their lower LMA and higher LWC. This demonstrates that not all physiological aspects of germinant tolerance are more vulnerable to thermal extremes than older life stages, with certain leaf traits potentially playing an important role in early survival.