Why Study Soil-Plant-Water Relations?

An-Najah National University Faculty of graduated studies Department of environmental sciences Why Study Soil-Plant-Water Relations? Soil, Water & Plant Relationship 400555 Date: 8-6-2011 Dr. Heba Al-Fares

Introduction A . Population • Of the four soil physical factors that affect plant growth (mechanical impedance, water, aeration, and temperature) (Shaw, 1952; Kirkham, 1973), water is the most important. • Drought causes 40.8% of crop losses in the United States, and • excess water causes 16.4%; • insects and diseases amount to 7.2% of the losses (Boyer, 1982).

Water is lost from both the soil surface (evaporation) and the plant surface (transpiration), and is seldom optimal for maximum crop production in dry land (non-irrigated) agriculture.

People depend upon plants for food. Because water is the major environmental factor limiting plant growth, we need to study soil-plant-water relations to provide food for a growing population.

What is our challenge? The human population growth curve

B. The “Two-Square-Yard Rule” • The population is limited by the productivity of the land. • There is a space limitation which is a space of two square yards per person. • The sun’s energy that falls on two square yards is the minimum required to provide enough energy for a human being’s daily ration.

Movement of water through the soil-plant-atmosphere continuum The movement of water through the SPAC is divided into three parts: 1) water movement in the soil and to the plant root; 2) Water movement through the plant, from the root to the stem to the leaf; and 3) water movement from the plant into the atmosphere

What do plant growth curves look like? • A. The Importance of Measuring Plant Growth and Exponential Growth • Equations describing plant-growth curves demonstrate how we can quantify, and thus predict, plant growth. • Because water is the most important soil physical factor affecting plant growth, it is important to quantify plant growth to determine effects of water stress. • We first consider the growth of the bacterium E.coli.

If the start of our observations, at the time 0 min, there is 1 cell. When 20 min have elapsed there are 2 cells. When 40 min have elapsed there are 2 × 2 = 22 cells. • if N denotes the number of cells present at the moment when t minutes have elapsed, then the relation we seek is given by the equation N = 2t/20. “Linnaeus showed that an annual plant would have a million offspring in twenty years, if only two seeds grew up to maturity in a year.” X = 220 • where X is the number of offspring from the plant in twenty years.

B. Sigmoid Growth Curve • The S-shaped, or sigmoid, curve is typical of the growth pattern of individual organs, or a whole plant, and of populations of plants

The Role of Water in Plant Life • Water comprises more than 80% of the living and growing cells of most plants. • All actively growing plants have continuous liquid phase from soil to leaf. • Growing plants need large amounts of water • Water lose through leaves – transpiration- in dry climates, weight of water lost may be 100s or 1000s of time dry weight of plants. • Water loss through stomata. If these partially close to shut down some transpiration, it inhibits CO2 intake and slows photosynthesis. • Plant suction in the day might be so high that little growth takes place. Crop may make large portion of its growth at night.

The Role of Water in Plant Life • Number of units of water/unit of D.M. produced is called transpiration ratio. • Inverse of this ratio called water use efficiency

The Role of Water in Plant Life • Soil – the unconsolidated cover of the earth, made up of mineral and organic components, water and air and capable of supporting plant growth. Most important function: GROW PLANTS • As a medium for plant growth, soil performs four functions: • Anchors roots • Supplies water • Provides air • Furnishes minerals for plant nutrition • The pore space between the solids is taken up by water and air. • Air takes up part of the pore space not occupied by water. As the water increases, the air content decreases.

The Role of Water in Plant Life • Functions of Water in the Plant • Plants differ from animals because they are nutritionally self-sufficient, or autotrophic. • Water serves as a hydrogen donor and thereby as a building block for carbohydrates, which are synthesized by plants making use of sunlight. • Exchange of gases; uptake of CO2 and release of water vapour to the atmosphere (transpiration). • Plants live permanently in one place, so they have to remove water from the soil water reservoir in their immediate vicinity.

