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During natural leaf abscission, 2-year-old potted apple trees (Malus domestica Borkh. cv. MM.111 EMLA) were placed in a room at 6C for chilling [0,600, 900, or 1400 chilling units (CU)]. After each chilling treatment, respiration of shoot segments was measured as CO2 evolved and O2 consumed at 22C in several O2 concentrations. Respiration increased with oxygen concentration after all CU treatments. Carbon dioxide evolved at the several O 2 levels did not show a pattern related to CU, but O2 consumed decreased at a decreasing rate with additional CU. Respiratory quotient was 1, and fatty acids yield an RQ < 1; e.g., stearic acid respiration results in RQ = 0.7, and with palmitic acid, RQ = 0.36 (Noggle and Fritz, 1976). The RQ in dormant pea ( Pisum sativum L.) seeds increases from 0.4 to between 0.7 and 0.8 as seeds develop toward germination (Obrucheva and Kovadlo, 1985), which is thought to be due to oxidation of storage lipids early in the dormancy period. A similar r publication 25 June 1992. Accepted for publication 23 Nov. 1992. I e the valuable technical assistance of Robert Belding throughout this ost of publishing this paper was defrayed in part by the payment of page der postal regulations, this paper therefore must be hereby marked nt solely to indicate this fact. Soc. Hort. Sci. 118(4):509-512. 1993. s i t u a t i o n o c c u r s i n d o r m a n t a p p l e s e e d s ( D a w i d o w i c z Grzegorzcwska, 1989). where degradation of lipid bodiesdepends on chilling, which is necessary for germination. The RQ in apple buds during dormancy has not been reported, but Lui et al. (1991) found that lipase activity in apple buds increased greatly as chilling progressed. A recent study of low-oxygen storage of fully chilled, quiescent apple trees (Young and Blankenship, 1991) showed that O2 concentrations <5%, combined with 6.0C, inhibited budbreak for up to 35 weeks with no detectable effects on subsequent shoot growth. It is not known how O2 level affects respiration during dormancy and whether or not any effects would change with changes in respiratory substrate. The purposes of this study were 1) to measure respiration of apple shoot tissue during dormancy as both CO2 evolution and O2 consumption to calculate RQ values. and 2) to determine the effects of atmospheric O2 concentration on these respiration measures. Material and Methods Forty 1 -year-old rooted layers of Malling Merton 111 (MM. 111) apple [chilling requirement 1450 chill units (CU) (Hauagge and Cummins, 1991; Young and Werner, 1985)] were lifted from a commercial stoolbed (Treco, Woodburn, Ore.). All trees were pruned to 60 cm above the root collar, and all lateral branches were removed. Trees were potted in 3.5-liter containers in damp calcined clay (Turface) and placed in a room at 5 ± 2C. After each chilling treatment (0, 600, 900, or 1400 CU), 10 trees were removed and terminal shoots cut into four 10-cm sections, excluding the top and bottom 10 cm. One of each of these sections was placed in each of forty 100-ml test tubes containing a piece of moist filter paper to prevent desiccation. Shoot sections were placed randomly into the various oxygen concentrations to randomize any variation due to acrotony and basitony. Groups of four test lubes were each thoroughly blushed with one of 10 gas mixtures containing O2 at 0.5%, 2%, 4%, 6%, 8%, 11%, 13%, 15%, 18%, or 21% ± 0.5%. Gas mixtures were made by metering compressed air (0.03% CO2, 21% O2, 78% N2) and compressed nitrogen (99.5% N 2, 0.5% O2) together in various ratios. The final mixtures were monitored each time the tubes were flushed. Tubes were then sealed with a rubber serum stopper and inverted into a pan of 22C water to prevent gas leaks and maintain a constant temperature. Shoot sections were equilibrated in these atmospheres for4 h, then tubes were reflushed with identical gas mixtures and sealed as before. A 1-ml gas sample was withdrawn through the serum

Respiratory Oxygen Response and Respiratory Quotient of Apple Stem Sections during Chilling