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in this way, as other lamp types cannot be switched on and off so often. The underlying physiology is that the active form of phytochrome (which is formed in the light) continues to act during the following dark interval, provided that this is not too long. Recommendations for chrysanthemums are that the cycle should be at least 10 minutes but not more than 30 minutes long, with the light given for 20 to 50% of the time. For example, 20% light in a 30-minute cycle is 6 minutes light followed by 24 minutes dark; the cycle is then repeated for the duration of the night-break. Unfortunately, as with many other aspects of biology, not all plants behave in the same way. Carnations show a very weak response to intermittent lighting even when the dark intervals are extremely short and cyclic lighting offers no obvious advantage for this crop.
Long-day plants In contrast to most SDP, flowering in many longday plants (LDP) depends not only on the duration of the dark period but also on the kind of light received during the day. These so-called light-dominant plants are less responsive to a night-break and require quite long exposures to light of several hours duration in order to achieve a long-day response. Having said this, the same quantity and duration of light given during the middle of the night is usually more effective than the same amount given at the end of the day. So, in practical terms, to promote winter flowering in these plants the most efficient schedule is to give light for several hours (usually 4-8 hours) each day, in the middle of the dark period. However, in some cases, lighting throughout the night from dusk to dawn gives the most satisfactory results. In carnations, for example, 'all night' lighting for 2-4 weeks results in a flush of bloom some 3-5 months later. Light-dominant plants have another important characteristic. Even when the duration of darkness is favourable, the kind of light during the daytime influences the response and, in order to be effective, a long day must contain some FR light. The reason for this is not fully understood, although it is thought that one particular type of phytochrome may be involved. This phytochrome has been shown to be particularly associated with
Science and the Garden
sensing far-red light. (Another type of phytochrome may be involved in the perception of daylength in SDP, where FR is not required.) Sunlight is rich in FR wavelengths (Table 10.3) and so, under natural conditions, only the duration of darkness is important for LDP. When using artificial light to increase the day-length, TF lamps, which have a high content of FR, are the lamps of choice. Unfortunately, TF lamps have a lower R/FR ratio than sunlight (Table 10.3) and, when used over several hours each day, can lead to the development of 'leggy' plants, with long, weak stems. Because of the reduction in light under glass and the higher temperatures, greenhouse-grown plants are often undesirably tall even in natural daylight. Recently, filters have become available that absorb FR light and so considerably increase the R/FR ratio of the light that is transmitted through them. This has the desirable effect of reducing internode elongation and producing a more compact habit of growth without the use of costly growth-inhibiting chemicals (see Chapter 8). Unfortunately, the removal of FR light means that flowering in 'light-dominant' long-day plants can be considerably delayed and may even be prevented entirely. So far, this problem has not been resolved. The amateur gardener needs only to be aware that the use of such filters to improve the habit of growth may pose problems for flowering in some long-day plants.
Day-length lighting in practice Only relatively low irradiances are necessary for day-length lighting because light is only acting as a signal for the pigment phytochrome and is not supplying an energy source for photosynthesis. Although species do differ somewhat in their sensitivity to light, photoperiodic lighting is usually installed at between 0.25 and 0.5 W m 2, which is adequate for most species. Supplementary lighting for photosynthesis can also be used to obtain long days and, in this case, a much higher irradiance is needed. It is important to take care to avoid overspill to other nearby plants if they are responsive to day-length. From the above discussion, it is evident that much remains to be discovered about the mechanism(s) through which plants 'see' day-