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Information about climatic conditions during plant growth is preserved by the oxygen-isotope composition of biogenic silica (phytoliths) deposited in grasses. The oxygen-isotope composition of phytolith silica is dependent on soil-water delta(18)O values, relative humidity and evapotranspiration, and temperature during plant growth. Phytolith and plant-water delta(18)O values for C3 (A. breviligulata) and C4 (C. longifolia) grasses from natural and greenhouse sites in southwestern Ontario were used to compare the isotopic fractionation between biogenic silica and water in various parts of these living plants. For non or weakly transpiring tissues (rhizomes, stems, sheaths) in both grass species, the Delta(18)O(silica-plant water) remained constant at similar to 34 parts per thousand, and the delta(18)O and delta D values of plant water collected from pre-dawn and mid-day samplings showed little variation. These plant waters were only slightly enriched in O-18 and D relative to water provided to the grasses. Isotopic temperatures calculated from the silica and plant-water isotopic data matched measured growing temperatures for the region. By comparison, the upper leaf water was extremely enriched in oxygen-18 and deuterium at maximum rates of transpiration relative to water from non-transpiring tissues, as were the calculated, steady-state values for leaf-water delta(18)O and delta D. Silica produced in the transpiring tissues (leaf, inflorescence) has higher delta(18)O values than silica from non-transpiring tissues, but the enrichment is modest compared to upper leaf water under mid-day conditions. Leaf phytoliths have formed from plant water typical of average conditions in the lower leaf, where the extreme O-18-enrichment is not encountered. C. longifolia was also collected from Alberta and Nebraska, where growing conditions an different from southwestern Ontario. Phytoliths at all three sites have a similar pattern of delta(18)O values within the plants, but the isotopic separation between leaf and stem silica increases from 4 to 8 parts per thousand as average relative humidity decreases. The difference between actual growing temperature and that calculated using measured delta(18)O values for stem silica and local meteoric water became progressively larger as relative humidity decreased, likely because of evaporative O-18-enrichment of soil water. Such effects are most pronounced in arid environments and pertinent in grasslands where much of the active rooting zone can be situated at the shallower depths most affected by the O-18-enrichment of soil water. Copyright (C) 2000 Elsevier Science Ltd.