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Chapter V
Energy and Material metabolisms in plant
Section 2 Water metabolism
The importance of water in plant life
1.Water content and its states in plant
Water content in plant is about 70% ~90%
It always depends:
⑴ in different plant species:
e.g. water hyacinth (水浮莲)>90%
lichen (地衣) only 6%
⑵ The same plant in different growth enviroments:
e.g. shade plants > arid and sunbaked plants
⑶ in different organs of the same plant
e.g. seedling > bole (树干) > dormant bud > dormant seed
70~90%
35~70%
40%
5~15%
States : bound water and free water
Bound water: is absorbed by the components of protoplasm
and can not move freely.
Free water:
isn’t absorbed , can move freely, participate in
metabolism and is used as solvent for different metabolisms.
The ratio of bound water to free water is one of the physiological
indexes indicating growth situation and resistance of plant to
adverse growth conditions.
2. Physiological and ecological roles of water in plant
physiological action:
⑴ a main constituent of protoplasm
⑵ substrate for plant metabolism,
e.g. photosynthesis and respiration
⑶ a good solvent for absorption and transportation of
substances
⑷ maintain the natural shape of plant
⑸ adequate water is required for cell division and the
growth
ecological roles：
⑴ Adjusting plant temperature
High specific heat (比热) — maintain constant temperature
High heat of vaporization（高汽化热）— reduce plant temperature
to prevent the harm
High dielectric constant（介电常数）— is good for ionic dissolution
⑵ Water has high permeability to visible light
⑶ Water adjusts the plant living environment
Definition of water potential
Water potential (ψw ) 水势：
The potential energy of water per unit volume relative to
pure water in reference conditions. Water potential quantifies
the tendency of water to move from one area to another due to
osmosis, gravity, mechanical pressure such as surface tension.
一个系统中，水的总能量中可以用于作功的能量
（自由能）的大小即为水势。
Generally, the water potential of pure water at an atmosphere
and 0 ℃ is defined as 0 Pa.
When there have some solutes, the solute particles in solvent
can reduce the free energy of water. So the free energy in
solution is lower than that of pure water.
The value of water potential is negative, and the denser its
concentration is, the lower its water potential will be.
通常将纯水在1个大气压和0℃下的水势定为0 Pa，
当水中有溶质存在时，溶液中的溶质颗粒降低了水
的自由能，所以溶液中水的自由能要比纯水低，溶
液的水势为负值，溶液越浓，水势越低。
Water potential of plant cell
Water move in or out from cells following the gradient of
water potential. Plant cells are a complicated system, its water
potential is determined by several factors, including:
⑴ solute potential, (ψs ) 溶质势 its value is always negative
⑵ pressure potential, (ψp) 压力势 mostly positive
⑶ matrix potential, (ψm) 衬质势
always negative
Ψw = ψs + ψp +ψm
Ψw = ψs + ψp ( when have a mature vacuole )
ψw== ψm ( dry seed )
Solute potential ( ψs )溶质势/渗透势: also named as osmotic
potential. is due to the existence of solute that reduce the
free energy of water, so its value is lower than that of pure
water. Value is negative.
由于溶质颗粒的存在降低了水的自由能，因而其水势低于纯
水的水势。
恒为负值.
Osmosis
渗透作用
Osmosis：water move from a hypotonic (低渗)system to a
hypertonic (高渗）system through a semipermeable membrane
Pure water
Sweet water
Semipermeable
membrane
Ψw values for several common compounds
solution
Ψw / Mpa
Pure water
0
Hoagland nutrient solution
-0.05
seawater
-2.50
1mol·L-1 sucrose
-2.69
1mol·L-1 KCl
-4.50
A plant cell is a kind of osmosis system
The protoplasm layer, including plasma
membrane, protoplasm and vacuole
membrane, of a mature cell is to be
equivalent to a semipermeable membrane.
Thus vacuolar sap (cell sap) and
extracellular fluid constitute an osmotic
system due to the semipermeable
membrane role of protoplasm.
protoplasm layer
water transport between plant cells （Ψw， MPa）
A
B
-0.8
-0.6
C
-0.4
Water always move from a
higher water potential system
to a lower water potential
system, untill their water
potentials are same.
1.When in a higher Ψw solution, a cell will soak up water.
2）When in a lower Ψw solution, cell will dehydrate.
3）When in a solution with equal water potential, cell will soak
and dehydrate the same quality of water.
Pressure potential (Ψp ) 压力势
由于细胞壁压力的存在而引起的水势增加值。
一般情况下，压力势为正值； 质壁分离时，压力势为零；
cell with turgor pressure
cell with decreased turgor pressure
Matrix potential (Ψm)衬质势：
衬质是指表面能够吸收水分的物质（蛋白质
、淀粉 、 纤
维素）。由于细胞胶体物质亲水性和毛细管对自由水的束缚
而引起的水势降低值。
恒为负值
未形成液泡的细胞有一定的衬质势（如干燥种子），已
形成液泡的细胞衬质基本为水饱和，衬质势绝对值小（趋于
零），可忽略不计
故具有液泡的成熟细胞，其水势可简化为：
Ψw = Ψs + Ψp
（ 水势 = 渗透势/溶质势 + 压力势）
Water absorption of plant cell
植物细胞对水分的吸收※
吸
水
方
式
Osmotic water absorption 渗透性吸水（具液泡的成
熟细胞）
最主要的吸收方式
water imbibition 吸胀性吸水（未形成液泡的细胞）
即吸胀作用。通过细胞中亲水胶体吸水膨胀的现象
metabolic water absorption 代谢性吸水（直接耗能）
耗能跨膜吸收有机溶质和无机离子以增加渗透势方式
Water absorption in plant root system (active
and passive ）
Active water absorption
Plant root uptakes water from outside environment just
due to the metabolic activity of plant root system.
