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Chapter 5 The nutrients translocation in the plant The short distance transports of nutrients There are two main steps nutrients translocation in plant after nutrients absorbed by the root. One is the centripetal transport from the rhizoplane through the cortex tissue of the roots towards the xylem vessels of the central cylinder. Another one is the vertical transport from from the root towards the leaves. The short distance transports of nutrients (1)短距离运输(short distance transport) 也叫横向运 输(the centripetal transport),或径向运输。指养分由 根的外表皮,穿过皮层进入中柱的过程。 It is the centripetal transport from the rhizoplane through the cortex tissue of the roots towards the xylem vessels of the central cylinder中柱. There are two pathways of ions transportation in the centripetal process. 即质外体途径(Apoplastic pathway)和共质体途径 (Symplastic pathway)。 Symplastic Apoplastic The apoplast (apo, greek= away from the plasma) is the continuous system of water and air filled spaces of the cell wall and is in close contact with the soil medium, the contact often being enhanced by numerous root hairs. Water free space(水分自由空间) and Donnan free space( 杜南自由空间) Cations interact with the fixed anions in the cell wall (pectin and proteins) and hence their motion is much restricted. The part of the apoplast is also called apparent free space (表观自由空间)as this space appears to be free for diffusion. ‘Apoplast’: cell walls & spaces between cells (‘intercellular spaces’); filled with ‘air’ & water Cell walls 外皮层 根被层或表皮层 It should be borne in mind that the effect of metabolism on water uptake and retention is an indirect one 中柱鞘 导管分子 Water may also travel through the root cortex via the cellular pathway including membrane transport (transmembrane pathway) or through plasmodesmata 胞间连丝(symplastic pathway) The cytoplasm of one plant cell is generally connected to the cytoplasm of neighboring cells by numerous plasmodesmata. This cytoplasmatic continuum is called the sympalst. Symplast共质体 and apoplast质外体 symplastic route (intracellular) apoplastic route (extracellular) Part cross-section of primary root - two pathways for movement of water & nutrients Within cells ‘Symplastic ’ pathway Between cells ‘Apoplasti c’ pathway The exodermis as a barrier (some species) 皮层 ‘Symplastic’ pathway 中柱 早期后生木质部 末期后生木质部 韧皮部 内皮层 凯氏带 Uptake blocked 外皮层 表皮层 Symplast共质体 and apoplast质外体 • However, in practice it is probably only the outermost cells of the epidermis (including root hairs) and cortex which take up nutrients because of the depletion zones. Thus, by the time that the outermost cells have removed ions from the apoplastic solution, the concentration in the cell wall of inner cells of the cortex must be very low. • Measurement of electrochemical gradients in cell profiles across the root show that uptake of K+ into cells may be active or passive, but the final transfer into the xylem is passive. • CI- into cell is active, but is passive into xylem The short distance transports of nutrients N、P、K, Mg等养分主要以质流的方 式通过共质体(symplast)途径运输;运输 过程中,有部分养分进入液胞(Vacuole); Ca、Si、Na、Fe(?)主要通过质 外体(poplast)途径运输。 Ca 在运输过程中容易被细胞壁上的 负电荷吸附。 The relationships of potassium content in tissue with K concentration is cytosol and vacuolar Cytosol Vacuole The effects of supplying Ca concentration on the Ca content and growth of dicotyls and monocotyledons (Loneragan) Ca concentration (mM) rye tomato rye tomato 0.8 2.5 10 Relative growth rate 43 100 94 3 19 52 Ca content(mg/g DM) 0.6 0.7 1.5 2.1 1.3 3.0 100 1000 94 100 93 80 3.7 12.9 10.8 24.0 Long distance transport of mineral nutrients In higher plants, an adequate transport of minerals between sites of uptake and production (sources) and site of consumption (sink) is essential. The most important pathways for long distance transport are the vascular tissue(导管组织) of the xylem(木质部) and phloem(韧皮部) Long distance transport of