The biochemical changes occurring during the natural senescence of apple leaf tissue (Pyrus malus L., Golden Delicious) coincided with specific changes in the environment. Protein, sugars, and total nitrogen began declining in leaf tissue when the daylength first became less than 14 hours in the second week of August. The activity of triose phosphate dehydrogenase declined shortly afterwards, while the activities of malate dehydrogenase, glutamic dehydrogenase, and aspartate aminotransaminase increased. Chlorophyll, DNA, RNA, and fresh weight began declining when the daylength first became less than 12 hours at the end of September. At the same time sugars and the activities of RNase, polyphenol oxidase, and proteolytic enzymes began increasing. Protein synthesis, total nitrogen, and the activities of malate dehydrogenase, glutamic dehydrogenase, and aspartate aminotransaminase began declining rapidly and amino acids began to accumulate after the first frost of the year. RNase, polyphenol oxidase, and proteolytic activity reached their highest specific activities after the first frost.The process of senescence has been studied in detached leaves, leaf discs, and attached leaves. In general, senescence is characterized by losses of total nitrogen, protein, chlorophyll, RNA, DNA, and dry weight (3,16,28). During leaf-disc senescence, these changes occur immediately and proceed rapidly accompanied by an accumulation of amino acids and a great reduction in protein and RNA sy,nthesis capacities (3, 28). Unlike leaf discs, the attached leaves of deciduous trees undergo a gradual, orderly, and genetically controlled pattern of senescence in which the loss of constituents occurs at different times and rates (22,29). Old leaves are able to synthesize protein almost as well as young leaves (21, 24) and do not accumulate amino acids until quite late in the senescence process (22).The initiation and duration of natural senescence is regulated by environmental as well as internal factors. The most critical environmental factors are photoperiod, temperature, and fertility of the soil (12,15 Of major concern is the conservation of leaf nitrogen during senescence. Apple trees, for instance, lose 40 to 80% of their leaf nitrogen (based on percentage of dry weight) before the leaves fall or are killed by freezing temperatures (16, 18). The nitrogen thus conserved from the leaves becomes part of the trees' reserves, which are required for the first flush of growth and flowering in the spring (4, 25). A severe drop in temperature or an early frost inhibits leaf abscission and prevents nitrogen removal from the leaf (1).To gain a better understanding of the factors that initiate and limit the amount of nitrogen conserved from the apple leaf tissue, we have studied the changes that occur in several biochemical constituents and enzymatic activities during the natural senescence of apple leaves. MATERIALS AND METHODSMid-shoot leaves were collected during the summers and autumns of 1969, 1970, and 1971 from six 10-year-old Golde...

Quantitative and qualitative changes in proteins and ethanol‐soluble nitrogen were followed in senescing leaf and bark tissues of ‘Golden Delicious’ apple trees (Malus domestica Borkh.). While senescing leaves lost 46% of their proteins, total bark protein increased 240% during senescence. However, the protein nitrogen gain in bark tissue was about the same as the protein nitrogen loss in leaf tissue per unit fresh weight of tissues. The pattern of bark protein accumulation appears to be gradual from early August to November and sequential from lower to higher molecular weight species of proteins. The final electrophoretic profile of total bark proteins was established at the later stages of senescence. By late November, 89% of the nitrogen in the bark tissue was found in proteins with 11% in the ethanol‐soluble fractions. The total protein content of dormant bark tissue was 3.5% per gram dry tissue. Fractionation of the total bark proteins by DEAE‐cellulose chromatography indicated that the final upsurge of bark proteins observed in November was associated primarily with one group of proteins (Peak III).

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Separation of the bark storage protein into three groups was achieved with minimal contamination from interfering cbromophores, using diethylaminoethyl cellulose column chromatography. Two of these groups were shown 10 contain neutral sugars in close association with the proteins. All three groups disappeared rapidly after the onset of growth. Polyacrylamide gel electrophoresis of the storage proteins showed the presence of eight prominent proteins. Ammo acid analysis of hydrolyzates of the proteins indicated that a large percentage of the protein nitrogen was in the form of basic amino acids.