Development of the Wheat Plant - Wheat Science and Trade - page 37

Fig. 2.2 Winter wheat shoot apex developmental sequence correlated with developmental stages (Large 1954) for conditions
with no stresses. The timeline is presented as thermal time (TT, in growing degree-days, GDD, using 0 ºC base and
Method 1 of McMaster and Wilhelm 1997) and number of leaves (#LVS). Question marks indicate areas of uncertainty
or signifi cant variation among cultivars. See Table 2.1 for descriptions of developmental stages. (Adapted from McMaster
et al., 1992b.)

An important transition of the shoot apex is
from leaf to spikelet primordium initiation, and
this occurs at the double ridge stage. The general
rule is the rate of spikelet primordia initiation is
about two to three times greater than leaf primor-
dia initiation (Baker and Gallagher 1983a,b; Hay
and Kirby 1991). Extensive physiological research
has examined the environmental factors infl uen-
tial in determining when this switch occurs. Of
these factors, the vernalization requirement and
photoperiod sensitivity of the genotype play a
critical role in the switch.

Several vernalization genes ( VRN ) have been
identifi ed as further discussed in Chapter 3, and
their presence or absence determines the vernal-
ization requirement for the genotype. The physi-
ological conceptual model is that cool temperatures
are needed to satisfy the vernalization require-
ment and that vernalization is a process of accu-
mulating days with cool temperatures until a
suffi cient period of time with cool temperatures
has occurred. Winter wheat cultivars with a high
vernalization requirement may require 6 weeks or
more of cool temperatures to complete their ver-