Abstract: Aims More frequent extreme high temperature events cause threats to the growth and survival of plants under global climate change, thus accurate evaluation of physiological heat tolerance across plant species is essential for species conservation and adaptability prediction. Magnoliaceae, as a typical basal angiosperm lineage, is one of the representative lineages in subtropical evergreen forests in China, with high ecological and evolutionary significance. However, the photosynthetic heat tolerance of Magnoliaceae plants is still unclear. Methods In this study, 23 species of Magnolicaeae were selected to measure their photosynthetic heat tolerance using the chlorophyll fluorescence method. For each species, leaf morphological traits were measured and climatic temperature niche across their geographic distribution areas were calculated, in order to analyze the relationships between photosynthetic heat tolerance, leaf morphology and temperature niche. Important findings The temperature that causes 50% decrease of the maximum photochemical quantum efficiency of photosystem II (T50) ranged from 46.1 to 56.7 °C. T50 was positively correlated with leaf area, but showed no significant correlation with leaf thickness. Evergreen species had higher T50 than deciduous species. T50 was positively correlated with mean annual temperature and mean temperature in the coldest month, but was not correlated with mean temperature in the warmest month. A weak phylogenetic signal was found in T50. Our results suggested that photosynthetic heat tolerance of Magnoliaceae species were generally strong, which was influenced more by leaf morphology and environment conditions, rather than phylogeny. Furthermore, future extreme high temperature events may not enhance plant heat adaptations, instead they may severely threaten Magnoliaceae plants living in warmer areas.

Key words: photosynthetic heat tolerance, Magnoliaceae, heat stress, temperature niche, leaf morphology, leaf habit