How do Crop Plants Extract Water? | Botany

In this article we will discuss about how crop plants extract water from soil and atmosphere for their growth.

Soil Water Availability:

Plant roots grow into moist soil and extract water until a critical water potential in the soil is reached. Water that can be extracted from the soil by plant roots, termed available water, is the difference between the amount of water in the soil at field capacity (water held in the soil against the gravita­tional force) and the amount of water in the soil at permanent wilting percent­age (the percentage of soil moisture at which a plant will wilt and not recover in an atmosphere of 100% relative humidity).

The availability of soil moisture is affected by the colloidal property (i.e., the surface area of soil particles). A clay loam soil holds about 20% of its weight as available moisture, whereas a coarser textured soil, such as fine sand, only holds about 7%. On a soil volume basis, the clay loam at field capacity will hold about 17 cm available water, per meter of soil depth whereas the fine sand will hold less than 8 cm. A fine-textured loam soil at field capacity can supply about 25 cm (10 in.) of water to a plant that has roots extending 1.5 m (5ft) into the soil.

Soil water potential (ψ_soil) in agricultural soils is primarily affected by matrix potential (ψ_m) and secondarily by solute potential (ψ_s). Soil water poten­tial can be related to field capacity and permanent wilting percentage. At field capacity the ψ_soil is about —0.1 to -0.3 bars.

Permanent wilting percentage varies among crop plant species (— 15 to —50 bars) but is often arbitrarily set at —15 bars. The water potential at the permanent wilting point is of minor importance, since over 70% of the available water has been removed from a soil at -5 bars and very little water is available from —15 to —30 bars.

Water Uptake and Movement:

Air usually has an extremely low water potential, compared with plants or soils. Since a living leaf usually has a water potential greater than -15 bars, there is a steep energy gradient and continual movement of water as vapor from the leaf to the air. When there is no loss of water from the plant to the air (e.g., at night), the ψ_Plant, would come close to equilibrium with the ψ_soil.

When the stomata are open, the water loss from the leaf is continual, which lowers the ψ_leaf, below that of the ψ_petiole. Since water moves from a high ψ_w to a low ψ_w, water will flow from the petiole to the leaf. This water flux reduces the ψ_petiole, which was at equilibrium with the ψ_stem, so water moves from the stem to the petiole. This energy gradient is continuous down to the root and soil.

In other words, the system develops a ψ_w gradient from the soil to the air. Rates of water absorption and movement through the plant can be affected by the amount of soil moisture, root to soil contact, plant and soil resistances to water flux, and the ψ_w gradient.

Figure 4.4 is a schematic illustration of changes in the ψ_w of a soil-plant system with a 5-day drying period. When adding water to the soil and allowing the soil to lose gravitational water, the ψ_soil is approximately -0.3 bars. At night the stomata are closed so water moves into the plant and creates ψ_soil and ψ_plant equilibrium. During the day, the stomata are open and transpiration occurs.

As water is lost from the leaf the ψ_leaf becomes reduced, initiating a ψ_w gradient. This provides the energy differential to cause water movement from the soil to replace the water lost by transpiration. At night stomata close and transpiration is reduced to near zero. However, water will continue to flow in the system until the ψ_plant, and ψ_soil again approach equilibrium. As water is removed from soil by plants, the ψ_soil becomes lower and the ψ_leaves become relatively lower, establishing a ψ_w gradient for continual uptake.

The ψ_leaves on day 4 (Fig. 4.4) drop to – 15 bars and remain there, indicating that transpira­tion is being reduced by stomatal closure, which is usually accompanied by temporary leaf wilting.

On day 5 the water potential of leaves, roots, and soil go below a water potential of —15 bars. This usually indicates that the water available to the plant is not enough to prevent wilting and recovery will not occur unless water is added to the soil.

Soil moisture dropping to permanent wilting percentage in 5 days (Fig. 4.4) indicates a very limited soil volume and a root system in close contact with the total soil volume. Under most field conditions, the soil volume per plant is larger, allowing for a much slower reduction of soil water content. In the field the water content is not uniform throughout the soil profile; while roots extract moisture from one area, they are extending into new areas of the soil that may have a high ψ_w.

In this manner, the plant is often able to stay at a ψ_w higher than the average ψ_soil. However, as the volume of moist soil diminishes, the plant will need large ψ_w gradients for roots to absorb enough moisture to satisfy transpirational loss. This is a gradual process in the field and may last for weeks in medium- to low- textured soils, which allows the plants to become acclimated to lower water potentials. Under limited soil volumes, the change in ψ_w is rapid and the plant has less chance to adjust to a lower ψ_w.

