Root hair - Wikipedia
Water balance of plant To prevent leaf desiccation, water must be absorbed by the roots, and transported through the plant body. .. Figure Root hairs intimate contact with soil particles and greatly amplify the surface area used . The transpiration ratio measures the relationship between water loss and carbon gain. In particular, we expect that root hairs and mucilage optimally connect the roots to the soil .. And what effect do these properties have on soil plant water relations? The average annual water budget for 30 years shows inches of .. To test the reliability of an earlier study showing embolism refilling in roots at. The soil water balance is calculated for the effective rooting depth as: .. This is a major reason that functional relationships and the order of calculations in difference models are Soil nutrient status can affect root hair formation and growth.
The vapour pressure over a solution at atmospheric pressure is influenced by solute concentration and mainly by temperature. In principle we can assume that the substomatal air space of leaf is normally saturated or very nearly saturated with water vapour.
On the other hand, the atmosphere that surrounds the leaf is usually unsaturated and may often have a very low water content. This difference in water vapour pressure between the internal air spaces of the leaf and the surrounding air is the driving force of transpiration. On its way from the leaf to the atmosphere, water is pulled from the xylem into the cell walls of the mesophyll, where it evaporates into the air spaces of the leaf. The water vapor than exits the leaf through the stomatal pore.
The movement of liquid water through the living tissues of the leaf is controlled by gradients in water potential. However, transport in the vapor phase is by diffusion, so the final part of the transpiration stream is controlled by the concentration gradient of water vapor. Almost all of the water lost from leaves is lost by diffusion of water vapour through the tiny stomatal pores. In most herbaceous species, stomata are present in both the upper and lower surfaces of the leaf, usually more abundant on the lower surface.
In many tree species, stomata are located only on the lower surface of the leaf. The driving force for transpiration is the difference in water vapour concentration Transpiration from the leaf depends on two major factors: In contrast to the volume of the air space, the internal surface area from which water evaporates may be from 7 to 30 times the external leaf area. The air space in the leaf is close to water potential equilibrium with the cell wall surfaces from which liquid water is evaporating.
The concentration of water vapor changes at various points along the transpiration pathway from the cell wall surface to the bulk air outside the leaf. The second important factor governing water loss from the leaf is the diffusional resistance of the transpiration pathway, which consists of two varying components: The resistance associated with diffusion through the stomatal pore, the leaf stomatal resistance.
The resistance due to the layer of unstirred air next to the leaf surface through which water vapor must diffuse to reach the turbulent air of the atmosphere.
This second resistance is called the leaf boundary layer resistance. Some species are able to change the orientation of their leaves and thereby influence their transpiration rates.
Many grass leaves roll up as they experience water deficits, in this way increasing their boundary layer resistance. Stomatal control couples leaf transpiration to leaf photosynthesis Because the cuticle covering the leaf is nearly impermeable to water, most leaf transpiration results from the diffusion of water vapor through the stomatal pore.
The microscopic stomatal pores provide a low-resistance pathway for diffusional movement of gases across the epidermis and cuticle. Changes in stomatal resistance are important for the regulation of water loss by the plant and for controlling the rate of carbon dioxide uptake necessary for sustained CO2 fixation during photosynthesis. At night, when there is no photosynthesis and thus no demand for CO2 inside the leaf, stomatal apertures are kept small or closed, preventing unnecessary loss of water.
Leaf can regulate its stomatal resistance by opening and closing of the stomatal pore. This biological control is exerted by a pair of specialized epidermal cells, the guard cells, which surround the stomatal pore. The cell walls of guard cells have specialized features Guard cells are found in leaves of all vascular plants. In grasses, guard cells have a characteristic dumpbell shape, with bulbous ends Figure 1. These guard cells are always flanked by a pair of differentiated epidermal cells called subsidiary cells, which help the guard cells control the stomatal pores.
Subsidiary cells are often absent, the guard cells are surrounded by ordinary epidermal cells. A distinctive feature of guard cells is the specialized structure of their walls. The alignment of cellulose microfibrils, which reinforce all plant cell walls and are an important determinant of cell shape, play an essential role in the opening and closing of the stomatal pore.
Environmental factors such as light intensity and quality, temperature, leaf water status, and intracellular CO2 concentrations are sensed by guard cells, and these signals are integrated into well-defined stomatal responses. The early aspects of this process are ion uptake and other metabolic changes in the guard cells. Water relations in guard cells follow the same rules as in other cells.
As water enters the cell, turgor pressure increases. Such changes in cell volume lead to opening or closing of the stomatal pore.
Subsidiary cells appear to play an important role in allowing stomata to open quickly and to achieve large apertures. The transpiration ratio measures the relationship between water loss and carbon gain The effectiveness of plants in moderating water loss while allowing sufficient CO2 uptake for photosynthesis can be assessed by a parameter called the transpiration ratio.
This value is defined as the amount of water transpired by the plant divided by the amount of carbon dioxide assimilated by photosynthesis.
