Experiments on Photoperiodism, Dormancy, and Leaf Age and Abscission in Sugar Maple

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Abstract
A seedling population of sugar maple (Acer saccharum) was pot-grown for 7 yrs. in the Univ. of Chicago greenhouses and gardens with variables in length of photoperiod and amt. of winter chilling. Loss of green color and leaf abscission are basically conditioned by the naturally decreasing autumnal photoperiod. Constant photoperiods of 16, 12, and 8 hrs., imposed in the greenhouse on Sept. 10, resulted in delayed senescence and abscission (up to 5 mos. for youngest leaves on 16-hr. photoperiod). In all expts. loss of green color preceded abscission. Leaf abscission occurred eventually on any of the photoperiodic treatments employed, but the chronological age of leaves 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. Completion of bud rest and budbreak were studied in relation to winter chilling. In general, the population tested apparently required several hundred hours of chilling to complete bud rest. Some plants eventually grew without chilling, but some unchilled plants remained dormant for 15-18 mos. and then grew after chilling. After chilling outdoors until Dec. 6, populations brought into a warm greenhouse at intervals until Feb. 15 showed proportionately earlier bud-break the longer the exposure, until the last week in Jan. when rest was apparently nearly completed. 23 hrs. of additional exposure to temps. of 10[degree]-10[degree]F. On Dec. 13-14 advanced bud-break 2 wks. over those brought in the day before. Duration of terminal growth was similar for first flushes, irrespective of photoperiod, but, in 1942, 70% and 23% of plants on 20-hr. and natural photoperiods, respectively, made a 2d flush, whereas none did on 9-hr. photoperiod. Terminal buds became dormant on photoperiods ranging from the natural short days of March and Nov. to the longest ones of June, and on 9- and 20-hr. constant photoperiods. The development of bud rest under natural conditions is thus not decisively controlled by photoperiod.