Photoperiod Effects

The early work (1930s) on photoperiod effects was directed mainly toward control of autumnal leaf senescence and shedding in woody plants and the culture of plants in controlled environments.

Autumnal senescence occurs at a time of decreasing day length, and therefore, short days (SDs) would be expected to cause leaf senescence/shedding in trees and herbaceous plants if they are photoperiodic. In many, but apparently not all species, autumnal senescence is induced by SDs (Matzke, 1936; Jester and Kramer, 1939; Ashby, 1950; Olmstead, 1951; Krizek et al., 1966). Interestingly, street lights delay leaf senescence and shedding in many tree species (Matzke, 1936). The effect of this night interruption tends to be greater close to the light source and more pronounced in areas with milder winters (Larry Nooden, personal observations). Under continuous days in a glasshouse, some leaves on red maple saplings lasted almost 2 years, far beyond their natural longevity (Jester and Kramer, 1939). Since SDs with interrupted nights behave like long day (LD) photoperiods, the SD-photoperiod effects are not due simply to light dosage (Ashby, 1951; Krizek et al., 1966).

LD photoperiods can also promote leaf senescence and plant death in some species (Schwabe, 1970; Khan and Padhy, 1978; Kang and Cleland, 1990; Trippi and Brulfert, 1973; Matta etal., 1979; Kar, 1986). Longer day lengths may promote senescence proportionately more in excised rice leaves (Kar, 1986) and this suggests possible phototoxicity; however, in butterfly flower leaves, day lengths beyond 14 h promote less (Matta et al., 1979). In Arabidopsis, LDs promote leaf senescence at 300 ^m m-2 s-1; however, when the light intensity is reduced to 180 pm m-2 s-1, LDs do not promote leaf senescence (Nooden et al., 1996). Here, what looked like a photoperiod effect turned out to be a light-dose effect.

Because the reproductive structures commonly trigger leaf senescence (correlative controls) and photoperiod often controls their development, the effect of photoperiod on leaf senescence could be indirect, via the reproductive structures. This does not seem to be the case in cocklebur (Krizek et al., 1966), but it may apply in other monocarpic plants where the reproductive structures play such a dominant role (Nooden, 1980, Chapter 15 of this volume). Nonetheless, photoperiod can also alter senescence in detached leaves (Schwabe, 1970; Misra and Biswal, 1973; Kar, 1986; Kang and Cleland, 1990) and therefore can also act directly on the leaves.

Another important effect of flower-inducing photoperiods is the cessation of vegetative growth as the plants shift their resources to reproductive development (Nooden, 1980). For example, LDs induce apex senescence in certain pea varieties (Proebsting and Davies, 1978). The result is that "worn-out" leaves, etc. are not replaced, and eventually this could contribute to the death of the plant.

In early studies on the effects of controlled environments on plant development, it was noticed that some species grow poorly or even die under continuous days. For example, continuous illumination injures geranium, tobacco, coleus and potato plants, while it actually kills tomato plants (Arthur et al, 1930; Wheeler and Tibbitts, 1986). Others such as winter wheat actually grow better under continuous illumination, and many species are not significantly affected. The detrimental effects of continuous illumination may be photope-riod effects or phototoxicity; however, in tomato plants, it has been ascribed to interference with the plants' endogenous rhythms (Highkin and Hanson, 1954). Generally, the pho-toperiod effects on leaf senescence have not been tested to determine if they are due to insufficient light (SDs) or excess light (LDs); however, most are probably true photoperiod effects as opposed to light-dose effects as in Section III above.

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