Photoperiodism is one of the most significant and complex aspects of the interaction between plants and their environment. The word itself is derived from the Greek roots for 'light' and 'duration of time', and can be defined as responses to the length of the day that enable living organisms to adapt to seasonal changes in their environment. Such a response could clearly confer selective advantages to the organism. It can be used as a means of anticipating, and consequently preventing, the adverse effects of a particular seasonal environment. For example, at high latitudes autumnal short days precede winter low temperatures. The shortening autumn days act as a signal to induce bud dormancy and cold hardiness, responses which enable the plant to survive the unfavourable winter environment. Similarly, in some desert species dormancy is induced by the long-day conditions which accompany the unfavourable environment of water stress (Schwabe and Nachmony-Bascombe, 1963). A photoperiodic response can enable a plant to occupy an ecological niche in space and time. For example, a response to short days can enable a woodland plant to flower and seed before the dense leaf canopy is formed. Even in tropical latitudes where the seasonal daylength changes are small, many plants are photoperiodic, using daylength to synchronise reproductive or other activities with seasonal events such as dry or rainy periods. Synchronisation through photoperiodic sensitivity can confer advantages independently of whether flowering is matched with a particular favourable environment. In particular, coincident flowering in individuals of a species increases the chances of outbreeding and thus genetic recombination.

The broad definition of photoperiodism given above, while adequate for ecophysio-logical questions, requires refining when one considers the mechanisms in the plant which give rise to photoperiodic behaviour. Perhaps the most useful proposal is that of Hillman (1969a), who defined photoperiodism as a response to the timing of light and darkness. Implicit in this definition is that total light energy, above a threshold level, is relatively unimportant, as is the relative lengths of the light and dark period. What is important is the timing of light and dark periods or, to think of it another way, the times at which the transitions between light and darkness take place. Implicit in this definition is that as long as light is above a particular threshold, so that it is perceived as light by the plant, the actual level is relatively immaterial. Because plants respond directly to light through photosynthesis and photomorphogenesis, it is to be expected that varying combinations of light and dark periods might directly affect plant development. Where this happens, the response is likely to be quantitatively related to the durations of the light and dark periods over a range of values. This type of response is distinct from photoperiodism where development or metabolism proceeds in one of two alternative states depending on the relationship between the daylength received and a threshold or 'critical' daylength. It is not enough, therefore, to show that changing the daylength has an effect on a particular response for that response to be considered to be photoperiodically regulated. This book is concerned with true as opposed to quasi-photoperiodic processes and we have tried where possible to limit description and discussion to the former. However, experimental design frequently fails to discriminate between the two, particularly with respect to some of the more marginal processes discussed in Chapter 13.

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