Osmotic Adjustment:

Much of the research on water stress effects on plants has been performed on tissue excised from plants or on plants grown in pots with restricted soil volumes. There is increasing evidence that plants grown in pots respond dif­ferently to water deficits than those grown under field conditions; plants grown with small soil volumes experience water stress more rapidly than under usual field conditions. The root density is likely to be high throughout the soil volume, water extraction from the whole soil profile is uniform, and the drying cycle is relatively fast.

The roots of field-grown plants usually grow in large soil volumes. High root densities occur in the upper soil profiles where water is extracted rapidly, but as water becomes limiting in the upper soil profile, roots expand into lower soil profiles where water is more abundant. Thus in field-grown plants the development of stress during a drying cycle is much more gradual, the possibil­ity for overnight recovery of is much greater, and the plant has time to adapt to the developing stress.

Plants grown in growth chambers show rapid reduction in leaf expansion, starting at a leaf ψ_w of -2 to -4 bars and photosynthesis at -6 to -12 bars, while field data show rapid leaf expansion rates at —8 to —10 bars. Water movement in plants is deter­mined by ψ_w, but physiological processes affected by water availability are better predicted by ψ_w components.

Solute potential (ψ_s) and turgor pressure (ψ_p) must be known in order to evaluate the effect of water stress on physiologi­cal processes. The primary factor affecting growth or cell expansion is ψ_p, which can vary considerably at any ψ_w because it is the positive value equal to the negative values of ψ_s and ψ_m.

For example, when using equation (4.1) a cell could have the following ψ_w:

However, if (due to starch decomposition or potassium movement) the solute level should increase in the cell, water would diffuse into the cell, increasing the ψ_p even though the ψ_w may be decreasing. This is called osmotic adjustment.

Research on the expansion of maize and sorghum leaves by Acevedo et al. (1979) illustrates osmotic adjustment. They measured ψ_p and ψ_s of expanding leaves during a 24-hr period and found that leaf expansion occurred rapidly late in the day due to increased ψ_P even though the ψ_w was below —6 bars.

The increase in the ψ_p resulted from a drop in the ψ_s, due to sugar accumulation in expanding cells. Shaded leaves did not accumulate sugars and had reduced leaf expansion, and little leaf expansion occurred at night because of low temperature. Even though leaf expansion occurred under low ψ_w, irri­gated plants still had greater expansion during the morning period than did leaves of non-irrigated plants.

Plants grown in small pots, having restricted root systems and rapid development of water deficits, do not seem to be able to create the osmotic adjustment found in plants grown in the field. It cannot be assumed, however, that all crop species can generate osmotic adjustment under field conditions. More research must be done before this phenomenon is well understood.

Stomatal Response to Moisture Stress:

Stomatal opening results from an increase in turgor pressure of guard cells in relationship to surrounding cells. This turgor is in response to environ­mental stimuli, sometimes an influx of potassium ions, affecting osmotic ad­justment. Light, low CO₂ concentrations, adequate water, and low levels of ABA are the factors needed to stimulate the influx of potassium ions into guard cells. Therefore water stress, which can reduce stomatal opening, may be mediated through ABA.

Stomatal response can be different in plants grown in small pots and those grown under field conditions. Figure 4.14 shows a differential response of CO₂ uptake to leaf ψ_w. The measurements for maize and sunflower were made on plants with reduced soil volumes. Stomatal closure had to be the factor reduc­ing photosynthesis because transpiration (stomatal resistance) was reduced to the same degree as CO₂ intake. In potted plants, stomata start closing at a leaf of about -8 bars.

However, work by Sung and Krieg (1979) showed that in sorghum and cotton, CO₂ uptake started a decline at a leaf ψ_w of -30 bars. It is apparent that stomata under field conditions, in which the drying cycle occurs over a period of weeks instead of days, are often able to stay open at a much lower ψ_leaf.

In some species under field conditions, the stage of development in­fluences the stomatal opening. Research on maize and sorghum by Ackerson and Krieg (1977) showed that in the vegetative stage a low ψ_w would cause stomatal closure under sunlight. Stomatal closure was never complete under sunlight because the highest leaf resistance was around 10 s . cm^-1 at a ψ_w of -20 bars, whereas in the dark the resistance increased to 30 s . cm^-1.

During the reproductive stage, both maize and sorghum showed no change of leaf resistance with changes in leaf ψ_w. Thus stomatal control showed a complete insensitivity to moisture stress during the reproductive stage of growth. Under these conditions, it is difficult to determine what does limit water loss from plants. Plants may be able to create internal resistances that limit the ability to transpire.

Stomatal behavior is different in different environments, at different stages of development, at different leaf positions on the plant, and in different crop species. Further study is needed to better understand the factors affecting plant responses to different water regimes.