Seeds seek signs from shaded soil
In the Biblical
parable of The Sower, some of the
seeds fell on stony ground. They germinated successfully but the seedlings did
not survive because they withered in the heat of the sun. Germination in the
wrong place clearly has disastrous consequences. For some species, mechanisms
have evolved to ensure that this does not happen, as described by Kos and Poschlod (Regensburg, Germany, pp.
667–675) in respect of plants occupying a particular niche in the xeric
Kalahari savannah. Plants that grow under the canopy of Acacia erioloba trees form a well-defined subset of the local flora
in this ecosystem. Many of them are dispersed by birds, which shed the seeds
from their perches in the trees. Seeds germinate in the shade of these ‘nurse’
trees but do not do so in the open ground around the trees. Soil temperatures
at a depth of 10 mm were measured every 2 h for 3 months under the canopy and
in the open ground (matrix). Two features were very clear. First, the minimum
soil temperatures of the two habitats were very similar; second, the maximum
soil temperature was several degrees higher in the matrix than under the
canopy, which meant that the daily temperature fluctuation (DTF) was much
greater in the matrix. Seeds of canopy and matrix species were then set to
germinate under various soil temperature regimes. Here we focus on regimes that
mimicked the two habitats. Again the results were clear. Matrix perennials and
annuals germinated equally well under both ‘habitat-regimes’. Presumably there
are other factors that prevent the growth of these species under the canopy. By
contrast, for all the canopy perennials and some of the annuals, the matrix
soil temperature regime was inhibitory to germination, in some species very
strongly so. These species thus avoid unfavourable growth conditions by not
germinating in the wrong place – unlike the seeds in the parable!
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
