In this essay we will discuss about the process of water absorption by plants.
Essay Contents:
- Essay on Soil-Plant-Atmosphere Continuum
- Essay on Conductance Pathway in Space
- Essay on Root Characteristics
- Essay on Moisture Extraction Pattern
1. Essay on Soil-Plant-Atmosphere Continuum:
Water in the soil-plant-atmosphere system moves in response to the differences in energy status of water from one region of the system to the other and always from a region of higher potential to that of a lower potential.
Major areas of plant-water relationships in irrigation water management are water absorption and water loss or evapotranspiration.
The soil-plant-atmosphere continuum (SPAC) is the pathway for water moving from soil through plants to the atmosphere and back (Fig 5.5).
The transport of water along this pathway occurs in components, variously defined among scientific disciplines:
1. Soil physics characterises of water in the soil in terms of tension
2. Physiology of plants and animals characterises of water in organisms in terms of diffusion pressure deficit
3. Meteorology uses vapour pressure or relative humidity to characterise atmospheric water.
The SPAC integrates these components and is defined as a concept recognising that the field with all its components (soil, plant, animals and the ambient atmosphere taken together) constitutes a physically integrated, dynamic system in which the various flow processes involving energy and matter occur simultaneously and independently like links in the chain. This continuum hypothesis characterises the state of water in different components of the SPAC as expressions of the energy level or water potential of each.
Soil-plant-water relationships deal with those physical properties of soil and water that influence the movement, retention and use of water by the plants that must be considered to plan for an efficient irrigation system.
Availability of soil moisture is not a property of the soil alone, but indeed a combined function of the soil, the plant and the climate.
In principle, the rate of water uptake by the plants depends on the ability of the roots to absorb water from the soil with which they are in contact, as well as on the ability of the soil to supply and transmit water towards roots at a rate sufficient to meet transpiration and growth requirements. These variables, in turn, depend on plant, soil and weather conditions.
As such, an understanding of the relationships between the soil, water and plants (SPAC) is essential for efficient use of irrigation water.
2. Essay on Conductance Pathway in Space:
Water movement from soil to atmosphere can be summarised as indicated below:
The rate of movement of water in the plant is affected by conductance of the pathway. A drop in energy status of leaf water due to loss of water through transpiration is transmitted through stem to roots in contact with soil water. This set up a gradient needed for flow of water from soil through root and stem to plant leaf. Flow of water in SPAC is, generally, expressed in terms of resistance, which is the reciprocal of conductance.
q = (Ψs – Ψl)/Rl
where, q = Rate of water uptake
Ψs = Water potential in soil
Ψl = Water potential in leaf
Rl = Total resistance offered to water flow by the stem.
The total resistance from soil and leaf has three components arranged in series.
q = (Ψs – Ψl)/(Rsoil + Rroot + Rshoot)
When the water content of leaf is not changing, rate of uptake equals the rate of transpiration (T).
Substituting T for q we get:
Ψl = Ψs – T (RSoil + Rroot + Rshoot)
This would show that W] does not have a unique relation with Ws. Relative importance of plant and soil resistance in water uptake by roots appears to be a matter of dispute. However, it is well known that conductance of soil decreases or the resistance increases sharply with decrease in soil wetness.
Water absorption by roots is dependent on the supply of water at the root surface. For this, either the water should move to the root surface or the roots grow into the soil mass. As the soil dries out, the rate of water movement in the soil decreases rapidly. As such, the root system must expand continuously in search of moisture to replace transpiration losses in the absence of timely rainfall or irrigation. Hence, all the factors that affect root growth also affect absorption of water by plants.
The movement of water through the root and the plant system (xylem) to leaves is initiated and controlled by transpiration from the leaves in response to the water potential gradient extending from soil-water through the plant to atmosphere. (Fig. 5.6)
3. Essay on Root Characteristics:
Root systems in the field are seldom uniform with depth. Root penetration is seriously affected by a hard pan or compacted layer in the soil profile. In a shallow soil, roots may be confined to a thin layer of soil irrespective of their usual pattern. Similarly, high water table limits normal root growth. Crops with extensive and dense roots can utilise soil moisture more effectively and to a lower residual soil moisture than crops with sparse and shallow roots (Fig 5.7).
