Ecology of Water Relations Advanced article in Plants Article Contents . Introduction Yoseph Negusse Araya, The Open University, Milton Keynes, UK . Water Uptake and Movement through Plants . Water Stress and Plants Water is an important resource for plant growth. Availability of water in the soil determines . Plant Sensing and Adaptation to Water Stress the niche, distribution and competitive interaction of plants in the environment. Distribution of Plants in Response to Water Regime Introduction doi: 10.1002/9780470015902.a0003201 Importance of water for plants Moisture Water typically constitutes 80–95% of the mass of growing 8 plant tissues and plays a crucial role for plant growth (Taiz 7 and Zeiger, 1998). Plants require water for a number of 6 physiological processes (e.g. synthesis of carbohydrates) 5 and for associated physical functions (e.g. keeping plants turgid). 4 Water accomplishes its many functions because of its 3 Moisture index 2 unique characteristics: the polarity of the molecule H2O (which makes it an excellent solvent), viscosity (which 1 makes it capable of moving through plant tissues by 0 capillary action) and thermal properties (which makes it Forest Woodland Grassland Desert capable of cooling plant tissues). Total net productivity 1 Plants require water, soil nutrients, carbon dioxide, ox- − 1000 year ygen and solar radiation for growth. Of these, water is most 2 − often the most limiting: influencing productivity (Taiz and m 1 800 Zeiger, 1998) as well as the diversity of species (Rodriguez- − Iturbe and Porporato, 2004) in both natural and agricul- 600 tural ecosystems. This is illustrated graphically in Figure 1. 400 How does water affect ecology of plants? 200 In order to understand the ecology of plant–water rela- 0 Total net productivity g Total tions it is important to understand from where and how Forest Woodland Grassland Desert plants acquire water in their environment (the latter is dis- cussed in the section on water uptake and movement Plant species diversity through plants). 1 Unlike animals, which are capable of wandering around to forage for resources, plants are for the most part sta- 0.8 Wiener index Wiener tionary, depending on the availability of nutrients in their − 0.6 surrounding environment (soil and/or atmosphere). Of these two sources of resources, i.e. soil and atmosphere, the 0.4 soil is by far the major and more accessible reservoir. Con- sequently, the soil is the primary store and regulator in the 0.2 water flow of ecosystems, by intercepting precipitation Diversity/Shannon 0 input and controlling its use by organisms (Rodriguez-It- Forest Woodland Grassland Desert urbe and Porporato, 2004). Figure 2 summarizes the soil and plant–water interrelationship. Figure 1 Moisture, total net productivity and plant species diversity of Soil moisture availability is dependent on the soil particle selected vegetation communities, along an elevation gradient from Santa Catalina Mountains, Arizona (after Whittaker and Niering, 1975). The size distribution (also called soil texture) and arrangement elevation gradient ranges from 1000 to 3000 m above sea level. The of these particles (soil structure). The soil texture and struc- moisture index relates to precipitation ranges of 190 mm per annum ture influence the size of soil pores where water is held by (moisture index 8) and 850 mm per annum (moisture index 1). ENCYCLOPEDIA OF LIFE SCIENCES & 2007, John Wiley & Sons, Ltd. www.els.net 1 Ecology of Water Relations in Plants is ultimately lost to the atmosphere during transpiration. Plants This cycle is referred to as the soil–plant–atmosphere con- tinuum (SPAC). See also: Plant–Water Relations Organic matter Diffusion and return gas exchange When water moves through the SPAC, it travels through Plant different mediums (including cell wall, cell membrane and uptake air spaces) at different distances, which utilize different Soil nutrients Plant Plant Soil aeration modes of transport. uptake water loss There are three principal modes of water transport: diffusion, mass flow and osmosis. In diffusion, water mole- cules move spontaneously from regions of high concen- Mineralization Air-filled pore tration to regions of lower concentration, i.e. along a and transport Soil moisture space concentration gradient. This movement is rapid over a short distance and thus drives short-distance transport, for Figure 2 Schematic summary of the processes that influence the example between cells and during the loss of water to the relationship between plants and soil water. atmosphere from leaf stomata. In mass flow, groups of water molecules move under an 100 external force, such as a build-up of pressure that forms a 1000 gradient. Mass flow of water is the predominant mode by which long-distance transport of water in stems is accom- plished. It also accounts for much of the water flow though 100 50 the soil and through the cell walls of plants. ace (% volume) p The third mode, called osmosis, is movement of water molecules through a semipermeable membrane, an exam- ore s 10 p ple of which is the cytoplasmic membrane. Osmosis occurs spontaneously in both short- and long-distance transport Soil water potential (kPa) 10 as a response to driving forces of concentration (as in 0 Air-filled diffusion) and pressure gradient (as in mass flow). 