Mangroves are renowned for their ability to thrive in harsh, saline environments, but new research reveals that their leaves exhibit some peculiar properties. A study by Guo-Feng Jiang and colleagues, published in Annals of Botany, examined the leaf anatomy of 13 mangrove species across China. The researchers found that while mangrove genomes are tiny, their leaf cells are oversized. Leaf anatomy also varied in response to climate, with higher temperatures linked to smaller epidermal cells and higher densities of veins and stomata.
Mangroves are well adapted to life in an environment other plants could find hostile. For example, they have a high degree of salt tolerance to cope with the ocean and aerial roots that allow oxygen absorption in waterlogged soils. They also have small genomes, which have been connected to stress tolerance in past research.
Small genomes for plants allow them to reduce their minimum cell size, and it can also allow a greater cell packing density. This gives plant leaves the practical advantage of improving photosynthesis by allowing more carbon dioxide to diffuse mesophyll cells in the same leaf volume.
However, their leaf anatomy remains surprisingly flexible. Leaf cell size is not rigidly defined by genome size, as in other plants, but instead varies with climate. Jiang and colleagues found that colder sites had larger epidermal cells, while warmer sites had smaller cells and higher vein and stomatal densities, possibly to facilitate increased transpiration.
Intriguingly, mangrove stomata, the air holes in leaves that allow gases in and out, were larger than epidermal cells, unlike most plants. Unlike the usual trend, stomatal size decreased with increasing genome size. The researchers suggest this may relate to the mangroves’ efficiency in stomatal opening and closing, helping them carefully regulate water loss.
The densities of veins and stomata were also co-ordinated, but not in the typical way. Usually, smaller epidermal cells allow higher stomatal densities, but the correlation was independent of epidermal cell size. The researchers propose this could indicate novel adaptations in mangroves for optimising gas exchange and water transport.
Jiang and colleagues argue that their results show there is something odd about the allometry, the relationship of plant size and shape, in mangrove species compared to other plants. They write:
Our analysis of 13 mangrove species, four of which occurred at more than one site, provides strong evidence that the allometry of cells and tissues in mangrove leaves is distinct from other C3 angiosperm species. Our results highlight that while mangroves exhibit some of the same trait relationships exhibited by non-mangrove angiosperms, they deviate in some potentially important ways, most notably that they have unusually small epidermal pavement cells and large guard cells. Despite these deviations from other angiosperms, mangroves nonetheless attained similar maximum theoretical stomatal conductance. Because leaves are composed of multiple cell types and because genome size limits only minimum cell size, there can be numerous combinations of final cell sizes and packing densities that allow for variation in leaf structure that lead to similar maximum potential gas exchange. Understanding the implications of these differences could shed further light on how the unique selective pressures of the mangrove habitat have resulted in novel anatomical and physiological adaptations.Jiang et al. 2023
The flexibility of mangrove leaf anatomy and its divergence from typical trends reveals a new perspective on how these plants are exquisitely tuned to their environment. While mangroves achieve a similar maximum stomatal conductance to other trees, the mechanisms and trade-offs that underpin this are distinct. Their anomalous vein-stomata coordination and stomata larger than epidermal cells hint at structural and physiological specialisations that will produce plenty of fruitful research in the future.
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Jiang, G.-F., Li, S.-Y., Dinnage, R., Cao, K.-F., Simonin, K.A. and Roddy, A.B. (2023) “Diverse mangroves deviate from other angiosperms in their genome size, leaf cell size and cell packing density relationships,” Annals of Botany, 131(2), pp. 347–360. Available at: https://doi.org/10.1093/aob/mcac151.