September 2020 - 27

CONTROLLED ENVIRONMENT AGRICULTURE
FeED 16N-4P-17K; J.R. Peters). Seedlings were
grown inside an environmental growth chamber
at a constant air temperature of 72° F and a
16-hour day with a photosynthetic daily light
integral (DLI) of 14 mol·m-2
·d-1
.
Two weeks after sowing seed, seedlings were
transplanted into 5.5-inch containers filled
with a commercial soilless peat and perlite
substrate and irrigated with 150-ppm nitrogen
from a nitrate-based fertilizer (15-5-15; Peters
Excel). After watering plants in with fertilizer,
they were placed back into one of six different
environmental growth chambers maintained at
one of six different constant air temperatures:
46, 55, 64, 73, 82 or 91° F. The DLI was again
maintained at 14 mol·m-2
·d-1
.
Three weeks (arugula and pac choi) or four
Figure 1. This non-linear, asymmetrical curve represents the generalized response of plant development
to air temperature. Cardinal temperatures include the base (Tbase
and Topt
), optimal (Topt
temperatures, and the linear range is between Tbase
.
) and maximum (Tmax
)
weeks (Swiss chard and kale) after placing
plants into temperature treatments, data were
collected. Prior to destructive harvesting, gas
exchange (photosynthesis, conductance and
transpiration), chlorophyll fluorescence (a
measure of stress), and relative chlorophyll
concentration were measured, as were plant
height and width.
Shoots were harvested and immediately
weighed to determine fresh shoot weight (yield),
after which time the number of unfolded leaves
were counted and the leaf area measured.
Shoots were then dried and the dry weight was
recorded after three days. To determine the
air temperatures corresponding to Tbase
Tmax
, Topt
,
, and the linear range, data were analyzed
using non-linear regression analyses, and
these models were used to identify cardinal
temperatures for each species.
THE RESULTS
The vast majority of arugula, kale, pac choi
Figure 2. Arugula, kale, pac choi and Swiss chard three (arugula and pac choi) or four weeks (swiss chard
and kale) after growing in environmental growth chambers maintained at 46, 55, 64, 73, 82 or 91° F.
and swiss chard responses (Figure 2) displayed
the general non-linear, asymmetric response to
air temperature demonstrated in Fig. 1. However,
the specific air temperatures corresponding
to Tbase
and the responses. For example, the Topt
greatest. In between the Tbase
and Topt
is linear
range, where plant development increases or
decreases linearly as temperature increases
or decreases, respectively. The actual air
temperature for these important, or cardinal,
temperatures are species-specific, varying with
each crop.
Although specialty greens are increasingly
popular, there is little information on the
environmental and cultural requirements for
these species, including their responses to air
temperature. We conducted research to quantify
the response of arugula, kale, pac choi and
Swiss chard to air temperature and determine
their cardinal temperatures.
THE EXPERIMENT
Arugula, 'Starbor' kale, 'Win-Win' pac choi
and 'Fordhook Giant' swiss chard were sown in
162-cell phenolic foam propagation sheets with
a top groove (Oasis HortiCubes XL; SmithersOasis)
placed in 1020 flats with drainage
holes. Immediately after seeding and every day
throughout the experiment, flats were irrigated
100-ppm nitrogen from a complete, balanced,
water-soluble hydroponic fertilizer (Jack's Hydro
, Topt, and Tmax differed for each species
for
photosynthesis for arugula and kale were 61.9
and 60.4° F, respectively, while for total leaf area
the Topt
were 72.1 and 75.7° F, respectively.
For commercial greenhouse and controlled
environment food crop producers, yield is
the most important aspect of production. For
specialty leafy greens, that is the leaf fresh
weight (we included stems in measurements for
arugula, pac choi and Swiss chard, but not for
kale), and while each species will have a number
of different cardinal temperatures, we think the
Topt
for fresh weight are the most useful to base
management air temperature management
decisions off of.
GREENHOUSE PRODUCT NEWS 27

September 2020

Table of Contents for the Digital Edition of September 2020