Adaptation and Acclimation of Gametophytic Traits to Heat Stress in a Widely Distributed Wild Plant Along a Steep Climatic Gradient.

Climate change-induced heat waves often reduce seed yields and quality via high-temperature effects in the gametophytic phase. Yet, in contrast to model and crop species, the ability of pollen and ovules to adapt or acclimate to heat stress in wild plants remains poorly understood. To address this gap, we examined the adaptation and acclimation potential of six gametophytic traits in 11 wild populations across a temperature gradient in Europe. First, we cultivated plants in a common garden to reveal differences in gametophytic traits indicative of adaptation. Next, we assessed their acclimation potential by subjecting flowering plants to two chronic heat stress (CHS) treatments: moderate (35°C/30°C) and severe (40°C/35°C) for 18 days. Also, we estimated the CHS effects on seed quantity and quality. The common garden experiment showed no intraspecific variation in gametophytic traits across the temperature gradient, suggesting these traits may not influence reproductive adaptation to local habitats. During CHS, the female gametophyte was less temperature-sensitive than the male. Moderate CHS led to larger ovaries with more large-sized ovules, while severe CHS reduced ovule numbers but increased their size. Both CHS treatments decreased pollen grain numbers, size, and anther length, with severe CHS causing greater reductions. These reductions in gametophytic traits led to lower seed yield and quality. Under both CHS treatments, acclimation potential did not vary along the temperature gradient, except for pollen size under severe CHS, which was larger in warmer climates. Our findings revealed the lack of adaptation and acclimation mechanisms in the gametophytic traits (except for pollen size) of wild populations along the temperature gradient. These findings suggest that plants may rely on alternative strategies, such as shifts in gametophyte physiology and biochemistry or flowering phenology, to respond to thermal stress associated with heat waves.