Budbreak and root and shoot extension growth of apple trees (Malus domestics Borkh. MM.111) were affected by exposure to 500 hours of higher temperatures (15, 20, or 30C) during the first, second, or third 500 hours of a 1500-hour, 5C chilling period. Exposure to 15C during the third 500 hours had a significantly positive effect on budbreak, 20C at this time had no effect, and all other treatments had a negative effect on budbreak and new root and shoot growth.
During natural leaf abscission, two-year-old, potted apple trees (Malus domestica cv. MM.111 EMLA) were placed in a cold room at 6C for chilling. At 0, 600, 900, and 1400 chilling hours (CH), ten trees were removed and terminal shoots cut into four 15 cm sections. These sections were randomly placed in forty 10 1 test tubes at 20C and subjected to ten oxygen levels from 0.5 to 21% O2 with four reps each by flushing tubes with mixtures of air and nitrogen prior to sealing. Tubes remained sealed for 4 hr then reflushed and resealed for another 4 hr after which 1 ml gas samples were drawn and CO2 and O2 levels measured on a gas chromatograph. Respiration decreased with oxygen level at all CH. CO2 evolved did not show a pattern related to oxygen level, but O2 consumed decreased at a decreasing rate with additional CH. Respiratory quotient was below one at 0 and 600 CH and equal to one at 900 and 1400 CH, indicating a possible shift in respiration substrate with chilling.
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 O2 levels did not show a pattern related to CU, but O2 consumed decreased at a decreasing rate with additional CU. Respiratory quotient was <1 at 0 and 600 CU and equal to 1 at 900 and 1400 CU, indicating a possible shift in respiratory substrate with chilling.
Dormant apple trees (Malus domestics Borkh., cv. MM. 111) were chilled at SC for O, 500, 1000, or 1500 hours and then forced at 10, 20, or 30C for 21 days. Budbreak and root growth were recorded after forcing, and shoot and root respiration was measured at 5, 10, 20, and 30C to determine Q10 and energy of activation values. Budbreak, root growth, and respiration generally increased with chilling and forcing temperature. The Q10 of shoot respiration increased significantly with increasing chilling when measured before forcing; however, after forcing, Q10 decreased with chilling. Root respiration Q10 was not as influenced as shoot respiration by chilling either before or after forcing. Energy of activation for shoot and root respiration decreased significantly with chilling after forcing at each temperature.
Michael Parker and Eric Young
Controlling vegetative growth resulting from a long growing season in the southeast is difficult while trying to promote early fruiting. This study was initiated in 1990 to evaluate higher density leader training techniques, cultivar interactions, and the benefits of pre-plant fumigation on apple replant sites. Another objective was to evaluate these management parameters in four regions with very different climates and elevations. The training techniques evaluated were, weak leader renewal, bending of the leader during the growing season, and partial terminal leaf removal every 10 inches of leader growth (without injuring the apical meristem). The cultivars used were Jonagored, Spur Gala, and Red Fuji, all on Mark rootstock. First and second year branching was not significantly different between the various training techniques. Yields during the third year did not appear to differ between the leader training techniques. Pre-plant fumigation appeared to be beneficial in increasing tree growth in only two of the four sites.
Eric Young and S.M. Blankenship
Three percent oxygen significantly delayed and reduced budbreak of fully chilled apple (Malus domestica Borkh.) trees in a greenhouse. When ambient oxygen levels were restored, budbreak occurred normally. Apple trees stored under 3% ± 1% oxygen at 6C for 35 weeks had no detectable bud development in storage. Budbreak and subsequent shoot growth were normal after the trees had been removed from storage.
Michael A. Arnold and Eric Young
Bare-root Malus × domestica Borkh. seedlings were chilled for 0, 600, 1200, or 1800 hours at 5C (CH). Seedlings were then placed with roots and/or shoots in all combinations of 5 and 20C forcing conditions (FC) for up to 21 days. Virtually no growth occurred at 5C FC. When the whole plant was forced at 20C, all measures of root and shoot growth increased in magnitude, occurred earlier and at a faster rate with increasing CH. Thus, roots and shoots responded similarly to chilling. When shoots or roots were subjected to 20C FC, while the other portion of the plant was at 5C, the responses were reduced in magnitude and delayed. However, the overall growth enhancement by chilling was not negated. Root and shoot growth enhancement by chilling appeared to be increased if the other portion of the plant was actively growing also, but not dependent on it. Growth of adventitious shoots on roots (root suckers) was greatly enhanced with increasing CH on plants subjected to 5C shoot and 20C root FC. While total root and shoot bark protein levels on a per-seedling basis were similar, protein concentrations were lower in root bark than in shoot bark. During chilling, total protein per seedling generally increased until just before the time that chilling requirements for vegetative budbreak were satisfied. Protein degradation then began, resulting in lower protein levels through 2300 CH. Rapid protein breakdown (1200 to 1800 CH, roots; 1000 to 1800 CH, shoots) occurred at about the same time that root (1000 to 1800 CH) and shoot (800 to 1800 CH) growth responses to chilling were increasing. Warm FC resulted in increased protein breakdown with increased CH and forcing time.
Tania K. Dautlick and Eric Young
One-year-old potted `Jonagold' apple trees on M. 9 EMLA rootstock were chilled in a cold room at 5C for 1320, 3288 or 4464 hours. They were then placed in a 22C greenhouse for forcing. Rate of dark respiration of buds and internodes was measured in vivo using the LICOR LI 6200 portable photosynthesis system. Measurements were taken on each tree at several distances from the shoot apex during the forcing period. The new apical growth was removed from half of the trees when apical growth was 2-4cm. Removal of apical growth did not appear to influence respiration. Respiration of both buds and internodes generally decreased with distance from apex. As budbreak occurred or when trees received excess chilling hours, the effect of distance from apex on respiration decreased. Excess chilling also resulted in greater and more rapid budbreak.
Michael L. Parker and Eric Young
Apple (Malus domestica Borkh.) cultivars Spur Galagored (#42) (`Gala'), Jonagored (`Morren's'), and Red Fuji (B.C. #2) (`Fuji') on Mark rootstock and `Gala' on Malling 26 EMLA (M.26) and Malling 9 EMLA (M.9) were planted in the four major apple-production regions of western North Carolina. Three leader management techniques, weak leader renewal, snaked leader, and heading with partial terminal leaf removal (H + PTLR), were applied to five-tree plots beginning the spring after planting. Leader management techniques, weak leader renewal or H + PTLR, which involved dormant pruning or vegetation removal and an interruption in vegetative vigor, reduced total branching and yield during the third year. Fumigation with methyl bromide increased lateral branching and yield in the third year. No significant yield differences were detected for `Gala' grown on M.9, M.26, or Mark rootstocks. Trees grown in the most western region of the state, Haywood County, had smaller trees and reduced yields compared to the other three regions due to a shorter growing season.