In situ O ₂ dynamics in submerged Isoetes australis: varied leaf gas permeability influences underwater photosynthesis and internal O ₂

Abstract

A unique type of vernal pool are those formed on granite outcrops, as the substrate prevents percolation so that water accumulates in depressions when precipitation exceeds evaporation. The O ₂ dynamics of small, shallow vernal pools with dense populations of Isoetes australis were studied in situ, and the potential importance of the achlorophyllous leaf bases to underwater net photosynthesis (P _N) and radial O ₂ loss to sediments is highlighted. O ₂ microelectrodes were used in situ to monitor pO ₂ in leaves, shallow sediments, and water in four vernal pools. The role of the achlorophyllous leaf bases in gas exchange was evaluated in laboratory studies of underwater P _N, loss of tissue water, radial O ₂ loss, and light microscopy. Tissue and sediment pO ₂ showed large diurnal amplitudes and internal O ₂ was more similar to sediment pO ₂ than water pO ₂. In early afternoon, sediment pO ₂ was often higher than tissue pO ₂ and although sediment O ₂ declined substantially during the night, it did not become anoxic. The achlorophyllous leaf bases were 34% of the surface area of the shoots, and enhanced by 2.5-fold rates of underwater P _N by the green portions, presumably by increasing the surface area for CO ₂ entry. In addition, these leaf bases would contribute to loss of O ₂ to the surrounding sediments. Numerous species of isoetids, seagrasses, and rosette-forming wetland plants have a large proportion of the leaf buried in sediments and this study indicates that the white achlorophyllous leaf bases may act as an important area of entry for CO ₂, or exit for O ₂, with the surrounding sediment.