mineral nutrients • 木质部 xylem transport • 单向运输 unitransport: water and nutrients are taken up from the root medium and translocated towards the upper plants parts by the xylem • 装载(Xylem loading) :K+、NO3- 、CI-等离子 为被动(passive) Long distance transport of mineral nutrients • 蒸腾作用(Transpiration) • The relatively high rate of water flow along the xylem vessels in an uptake direction cause a rapid translocation of solutes dissolved in the xylem vessels. • It is generally accepted that transpiration is the main diving force not only for water transport, but also for nutrient translocation in the xylem. Long distance transport of mineral nutrients • 离子的交换吸附 interception exchange • The transport mechanism for solutes in the xylem sap is predominantly one of mass flow, where cations and particularly divalent cations may be adsorbed to cell walls surfaces and exchanged for other cations. The effect of other cations in root exudate on the long distance translocation of 45Ca in bean stem without root(24h) (μe/g DM)(Jaeoby,1967) Supply to the cut stem Part of plant 45CaCI 2 45CaCI +Ca2++Mg2+ 2 +K++Na+ 45CaCI +root 2 Primary leaves 0.04 4.7 1.8 12~18cm stem 7 19 11 8~12cm stem 28 56 40 4~8cm stem 84 57 61 0~4cm stem 159 81 81 exudate Long distance transport of mineral nutrients 传递细胞的作用( function of transfer cells): Some inorganic nutrients can be taken up rapidly from cells adjacent to the xylem vessels. They thus decrease in concentration as they are transported along the xylem vessels. This is true for the major plant nutrients , such as NO3-, H2PO4-and K+; On the other hand, other nutrients are absorbed relatively slowly in the xylem sap by adjacent cells. Long distance transport of mineral nutrients Some Solutes in the xylem saps are translocation from the xylem into phloem cells through the transfer cells, and storage in the cells or transport back to the roots. Transfer cells along the phloem which with their invaginations( 凹入) increase the adjacent cell surface and thus also the lateral movement of ions into the adjacent phloem parechyma cells. 木质部(Xylem) 韧皮部 P X 韧皮部( Phleom) 转移细胞(Transfer cell) T 木质部与韧皮部之间养分转移示意图 Long distance transport of mineral nutrients 释放与分泌 (secretion) In some plants, the NO3- concentration is decreasing, and amino acids contents is increasing in the sap as solutes transport from root the shoot in the xylem. 如豆科作物,在沿木质部迁移时,NO3-N含 量减少,氨基酸含量增加 • Soil-Plant-Atmosphere continuum Cohesion of water molecules to one another and adhesion to xylem walls by hydrogen bonds The composition of xylem • The xylem sap is rather dilute solution, which is made up largely of inorganic ions and also of amino acids. • Different techniques have been used in the collection of xylem sap which have result in variations in the ion concentrations that have been obtained. Composition of xylem sap in Ricinus communis after applying the root exudation and the root pressure chamber technique (Schurr and schulze 1995). Intact plant(mol/m3 ) Root exudation(mol/m3 ) Root exudation/intactplant K 6.6 17 2.6 Ca2+ Mg2+ 1.8 0.7 5 4 2.8 5.5 - 4.7 0.2 20 1.8 4.3 7.5 PO4 CI- 3- 0.2 0.09 4 0.4 27.7 4.6 Gln 0.57 5 8.8 Arg 0.004 0.2 16.5 Glu H+ 0.009 0.00031(pH6.5) 0.06 0.0036(pH5.4) 23.3 11.6 Nutrients + NO3 SO4 2- Exudation rates averaged to 20μ L/min in the root exudation technique and transpiration rate at 450μ L/min in average The composition of xylem • The factors which impact on the amplitude of the diurnal pattern: • Transpiration nutrient uptake (K+) Lateral exchange of cations (Ca2+) • Taken up by the cell adjacent cells to the xylem vessels(H2PO4- and K+) • Phloem cycling 韧皮部(phloem) The phloem is the tissue that mainly translocates organic compounds, the products of photosynthesis and amino acids from mature leaves to areas of growth and storage driven by a turgor pressure gradients. 