Rooting depth of annual field crops on deep well drained soils (Table 5.2) range from 0.30 to 2.0 m. In general, the root zone depth of crops on clayey soils is reduced by 25 to 35 per cent and on sandy soils increased by 25 to 35 per cent.
Table 5.2: Rooting depths (m)of annual field crops on deep well drained soils
The soil depth from which the crop extracts most of the water needed to meet its evapotranspiration requirements is known as effective root zone depth. It is also called as design moisture extraction depth, the soil depth used to determine irrigation water requirements for design.
It is the soil depth in which optimum available soil moisture level must be maintained for high productivity of crops. If two or more crops with different rooting characteristics are to be grown together, the design depth should be that of the crop having the shallower root system. In the absence of crop moisture extraction data for design, information in Table 5.2 can be used as a guide.
4. Essay on Moisture Extraction Pattern:
For most plants, concentration of absorbing roots is greatest in upper part of the root zone and near the base of plants. Extraction of water is most rapid in the zone of greatest root concentration and under favourable environmental conditions. In general, plants growing in uniform soil with adequate available soil moisture have similar moisture extraction pattern.
Usual moisture extraction pattern shows that about 40 per cent of the extracted moisture comes from upper quarter of the root zone, 30 per cent from second quarter, 20 per cent from third quarter and 10 per cent from fourth bottom quarter (Fig 5.8).
This general pattern of extraction slightly varies with irrigation frequency. Higher the frequency, greater the moisture extraction from first quarter of the root zone. Low frequency irrigation leading to depleting soil moisture results in more moisture extraction from lower quarter of the root zone soil depth.
Optimal soil moisture for crop growth varies with the stage of crop growth. Certain periods during the crop growth and development are most sensitive to soil moisture stress compared with others. These periods are known as moisture sensitive periods. The term critical period is commonly used to define the stage of growth when plants are most sensitive to shortage of water.
Inadequate water supply during moisture sensitive periods will irrevocably reduce the yield and provision of adequate water and other management practices at other growth stages will not help in recovering the yield lost. Inadequate water supply during tillering stage limits tiller production and hence the number of panicles or ear heads per plant or unit area.
Similarly, stress at panicle initiation leads to reduction in number of grains per panicle. Reduced tiller number per plant or grains per panicle due to moisture stress cannot be compensated by adequate water supply or other management practices at later stages of crop growth.
Moisture sensitive (critical) periods of crops (Table 5.3) indicate necessity for adequate available soil moisture during reproductive stages of crops. Water shortage during germination and emergence affect stand establishment. Shortage of water during vegetative stage, generally, has little effect on final yield of crops unless it is very severe. Moisture stress at heading and flowering stages reduces grain formation while that during grain development leads to shriveled grains with low test weight.
Cereals are highly sensitive to soil moisture stress during panicle initiation and flowering. In the case of rice crop, highest yield reduction (70%) occurs due to stress at booting (reduction division) stage followed by that at early heading (64%). Soil moisture stress at crown root initiation stage of dwarf wheat leads to about 40 per cent reduction in grain yield, while that at tillering reduces the yield by about 25 per cent. Legumes are sensitive to stress at flowering and pod development.
All the stages of growth are equally sensitive to soil moisture stress for crops where vegetative parts are of economic importance. Total growth and yield of perennial plants are the summation of effects of stress at each growth stage. However, adequate water supply is essential at flower bud initiation, flowering and fruit set. Flower bud formation, however, increases due to restricted water supply prior to flower bud initiation in the case of citrus and mango.
For realising maximum benefit from the scarce irrigation water, irrigations are to be scheduled at moisture sensitive periods by withholding irrigations at other periods of lesser sensitivity. Such irrigation schedules along with improved management practices increase the water-use efficiency in crop production.