Submitted to: Journal of Experimental Botany
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/11/2005
Publication Date: 11/1/2005
Citation: James, J.J., Alder, N.N., Muhling, K.H., Lauchli, A.E., Shackel, L.A., Donovan, L.A., Richards, J.H. 2005. High apoplastic solute concentrations in leaves alter water relations of the halophytic shrub, sarcobatus vermiculatus. Journal of Experimental Botany. 57(1):139-147. Interpretive Summary: Water relations models are used to evaluate competition between plants, to assess drought tolerance of crop and wildland plant species, and to develop more efficient irrigation systems. We used a native, salt-tolerant shrub to test some commonly held assumptions about plant and soil water interactions. Contrary to current thinking, results suggest that plants can maintain high salt ion concentrations in leaf intercellular spaces. The results can be used to refine plant water relations models and will help scientists gain a better understanding of plant and soil water interactions of native and crop plants.
Technical Abstract: Predawn plant water potential (Yw) is expected to equilibrate with the Yw of the wettest soil layer accessed by roots. Although this equilibrium assumption provides the basis for interpreting a number of physiological and ecological parameters much work suggests predawn plant Yw is often substantially more negative than root-zone soil Yw. For many halophytes even when soils are well-watered and nighttime shoot and root water loss eliminated, predawn disequilibrium (PDD) between leaf and soil Yw can exceed 0.5 MPa. We used a model desert halophyte, Sarcobatus vermiculatus, to test the predictions that low predawn solute potential (Ys) in the leaf apoplast is a major mechanism driving PDD and that low Ys is due to high Na+ and K+ concentrations in the leaf apoplast. Measurements of leaf cell turgor (Yp) and solute potential (Ys) of Sarcobatus plants grown under a range of soil salinities demonstrated that predawn symplast Yw was 1.7 to 2.1 MPa more negative than predawn xylem Yw, indicating a significant negative apoplastic Ys. Measurements on isolated apoplastic fluid indicated that Na+ concentrations in the leaf apoplast ranged from 80 to 230 mM, depending on salinity, while apoplastic K+ remained around 50 mM. Our water relations measurements suggest that without a low apoplastic Ys, predawn Yp may reach pressures that could cause cell damage. We propose that low predawn apoplastic Ys may be an efficient way to regulate Yp in plants that accumulate high concentrations of osmotica or when plants are subject to fluctuating patterns of soil water availability.