A seedling population of sugar maple (Acer saccharum Marsh.), derived from two native seed trees in southeastern Wisconsin, was grown for 7 years (1941-1947) in pots in the University of Chicago greenhouses and gardens. The experimental variables were mainly length of photoperiod and amount of winter chilling, studied particularly in relation to leaf senescence and abscission, onset and completion of bud rest, budbreak, and amount of terminal growth. 2. It was concluded that in this species the loss of green color and leaf abscission under natural conditions are basically conditioned by the naturally decreasing autumnal photoperiod. In the various years and experiments leaves expanding from as early as February to as late as August were mostly abscised in October and November if the plants were maintained on natural photoperiod, either indoors or out. Chronological age of leaves had a limited relation to time of senescence and abscission, youngest leaves persisting a few days longer. Constant photoperiods of 16, 12, and 8 hours, imposed in the greenhouse on first-year seedlings on September 10, resulted in delayed senescence and abscission (up to 5 months for youngest leaves on 16-hour photoperiod) compared with plants outside on natural photoperiod. There was considerable variation in time of these events within any one treatment, but in general the amount of delay was positively related to length of constant photoperiod. A change from 16- to 8-hour photoperiod in December accelerated senescence and abscission as compared with plants remaining on 16-hour photoperiod. In second-year plants leaves of similar chronological age persisted longer when they had expanded and had been maintained inside on 20-hour than on 9-hour light periods; but those on natural photoperiod lost their green color and were abscised in October and November,generally earlier than those on either constant photoperiod.In all experiments loss of green color always preceded abscission. These results all suggest that the natural photoperiodic mechanism leading to senescence and abscission may involve shifting gradients of abscission inhibitors and/or activators conditioned in a complex fashion by changing light-dark ratios in the photoperiodic cycle. Thus, leaves were eventually abscised on any of the photoperiodic treatments employed, but their chronological age had much more influence on the length of their persistence when they were on constant than when on natural photoperiods. Presumably decreasing photoperiods critically condition the aging process leading to abscission. 3. Completion of bud rest and budbreak were studied in relation to winter chilling, as modified by the effects of past history and current photoperiod. In general, the population tested apparently required several hundred hours of chilling to complete bud rest and to allow bud-break at the normal time. This need varied from plant to plant both genetically and in relation to depths of rest as conditioned by past history. Some plants eventually grew without chilling, but some unchilled plants remained dormant for 15-18 months and then grew after chilling. After no or inadequate winter chilling in 1941-1942, exposure to 20-hour photoperiod after April 28 or to natural long days of spring and summer induced bud-break in higher percentages of plants, and more vigorously, than did a 9-hour photoperiod. Only 35% of unchilled plants eventually grew when on 9-hour photoperiod; but 64% and 54% had grown by September 15 on natural and 20-hour photoperiods, respectively. Thus, in some plants, but not all, inadequate or no chilling may be partially compensated for by long photoperiods. A 16 hour photoperiod imposed in September, however, did not stimulate any newly formed buds to grow. In some, but not all, cases plants inadequately chilled or unchilled were more delayed in bud-break if they had continued growth later, or had green leaves persisting longer, in the preceding growing season. After chilling outdoors until December 6, populations brought into a warm greenhouse at intervals until February 15 showed proportionately earlier bud-break the longer the exposure, until the last week in January when rest was apparently nearly completed. The effect of additional exposure in accelerating bud-break was more pronounced in December than in January. The most interesting effect in such acceleration resulted from 33 hours of additional exposure to temperatures of 10⚬-18⚬ F.on December 13-14. Mean date of bud-break for plants receiving this additional exposure was advanced 2 weeks over those brought in the day before. There was some evidence that winter injury was more likely to occur after rest was completed. Chilling requirement is undoubtedly a factor in the horticultural distribution of this species. Its importance in natural distribution can only be suggested until other native populations are tested for possible geographic variation in chilling needs. 4. Duration and extent of terminal growth were related to photoperiod .Individual leaves expanding on 20-hour or long natural photoperiods were larger than those expanding on 9-hour photoperiod. Internodal extension was not measured. Duration of terminal growth was similar for first flushes, irrespective of photoperiod, but, in 1942, 70% and 23% of plants on 20-hour and natural photoperiods, respectively, made a second flush, whereas none did on 9-hour photoperiod. Long days are temporarily stimuli to extension for buds not in deep rest. 5. Terminal buds became dormant on photoperiods ranging from the natural short days of March and November to the longest ones of June, and on 9- and 20-hour constant photoperiods. The development of bud rest under natural conditions is thus not decisively controlled by photoperiod, even though some results suggest that daylength may contribute secondarily-as, for example, in the lack of a second flush of growth in any plants on 9-hour photoperiod.
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C. E. Olmsted (1951) studied this question.