Water Relations of Coral Cay Vegetation on the Great Barrier Reef: Water Potentials and Osmotic Content

Abstract

Foliage of 18 species from One Tree Island, a coral cay at the southern extremity of the Great Barrier Reef, contained abundant K⁺, Na⁺, Mg²⁺, Ca²⁺ and Cl^-, although very high NaCl levels characteristic of halophytes were found only in Sesuvium portulacastrum. Four species contained large quantities of compatible solutes: glycinebetaine in Achyranthes aspera, proline in Sesuvium, Scaevola taccada and Melanthera biflora, and β -dimethylpropiothetin in Melanthera. The ability to make such S-containing compatible solutes as the last mentioned may be especially important for maritime species. Most of the calcium and a large proportion of the magnesium were in insoluble form in Melanthera, Argusia argentea and Pisonia grandis; leaves of all three contained crystals which were shown to be Ca-rich in Pisonia. It is suggested that control of calcium levels may be particularly important in plants of highly calcareous coral cays. Predawn water potentials (Ψ) in Argusia, Pisonia and Melanthera averaged between - 0.3 and -0.8 MPa in wet weather, indicating high soil Ψ there was a daily fall of 0.5 - 1.O MPa in plant Ψ. During drought, predawn Ψ was lower in Argusia. Scaevola and Pisonia, and the daily Ψ fluctuation was small, presumably because of stomatal control of water loss. No evidence of osmotic adjustment was found in these three species. In Melanthera in drought, predawn Ψ became very low (often below -2.0 MPa) but the large daily fluctuation was still present, resulting in extremely low midday Ψ. From this and the large accumulation of solutes, especially at dry sites, it is suggested that Melanthera is capable of some osmotic adjustment.