For plants in which the first stable product of carbon fixation is a 3-carbon compound C3 plantsas many as molecules of water are lost every molecule of CO2 fixed by photosynthesis, giving a transpiration ratio of Plants in which a 4-carbon compound is the first stable product of photosynthesis C4 plantsgenerally transpire less water per molecule of CO2 fixed than C3 plants do.
A typical transpiration ratio for C4 plants is about Plants with crassulacean acid metabolism CAM photosynthesis the transpiration ratio is low, values of about 50 are not unusual. Plant water status The water status of plant cells is constantly changing as the cells adjust to fluctuations in the water content of the environment or to changes in metabolic state.
The plant water status is dependent on: Water potential is often used as a measure of the water status of a plant. Plants are seldom fully hydrated. During periods of drought, they suffer from water deficits that lead to inhibition of plant growth and photosynthesis.
Several physiological changes occur as plants experience increasingly drier conditions Figure 1. Cell expansion is most affected by water deficit. In many plants reductions in water supply inhibit shoot growth and leaf expansion but stimulate root elongation. Drought does impose some absolute limitations on physiological processes, although the actual water potentials at which such limitations occur vary with species.
Thus, physiological responses to water availability reflect a trade-off between the benefits accrued by being able to carry out physiological processes e. Water stress typically leads to an accumulation of solutes in the cytoplasm and vacuole of plant cells, thus allowing the cells to maintain turgor pressure despite low water potential. Some physiological processes appear to be influenced directly by turgor pressure.
However, the existence of stretch-activated signalling molecules in the plasma membrane suggests that plant cells may sense changes in their water status via changes in volume, rather than by responding directly to turgor pressure.
Influence of extreme water supply Plant growth can be limited both by water deficit and by excess water. Drought is the meteorological term for a period of insufficient precipitation that results in plant water deficit. Excess water occurs as the result of flooding or soil compaction. The deleterious effects of excess water are a consequence of the displacement of oxygen from the soil.
The relative humidity of the air determines the vapour pressure gradient between the leaf stomatal cavity and the atmosphere, and this vapour pressure gradient is the driving force for transpirational water loss.
When a soil dries, its hydraulic conductivity decreases very sharply, particularly near the permanent wilting point that is, the soil water content at which plant cannot regain turgor upon rehydration.
Redistribution of water within the roots often occurs at night, when evaporative demand from leaves is low. Water-deficient plants tend to become rehydrated at night, allowing leaf growth during the day. But at the permanent wilting point, water delivery to the roots is too slow to allow the overnight rehydration of plants that have wilted during the day. Thus, decreasing soil water conductivity hinders rehydration after wilting. Water deficit is stressful, but too much water can also have several potentially negative consequences for a plant.
Flooding and soil compaction result in poor drainage, leading to reduced O2 availability to cells. Flooding fills soil pores with water, reducing O2 availability.
Dissolved oxygen diffuses so slowly in stagnant water that only a few cm of soil near the surface remain oxygenated. At low temperatures the consequences are relatively harmless.
Flooding sensitive plants are severely damaged by 24 hours of anoxia lack of oxygen. The yield of flooding-sensitive garden-pea Pisum sativum may decrease by fifty percent.
Corn is affected by flooding in a milder way, and is more resistant to flooding. It can withstand anoxia temporarily, but not for periods of more than a few days. Soil anoxia damage plant roots directly by inhibiting cellular respiration. At a given root system size, up to three-fold variation in whole-plant biomass was found among rice accessions under P deficiency, indicating that genotypes differed in how efficiently their root system acquired P to support overall plant growth Mori et al.
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Conventional breeding showed significant progress in developing P-efficient cultivars, particularly for soybean in China Wang et al. But success with marker-assisted breeding has been limited due to significant environmental effects on traits influencing PAE resulting in most QTL identified making small contributions to overall P efficiency.
Developing crop genotypes tolerant to acid soil conditions is an ecologically friendly, energy-conserving and economical solution for resource-poor farmers in the tropics. Genetic variation exists for acid soil adaptation among crops and genotypes within a crop.
Field screening for Al resistance would seem to be the most desirable approach, because it best approximates the intended cropping environment Haling et al. In practice, however, reliable ranking of genotypes in the field has been difficult. This is mainly because exchangeable Al levels are not uniform and environmental factors may interact with soil Al to mask the expression of Al resistance.
Thus, it is necessary to combine field with greenhouse screening techniques based on physiological traits of Al resistance Rao, Despite the rhizotoxicity of Al being identified over years ago, there is still no consensus regarding the mechanisms whereby root elongation rate is initially reduced. At soil pH values of 5 or below, toxic forms of Al are solubilized and excess levels of toxic Al inhibit root growth and function Delhaize and Ryan, ; Horst et al.
The primary and earliest symptom of Al toxicity is a rapid within minutes inhibition of root elongation Ryan et al. The distal part of the transition zone in the root apex was identified as the primary site of action of toxic Al ions Sivaguru and Horst, Callose formation in root apices of maize is an excellent indicator of Al injury Eticha et al.
Al exclusion is an Al resistance mechanism based particularly on exudation of Al-chelating organic compounds e. There is a need to define the precise mechanism of Al-induced inhibition of root elongation.
