Water content and uptake
The shape of the plant is determined by its water content, for when the
water content declines, wilting occurs and the plant begins to lose its shape
and begins to droop. Wilting occurs initially in newly developing tissue
that has not yet developed a firm cellular structure. There may be condi-
tions where water uptake and movement within the plant are insufficient
to keep the plant fully turgid, particularly when the atmospheric demand
is high and/or when the rooting environment (temperature, aeration, and
water and salinity levels) is such that it restricts the uptake of water through
the roots. In general, field-grown plants are less sensitive to water stress
than are plants grown in controlled environments, which may partially
explain why plants in the greenhouse are particularly sensitive to water
stress, which in turn significantly impacts growth rate and development.
Water is literally pulled up the conductive tissue (mainly in the xylem)
by the loss of water from the leaves of the plant by a process called “tran-
spiration,” which takes place mainly through open stomata located on leaf
surfaces as well as through lenticels and the cuticle. To understand this
process, visualize a continuous column of water from the root cells up
to atmospherically exposed leaves; the rate of water movement is driven
by a water potential gradient between the leaves and the surrounding
atmosphere. Transpiration has two important effects: It reduces foliage
temperature by evaporative cooling (as plant leaves absorb solar energy,
most of the absorbed energy is converted into heat), and it provides the
physical force for the translocation of elements from the rooting environ-
ment up into the upper portions of the plant.
Leaves exposed to direct solar radiation will rise in temperature if
water movement up the plant is restricted. Leaf temperature affects rates
of photosynthesis, respiration, and plant growth. The amount of water lost
by transpiration will depend on the difference in vapor pressure between
the leaf and ambient air. Leaf and air temperatures impact gas diffusional
rates; hence, rates of photosynthesis and leaf respiration all decrease with
increasing leaf temperature. The rate of transpiration increases signifi-
cantly with increasing movement of air over the leaf surfaces at similar
stomata aperture openings. In addition, water lost by transpiration is
determined by a complex relationship that exists between air temperature
and relative humidity as well as the taxonomic classification and ontoge-
netic age of the plant organ.
In order for water to enter the roots, the roots must be fully functional.
Water absorption by plant roots declines with decreasing temperature,
decreases with increasing ion content of the water surrounding the root,
and decreases with decreasing O 2 content of the surrounding root mass
environment (Table 2.2).
Temperature is another important factor that influences root growth,
as well as the absorption of water and essential element ions. The opti-
mum root temperature will vary somewhat with plant species, but in gen-
eral, root temperatures below 68°F (20°C) begin to bring about changes
in root growth and behavior. Below the optimum temperature, there are
reduced growth and branching, leading to coarser looking root systems.
Absorption of both water and ions is also slowed as the permeability of
cell membranes and root kinetics are reduced with decreasing tempera-
ture. Translocation in and out of the root is equally slowed at less than
optimum root temperatures (68°F to 86°F [20°C to 30°C]). When root tem-
peratures are below the optimum (as well as just being less than the air
temperature), plants will wilt during high atmospheric demand periods,
and elemental deficiencies will appear. Ion absorption of the elements P,
Fe, and Mn seems to be more affected by low temperature than that of
most of the other essential elements, major, and micronutrients. It should
also be noted that the viscosity of water decreases with decreasing temper-
ature, which in turn affects water movement in and around the plant root.
The maximum root temperature that can be tolerated before signifi-
cant reduction in root activity occurs is not clearly known. Roots seem to
be able to tolerate short periods of high temperature. Roots are fully func-
tional at 86°F (30°C) and probably can withstand temperatures up to 95° ̊F
(35° ̊C). However, the current literature is not clear as to the exact limits of
the optimum temperature range for best plant growth.
In order to avoid the hazards of either low or high temperatures, the
roots and rooting medium should be kept at a temperature between 68°F
and 86°F (between 20°C and 30°C). Reduced growth and other symptoms
of poor nutrition will appear if root temperatures are kept at levels below
or above this recommended temperature range.
Aeration is another important factor that influences root and plant
growth. Oxygen (O 2 ) is essential for cell growth and function. If not avail-
able in the rooting medium, severe plant injury or death will occur. The
energy required for root growth and ion absorption is derived by the pro-
cess called “respiration,” which requires O 2 . Without adequate O 2 to sup-
port respiration, water and ion absorption cease and roots die.
Oxygen levels and pore space distribution in the rooting medium
will also affect the development of root hairs. Aerobic conditions, with
equal distributions of water- and air-occupied pore spaces, promote root
growth, including root hair development.
If air exchange between the medium and surrounding atmosphere is
impaired by overwatering, or the pore space is reduced by compaction,
the O 2 supply is limited and root growth and function will be adversely
affected. As a general rule, if the pore space of a solid medium, such as
soil, sand, gravel, or an organic mix containing peat moss or pine bark, is
equally occupied by water and air, sufficient O 2 will be present for normal
root growth and function.
In hydroponic systems where plant roots are growing in a standing
solution or a flow of nutrient solution, the grower is faced with a “Catch-
22” problem in periods of high temperature. The solubility of O 2 in water is
quite low (at 75°F, about 0.004%) and decreases significantly with increas-
ing temperature, as is illustrated in Figure 2.2. However, since plant
respiration, and therefore O 2 demand, increase rapidly with increasing
temperature, attention to O 2 supply is required. Therefore, the nutrient
solution must be kept well aerated by either bubbling air or O 2 into the
solution or by exposing as much of the surface of the solution as pos-
sible to air by agitation. One of the significant advantages of the aeroponic
system (see pp. 108) is that plant roots are essentially growing in air and
therefore are being adequately supplied with O 2 at all times. Root death,
a common problem in most NFT systems and possibly
other growing systems as well, is due in part to lack of adequate aeration
within the root mass in the rooting channel.
In soil and soilless rooting media, a greater root mass can contribute
to increasing absorption capacity, while in a hydroponic growing sys-
tem, root mass is less a contributing factor. The nutritional status of a
plant can be a factor, as a healthy actively growing plant will supply the
needed carbohydrates required to sustain the roots in an active respira-
tory condition.
It is generally believed that most of the water absorption by plant
roots occurs in younger tissue just behind the root tip. Water movement
across the root cortex occurs primarily intercellularly, but can also occur
extracellularly with increasing transpiration rate.
As water is pulled into the plant roots, those substances dissolved
in the water will also be brought into the plant, although a highly selec-
tive system regulates which ions are carried in and which are kept out.
Therefore, as the amount of water absorbed through plant roots increases,
the amount of ions taken into the root will also increase, even though a
regulation system exists. This partially explains why the elemental con-
tent of the plant can vary depending on the rate of water uptake. Therefore,
atmospheric demand can be a factor affecting the elemental content of the
plant, which can be either beneficial or detrimental. In addition, many
other water-soluble compounds in the rooting medium might be brought
into the plant and enter the xylem.