Abstract
When 19-day-old Helianthus annuus plants were placed horizontally with one cotyledon oriented downward and the other upward, the senescence of these cotyledons, especially of the former one, was greatly accelerated compared with that of upright ones. Losses of fresh weight, chlorophyll and free polyamines, and increased activity of proteases, appears in the cotyledons of horizontal plants much earlier than in upright ones. Necrosis of the control, upward and downward oriented cotyledons occurred 33, 28 and 23 days, respectively, after the beginning of the experiment. The differences in the rates of senescence of the cotyledons are discussed in relation with the negative geotropism of the horizontally placed plants.
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