Greenhouse Canada

Growing Media Inputs
Special Series in the rootzone #1: Water uptake


December 3, 2009
By Andrew Lee

Topics

In the first of six articles for Greenhouse Canada, Grodan® crop consultant Andrew Lee provides an insight into the physiological process of water uptake by plants and describes how the rootzone and aerial environment interact to drive this within the greenhouse.

In the first of six articles for Greenhouse Canada, Grodan® crop
consultant Andrew Lee provides an insight into the physiological
process of water uptake by plants and describes how the rootzone and
aerial environment interact to drive this within the greenhouse.

Quite simply, water moves through the plant from the roots to leaves
within structures called xylem vessels, a process that is governed by
transpiration. Of the quantity of water absorbed by a plant, around 90
per cent is transpired while only 10 per cent is used for growth.

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To put this into perspective, a cubic metre of greenhouse air at 20˚C
(68˚F) can hold a maximum of 17 grams of water. An actively growing
crop can transpire as much as 4.5 litres of water per square metre on a
sunny day of 2000 J/cm2. Water evaporated from the leaf in this way
acts to cool the greenhouse climate in the same way as a high pressure
fogging system. Indeed, the temperature of a transpiring leaf can be
2-6˚C (4-11˚F) lower than a non-transpiring one. This is why during the
summer months it is essential to have a good quality root system and
optimum leaf area index to achieve sufficient cooling capacity and
thereby maximize production and fruit quality.

However, transpiration by the crop, because it adds so much moisture to
the air, can create problems at other times of the year when
ventilation is limited, or in periods of dark and mild weather, by
increasing humidity levels beyond desired limits set by the grower.
When the greenhouse climate is humid, it is essential that the rootzone
is managed correctly to avoid the onset of plant (disease) and fruit
quality issues.

Understanding how the rootzone and aerial climates interact with each
other is a fundamental requirement for any grower; only when these
environments are working in balance is it possible to maximize returns.


TRANSPIRATION

Transpiration begins with the evaporation of water through the
stomata (tiny pores on the underside of the leaf) when they are open
for the passage of carbon dioxide and oxygen during photosynthesis.

This moisture is then replaced by water from adjacent cells located
directly behind them. Water subsequently moves into these cells from
the xylem vessels located within the leaf. As water moves into the
leaf, it pulls on the column of water held within the xylem all the way
down to the roots. This draws the xylem walls inward, creating a
negative pressure and results in water moving into the root and up
toward the leaves.


ROLE OF THE STOMATA IN TRANSPIRATION

Evaporation through open stomata is the major route of water loss in
the plant. The stomata must open for the passage of carbon dioxide and
oxygen during photosynthesis, however a balance must be maintained
between the gain of carbon dioxide and the loss of water. The plant
achieves this balance by regulating how wide the stomata open.

Opening and closing of stomata is stimulated by light. Other parameters
can influence the rate of transpiration such as heat and not least,
relative humidity or more precisely, from a plants perspective, the
vapour pressure deficit (VpD), defined as the difference between the
vapour pressure inside the stomata and the vapour pressure of the
greenhouse air. Consequently, changes in the aerial environment –
light, heat and humidity – will influence the time that transpiration
starts and rate of transpiration during the day. This will impact on
how the rootzone is managed.

Stomata open when light strikes the leaf surface in the morning. In
greenhouse situations we see transpiration or plant activity starting
at approximately 150-200W/m2 outside radiation. This is clearly shown
in Figure 1.0 by the difference in surface temperature, as a result of
evaporative cooling, between a tomato leaf and a non-transpiring leaf
sensor. The first irrigation of the day should be timed to coincide
with this.

This relationship also has implications for the minimum pipe strategy
in the morning. Astute growers reading this article will now understand
why the minimum pipe strategy is reduced on light, not time, in the
range 200-400W/m2, depending on greenhouse structure. Using a minimum
pipe above 400W/m2 under these circumstances would just cost the grower
extra money as the plant is already activated by the sun. However,
there is a notable exception. When the rootzone is cold, 12˚C (53.6˚F),
there can be a delay in transpiration of up to two hours compared to a
rootzone temperature of 17˚C (62.6˚F). In these situations, the start
time of irrigation and minimum pipe strategies should be adjusted.

The rate of transpiration during the day will be governed by how active
the climate inside the greenhouse is, i.e., the higher the temperature
and lower the relative humidity the greater the rate of transpiration.
I will briefly describe two contrasting situations.

rootzone_fig1
Figure 1.0: Relationship between the temperature of a tomato leaf and
plant activity sensor (PA Sensor) in relation to outside radiation at
the start of the day. (Source: Peter Stradiot, Innogreen)


SUNNY DAY
During the day, if the absorption of water by the roots is less than
the rate of transpiration loss of cell, turgor occurs and the stomata
will close to prevent the plant from wilting. This immediately reduces
the rate of transpiration, as well as photosynthesis, resulting in a
potential loss of fruit quality and production. Plant (and air)
temperature will increase as a result leading to higher and higher
respiration rates until the plant effectively burns itself up. For this
reason it is important to maintain the quality of the root system,
especially from over-wintered crops entering the spring.

It is also advisable under high light conditions (>1000 J/cm2/day)
to link irrigation volumes to the radiation sum (Figure 2.0). Indeed,
in extreme circumstances, growers can use the water uptake of their
crop as an indication of how well the crop is performing and thereby
adapt their venting and screening strategies accordingly. In this
respect it is important to remember that water uptake by the crop
should not decrease when using these tools. Excessive screening reduces
light penetration, and light, quite simply equals production!

rootzone_fig2
Figure 2.0: Irrigation strategy over two sunny days for a pepper crop
in Holland – substrate Grotop Expert. Data generated using Grodan®
Water Content Meter.
  Water content = black line.
Substrate EC = red line.
Blue bars = total volume of water applied and drain achieved.

