The relationship between phytolith- and plant-water delta18O values in grasses

Information regarding climatic conditions during plant growth is preserved by the oxygen-isotope composition of biogenic silica (phytoliths) deposited in grasses. The O-isotope compositions of phytoliths and the plant water from which they precipitate are dependent on soil-water delta18O values, relative humidity, evapotranspiration rates, and temperature. Plant water and phytoliths from two grass species, Ammophila breviligulata (C3) and Calamovilfa longifolia (C4) at Pinery Provincial Park in southwestern Ontario, Canada, were examined to determine the variability in their delta18O values. Stem water was unfractionated from soil-water in oxygen isotopic composition and the delta18O values of stem silica provide a good proxy for the soil water available to roots during the growing season. Greater spatial and temporal variation in the delta18O values of water in the top 5 cm of the soil, and their enhanced sensitivity to evaporative 18O enrichment, are reflected in the generally higher delta18O values of water in the shallow roots and rhizomes of these grasses. Water within the sheath and lower and upper leaf tissues experiences continual evaporation, becoming progressively enriched in 18O as it moves towards the tip of the leaf. However, the water from which leaf silica precipitates has not acquired the extreme 18O enrichment predicted using steady-state models, or measured for midday or average daily leaf water. Possible explanations for this behaviour include preferential deposition of silica at night; the existence of a secluded water fraction within the leaf, which experiences smaller diurnal variations in isotopic composition than leaf water at sites of evaporation; kinetic isotope effects during rapid precipitation of leaf silica; and incomplete exchange between the oxygen in the silicic acid and the leaf water.