Plant Transpiration & Nutrient Absorption PDF

Because of the unbroken stream of water in the plants from the roots to the leaves, then the water that leaves the intercellular spaces is replenished continuously. Because plants are constantly losing water and it is so essential to the plant’s metabolic activities, then growing plants depend on t...

Because of the unbroken stream of water in the plants from the roots to the leaves, then the water that leaves the intercellular spaces is replenished continuously. Because plants are constantly losing water and it is so essential to the plant’s metabolic activities, then growing plants depend on the continuous stream of water entering and leaving at all times. For this reason, water must always be available to their roots. The only way the plant can control water loss on a short-term basis is to close the stomata. Plants do this when they are subject to water stress. Plants must respond to both the need to conserve water and to the need to admit CO2. The stomata open and close because of changes in the water pressure of their guard cells. The guard cells are the only epidermal cells with chloroplasts and for this reason and their shape they stand out. They are thicker on the side of the stoma opening. The ion most important in controlling the turgidity of guard cells is the potassium ion, large amounts of which are held in the cells surrounding the guard cells. Photosynthesis (ATP) in the guard cells drives the active transport of K+ ions into and out of the guard cells. Abscisic acid plays a primary role in allowing the K+ ions to pass rapidly out of the guard cells and therefore, in causing the stomata to close.  External factors affecting the velocity of transpiration. 1. Relative humidity. Difference in water vapor concentration between the atmosphere and that in the interior of the cell. 2. Humidity in the soil. More water in the soil, then greater absorption by the roots, in consequence there is a higher water potential in the whole plant and specifically in the guard cells and when the guard cells are turgid, then the stomata opens hence greater transpiration. 3. Concentration of CO2 in the atmosphere. High concentration of CO2 can completely close the stomata. The physiological significance to this response is based on the fact that the plant tries to establish a balance between photosynthesis and transpiration. 4. Illumination (light). An increase in light intensities opens the stomata and hence increases transpiration. In the absence of light, photosynthesis is not possible, hence no CO2 is needed, so the stomata close to avoid the loss of water. 5. Temperature. In general, high temperatures favour transpiration. When the temperature rises, the relative humidity decreases sharply, hence the diffusion gradient increases. When the temperature increases, the diffusion coefficient of water vapor also increases, which o favours transpiration. Temperatures greater than 40 C in general favour the closure of the stomata. 6. Wind velocity. There is not a direct effect on the functions of the stomata, but it has a great effect on transpiration because it affects the humidity gradient. Other factors regulating transpiration include dormancy at times when water is in short supply. The deciduous habit, which is common in plants grown in severe drought or in the wintertime. Functions of transpiration 1. The cooling of the leaves and the whole plant. 2. The plant also uses transpiration to concentrate minerals that the plant absorbs in a diluted form from the soil. 3. Transpiration is one of the essential factors in the ascension of water through the xylem. This ascension plays an important role in the distribution of nutrients in the plant.  Absorption of nutrients by the roots Plants normally absorb nutrients through their roots. With the exception of C (photosynthesis) and partially O2, the rest of the essential elements are captured in the soil by the plants through its rooting system. Absorption rarely takes place in the form of the salts, but rather in the form of the individual ions. 2+ 2- Example: CaCO Ca + CO 3 3 The presence of an element in the soil is not indicative of its availability to the plant. Only those that are in the soluble form are available or by ionic exchange with the soil. This exchange is primarily cationic because of the predominance of the superficial negative charge in the inorganic (clay) and organic (humus) parts of the soil. Cation Exchange Capacity (CEC) Ion absorption by passive transport In passive transport, diffusion of ions occurs in favor of the chemical potential or electrochemical potential gradient without the need for external energy expenditure. A substance tends to move towards the regions of lower chemical potential. In the case of water, since it is a molecule without a charge, the term electric does not enter its chemical potential. In the case of ions, it’s different because of the presence of the charge. 