Runner Formation

The cultivated strawberry (Fragaria X ananassa Duch) has a perennial habit and forms stolons and daughter offsets under LD conditions. Stolons, bearing daughter runner plants, arise in axillary positions and can be regarded as branch crowns in which the first two internodes become extremely elongated. Stolon initiation begins in the spring after winter chilling and is terminated by SD in the autumn. In cultivars where daylength regulates flower initiation, plants require SD for floral initiation. Under a wide range of environmental and cultural treatments vegetative growth, including runner formation is correlated with flower inhibition (Guttridge, 1969). The critical daylength for flower initiation is very similar for that for the inhibition of runner formation (Fig. 12.5) suggesting the possibility of a common regulatory system. Further evidence is that the inhibition of flowering or promotion of runnering by night breaks in a long dark period required one hour with incandescent light, but 20 or 30 min was either less effective or without any effect at all (Borthwick and Parker, 1952).

In general, suppression of flowering by long photoperiods is only effective using light sources which have an appreciable proportion of their energy in the FR part of the spectrum. Flowering in strawberry is therefore inhibited by LD in a typical 'lightdominant' manner rather than promoted by SD as a dark-dominant response. The promotion of runnering also seems to be a light dominant response. A light regime of 10 h fluorescent and incandescent light at a ratio of approximately 30:1 followed by 8 h low intensity incandescent was more effective than 18 h of the combined source on its own (Guttridge, 1969). As with onion bulbing, we would expect that a lightdominant response with sensitivity to FR would be mediated by phytochrome A. Strawberry is another species in which it is now possible to carry out genetic

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FIG. 12.5. Daylength dependence of runnering (promoted by LD) and floral initiation (promoted by SD) in strawberry. Data of Guttridge (1969).

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Photoperiod / h

FIG. 12.5. Daylength dependence of runnering (promoted by LD) and floral initiation (promoted by SD) in strawberry. Data of Guttridge (1969).

transformation. An everbearing cultivar, Rapella, has been transformed with rice phytochrome A under the control of a constitutive promoter. The transgenic plants show greatly enhanced runnering, consistent with runnering being promoted by a lightdominant response mediated by phytochrome A (Thomas, unpublished observation).

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