韧皮部(phloem) Phloem-mobile mineral nutrients are also transport in the phloem and are transported both in an upward and downward direction----bidirectional. Basically the transport in the phloem is driven by a turgor pressure gradient from the source area of plant with low solute potential to several sink areas with a high solute potential. • The phloem comprises of a number of different cell types of which the sieve elements (SE) and companion cells (CC) are the most important. These are living cells although SE have lost some organelles (e.g. the vacuole) and others are modified (e.g. mitochondria and plastids). • • The SE and CC are connected by plasmodesmata (modulated pores connecting the cytosols of adjacent cells). The SE form the translocation pathway and are separated by a sieve plate containing pores. A collection of SE forms a sieve tube. Composition of phloem of castor oil plant (Ricinus communis蓖麻) (from Hall and Baber 1972) Dry matter Sucrose(蔗糖) Reducing sugars(还原糖) Amino acids(氨基酸) 100-125mg/g 234-304mol/m 3 35.2 mol/m3 3 Keto acids(酮酸) Phosphate(H2 OP4 -) 15-24 mol/m 2.5-3.8 mol/m3 Sulphate(SO4 2-) 0.3-0.5 mol/m3 Chlor ide9CI) 10-19 mol/m Nitrate(NO3 -) 3 - 3 Bicarbonate(HCO3 ) 1.7 mol/m Potassium(K) Sodium(Na) 60-112 mol/m 2-12 mol/m3 Calc ium(Ca) Magnesium(Mg) 0.5-2.3 mol/m 4.5-5.0 mol/m3 Ammonium(NH4 +) 1.6 mol/m3 Auxin(生长素) Gibberellin(赤霉素) 0.06 mol/m 0.0067 mol/m3 Cytokinin(细胞分裂素) 0.052 mol/m ATP 0.4-0.6 mol/m pH Solute potential(溶质势) 8-8.2 mol/m3 Conductance(导度) Viscosity(粘度) 3 3 3 3 3 -1.4—1.5MPa 13.2mS 1.34cP at 20℃ Cycling of mineral nutrients between phloem and xylem transport The phloem and the xylem are not directly linked to one another. Thus in the translocation between two pathways, water and solutes must pass through the connecting tissues (such as transfer cell). Phloem absorbs water from the surrounding tissues which in turn obtain water from xylems. On average about 5% of water transported in an upward direction in the xylem is retranslocated via phloem to the lower plant part(Zimmermann,1969). Cycling of mineral nutrients between phloem and xylem transport • Research in the last decade has shown that continuous nutrients cycling i.e. the retranslocation of nutrients in phloem from the shoot to the root and the cycling i.e. the translocation of cycled nutrients back in the xylem to the shoot is of great importance for nutrients which show a high phloem mobility, such as for nitrogen and potassium, phosphorus, sulphur and magnisium (Marschnaer, 1996,1997) Table K+ cycling and recycling in castor bean and NaCIstressed white lupin relatively large amount of Na+ are cycled back in the phloem to the roots (Jeschke and Pate,1991; Jeschke,1987) Proportion of total uptake path White lupin Caster bean K Na K Na Xylem import to the leaf 96 45 138 11 Xylem export from the leaf Phloem transport to the root 72 59 33 33 93 85 9 9 Cycling through the root Total uptake (mmol per plant) 39 1.07 1.23 78 2.88 0.48 Cycling of mineral nutrients between phloem and xylem transport • Major function of the cycling • Cover nutrient root demand for root growth (N and S). • Driven force in the xylem and phloem (K) • Counteraction of toxic ion in source leaves (CI) • Maintainance of cation-anion balance (K and organic acids) • Act as signal to control nutrients uptake pool NO3- NH4+ xylem NO3- pool NO3- NH4+ NO3- NH4+ Amino acids Amino 氨基酸 acids proteins Amino acids Amino 氨基酸 acids proteins leaves phloem root Soil solution Model of cycling of nutrients in plant CO PEP Malate K K + phloem Malate K HCO K+ NO malate NH K + shoot NO xylem KNO pyruvate root K+ NO Models of K cycling in the xylem and phloem with NO3- and malate transport in plant Redistribution of mineral nutrients in the phloem • As leaves and root age, some of their nutrients are set free and retranslocated to the young growing leaves, roots, fruits, or storage organs. • Redistribution is remobilization and the