Water is an important constituent of all plants. • Root, stem and leaf of herbaceous plants consist of 70–95% water. • In contrast, water comprises only 50% of ligneous tissues, and finally dormant seeds contain only 5–15% water. water has a unique physiological importance in the life of plants – for CO2 assimilation, for biochemical transformations and for the transmission of impulses and signals.

As a chemical agent it takes part in many chemical reactions, for instance in assimilation and respiration. It is a solvent for salts and molecules, and mediates chemical reactions. • Water is the medium of transport for nutrient elements and organic molecules from the soil to the root and the means of transport of salts and assimilates within the plant. • Stimulation and motion of organelles and cell structures, cell division and elongation are examples of processes controlled by hormones and growth substances, and water is the carrier of these messengers, enabling the regulatory system of the plant.

Water confers shape and solidity to plant tissues. • The hydrostatic pressure in cells is dependent on their water content, and permits cell enlargement against pressure from outside, which originates either from the tension of the surrounding tissue or from the surrounding soil. • The large heat capacity of water greatly dampens the dailyfluctuations in temperature that a plant leaf might undergo, due to the considerable amount of energy required to raise the temperature of water. • The vapour that transpires from leaves causing cooling due to evaporation

Availability of Soil Water to Plants Water moves into the plant whenever suction in the water in the plant is greater than that in the water in the soil. Most plants withdraw water from soils until soil moisture reaches about 15 bars. • Fine textured soils hold more water than sands at field capacity • Fine textured soils are less droughty

Depending on soil texture, which is determined by the particle-size distribution, soils will vary in water content at field capacity and at the permanent wilting point. Both characteristic values enclose the plant-available water content. Silt loam soil contains the maximum of available water. The water at the permanent wilting point is not available to plants. The fineness of texture increases with the silt and clay content, presented as approximate percentages.

Water Requirements of Crop Plants • The rate at which water if available would be removed from the soil and plant surface is potential evapotranspiration (PET). • The ratio of evapotranspiration (ET) to (PET) gives figure called relative evapotranspirationor crop coefficient (Kco). • Energy is required to evaporate water from soil and to cause plants to transpire. • Crops utilize only 1 to 2% of energy received. Utilization of energy may become the next limiting factor when moisture is adequate and good cropping practices are followed. • Many crops have critical stages of growth when a water deficient will cause unusually large reduction in yield.

Adaptation Strategies of Plants to Overcome Water Shortage • According to the presence and supply of water, ecologists divide terrestrial plants into • hygrophytes, • mesophytes and • xerophytes.

Hygrophytes • Hygrophytes are plants that thrive in generally humid habitats, where there is no shortage to the water supply throughout the growing season. • In temperate zones, in addition to these plants with a humid biotype, there are many shade-loving herbaceous forest species that also belong in this category.

Xerophytes • Xerophytes are adapted to water shortage, which may occur regularly and may persist over long periods of time. • Anatomical and physiological specialization has taken place to meet the requirements of these plants so that they can survive extended periods of drought. • To this group belong succulent plants that establish an internal water reservoir for use during drought, thereby postponing desiccation. • Another group of xerophytic plants are able to endure considerable water loss from their tissues without losing their ability to survive.

Mesophytes • Mesophytes fit in between these two extremes. • Many plants from temperate climates belong to this group, but the cultivated plants from those regions are also included. • The latter cannot endure an extreme form of arid climate without being irrigated. • However, for short periods of water shortage they are well prepared. • When water supply falls short, they can reduce their transpiration rate dramatically and modify other processes.

How do plants react to water shortage? , and • what kind of strategies have they developed with respect to drought resistance?

Drought escape • Those plants that are adapted to drought escapewill germinate from dormant seeds only when there is abundant rainfall. • Afterwards they can manage with a limited supply of water because they can terminate vegetative growth and become reproductive after a very short life cycle of just a few weeks, even ending with mature seed. • Subsequent dry periods are escaped through seed dormancy.

