Freshwater plants, which evolved from terrestrial ancestors, exchanged water as a limiting factor in their environment for a potential carbon shortage due to low rates of carbon dioxide diffusion into the leaves. Adaptations made to accommodate this limitation include thin cuticles, a lack of stomata and sub-stomatal spaces, and chloroplasts in the epidermal cells. Some freshwater plants also have multiple carbon dioxide-concentrating mechanisms (CCMs), including bicarbonate (used by about 50%), CAM (about 8%), and C4 (about 4%). For plants with multiple CCMs, the switch from one to another can be induced by a change in carbon dioxide concentration, with CAM or C4 kicking in when CO2 is low. However, the anatomical changes that leaves undergo in response to being grown in different CO2 concentrations have never been studied.
In a recent article published in Annals of Botany, Shijuan Han and colleagues studied the leaf anatomy of the freshwater plant Ottelia alismoides. This plant is unique in that it is the only known species with three different CCMs and might therefore be expected to have unusual leaf anatomy as a result. The authors used light and transmission electron microscopy to study the differences between mature O. alismoides leaves that had been grown in either high or low CO2 concentrations.
At non-limiting CO2 levels, the researchers found that the plants used C4 photosynthesis, while at low CO2, both C4 and CAM were used. The Kranz anatomy normally associated with C4 photosynthesis, which serves as a CO2 concentration site, was absent in the leaves, which were composed of epidermal and mesophyll cells that both contain chloroplasts, as well as a large air space. The number of chloroplasts was higher in the epidermal cells than the mesophyll cells. The presence of chloroplasts in the epidermis is an arrangement that is rare in terrestrial angiosperm species, but common in aquatics. Structurally, the leaves were similar at different CO2 concentrations, but at the higher concentration, the thickness of the epidermal layers, the mesophyll, and the internal air space was higher.
The mesophyll cells of O. alismoides have a high starch content, suggesting that the cells are either the site of high rates of photosynthesis resulting from decarboxylation, or that they are used for starch storage. The discovery of high numbers of mitochondria grouped about the mesophyll chloroplasts in leaves grown in low CO2, however, points to the former. “In this dual-cell model, the O. alismoides epidermal cells would then perform the function of the terrestrial C4 mesophyll cells by producing a C4 product and the O. alismoides mesophyll cells would perform the function of the terrestrial C4 bundle sheath cells in decarboxylating the C4 product,” the authors explain. This model allows C4 photosynthesis to take place without Kranz anatomy.
“However, further studies are needed to conclude definitively if O. alismoides has dual-cell C4 with the mesophyll cells representing the site of decarboxylation. Work is underway to test this by locating key photosynthetic enzymes in the epidermal and mesophyll cells.”