- THE FLORICULTURE VEGETATIVE CUTTING INDUSTRY 153
The general recommendation is to provide indirect or diffuse light
by use of shade curtains, resulting in a uniform photosynthetic photon
flux (PPF) of 120–200μmol⋅m−^2 ⋅s−^1 or a DLI of 3–8 mol⋅m−^2 ⋅d−^1 dur-
ing Stage 2 (Blanchard et al. 2006; Faust 2011). Supplemental lighting
may be necessary to achieve this target DLI during the winter months
in northern latitudes.
D. Root Development (Stage 3)
Root primordia may be formed at several sites on an herbaceous cut-
ting after excision, including near phloem tissues, bud or leaf gaps on
the stem, or in callus formed at the basal end of a cutting (Lovell and
White 1986), although this site can vary among species (Lovell and
White 1986; Hartmann et al. 2002). Generally, dedifferentiated cells in
the vicinity of the vascular cambium and phloem divide, become meris-
tematic, and initiate adventitious roots (Hartmann et al. 2002).
Root initiation generally occurs 5–8 d after cuttings are placed in
propagation, and the development of the initiated adventitious roots
continues. The growth and development of roots is strongly related to
substrate temperature (Gregory 2006) and photosynthetic light (Lopez
and Runkle 2008a). The rooting response to temperature follows a gen-
eral quadratic curve, with base, optimum, and supra-optimal tempera-
tures in relation to root growth (Gregory 2006). Additionally, a supply
of carbohydrates to the developing roots is important for root devel-
opment (Hartmann et al. 2002); therefore, current photosynthesis must
supply carbohydrates to the developing roots (Davis 1988). While DLI
from3to5mol⋅m−^2 ⋅d−^1 during Stages 1 and 2 propagation may be bene-
ficial for minimizing stress and developing callus at the base of cuttings,
DLI from 5 to 10 mol⋅m−^2 ⋅d−^1 is required to promote subsequent root
development during Stage 3 (Loach 1988). However, DLI greater than
10 mol⋅m−^2 ⋅d−^1 during Stage 2 and 3 can be detrimental due to exces-
sive evapotranspiration and subsequent wilting of cuttings.
During Stage 3, cuttings absorb water from the substrate with the
newly formed roots. At this point, the water absorbed by the roots
replaces that lost from transpiration and therefore mist can be discon-
tinued or greatly minimized. At this point, only a very small amount of
water can be squeezed from the substrate, as it should be substantially
drier than in Stage 2. During Stage 3, the general recommendation is to
increase fertilization up to 100 mg⋅L−^1 N after root emergence.
During Stage 3, DLI has been shown to have a profound impact on the
root and shoot growth, stem elongation, and development of cuttings of
various species including: bacopa, Gypsophila(Islam and Willumsen