Drought escape • Among cultivated plants, the short-lived two rowed barley is a drought escaper. • Groundnut and cowpea are classed in this group along with the C4 plants from the different species of millet. • All of these crops reach maturity, although annual precipitation may not exceed 250–300 mm

drought avoidance. • Plants at adapted todrought avoidance may avoid or at least retard desiccation of their tissues by increasing water uptake, reducing water loss, or by enhancing the internal storage of water. • Like the first group these plants maintain a water balance that is largely in equilibrium. • They belong to the hydrostable or homoiohydric species.

Water savers: • Many of these plants are succulents and can save a large volume of water within parenchymatous tissue when the very short periods of rainfall occur. • Quite a number of species in the family Cactaceae belong to this group. • Cacti, as well as plants of the families Crassulaceae and others are representatives of a group that demonstrate CAM. • These CAM plants effect a unique physiological adaptation to water shortage. • During the night, however, they will be opened for CO2 assimilation and accumulation in the form of organic acids, which during the daytime supply CO2 again for producing carbohydrates by photosynthesis

There are also water savers among C3and C4 plants • In many cases the plants possess distinct anatomical features such as stomates that are deeply sunk into the epidermis, thick and leathery or fleshy leaves, small leaves, leaves with waxy coatings over the cuticle and leaves with a felt-like cover of fine hairs. • Some of the water savers restrict water loss during dry periods by rolling or folding their leaves

Water spenders • These plants raised water through deep rooting system during the night from deep layers to more shallow ones, where the water was released from the roots into the surrounding soil. • This ‘hydraulic lift’ enables plants to make use of a larger water supply during the day for transpiration and for CO2 assimilation. • Water spenders include esparcet. • This is a perennial deep-rooted forage legume, adapted to calcareous soils and native to Mediterranean regions

Drought tolerance • Plants relying on this strategy are able to tolerate a certain level of tissue desiccation. • During phases of desiccation they limit their vital functions quite considerably. • The plants are said to be hydrolabile

Osmotic adjustment • The capability of solute accumulation is termed osmotic adjustment. • When desiccation develops slowly over time, many plants are able to accumulate inorganic ions or organic compounds, such as sugars, alcohols and amino acids, in their tissues. • The solutes are concentrated in the cytoplasm and vacuoles, but the water content of the cells is maintained at a more or less stable level. • By osmotic adjustment plants guard against a loss of turgidity. • This adjustment will allow the plant to survive periods of drought more vigorously and for longer periods of time, and can allow the extraction of water from soil • Sorghum is considered as a crop species characterized by a strongly developed drought tolerance compared with other crops. • soybean are capable of osmotic adjustment, and the same is true of other grain legumes and sugarbeet

Water and Net Primary Production • There is a well defined relationship between water use and the amount of dry matter produced. • the net primary production, i.e. gross primary production minus respiration

Water, temperature and radiation • Factors influence plant growth and can regulate net primary production through • Net assimilation rate (NAR, rate of growth per unit of leaf area). • A small biomass will result in a small leaf area index (LAI, total green area of one side of a leaf as a ratio of one unit of soil surface area). • A small LAI is the second cause of reduced productivity. • The Crop Growth Rate (CGR) is the rate of growth per unit of soil surface area. • CGR = NAR × LAI (1.1) • Equation 1.1 establishes that the productivity of a crop stand is dependent on the photosynthetic net productivity of the single leaf and of the size of the total leaf canopy

Relationship between net primary production of terrestrial forests and annual precipitation as a rough index of the level of available water.

Seed yield of groundnut as related to water use. The water use includes the transpiration of the crop and the evaporation from the soil. Lysimeter studies in Georgia and Florida, cultivar is Florunner (after Boote and Ketring, 1990).

Influence of temperature on the rates of gross photosynthesis, respiration and net photosynthesis (A) as well as on growth rate (B). The three cardinal points for temperature are the minimum, optimum and maximum values (Tmin, Topt and Tmax). (Schematic after Pisek et al., 1973 for (A) and Fitter and Hay, 1981 for (B).)