50 100 These driving forces of water movement of both osmotic Soil water content (% volume of pore space) (concentration) and mass flow (pressure) origin are collec- Figure 3 Soil water availability and soil aeration availability for two tively known as water potential. Water potential is meas- representative sandy (solid line) and clayey (broken line) soils. Soil water ured in units of pressure or suction, i.e. force per unit area contents on volume basis is shown against soil water potential (suction) and required to move a specific amount of water. The most against air-filled pore space (volume of pore space not occupied by water). common unit used for studying soil water potentials in the capillary forces. Soils with fine-sized particles, like clay hold field is kilopascals (kPa). more water than soils dominated by coarse grain particles of The movement of water in the SPAC is thus dependent sand. However, this does not mean that all the water in on differences in water potential between surrounding soil fine-particle sized soils is available for plant uptake. This is and plant or atmosphere. Often, the water potential gra- because the capillary forces holding water in the pores of dient is directed from the roots towards the shoot, as tran- fine-textured soils are so powerful that the plants struggle to spiring leaves exposed to the atmosphere have the lowest water potential. However, under situations when the soil is extract any water. Figure 3 shows soil water and soil aeration availability for different soil texture classes. too dry this water potential gradient could be reversed, Soil moisture availability primarily influences plants by resulting in loss of water from plant roots to the soil. Also any environmental factors that influence the transpiration two routes (see Figure 2), either by being directly limiting as a resource, or indirectly by filling pore spaces in soil of the leaf stomata, e.g. wind or increase in temperature and thereby excluding air, causing oxygen availability to may further decrease the leaf water potential further, become limiting for the activity of plant roots. This is ex- speeding up water loss. plained further in the section on water stress and plants. Water Stress and Plants Water Uptake and Movement through In addition to an adequate level of water in their tissues, Plants plants also require a continuous flux of water to perform vital processes such as photosynthesis and nutrient uptake. Water is constantly moving from the soil, into plant roots, Water for these is not always available in the right quan- and through the xylem tissues of the stem to leaves where it tity and quality at the right time. This imbalance in water 2 Ecology of Water Relations in Plants supply and plant requirements results in plants undergoing that senses water stress is crucial to the initiation of occasional or, in some cases acute, water stress. defensive processes. See also: Plant Stress Physiology There are two types of water stresses that plants expe- rience. One is when water is not available in sufficient quantity – hence referred to as water-deficit, while the Plant sensing of soil drying second one is that when water is available – but in excess, called waterlogging. Water-deficit is the most common form of water stress Water-deficit affects plants through decrease of leaf studied in relation to sensing of impending soil drying by water potential, which in turn entails loss of cell turgor and plant roots and the subsequent communication to shoots. stomatal closure. This results in decrease of transpiration In this connection, signals of a chemical nature have and photosynthesis, which subsequently leads to reduced received a lot of attention, as they are suited for rapid growth and if it persists, wilting. On the other hand, water- communication between plant tissues. logging occurs when a large proportion of the pore spaces A well-known chemical signal of impending water stress in the soil are occupied by water. This means the diffusion originating from exposed roots is abscisic acid (ABA). of oxygen and gas exchange between the soil, plants and ABA is synthesized by dehydrating roots in nongrowing atmosphere is limited. The result of this is decreased root tissues as well as in apices, and in the cortex (Hartung growth and functioning, which negatively affects plant and Davies, 1991). An increase in ABA concentration in growth and survival. response to an increase in soil drying is known to initiate Plants start suffering the consequences of water stress water-saving measures like reduction in transpiration rate when certain thresholds for water-deficit and waterlogging and conductance (e.g.