are you growing puny plants?
Relative humidity or RH, is a measure
of the amount of water (vapor) in the air
relative to the saturation point, expressed
as a percentage. When the air is saturated
with water, the value at a given temperature would be 100%. Two integral
components of RH are temperature and
the total amount of moisture present.
Air is able to hold more moisture as the
temperature decreases. Likewise as temperature increases, the carrying capacity
of moisture in air decreases. This concept
will have important ramifications that will
be discussed later.
So why does relative humidity matter
when we are talking about stomatal opening and closing? Life exists in a constant
battle to reach equilibrium. Chemicals
at a high concentration will want to
move to areas of lower concentration
to maintain this equilibrium. Water is
a chemical compound and as such will
want to diffuse or move accordingly. This
is where the plant stomata come into
play. For example, if the relative humidity surrounding the leaf is low, then this
gradient will want to pull water from the
plant. This is why plants, especially grown
in containers, can dry out quickly and
wilt when the wind is blowing and the
relative humidity is low. This stimulus
(along with darkness) can trigger the
closure of the stomata. This may sound
great, especially when you think about
plants under drought or water
stress, but this also significantly
reduces uptake of carbon dioxide, which reduces photosynthesis. A reduction in the rate
74
Maximum Yield USA | February 2014
“The lack of water uptake
by growing plants in a
humid environment would
directly reduce nutrient
uptake, leading to reduced
growth and nutrient
deficiencies over time.”
of photosynthesis is obvious as there will
be a stunting of growth and an overall
lack of vigor. If this scenario played out
continuously, it would result in energy
starvation and plant death.
On the other hand, exposing plants
to relative high humidity will have the
opposite effect as stomata will remain
open. However, while you might think
that growing plants in a humid environment would cut back on watering needs
because that pull to lose water is eliminated or significantly reduced, there are
three main problems associated with
growing plants in high humidity. Plants,
like people, can use evaporative cooling
as a means of regulating internal temperatures during adversely hot days. While
some plants possess coping mechanisms,
in general, hot temperatures coupled
with high humidity will lead to reduced
growth. Another reason high humidity is not ideal for plant growth
is directly tied to pathology. Fungal spores,
like seeds,