In Temperate Zone woody plants, SDs induce growth cessation and bud dormancy. However, when SDs are extended with light extensions of appropriate spectral composition, plants perceive them as LDs. Such perception depends on the light requirement of those plants under consideration. Latitudinal ecotypes of northern tree species have different responses to light quality (Håbjørg, 1972; Junttila and Kaurin, 1985; Clapham et al. 1998), which could indicate differences in composition of their phytochrome systems. In experiments using light sources enriched in R or FR light, differences in FR light requirement by latitudinal ecotypes have been indicated. For example, in Salix pentandra, high R/FR ratio during the night extension did not produce a LD effect in a northern ecotype but did so in a southern one (Junttila and Kaurin, 1985). Moreover, over-expression of phyA changed (shortened) the critical photoperiod of hybrid aspen (Olsen et al. 1997). Such responses have not been tested, however, with monochromatic lights or defined R/FR ratios. Moreover, the responses of Betula pendula ecotypes upon extension of the PAR light with R, FR or various ratios of R and FR have not been studied. Although phytochrome genes have been cloned from both conifers and deciduous species, so far there is no information about possible differences between photoperiodic ecotypes at the phytochrome level. This study compared the vegetative growth responses of latitudinal ecotypes of B. pendula to monochromatic R and FR, and to various combinations of R and FR. Using special diodes that emit monochromatic light, we studied the effect of red (R), far-red (FR) and R/FR combinations on shoot growth of latitudinal ecotypes of B. pendula.

When a 12 hrs PAR (110 µmol m^-2 s^-1) was extended with R, FR or R/FR ratios, lower intensities of monochromatic lights could not prevent growth cessation. At 25 µmol m^-2s^-1, FR compared to R enhanced stem elongation in all ecotypes. FR at about 9.5-25 µmol m^-2s^-1 sustained continuous elongation of shoots of all ecotypes compared to R. However, the combined effect of R and FR at various ratios and at an intensity of 25 µmol m^-2s^-1 had a significantly higher effect. This clearly showed the combined effect of phyA and phyB in all B. pendula ecotypes.

CLAPHAM, D.H.; DORMLING, I.; EKBERG, I.; ERIKSSON, G.; QAMARUDDIN, M. and VINCE-PRUE, D. Latitudinal cline of requirement for far-red light for the photoperiodic control of bud set and extension growth in Picea abies (Norway spruce). Physiologia plantarum, 1998, vol. 102, no. 1, p. 71-78.

HÅBJØRG, A. Effects of light quality, light intensity, and night temperature on growth and development of three latitudinal populations of Betula pubescens. Meldinger Norges Landbrukskole, 1972, vol. 51, no. 44, p. 1-17.

JUNTTILA, O. and KAURIN, Å. Climatic control of apical growth cessation in latitudinal ecotypes of Salix pentandra L. In: KÅURIN, Å.; JUNTTILA, O. and NILSEN, J. eds. Plant Production in the North. Norwegian University Press, Oslo, 1985. p. 83-91.  

OLSEN, J.E.; JUNTTILA, O.; NILSEN, J.; ERIKSSON, M.; MARTINUSSEN, I.; OLSSON O.; SANDBERG, G. and MORITZ, T. Ectopic expression of phytochrome A in hybrid aspen changes critical day-length for growth and prevents cold acclimation. The Plant Journal, 1997, vol. 12, no. 6, p. 1339-1350.

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