Diurnal and seasonal stomatal regulation of water use efficiency in leaves of field-grown apple trees

Two-year-old field-grown 'Gold Spur' apple trees (Malus domestica Borkh.) were used to study the diurnal and seasonal stomatal regulation of water use efficiency under Shandong climatic conditions (wet summer and dry spring and autumn). In most cases, leaf photosynthetic rate (Pn) and stomatal conductance (Gs) showed similar diurnal changes. They reached their maxima in midmorning, then gradually declined for the rest of the day. Transpiration rate (Tr) increased rapidly after sunrise, peaked at midday, then declined until sunset. Water use efficiency (WUE) was high in early morning, declined rapidly, then remained low in the afternoon. Daily CO2 gain and maximum Pn were high in spring, depressed in midsummer, recovered in late summer, and then declined with senescence of leaves. This seasonal trend was mainly controlled by nonstomatal factors. Daily transpiration and maximum Tr were low in spring, peaked in midsummer, and then declined in autumn. Daily WUE was high in spring, declined to the lowest in midsummer, and then recovered in autumn. These seasonal trends were largely dependent on the seasonal change of daily maximum Gs. In spring and autumn daily maximum Gs was low, while the opposite was true during wet summer. This represents an example of how stomata adjust their aperture to coordinate CO2 gain, water loss, and energy balance in response to environmental signals.