1. Diffusion Diffusion is a spontaneous process by which the net movement of a substance occurs from one region to another in favor of its chemical potential. The final theoretical result is the uniform distribution of the chemical potentials in all the points of the system or until it reaches the diffusion equilibrium. Osmosis is a special case of diffusion through an ideal semi-permeable membrane. 2. Ionic exchange This is another physical-chemical mechanism that participates in the passive absorption of ions. Cell walls have a negative charge because of the predominance of the carboxylic group (-COO-), hence the exchange is primarily cationic. 3. Mass flow Because of transpiration, the suction through the leaves- xylem-roots is responsible for the passive movement of ions in the soil solution into the plant through the roots. Ion absorption by active transport In an active transport system, particles without charge are transported actively if its net movement is against the concentration gradient. Particles with electric charge are transported actively if its net movement is against its electrochemical gradient. This process against the gradient requires external energy that is supplied by cell metabolism. ADP + Pi ATP + H2O This reaction can occur via photophosphorylation and oxidative phosphorylation. It occurs in the chloroplast and mitochondria against + the gradient of H energy. 3- PO4 Hydrolysis of ATP  ATP + H2O ADP + Pi ATPase permits the conversion of chemical energy of ATP to potential concentration gradient (electrochemical potential). This is primary active transport, which is used for the secondary active transport of solutes. In primary active transport, the transport of ions or solutes is linked directly to the energy from the ATPase, while in the secondary active transport, the energy for the transport against the gradient doesn’t come from the energy released by the ATPase, but rather from the electrochemical potential that has been generated.  Transport by the phloem All multicellular organisms need a highly organized transport system in order to function. In unicellular organisms, it’s mostly by passive transport, following the concentration gradient or by active transport when the need arise to cross a membrane. In higher plants, for the transport of nutrients, two pathways are used, which are the xylem and the phloem and both form the vascular system. The principal cell types of the phloem are sieve cells or sieve-tube elements, which lack a nucleus at maturity. If sieve-tube elements are present, they are associated with specialized living, parenchyma cells called companion cells or transfer cells. Nature of the substance transported by the phloem The knowledge of the chemical nature of the substance transported by the phloem is interesting because: 1. It gives a better understanding of the metabolic relationship between the different parts of the plant during its development. 2. It can give some indications about the mechanism of transport. 3. The knowledge of which substance and which cannot be transported by the phloem. This is important for the adequate utilization of herbicides and fertilizers. Substances transported by the phloem 1. Carbohydrates In majority of the species, sugars, besides water constitutes the largest part of substance transported by the phloem. Sucrose is the dominant sugar. 2. Nitrogen substance Amino acids represent the most important fraction of the nitrogen substance transported by the phloem with glutamic acid, aspartic acid and asparagine are the most abundant. In general, all nitrogen substances of low molecular weight can be transported very easily by the phloem. Proteins are also detected in the phloem as well as proteins of enzymatic nature. 3. Organic acids and inorganic substances A considerable amount of organic acids have been detected in the phloem. Alpha-ketoglutaric acid, pyruvic acid, oxalacetic, fumaric, oxalic, malic, citric. They all appear in very low concentrations. Even though the phloem is considered as a conductor system for organic substances, we must not forget that it also transport water and inorganic ions in a comparable way as the xylem, but not identical. 4. Ions Potassium is the most predominant cation transported. Others include sodium, calcium and magnesium. The most important anions transported are: phosphate, sulphate, nitrate and bicarbonate. 5. Growth substance Plant growth regulators. Plant hormones. (Auxins, Cytokinins, Abscisic acid, Gibberellic acid, Ethylene and Brassinosteroids) The equilibrium between the concentration of promoters and inhibitors in the phloem can vary greatly with the change of environmental conditions. 