Does the prostrate-leaved geophyte Brunsvigia orientalis utilize soil-derived CO2 for photosynthesis?

A test was made of the hypothesis that the prostrate growth habit of the leaves of the geophyte Brunsvigia orientalis enables utilization of soil-derived CO(2) and is related to the presence of lysigenous air-filled channels characteristic of B. orientalis leaves.

Brunsvigia orientalis was sampled at a field site. Leaf anatomy, stomatal density, leaf/soil gas exchange characteristics and soil atmosphere and leaf delta(13)C isotope abundances were examined.

The leaves of B. orientalis have large lysigenous air-filled channels separating the upper and lower surfaces of the leaves. The upper surface comprised approx. 70 % of the leaf mass and 75 % of the leaf N (mmol g(-1)). Between 20 % and 30 % of the stomatal conductance and CO(2) assimilation was through the lower surface of the leaf. CO(2) efflux rates from the soil surface were up to 5.4 micromol m(-2) s(-1) while photosynthetic fluxes through the lower surface of the leaves were approx. 7 micromol m(-2) s(-1). However, the utilization of soil-derived CO(2) only altered the leaf delta(13)C isotope abundance of the prostrate leaves by a small amount. Using delta(13)C values it was estimated that 7 % of the leaf tissue C was derived from soil-derived CO(2).

A small proportion of photosynthetically fixed CO(2) was derived from the soil, with minimal associated transpirational H(2)O loss into the space between the leaf and soil. The soil-derived CO(2), taken up through the lower surface was probably assimilated by the palisade tissue in the upper surface of the leaf which was exposed to sunlight and where most of the leaf N was located. The occurrence of lysigenous air channels in the leaves may provide longitudinal strength without impaired transfer of CO(2) taken up through the lower surface to the upper surface.