Temperature
Plant growth and development is
primarily influenced by temperatures
at the growing points of plants (i.e.,
roots and shoot tips). When we are
discussing temperature, it is important
to understand that plant temperature
(not air temperature) drives physiological
responses in plants. Air temperature can differ by as much as
10° F from plant temperature, depending on your light source (HPS,
MH, or LED), light intensity, humidity, and air speed. For example,
HPS lights emit a large percentage of their energy in the infrared (IR)
range (800nm–1000nm) which is not photosynthetically active yet
significantly increases plant temperature. As a result, growers need
to decrease their air temperature set-point to counter the additional
radiant heat.
All crops have a species-specific base temperature, at which growth
and development will not occur. Above the base temperature, growth
and developmental rates increase with temperature until an optimum
temperature is reached. Above the optimum temperature, plant
development decreases. Light intensity primarily influences
the rate of photosynthesis, while plant temperature primarily influences
developmental rates. Net photosynthesis under increased PPFD will increase as temperatures
approach the optimum temperature for
the species of plant you are growing;
however, the optimum temperature
for photosynthesis depends on the
concentration of CO2 ; it is
important to understand that as you
increase temperature, you will also
change the morphology of the plant by
increasing developmental rates. The
ratio between light intensity and temperature
is known as the photothermal
ratio. If you choose to grow at warmer
temperatures, you need to ensure that
you are providing an adequate light intensity, or you may produce
plants that have increased internode distance, small stem caliper, and
an overall spindly growth habit.
The difference between day/night temperatures (DIF) will also significantly
influence plant morphology. For example, if your day/night
air temperature is 75°/65° F you have a +DIF of 10° F, which will promote
stem elongation of most crops. Alternatively, if you have a warmer
night temperature 65°/75 °F (day/night) you will have a -DIF, which
will suppress stem elongation. Depending on the growth habit of
your crop, you will need to find a balance between temperature and
light intensity to achieve your desired plant architecture.
RH and VPD
Relative Humidity (RH) is the amount of humidity present at a
given temperature and is expressed as a percentage. When air is
completely saturated, it has a RH of 100%. Temperature and RH are
the two main variables that influence water movement within a plant.
Evapotranspiration is a process plants use to cool leaf surfaces. As
the temperature of a leaf increases, plants will pull more water from
the growing media. Water is evaporated from the leaf surface and
as a result the leaf temperature decreases. Increasing the
temperature in your controlled environment will reduce your RH,
causing an increase in transpiration rates and water demand, while
decreasing your temperatures will increase RH, causing decreased
transpiration and water demand.
A good tool to use when growing
in a controlled environment is vapor
pressure deficit (VPD). VPD is a good
indicator of plant stress brought about
by either excessive transpiration
(high VPD values) or the inability to
transpire adequately (low VPD values). When the VPD is too low (humidity too high) plants are unable to evapo¬rate enough water to en¬able the transport of mineral nutrients (such as calcium), and in cases where VPD is extremely low, water may condense onto the plant and provide a medium for fungal growth and disease. Table 8 provides VPD val¬ues based on temperature and humidity. Generally, you will want to grow your plants in the optimum VPD range. However, during establish¬ment growth (especially vegetative cuttings), optimal VPD is around 0.3 - 0.5 kPa, which is outside of the opti¬mal range in our VPD table.
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i’ll hit that sweet spot alright 