Look to the activity (transpiration) of the crop on both mornings as
indicated by the change in slope of the water content line about 07:00
hrs and compare this to May 16 and May 17 (Figure 3.0).

rootzone_fig3

Figure 3.0: Irrigation strategy over two dark mild days for a pepper
crop growing in Holland – substrate Grotop Expert. Data generated using
Grodan® Water Content Meter.


Water content = black line.
Substrate EC = red line.

DARK MILD DAY
On a dark, mild day, transpiration will be low, therefore the start
and particularly stop times of irrigation should be adjusted
accordingly. This is easy to implement given today’s modern climate
computers used in combination with tools such as the Grodan® Water
Content Meter (WCM). On dark, mild days, the use of the minimum pipe
temperature settings (50-60˚C) for a few hours in the afternoon along
with some ventilation may be required to stimulate plant activity. This
ensures that essential nutrients are still supplied to the plant and
will also maintain the crop in the right generative balance.

I will explain how to steer plant balance using the rootzone in the
third article of the series, Understanding and steering the rootzone in
response to the six phase life cycle of a crop. In the meantime,
readers who wish to know more about the six-phase life cycle can visit
www.grodan.com.

However, it is important not to over-stimulate the crop with a large
temperature influence on the humidity pipe. This will often make the
relative humidity worse, by increasing transpiration rates, and will
also increase the risk of creating a weak crop. For humidity control, a
minimum pipe of 40˚C is normally sufficient – taking into account
today’s energy prices, a humidity pipe no higher than 45˚C should be
used. For example, the minimum pipe setting may be 35˚C with +10˚C
influence for humidity increase in the range 80-90 per cent.

If you are alarmed by these numbers, investigate this in the coming
winter and spring. Look to your graphics on the climate computer,
particularly the relation between air humidity and humidity pipe
temperature. I would anticipate that greenhouse humidity will not
change if the humidity pipe is 40˚C or 60˚C. The only difference will
be the heating bill!

It is important to remember that the only way to let moisture out of
the greenhouse is to open the vents, so keep heating and venting lines
in these situations close together – this will also keep the greenhouse
active. However, avoid aggressive venting when it is cold outside by
linking the vent strategy to outside weather conditions, because cold
air (<13˚C, 55.4˚F) falling on the heads of the crop can impact
negatively on transpiration.

It is also worth remembering that often on a dull day it will be the
maximum rest time (i.e., the maximum length of time between
irrigations) that will determine the total quantity of water supplied
to the crop (Figure 3.0). So in combination with late start and early
stop times and the minimum pipe strategy, make sure this is not too
short. I will describe how this can be achieved in the fourth article
within the series entitled, Interpreting information from a WCM to
implement a sound irrigation strategy. This will prevent fruit
physiological quality problems such as splitting and uneven colour. You
will know if the maximum rest time is too short because the substrate
EC most likely will decrease too far.


THE ROLE OF ACTIVE ROOT UPTAKE
A plant can also take up water even when it is not transpiring. This
phenomenon is normally referred to as “active root uptake” and results
in what crop consultants call “root pressure.” Root pressure is
strongest when transpiration ceases during the night or when plant
“activity” is low.


WHAT CAUSES ROOT PRESSURE?

On the surface of the root there is a single layer of cells that
contain transport proteins. These allow ions (i.e., Ca2+, K+) to cross
from the surrounding substrate into the roots. This active process
burns the sugars (via respiration) that are made during photosynthesis,
but more importantly it creates a concentrated sugary solution of ions
inside the root cell. Water then follows the flow of minerals into the
roots by a passive process called osmosis. The plant can do nothing to
stop this, but a grower can limit its potential negative impact on
fruit quality (i.e., splitting and radial cracking in tomatoes) with
correct rootzone management. I will discuss fruit quality issues and
the role of rootzone management in greater detail in the fifth article
of the series entitled, Rootzone management and the impact for fruit
quality.

In this respect, I always advise growers not to have an aggressive EC
reduction of the irrigation solution based on light intensity (W/m2)
and to stop the irrigation sometime before sunset. This ensures that
the substrate EC is not at its lowest when transpiration ceases,
because a higher substrate EC at night acts to limit the flow of water
via osmosis into the roots. I always remind growers that EC in the
substrate should be at its lowest when light intensity is at its
highest!

There are many factors that can influence root pressure and I have summarized these in Table 1.0.

Table 1.0: Factors influencing root pressure of a tomato crop

Factor Reason
Grafting root stocks for improved vigour   A larger root system can access more water.
Warm slab temperatures Increased respiration within the root system resulting in greater ion transport.
Low fruit load   Less water buffering capacity within the plant.
Cool or cloudy weather conditions  Less transpiration
Low substrate EC When transpiration ceases at the end of the day a
low substrate EC allows more water to pass into the roots via osmosis.

PREVIEWING FUTURE ARTICLES
In conclusion, the rootzone environment can be described as the
engine room of the crop. A good quality root system will allow the crop
to transpire. However, the time that transpiration starts and the rate
of transpiration during the day are governed by interaction with the
aerial environment. The rootzone climate needs to be managed
accordingly, in order to maintain optimum plant balance, production and
fruit quality. This can be achieved by understanding what
functionalities the substrate has and how the grower can take advantage
of these when developing an irrigation strategy on the climate
computer.

In the next article, I will define the key features of greenhouse
substrates, why they are important and how these are used by growers to
achieve specific objectives in rootzone steering.


Andrew Lee works for Grodan BV as Business Support Manager for North America and Export Markets. He is a PhD graduate from the University of London, England, and has been with Grodan® for the past nine years.


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