The senescence of oat leaves has been studied by following the loss of chlorophyll and protein and the increase of a-amino nitrogen, after detachment and darkening. Protein Senescence has long been regarded as an essentially degradative process, and the rapid onset of proteolysis in isolated and darkened leaves has been documented since 1930 (3,28,29 found, inter alia, that the amino acid L-serine enhances senescence, thus acting in the opposite direction to cytokinin (22, 26). In explanation it was suggested that the serine might become incorporated into the active center of one or more proteolytic enzymes participating in the senescence process.The present investigation was therefore undertaken, firstly, to find out more in general about the progress of senescence in the oat leaf, and about the antagonism between L-serine and kinetin; secondly, and more specifically, to see whether there was evidence that proteolytic enzymes are, in fact, synthesized in the senescing leaf, and if their amount increases with time.A preliminary account of some of the results has been presented earlier (12).MATERIALS AND METHODS General Procedure. Oats (cv. "Victory") obtained from the U. S. Department of Agriculture were husked, soaked, and planted in vermiculite. They were grown 50 cm below a bank of eight 20-w cool white fluorescent lights giving 6.4 X 10-2 watts/cm2 at about 23 to 25 C for 7 days. The first leaves, which were then about 12 cm long, were cut off and the apical 3 cm placed on slides over moist filter paper in Petri dishes, a minor modification from the procedure previously used (25, 26), which in turn was based on work of Gunning and Barkley (6). Usually a single 10 ,ud drop was placed in the center of each leaf, as was previously done for the kinetin bioassay (25). This drop contained the test substance together with Mcllwain buffer, pH 4.7, diluted 1: 10, and 0.2% Tween 80. The isolated leaves were left for 72 hr (or occasionally for 96 hr) in darkness, at which time about 50% of the chlorophyll had disappeared. The chlorophyll and a-amino nitrogen of groups of five leaves were then extracted into boiling 80% alcohol for 30 min, the solution was made to 10 ml, its absorbance at 665 nm was read for the chlorophyll content, and an aliquot was taken for determination of a-amino nitrogen by the Moore and Stein method (16). The extracted leaves were then washed twice with 80% alcohol, the protein was solubilized with I N NaOH and determined by Miller's (14) modification of the Lowry et al. method (9). Initial values of chlorophyll, a-amino nitrogen, and protein were determined on a leaf sample at the beginning of each experiment, and the results below are expressed as a percentage of these initial values.Analyses of variance were carried out on each experiment except the time course (Fig. 1)

When the first leaf of the oat (Arena sativa) seedling is detached and placed in the dark, yellowing and proteolysis take place rapidly. The earlier finding that D-serine promotes this process has led to a further study of It has been shown in preceding papers (9,12,14) that the senescence of the detached first leaves of A vena, involving both the bleaching of chlorophyll and the massive hydrolysis of protein, can be prevented either by cytokinin or by certain inhibitors of protein syntheses, especially cycloheximide. The process is promoted by darkness and can also be vigorously promoted by L-serine (12). In regard to this action of serine, there are several other points of significance: (a) the action is specific for the L-form, (b) similar, though weaker, action is exhibited by cysteine, threonine, alanine, and even glycine, but it is shown below that homoserine has no such effect, (c) arginine protects against this action of serine, although arginine alone shows little or no real effect on senescence.

Spikelets of Themeda triandra are dormant when mature and require an after‐ripening period in dry storage of approximately 12 months before full germination potential is realized. Successful germination of spikelets entails the splitting of the tough upper glumes by radicles. Dormany appears to result from a combination of embryo dormancy and mechanically resistant glumes. Glume removal from dormant spikelets increases germination while glume removal plus gibberellic acid increases germination even more. During the after‐ripening period, the growth potential of spikelets and caryopses increases as measured by their ability to germinate in the presence of the osmoticum polyethylene glycol 6000. The inhibition of germination by decreasing osmotic potential of the germination medium significantly interacts with the promotion caused by gibberellic acid indicating that both factors affect germination by altering the growth potential of the embryos. The increase in growth potential during after‐ripening is probably related to the synthesis of gibberellin‐like substances. It is hypothesized that dormancy breaking during after‐ripening occurs because the growth potential of embryos increases and this consequently increases the ability of radicles to split the upper glumes during germination.