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Comparison between parapatric mangrove sister species revealed higher photochemical efficiency in subtropical than tropical coastal vegetation under chilling stress
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•The primarily subtropical Kandelia obovata is better adapted to chilling than its exclusively tropical sister species Kandelia candel.•Differences in the chilling response of K. obovata populations are driven by local adaptation to minimum temperature in the home site.•Local adaptation to temperature may contribute to the differentiation of mangrove populations and the divergence of mangrove species.
The distribution range of coastal plants with widespread marine seed dispersal across latitudes is often limited by their ecological adaptation to narrow temperature ranges. Such a narrow temperature range is a tradeoff between maintaining productivity at lower temperatures, and the ability to compete with more rapidly-growing species at higher temperatures. Kandelia candel and Kandelia obovata are sister mangrove species that display geographic divergence, with K. obovata being largely subtropical and K. candel being exclusively tropical. We leveraged on the parapatric distribution of these species to study the relationship between chilling tolerance and species distribution. Specifically, we compared the photochemical efficiency of photosystem II values of northern and southern populations of these species under chilling conditions. Kandelia obovata maintained significantly higher photochemical efficiency under chilling and more quickly recovered to pre-chilling levels than K. candel. Furthermore, the northern population of K. obovata maintained significantly higher photochemical efficiency than the southern population under chilling, while no intra-specific differences in chilling tolerance were found between K. candel populations. Hence, K. obovata is better adapted to the minimum temperatures encountered in the subtropics than K. candel, with populations at higher latitudes displaying higher chilling tolerance. These results underscore the importance of adaptation to different temperature brackets to the divergence of K. obovata and K. candel, and hint at the role of photosynthetic efficiency as a driving force behind the geographic divergence and speciation of coastal vegetation.