The Role of Water in Soil • Soil Genesis and Soil Functions • Water is of primary importance for soil genesis. • Without the action of water, soils would not develop. • Soils originate from parent rock. • The first step towards soil formation is the weathering of these rocks. • Water contributes to the processes of weathering through physical and chemical actions.

The Role of Water in Soil • The development of soil can be thought of as occurring in two phases… • Soil Genesis – the weathering of rock substrates by: • Mechanical forces • Chemical reactions

The Role of Water in Soil 2) Soil Formation – Hans Jenny (1941) characterized soil formation as a function of five independent variables: climate, organisms, topography, parent material, & time. • Organism include such elements as the soil microbial community, litter inputs, vegetation type. • Parent material largely determines chemical characteristics of the derived soils. • → The interaction of organisms & parent material with climate produce a soil with characteristic features.

Soil genesis • Soil genesis is accompanied by the formation of soil structure, which is essentially dependent on soil water. • Water causes the clay minerals to swell and shrink, and the soil matrix becomes subdivided by planes of weakness or by visible fissures.

Soil genesis • Also, ice formed by frost can separate the soil matrix into aggregates of characteristic size and form. • Without water there would be no transport processes in the soil. Water in the soil is seldom in a state of equilibrium. • Usually it is in constant motion within the soil profile. The reason is that the energy state of water, its potential, is generally not the same but varies between different locations within the profile.

Physical weathering • Physical weathering splits rocks and minerals, but their chemical composition is not basically changed. • Water is the main agent and works through frost action, but in arid regions differential thermal expansion of minerals can also split rocks. • The fragments formed are transported by surface water from higher elevations downhill into the valleys. • Here the fine debris is deposited in alluvial fans, burying the former bottom of the valley, and leveling the topographical features.

Chemical weathering • Chemical weathering breaks down minerals by hydration, hydrolysis and dissolution. • The disruptive force of water is greatly augmented by protons or hydronium ions (H3O+) that are derived from organic and inorganic acids. • In this way even the very insoluble silicates are finally broken down. • Increasing the temperature accelerates the kinetics of destruction.

Water moves cations, silicic acid, and iron and aluminium compounds solutes as well as colloidal solids deeper into the soil body or even beyond the soil into deeper strata. • Disintegration, displacement, precipitation and leaching are essential parts of the pedogenic processes, supported by water.

Soil water • Soil water is the most limiting factor for crop production in the world. • Only 45% of the earth's arable land receives adequate moisture for crop growth • Soil water carries nutrients to a growing crop and has a significant effect on aeration and temperature of the soil.

Soil Water One of the most important factors affecting crop production. • Water must be available to replenish that lost by evaporation and transpiration. • Soil water carries nutrients in solution to the growing crop. • Has significant effect on aeration and temperature conditions of the soil. • Seldom is the water content of soil at optimum value for maximum crop production.

How is Soil Water Classified? 1) Hygroscopic Water is held so strongly by the soil particles (adhesion), that it is not available to the plants. 2) Capillary Water is held by cohesive forces greater than gravity and is available to plants. 3) Gravitational Water is that water which cannot be held against gravity. • as water is pulled down through the soil, nutrients are "leached" out of the soil (nitrogen)

Soil water • There are certain limits for soil water. • Field capacity is when the soil pores are so full of water that the next drop will leach downward out of the rooting zone. • The opposite extreme is wilting point, the level at which plant roots can no longer take in water and turgor is lost (wilting). • The goal of a soil, water, plant continuum is to maintain the soil water between these extremes, allowing nutrient movement, aeration, and supplying water in excess of evaporation and transpiration (evapotranspiration). • Measuring plant available water and adjusting water levels with irrigation is another way mankind has tried to modify the environment to maximize food and fiber production

Why Study Soil-Plant-Water Relations?