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Stomatal conductance (g s ) of 'Hosui' Asian pear (Pyrus serotina Rehder) trees growing in lysimeters was characterized for trees in well-watered soil and after brief water deficit. The measures of water status used to interpret g s data were soil-water content, leaf water potential ( ψ l ), and instantaneous water use (trunk sap flow by the compensation heat-pulse technique). The diurnal course and range of g s values of well-irrigated Asian pear trees were similar to those reported for other tree fruit crops. Soil moisture at the end of a midsummer deficit period was 60% of lysimeter pot capacity, and diurnal ψ l reflected this deficit predawn and in the late afternoon compared to well-irrigated trees. The g s was sensitive to deficit irrigation during more of the day than ψ l , with g s values <3 mm·s -1 for most of the day; these were less than half the conductances of well-irrigated trees. The reduction of g s in response to a given soil-water deficit was not as great on days with lower evaporative demand. After a water deficit, g s recovered to predeficit values only gradually over 2 to 3 days. The low g s of trees in dry soil was the apparent cause of reduced transpiration, measured by trunk sap flow, and reduced responsiveness of sap flow to fluctuations in net radiation. Asian pears, also known as nashi in the Pacific Rim, are an important new crop in New Zealand (Caspari et al., 1993a). Information on their irrigation management is needed, and re- search has centered around a drainage lysimeter facility con- structed at Massey Univ. in 1987. Several aspects of Asian pear water relations have been studied, including the role of stomata. Aerodynamically rough, well-ventilated canopies such as for- ests or orchards are closely coupled to their environment. Conse- quently, a change in canopy conductance brought about by a change in stomatal conductance (g s ) can therefore cause an almost- proportional change in transpiration (Jarvis and McNaughton, 1986). Jones and Higgs (1989) used a model involving air tempera- ture, vapor-pressure deficit (VPD), and irradiance to predict apple (Malus domestica Borkh.) leaf g s . While 32% to 62% of g s variance could be explained, the model fit could be significantly improved by including the soil-moisture deficit. Reduced stomatal aperture due to soil-water deficit has been observed in many studies. When irrigation of potted peach ( Prunus persica (L.) Batsch) seedlings was stopped, there was no obvious effect on stomata for the first 3 days (Tan and Buttery, 1982). Thereafter, g s declined rapidly, closely following the decline in available soil moisture. Similar responses of stomata to reduced soil water have been reported for apple (Higgs and Jones, 1990), apple and sour cherry (P. cerasus L.) (Flore et al., 1985), and grape (Vitis vinifera L.) (Downton et al., 1987). The objective of this study was to characterize aspects of stomatal behavior of Asian pear in well-watered soil and after brief water deficit. Diurnal g s patterns are discussed in relation to leaf water potential (ψ l ) and transpiration as measured by the compen- sation heat-pulse technique.

Stomatal Conductance of Lysimeter-grown Asian Pear Trees before and during Soil Moisture Deficits