It is possible to distinguish between two types of photoperiod effect on stem growth or extension. The first is the situation where daylength modulates the rate of internode extension or the number of nodes in plants showing a single type of growth habit. In its most common manifestation stem extension is inhibited by SD in woody, perennial plants, as a prelude to the onset of dormancy. Stem elongation in woody plants is discussed in detail in Chapter 11 because it is so closely associated with dormancy. The second case is where photoperiod triggers the switch between a rosette and elongate habit, often in association with flowering.
In the herbaceous species Lycopersicon esculentum and Glycine max (Downs et al., 1958) and Cucurbita maxima (Zack and Loy, 1980) which do not enter dormancy, internode extension was greater when short days were extended with TF light as compared to when they were extended with fluorescent light. However, it is not certain that these are true photoperiodic effects. TF lamps establish a much lower Pfr/Ptot ratio than fluorescent light and for any species which shows a 'shade avoidance' response, stem extension rates will be higher under TF than fluorescent light irrespective of any photoperiod effect. The light treatments will also vary in the blue component, which may contribute to the difference in stem extension. A further confounding effect may be the so-called 'end-of-day response', where a light treatment which lowers the Pfr/ Ptot ratio immediately prior to the daily dark period results in greater stem extension and taller plants (see Chapter 3). The most effective treatment is FR which typically establishes a Pfr/Ptot ratio of 0.03, but any treatment which reduces Pfr/Ptot ratio in the range 0 to 0.85 will cause increased stem extension (Vince-Prue, 1975; Fig. 11.18; Gaba and Black, 1985). Extensions with tungsten filament could therefore also result in longer internodes than extensions with fluorescent light because they establish a lower end-of-day Pfr/Ptot ratio in addition to their effect during the photoperiod. Evidence for a photoperiodic effect on stem extension was found in Fuchsia hybrida cv Lord Byron where a R night-break resulted in longer internodes than SD controls (Fig. 13.5). The effect of R was maximal at about 6 h into darkness whereas the promotion in response to a short FR treatment decreased linearly as the length of time in darkness before it was applied was increased. The response to R was therefore not
Hours from beginning of darkness
FIG. 13.5. Response to night-breaks with R or FR given at different times during a dark period of 16 h. Plants of Fuchsia hybrida cv Lord Byron were grown in 8 h days and given 1 h white fluorescent (R) or FR at different times in the dark period. The plants treated with R after 4, 8 or 12 h flowered, other plants remained vegetative. After Vince-Prue, 1975.
simply the opposite of the response to FR and could be separated from the 'end-of-day' response.
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