Annals of Botany

Flat out to modify carbon ‘footprint’

Some growth forms adopted by plants are highly characteristic of particular habitats. Cramer et al. (Universities of Cape Town, South Africa and Western Australia, pp. 835–844) highlight the phenomena in respect of the winter-rainfall regions of South Africa. Many species of this habitat are geophytes whose aerial parts exhibit a very unusual prostrate growth habit causing the leaves to lie flat on the ground, a feature that is extremely rare in the rest of the world. What is the advantage of this prostrate growth form in this habitat? One possibility is that the plants may supplement photosynthetic fixation of atmospheric CO₂ with CO₂ from the soil. The authors have tested this with Brunsvigia orientalis, comparing photosynthesis in prostrate leaves with that in leaves that were fortuitously held aloft by surrounding vegetation. Leaf anatomy would certainly allow fixation of CO₂ derived from chemical and biotic processes in the soil. The underside of the leaves possesses about 22 % of the total leaf stomata and the lower and upper surfaces of the leaf are connected via extensive lysigenous air channels. These provide a route for CO₂ taken up through the lower surface to reach the cells exposed to the sun in the upper half of the leaf. Direct measurement of CO₂ uptake showed that between 20 % and 30 % entered via the lower surface in prostrate and in ‘propped-up’ leaves. However, comparison of δ¹³C abundances in CO₂ assimilated via the upper and lower surfaces of propped-up and prostrate leaves showed that only about 7 % of the photosynthetically fixed CO₂ in prostrate leaves came from the soil. This is certainly a measurable contribution to the plant’s carbon economy and, combined with the ability to assimilate CO₂ from the ‘air pocket’ beneath the leaf with little or no transpirational water loss, is very likely to be advantageous.

Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk