It has long been known that leaf senescence can be induced in many plant species by detaching leaves and placing them in the darkness. It recently has been shown that entire Arabidopsis plants placed in the darkness are not induced to senesce, as judged by visible yellowing and certain molecular markers. Here, we show that when individual Arabidopsis leaves are darkened, but not when entire plants are darkened, senescence is induced in the covered leaves. This induction of senescence is highly localized. The phenomenon is leaf age dependent in that it occurs more rapidly and strongly in older leaves than in younger ones, as is the case with many forms of induced senescence. Whole adult plants placed in darkness, in contrast, show delayed senescence, although seedlings lacking primary leaves do not. These observations imply that the light status of the entire plant affects the senescence of individual leaves. A model summarizing the results is presented.Leaf senescence is an active process regulated by exogenous and endogenous factors. An important exogenous factor is light. The interplay between light and senescence is complex, and many reports have been published describing both its senescenceinhibiting and -promoting qualities. The latter mostly have been reported in the context of relatively high light levels, and interpreted as the reactions of photosynthesis metabolically "aging" a leaf, and indirectly inducing its senescence (Biswal and Biswal, 1984; Noodén et al., 1996). Darkness is more commonly considered to be an inducer of senescence. Most of the work on darkness-induced senescence has been done on either detached adult leaves or the attached cotyledons or primary leaves of seedlings, however, which raises the question of the relationship of these results to the arguably more ecologically relevant system of shaded attached adult leaves. In fact, it has been shown in Arabidopsis that, by certain parameters, adult attached leaves are not induced to senesce when whole plants are placed in the darkness, although detached leaves are (Weaver et al., 1998).Here, we show that in Arabidopsis leaf senescence is not induced but is in fact inhibited when whole plants are placed in the darkness, whereas in contrast it is strongly accelerated when individual leaves are darkened while the rest of the plant remains in the light. These results demonstrate that the light status of the rest of the plant influences the senescence progression of the individual leaf. RESULTS Senescence Is Induced When Individual Leaves Are Darkened, But Not When Whole Plants Are DarkenedWhole Arabidopsis plants in their pots were placed in the darkness, or individual leaves were covered with cloth "mittens" (see "Materials and Methods"). Treatments were continued for either 2 or 5 d (control plants remained in continuous light) and all plants were harvested on the same day (d 0 in Fig. 1A). Consistent with previous results (Weaver et al., 1998), placing whole plants in the darkness did not induce senescence by most parameters measured: N...