Early
wake-up call elicits no response
Yam (Dioscorea
rotundata) tubers are an important crop in several parts of the tropics and
are grown in areas where the climate may permit more than one planting per
year. However, the growth cycle of’the tuber itself prevents this, as described
by Ile et al., Reading, UK and Ibadan,
Nigeria (pp. 497–504). Depending on where the
crop is grown, tubers are harvested either 180 days after planting (DAP) or
180–270 DAP (when the shoot becomes senescent). The authors point out that, in
contrast to potato, harvested tubers do not possess any surface buds, nor
indeed do they have any internal buds. They are therefore incapable of
sprouting. However, there is a layer of meristematic cells under the surface of
the tuber and it is in this layer that a ‘tuber germinating meristem’
differentiates, giving rise to the new shoot. The specific question addressed
by the authors was whether it is possible to induce earlier formation of buds
in the meristematic layer? The head region of each sampled tuber was cut
longitudinally into portions and the portions were assigned to different
treatments with plant growth regulators (PGRs): GA (two concentrations),
2-chloroethanol (two concentrations), thiourea and control. Typically, the
tuber germinating meristem appeared approx. 290 DAP and the treatments with
PGRs did not bring this forward. Indeed, with GA there was evidence for
inhibition of development. This long first phase of dormancy is thus likely to
be controlled by as yet unknown endogenous factors. However, once the tuber
germinating meristem had appeared, PGRs did have an effect on the timing of the
initiation of foliar primordia and the subsequent development of the shoot,
with 2-chloroethanol being especially effective in acceleration of these later
stages. The role of GA, however, remains unclear because of evidence that it
may delay, by re-imposition of dormancy, the outgrowth of the shoot.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