It has been a matter of debate whether the primary lesions of Al toxicity are apoplastic or symplastic Horst et al. Although there is evidence for symplastic lesions of Al toxicity, the protection of the root apoplast appears to be a prerequisite for Al resistance in Al-excluder and Al-accumulator plants. Recently, Kopittke et al.
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They also found that an alteration in the biosynthesis and distribution of ethylene and auxin was a second, slower effect. Their study demonstrated the importance of focusing on root traits related to cell wall composition as well as mechanisms involved in wall loosening to overcome the deleterious effects of soluble Al.
Differential genotypic response to Al stress is the basis of identification of new sources of Al resistance and contributes to improved understanding of the mechanisms of Al resistance. The analysis of spatial growth profiles revealed that the initial inhibition of root elongation by Al resulted from a generalized effect along the entire elongation zone Rangel et al. The kinetics of citrate exudation from root tips offered the most consistent explanation for the response in root elongation and Al uptake of both Al-resistant and Al-sensitive genotypes of common bean to Al treatment Rangel et al.
Al resistance was mainly dependent on the capacity to sustain citrate synthesis, thereby maintaining the cytosolic citrate pool that enables exudation. The initial Al-induced inhibition of root elongation in both Al-resistant and Al-sensitive genotypes was correlated with the expression of the 1-aminocyclopropanecarboxylic acid oxidase gene Yang et al.
The runner bean Phaseolus coccineus L. One selection, GQ, expressed excellent root development in all three evaluation systems, and appeared to offer a resistance mechanism that could be selected readily in any of these systems, and was crossed to the drought-resistant but Al-sensitive common bean line SER A derived line, ALB 91, expressed much of the root vigour of the Al-tolerant parent and has been used extensively in crosses Butare et al.
However, only very few progenies were similar to GQ; most expressed one or another trait of the drought-resistant parent Butare et al. Resistance to Al appears to be complex in Phaseolus coccineus and seems to be the result of a combination of traits that segregated among the progenies.
This probably indicates that multiple traits are required to confront an acid soil complex, of which Al resistance is one component. While resistance sources for individual stresses can be employed in breeding, it may be necessary to combine these multiple traits and subject breeding populations to relevant selection pressure under field conditions.
Brachiaria grasses are the most widely planted forage grasses in the tropics Miles et al. The very high level of Al resistance found in signalgrass was not associated with secretion of organic acids and phosphate at root apices Wenzl et al. Al resistance was related to less Al accumulation particularly in root hairs accompanied by an Al-induced increase of chlorogenic acid, indicating a possible role for chlorogenic acid as a primer for changes in root epidermal cell patterning that may contribute to Al hyper-resistance in signalgrass.
Interspecific Brachiaria hybrids are developed using B. Stapf palisadegrass with its high level of resistance to spittlebug, a major insect pest Miles et al. Implementation of a simplified version of a screening method using vegetative propagules Wenzl et al. Fifteen hybrids were identified with superior Al resistance. Intraspecific hybrids of B. In addition, these hybrids also have great potential to reduce nitrification in soil and emission of nitrous oxide to the atmosphere Rao et al.
Bulbous canarygrass is one of the most important sown perennial grasses used in south-eastern Australia due to its high productivity and drought tolerance Culvenor and Simpson, However, interactions among climate, soil acidity and grazing pressure can affect its persistence. The force for this entry of water is created in leaves due to rapid transpiration and hence, the root cells remain passive during this process.
During absorption of water by roots, the flow of water from epidermis to endodermis may take place through three different pathways: The mechanism of water absorption described earlier, in-fact belongs to the second category. However, a combination of these three pathways is responsible for transport of water across the root. External Factors Affecting Absorption of Water: Sufficient amount of water should be present in the soil in such form which can easily be absorbed by the plants.
Usually the plants absorb capillary water i. Other forms of water in the soil e. Increased amount of water in the soil beyond a certain limit results in poor aeration of the soil which retards metabolic activities of root cells like respiration and hence, the rate of water absorption is also retarded.
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Concentration of the Soil Solution: Therefore, absorption of water is poor in alkaline soils and marshes. Absorption of water is retarded in poorly aerated soils because in such soils deficiency of O1 and consequently the accumulation of CO2 will retard the metabolic activities of the roots like respiration. This also inhibits rapid growth and elongation of the roots so that they are deprived of the fresh supply of water in the soil.
Water logged soils are poorly aerated and hence, are physiologically dry. They are not good for absorption of water. This is probably because at low temp: There are two views regarding the relative importance of active and passive absorption of water in the water economy of plants. But according to Kramer the active absorption of water is of negligible importance in the water economy of most or perhaps all plants.
He regards the root pressure and the related phenomena involved in the active absorption of water as mere consequences of salt accumulation in the xylem of different kinds of roots. There are many reasons for regarding the active absorption as unimportant: Such plants may show even a negative root pressure i.
Two main arguments are against this view. Firstly, during periods of rapid transpiration the salts are removed from the root xylem so that their concentration becomes very low. Under such conditions the osmotic uptake of water cannot be expected to occur.