Lesson 26. Soil Water Extraction Pattern and Plant Response

26.1 SOIL WATER EXTRACTION PATTERN OF PLANTS

26.1.1 Water Extraction Pattern

Plants have normally a higher concentration of roots in the upper part of the root zone and near to their base. In a normal soil with good aeration and without restrictive layers, a greater portion of roots of most plants remains within 45 to 60 cm surface soil layers and most of the water needs of plants are met from this zone. As the available water from this zone decreases, plants extract more water from lower depths. When the water content of upper soil layers reaches the wilting point, all the water need of plants are essentially met from lower layers. Since there exist few roots in lower layers, the water extraction from lower layers may not be adequate to prevent wilting, although sufficient water may be available there.

When top layers of the root zone remain constantly kept moist with frequent irrigations, plants get most of their water need from the upper layers and a very little from the lower layers. In uniform soil profile with moist soil, all plants usually extract 40, 30, 20 and 10 per cent of the water need from the respective quarters of the root zone (Fig 26.1). The extraction pattern is normally positively correlated with the root distribution pattern.

Figure 26.1:   Mean design of soil water extraction pattern in soils adequately supplied with water and without restrictive layer in the root zone.

26.1.2 Design Water Extraction Depth

The design water extraction depth of crop refers to the soil depth from which the crop meets most of its water need. A greater part of the absorbing roots in concentrated in the design depth. Soil water is depleted most from this zone between two irrigations and the depleted water is replenished through irrigation. The depth of irrigation required to replenish the depleted water in the design water extraction depth may be called the design depth of irrigation.

Since the development of crop roots varies according to the water availability, soil conditions and crop culture, the design water extraction depths for various crops may be determined based on water extraction data in locality. This is important to achieve high water use efficiency. The purpose of irrigation is to provide adequate soil water for absorption and to help nutrient absorption. A high use efficiency of irrigation water is obtained when there is a minimum movement of applied water beyond the design depth. The interval between two irrigations and the design depth of irrigation are usually more for a crop with a greater design extraction depth in a given soil. Whenever two or more crops are grown together, the design depth of irrigation should be decided based on the crop having a shallower root system. In the absence of definite information on the actual design extraction depths of crops in an area, the quantity of water to be applied may be decided on the basis of design depths given in Table 26.1.

 Table 26.1: Design water extraction depths for crops (crops in very deep and well-drained soils)*

60 cm

90 cm

120 cm

180 cm

Cauliflower

Cabbage

Onion

Potato

Lettuce

Rice

Carrot

French bean

Garden pea

Chilli

Muskmelon

Tobacco

Wheat

Castor

Groundnut

Cotton

Tomato

Water melon

Maize

Sorghum

Sugar beet

Soybean

Pearl millet

Lucerne

Citrus

Apple

Grapevine

Coffee

Sugarcane

Safflower

 

*The depths may be increased by 25 to 33 per cent for sandy soils of uniform texture and reduced by 25 to 33 per cent for clayey soils (if crops are well adapted to these soils).  Source: Gandhi et al. (1971).

 26.2 WATER DEFICIT AND PLANT RESPONSES

Plants absorb water to do the normal function of nutrient absorption, transpiration and metabolic activities leading to growth and yield. When the available soil water is not enough to meet particularly the normal transpiration losses, a water deficit in plant is created interfering in many plant processes. As a result, the growth and yield are adversely affected and in severe cases the growth ceases and finally death may occur due to desiccation.

26.2.1 Soil Water Deficit and Plant Stress Conditions

All plants experience some amount of water stress during the growth period. The plant water stress may be severe when the soil water potential is low and environmental or plant factors interfere seriously with absorption of water. Hsiao (1973) reviewing the general effects of water deficit on various plant processes classified the level of water stress into the following categories.

Mild stress: A drop of relative water content of a plant (RWC) by 8 to 10 per cent compared to the value in a well-watered plant under conditions of mild evaporative demand of the atmosphere. This corresponds to a drop of plant water potential by -5 to -6 bars.

Moderate stress: A drop of RWC by 10 to 20 per cent compared to the value in a well-watered plant under conditions of low evaporative demand of the atmosphere. The drop of RWC corresponds to a fall of water potential by -12 to -15 bars.

Severe stress: A drop of RWC by more than 20 per cent compared to the value in a well-watered plant under conditions of low evaporative demand of the atmosphere. The drop of RWC corresponds to a fall of water potential by more than -15 bars.

Plant water stress may be classified into diurnal and cyclical water stress based on changes in stress occurring between two successive irrigations. The stress occurring during 24 hour-period of day and night referred to as diurnal stress. It increases with a rise of temperature during the daytime, reaches its peak at around 14 hours day time and then drops gradually attaining its lowest level early in the morning. It is directly related to the rate of transpiration that follows the diurnal temperature curve. The lag between absorption and transpiration is minimum at early morning and maximum at around 14 hours day. This is very often exhibited by plants showing signs of wilting during the hottest part of the day and recovering during the night and this condition of plant is known as temporary wilting and the soil water content at this stage is referred to as temporary wilting point. The stress that occurs gradually and increases progressively with advance of time after irrigation till the next irrigation is referred to as cyclical water stress. The available soil water decreases continually after irrigation owing to evapotranspiration till the subsequent irrigation creating the cyclical stress condition in plant. The stress becomes maximum just before the irrigation in the irrigation cycle and it disappears following irrigation.

