- THE FLORICULTURE VEGETATIVE CUTTING INDUSTRY 131
in order to increase the natural convection currents in the greenhouse
and to provide a cooler, less humid environment. Concrete floors also
reduce greenhouse humidity, which helps to prevent condensation
from forming on the inside of the glazing material during the night and
dripping condensate wetting the plants below; thus, concrete floors help
reduce pathogen pressure.
Heat stress can cause physiological problems. For example, poinsettia
stock plants of certain cultivars exposed to high temperatures (30◦C) fail
to develop viable axillary buds (Faust and Heins 1996). Often this phe-
nomenon is not noticed until the cuttings have been harvested, shipped,
and propagated. The crop fails to develop lateral shoots after the shoot
apex is removed. The plants are particularly susceptible to heat stress
when the individual stock plants are widely spaced, since this results
in higher plant temperatures. Once a stand of stock plants closes its
canopy together, evaporative cooling due to transpiration helps to min-
imize heat stress and improve axillary bud development.
- Daily Light Integral.Light plays a critical role in stock-plant produc-
tion, and daily light integral (DLI) measurements have become widely
adopted by the floriculture industry (Korczynski et al. 2002). Poin-
settia cutting stem caliper increased as the DLI increased from 4 to
10.6 mol⋅m−^2 ⋅d−^1 (Chong et al. 2014). This response to DLI can be an
important characteristic in the marketplace, since thin-caliper stems are
considered to be of inferior quality. The challenge faced in stock-plant
production is that the tallest stems in the canopy are harvested weekly,
and when those cuttings are removed, the next group of cuttings that are
exposed to direct sunlight will have just 1 week for the stem and leaf
tissues to mature prior to those cuttings being removing during the next
weekly harvest. This maturation process entails leaves becoming more
resistant to physical stresses and pathogen pressure in the postharvest
environment.
DLI affects the leaf-unfolding rate of shoots positioned in a dense
stock-plant canopy. Thus, DLI impacts that rate of progress of a shoot
toward becoming a harvestable cutting. For example, poinsettia cutting
yield increased from 2.5 to 6.0 cuttings/plant/week as DLI increased
from 2.2 to 21.0 mol⋅m−^2 ⋅d−^1 (Chong 2005). Improved cutting yield and
propagation performance resulting from higher DLI has also been doc-
umented for stock plants of heliotrope, New Guinea impatiens, petu-
nia, scaevola, and verbena (Verbena×hybrida) (Donnelly and Fisher
2001; Lopez 2007). The effect of DLI on unrooted cutting postharvest
and propagation performance is discussed in Section III.