PNL Volume 12 1980 RESEARCH REPORTS 61
Nozzolillo, C. University of Ottawa, Canada
Seeds of Pisum sativum ecotype arvense L. (PI 356980) normally germ-
inate to produce monopodial seedlings having a dominant apical meristem.
Secondary branches may develop when the young plant has produced several
nodes, but usually do not, at least not under greenhouse conditions. Some
seedlings are atypical, however, in that the stem tip dies at an early stage
of development, usually before the third internode is fully elongated, and
one or more branches then arise from the cotyledon axes, or from the axis
of the first or second cataphyll. It had been noted previously that the
numbers of such branched seedlings seemed to vary with germination temper-
ature. To test whether this was indeed the case and not simply a chance
selection of disparate seed lots, the F2 generations of each of six crosses
made by G. A. Marx (PNL 11:25-26, 1979) were divided into two parts, one
to germinate at a low temperature of 10°C or 13°C, the other at the higher
temperature of 20°C.
Seeds were soaked for 24 hours after surface sterilization with sodium
hypochlorite, scarified if not swollen by that time, and placed on a thin
layer of moist vermiculite in a 9 cm Petri plate. Germination, including
the soaking period, proceeded under a 16-hr photoperiod provided by fluor-
escent lamps. Seedlings were removed from the Petri plates as the epicotyls
emerged and planted into vermiculite, 5 per 4 in pot, for continued growth
in the same environment. The numbers of abnormal seedlings were counted
only when extension growth of the fifth internodes had been completed, by
which time an unequivocal distinction between normal and abnormal could
be made.
As Table 1 clearly shows, the tip death rate at 10°C or at 13°C ranged
from 0 to 9%, while at 20°C the range was from 10% to 32% and thus invariably
higher. The difference in response to temperature was not the same among
all the six familes and involvement of a genetic factor is suggested. Causes
of the premature death of the stem tip remain to be discovered. The fact
that only the primary apex is affected while the apical tips of secondary
branches develop normally indicates that it results from abnormal differen-
tiation of the epicotyl during seed development.