Water stress may also be categorized by visual symptoms in plants that show easily the signs of stress. The stress may be said as mild when plants exhibit signs of wilting during the hottest part of the day only. It is regarded as moderate when wilting occurs for a considerable period during the day time and plants recover during the night, and as severe when the plants wilt continuously and do not properly recover at night causing permanent leaf burning and ultimately death through desiccation.

Measurement of water stress in leaf is usually difficult. A measure of relative turgidity and leaf water potential does not always give a true picture. For practical purposes, an indirect measurement of soil water stress can profitably be used.

26.2.2 Plant Responses to Water Stress Conditions

The earliest effect of water stress is the reduction of cells growth and cell wall synthesis. This is followed by changes in various biochemical processes such as reduction in carbohydrate assimilation, protein synthesis and nitrate reductive activity, and accumulation of abscisic acis (ABA) and protein. Generally, water deficit leads to reduction in synthetic processes and activation of degradation processes.

Plant responses to water deficit are dependent on the degree and duration of water stress experienced, time of occurrence of stress in relation to plant stages, kind of plant and the type of plant produce wanted. Water stress affects the growth, yield and quality of produce in various ways. Plant processes such as root development, tiller formation, branching, flowering, seed formation, seed development are affected. Reduction in diameter of beet root and onion bulb, intermodal length of sugarcane, leaf area per plant in tobacco, flowering and fruiting in most plants, incomplete filling of grains in cereals, fruit drop and some such effects on many other crops are caused. The protein content of wheat grains and nicotine content of tobacco leaves increase with an increase in stress. If the duration of stress is brief, it may not cause a perceptible damage to certain types of crops such as grain crops, as they are able to compensate the digress caused by subsequent development under no stress condition. Yields of vegetables and fodder in which succulent vegetative parts are wanted, are depressed considerably even by a mild stress. An increasing stress for a longer period lowers the quality of vegetables, fodder and fruits significantly.

Occurrence of stress in certain plant stages when the cell division and differentiation are significant and plants undergo some significant changes in their growth behavior, affects growth processes adversely. A water deficit during crown root initiation stage in wheat, spike development stage in cereals and branching, flowering or seed development stages of crop plants in general is harmful and it depresses the growth and yield significantly.

Some amount of water stress is sometimes useful in increasing the water use efficiency. Imposing some stress by irrigating crops at a slightly longer interval in areas where irrigation water is scarce and costly can save water. This may however reduce the yield slightly, but definitely improve the water use efficiency. The water thus saved may be used to irrigate additional area that would provide additional crop production. Delaying the first irrigation for some days after germination in order to impose some amount of water stress, encourages deeper penetration of roots that enables the crops to explore water from deeper layers of soil and stand drought conditions better.

 26.3 SOIL WATER AVAILABILITY TO PLANTS

The availability of soil-water to plants is undoubtedly the most important aspect of the soil-water-plant relationship. Soils cannot retain water more than the field capacity under the well-drained condition. The volume of water absorbed by plants beyond the wilting point is very inadequate to meet the transpiration demand and for sustenance of plant life. The field capacity and wilting point are generally considered as the uppermost and lowermost limits of available soil water respectively. The soil water within these two limits is termed as available soil water, and the range of the available soil water between these two water constants is termed as available soil water range. The available soil water equals approximately to the capillary water.

The range of available soil water between the field capacity and permanent wilting point is subjected to criticism as some water beyond these two limits is also available to plants. Soil attains field capacity at about two or three days after irrigation or rainfall and during this period a part of the gravitational water is absorbed by plants. Again, some soil water is extracted by plants beyond the wilting point is however very insignificant. The rate at which soil water is available to plants between field capacity and wilting point is also controversial. Some workers consider that water is equally available to plants throughout this entire range and the plant growth is not affected. However, most of the studies show that the water is not equally available over the entire range as the growth declines after fall in soil water potential. Again, it has been observed that yield declines drastically when the available soil water falls below a particular point within this range. This point is referred to as critical soil water level or critical soil water tension for crop yield. Crops give optimum yield in most cases when the soil water is maintained from field capacity to 50 per cent of available soil water, and occasionally from field capacity to 25 per cent of available soil water.

The total water content of a soil does not give a true picture of the volume of water available to plants. A clay soil retains higher amount of water than a sandy soil at both field capacity and wilting point, but the amount of water available from these soils is not proportional to the actual water. However, the volume of water available is greater in heavier soils than in lighter soils. The volume of available soil water increases with the fineness of soil particles up to silt loam, but it declines with further fineness of particles. Abrol and Bhumbla (1968) stated that the available soil water function of the silt contents and the availability is maximum when the silt fraction of soil constitutes more than 50 per cent of the total silt plus clay fractions.

The upper region of the available soil water range provides the maximum amount of available soil water to plants. It is usually within the soil water tension of one to two atmospheres that most of the available water is released by soils. It may be noted that soil water content and its availability increase with decrease in soil water tension. Further, the water availability increase with an increase in soil depth to a certain level and then decrease with further depths (Abrol And Bhumla, 1968).

Last modified: Tuesday, 13 August 2013, 4:54 AM