phloem translocation of nutrients from source leaves. • It occurs under nutrients deficiency, during leaf senescence or in perennial plants in spring when nutrients are mobilized from the stem or root. Redistribution of mineral nutrients in the phloem • Not all nutrient elements retranslocation with equal case. • The retranslocation of plant nutrients via phloem under nutrient deficiency depends much on the capability of sieve tubes to take up nutrients rapidly. • The difference of ions in phloem mobility is also reflected in appearance of deficiency symptoms. Redistribution of mineral nutrients in the phloem • As the Ca supply to a plant organ mainly depends on transpiration intensity, the transpiration rate of a given plant organ is of particular importance in determining its Ca concentration. Where transpiration is low, Ca supply may be inadequate and Ca deficiency may thus result. Redistribution of mineral nutrients in the phloem • Fruit and storage organs generally have a lower transpiration rate than leaves. This gives rise to blossom end rot(脐腐病) in tomatoes, bitter pit in apples and blackheart in celery. Relative mobile of mineral nutrients in the phloem mobile Slowly mobile immobile N iron B P Mn Ca K Zinc Mg Copper Nutrients reusable and the deficiency symptoms Calcium is immobile in the phloem because Ca loaded into the sieve elements is inhibited, or Ca is deposited in the phloem by the high concentration Phosphate. The reason of Boron immobile in the phloem is uncertain. Nutrients reusable and the deficiency symptoms • The nutrients in any organs of parts of plant could be translocated to other organs or parts via phloem, and be reused, which is called reusable. The nutrient reusability is dependent on the capacity of its movement in the phloem. Such as N, P and K are mobile in the phloem, so they are the most reusable, but Ca and B are immobile in phloem and the lowest reusable. Nutrients reusable and the deficiency symptoms The deficiency symptoms of high reusable nutrients appears on the older part of plant firstly, but the symptoms of immobile nutrient appears on the younger parts of plants firstly. Nutrients reusable and the deficiency symptoms 氮、磷、钾和镁四种养分在体内的移动性大,因而,再 利用程度高,当这些养分供应不足时,可从老部位迅速及时 地转移到新器官,以保证幼嫩器官的正常生长。 N,P,K and Mg is rather mobile in throughout the whole plant, so their capability of retransloction and redistribution is higher. When external supply is inadequate, the nutrients in the older plant parts are mobilized and translocated via phloem into younger growing tissue to cover their growth requirement. Nutrients reusable and the deficiency symptoms 铁、锰、铜和锌通过韧皮部向新叶转移的比例及数 量还取决于体内可溶性有机化合物的水平。当能够螯合 金属微量元素的有机成分含量增高时,这些微量元素的 移动性随之增大,因而老叶中微量元素崐向幼叶的转移 量随之增加。 The translocation of iron, Mg, copper and zinc via phloem into younger leaves are determined the concentration of soluble organic substances (chelate) . The higher is concentration of chelate, the more mobilize of iron, Mg, copper and zinc, and the more of them translocated from older leaves to younger leaves. Total nutrients maturation anthesis seeds Vegetative organs Time (after germination) 禾谷类作物个体发育期间矿质养分分配的典型图解 Nutrients reusable and the deficiency symptoms Nutrients Reusability N、P、K、Mg Parts of deficiency symptoms Older leave S New leave Slowly mobile Fe、Zn、Cu、Mo New leave Very slowly mobile B、Ca New leave on the apical immobile mobile Conclude • There are two steps of nutrients translocation in plant: short distance and long distance transport • Apoplast pathways and symplast pathways in short distance transport • Xylem and phloem in long distance • Unidirectional transport in xylem and bidirectional transport in phloem • Cycling of mineral nutrients between phloem and xylem transport Conclude • Redistribution of nutrients in plants • Mobilization of nutrients in phloem and redistribution • Nutrients deficiency symptom and their reusable