6. Viral Particles 7. Other substances Vitamins like thiamine (B1), niacin (B3) and ascorbic acid. ATP. The transport of solutes depends directly or indirectly on the use of energy. It is suppose that this energy is supplied by ATP by the companion cells, which have a great number of mitochondria. Besides the natural substances, a series of synthetic compounds like herbicides, fungicides, insecticides and synthetic plant growth regulators are transported by the phloem. Intensity and velocity of the transport The intensity is expressed by the amount of solute transported by the unit time and the velocity is by the lineal distance travelled by unit time. 30-100 cm/h in C3 plants and more than 200 cm/h in C4 plants. Direction of the transport We have to consider three fundamental aspects: 1. Significance of the source and sink. During the whole plant’s life, there is a continuous transport of solutes from one point to the next. The movement is from the source to the sink. The source is the site of production and can also be a storage site if the availability of these compound exceeds its utilization. Examples of source would be: mature leaves, the seed’s cotyledon or endosperm in germination, reserve tissues of the stem, leaf or roots when it is shooting. The sink would be the site for reserved substance. Examples are: meristems, young leaves, seed’s cotyledon or endosperm in formation, reserve tissues of the stem, leaf or roots when it’s storing substance. Phloem is the via that unites the source and the sink. 2. Entrance of the solutes into the phloem. The first step of transporting solutes from the source to the sink is the active and selective entry of the solutes into the phloem. Sucrose is the main solute that is transported and is accumulated in the phloem reaching a higher concentration. Sieve tube elements are associated with companion cells (transfer cells). Sucrose does not pass directly to the sieve tube elements, but rather is it accumulated in the companion cells before. The accumulation of sucrose in the companion cells is done against the concentration gradient, hence requires the use of energy. The transfer is selective because sucrose can pass very easily, but glucose or fructose cannot. The membranes of the phloem contain ATPase. 3. Distribution of the solutes by the plants. There is a generalization that lower leaves export solutes to the roots and higher leaves export to the apical zones and intermediate leaves export in both directions. However in potato, where majority of the sink is below, then majority of the transport of solute is downwards. In tomato the fruits (sink) are located all along the stem. Solutes are transported to zones of priority and those are the zones of rapid growth like the buds, young leaves, growing roots, developing seeds and fruits or towards storage organs. The model of distribution of solutes is determined by the position of the source and the sink. Effect of environmental factors on transport The factors involve can affect the source, sink, or the vascular system or all three. 1. Light It has to do with the rate of photosynthesis, hence affecting the source. 2. Water potential One could be in an indirect form affecting the photosynthetic intensity and the other can be directly on the transport. Lack of water in sugarcane shows a decrease in photosynthesis by 18%, while the transport of sugars was decreased by 90%. Solvent Lack of water can lead to a higher viscosity of the solution to be transported. 3. Temperature o The optimum range of the transport of solute is 20- 30 C. Below 10 and above 40, transport is impeded considerably. Process depends on energy, hence photosynthesis for ATP. Theories on the mechanisms of transport by the phloem A. Passive mechanisms 1. Diffusion Movement by diffusion is too slow to be responsible for the transport of solutes through the phloem.  2. Pressure flow Great success and acceptance because of its simplicity. However, its simplicity in an apparently complicated system as the phloem raised some questions. Objections: Structural nature – Resistance by the sieve cells cytoplasm and that the sieve plates are always open. Physiological – bidirectional movement in the same sieve tube. B. Active mechanism 1. Electro-osmosis There are ions in the sieve plates maintained by metabolic activity. An electro-osmotic process would be possible if three basic requirements are met: A membrane should exist that have pores with electrical charge. The membrane should be semi-permeable allowing the free pass of ions. There should be a potential difference through the membrane that should be maintained for transport to take place.  It would permit the pass of K+, hence the membrane is negatively charged. The potential difference or the polarization is maintained by the movement of K+ where there is a higher concentration above the sieve plates and a lower concentration below the sieve plates. ATP supplies the energy for the circulation of the K+ ions. Objections: It’s not possible to explain the simultaneous flow of anions and cations. If the membrane is negatively charge, then how will anions pass? The bidirectional flow of solutes cannot be explained either. 2. Protoplasmic currents Movement from one end of the cell to the next. Objection: How does it pass through the sieve plates? Another is that this movement has only been observed in immature sieve cells. 3. Other hypothesis Filaments that pass through the sieve cells that have contraction, hence cause the solutes to flow. Objection: Lack of evidence that these structures exist.