Dew water-uptake pathways in Negev desert plants: a study using stable isotope tracers.

Dew is an important water resource for plants in most deserts. The mechanism that allows desert plants to use dew water was studied using an isotopic water tracer approach. Most plants use water directly from the soil; the roots transfer the water to the rest of the plant, where it is required for all metabolic functions. However, many plants can also take up water into their leaves and stems. Examining the dew water uptake pathways in desert plants can lend insight on another all water-use pathways examination. We determined where and how dew water enters plants in the water limited Negev desert. Highly depleted isotopic water was sprayed on three different dominant plant species of the Negev desert-Artemesia sieberi, Salsola inermis and Haloxylon scoparium-and its entry into the plant was followed. Water was sprayed onto the soil only, or on the leaves/stems only (with soil covered to prevent water entry via root uptake). Thereafter, the isotopic composition of water in the roots and stems were measured at various time points. The results show that each plant species used the dew water to a different extent, and we obtained evidence of foliar uptake capacity of dew water that varied depending on the microenvironmental conditions. A. sieberi took up the greatest amount of dew water through both stems and roots, S. inermis took up dew water mainly from the roots, and H. scoparium showed the least dew capture overall.

A comparison of clay colloid and artificial microsphere transport in natural discrete fractures.

Transport of monodispersed buoyant 1-mum latex microspheres, dense 1.34-microm montmorillonite particles, Li(+) and Br(-) was investigated in a naturally fractured chalk core with an average equivalent hydraulic aperture of 183 microm. Studied parameters were: tracer arrival time, C/C(0) values, mass recovery, size distribution and the impact of initial concentration. Breakthrough time of both colloidal tracers was faster than that of the soluble tracers. Significantly lower recovery and slightly slower breakthrough time were observed for the clay particles relative to the microspheres, apparently mainly due to the former's higher density, resulting in preferential gravitational settling of the clay particles. However, variable surface charge and nonuniform shape and size of the clay particles may also play a role in the observed differences. From the theoretical scale ratio, the time interval calculation seems to be a major factor in colloid recovery. Clay-particle size fractionation was observed (0.62 vs 1.34 microm at the outflow and inflow, respectively), and there was no significant influence of the initial concentration (100 and 500 mg/L) on transport properties. Our observations indicate that colloid density is a dominant property for their transport in fractures. This work emphasizes the need for caution when the results of studies in which buoyant colloids are used as tracers are extrapolated to natural systems in which clay colloids are present.