Seeing the light on the
slippery slope
I was
walking recently in the valley of the River Nidd in northern England and was
interested to observe the different growth angles of trees on the steep wooded
banks. Some were very nearly vertical while the growth direction of others was inclined in the direction of the downward slope. It is this topic that has interested Matsuzaki et al. from the University
of Tokyo (pp. 573–581). They have shown that, at a given
angle of slope, the inclination of the stems of trees varies according to species.
Of the four that they studied, Cryptomeria
japonica showed the least inclination and Quercus serrata the greatest. To investigate the
factors that regulate the angle of growth, seedlings were grown in individual
pots that were either kept vertical or inclined at 45o . For both treatments, seedlings were
illuminated either vertically from above or unilaterally
(light source at 90o to
vertical). This enabled the authors to evaluate the individual and the combined
effects of gravi- and phototropism on the growth of the seedlings. The stems of
all four species responded to both stimuli but, perhaps surprisingly, it was
the magnitude of the phototropic response that was correlated with the angle
away from vertical of trees growing on slopes. The authors conclude that even
after the cessation of elongation, the inclination of woody stems can be controlled
by phototropic responses. This work is important not only because it gives us
information about plant growth in general but also because it is relevant to
the amenity use of trees. The authors have obtained insights about what happens
to the above-ground parts of plants growing on slopes. It will be very
interesting now to link this with the work on the root systems of plants in
similar situations, such as that described by Chiatante’s group in their recent
publications (e.g. Annals
of Botany 97: 857–866).
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