Mechanism: The metabolic activity of roots causes ion
transportation and absorption, then induces a difference water
potential whinin or without plant cells. It pushes water goes
through epidermis, cortex and endoderm and goes into stele vessel
from outside environment, and then transports upword in plant.
Active water-absorption behaviours:
root pressure, bleeding and guttation.
Root pressure: a kind of power generated in plant root system
which make liquid stream transport from root to shoot. Root
pressure is caused by active transport of mineral nutrient ions
into the root xylem. Without transpiration to carry the ions up
the stem, they accumulate in the root xylem and lower the water
potential. Water then diffuses from the soil into the root xylem
due to osmosis. Root pressure provides a force, which pushes
water up the stem, but it always less than 0.1MPa and can only
make water column up about 10m.
root pressure
Active water-absorption behaviours:
root pressure, bleeding and guttation.
Bleeding: bleeding sap, determine the size of root pressure.
Guttation: The exudation of water from leaves as a result of root
pressure.
Sufficient soil moisture，wet weather，lower
transpiration , apex of leaf or edge of leaf.
Two evidences for existance of the plant root presure.
Guttation from barley plant leaves
（hydathode ）
bleeding
Root pressure is studied by
removing the shoot of a plant
near the soil level. Xylem sap
will exude from the cut stem for
hours or days due to root pressure.
If a pressure gauge (压力计) is
attached to the cut stem, the root
pressure can be measured.
2、Passive water-absorption in plant root system
Roots absorb water due to the transpiration of stem and leaf.
evidence：an ongoing stem and leaf transpiration can absorbs
water from anaesthetized (麻痹的) or dead root. Root is only be
used as a surface for passive absorption. The transpiration
drafting causes a decrease in root water potential and promotes
root absorb water from soil passively.
Transpiration: a process that water in plant body emits from
plant surface as a gaseity style. This is the main water-absorption
way for plant.
cuticular transpiration (≤5%)
Two ways
角质层蒸腾
stomata transpiration (95%) cuticular 气孔蒸腾
Under the same conditions, Whether 1mm2
area of free water or 1mm2 area leaf with
many stomatas have the same quantity of
transpiration？
3、Mechanism of stomatal transpiration —alveolus
diffusion （小孔扩散）：
Diffusion rate of gaseity water through alveolus surface
isn’t directly proportional to the area of alveolus, but is directly
proportional to the perimeter of alveolus.
气体通过小孔表面扩散的速率不与小孔的面积成正比，而
是与小孔的周长成正比。
Ralative area of
Ralative perimeter Ralative quatity of
alveolus
of alveolus
diffusion loss
小孔相对面积
小孔相对周长
扩散失水相对量
1.00
1.00
1.00
0.37
0.61
0.59
0.05
0.21
0.18
0.01
0.13
0.14
 when a big hole is scattered into a number of small holes
and gross area is the same, the perimeter will increases almost
10 times and the diffusion rate will also increase greatly.
 Stomatal space is about 10 times of its diameter, and will
have the smallest interfere between each diffused water
molecule and have the biggest edge effect.
 一个大孔分散为许多小孔,总面积相等时,周长增
加数十倍, 扩散速率增大;
 气孔间距为其直径的10倍左右, 扩散水分子间相
互干扰最小, 边缘效应最大。
Alveolus diffusion law
Transpiration rate through stomata
The area of stomata is always less than 1% that of the leaf area,
but the transpiration quantity through stomata is about 50% of
the quantity through plant leaf .
气孔面积一般不超过叶面积的1%，但通过气孔的蒸腾量却达到
叶片同样面积的蒸发量的50%以上。
Mechanism of stomatal movement:
Stomata can move —— open on daytime and close at night。
Reasons for opening and closing —— regulated by the water
potential of guard cells in leaves.
stomatal movement hypothesis:
1）starch-sugar conversion theory ( 淀粉—糖转化学说 )
2) inorganic ion uptake theory ；K+ (无机离子吸收学说)
3) malate production theory (苹果酸生成学说)
Mechanism ：osmoregulation of guard cells
—— imbibition and shrinkage
Starch-sugar conversion theory
illumination
darkness
photosynthesis in
guard cells
respiration in
guard cells
Consume CO2, pH rise
Produce CO2, pH decline
Hydrolysis activity of
amylophosphorylase rise
Synthesis activity of
amylophosphorylase rise
Starch is hydrolyzed
to G-1-P
G-1-P is synthesized
to Starch
Decline of
water potential
Increase of
water potential
Guard cells turgor pressure increase Guard cells turgor pressure decline
Stomatas open
Stomatas close
Malate production theory
Illumination/darkness
Photosynthesis/ respiration
in guard cells
Produce ATP and malic acid
ATPase on plasmalemma
hydrolyze ATP
malic acid dissociates
and prpduce H+
H+ is pumped out of guard cells and
K+ is pumped into of guard cells
Water potential declines
Guard cells turgor pressure increase
Stomatas opens
Inorganic ion uptake theory
K+ go in or out of the stomatal guard cells
3. Water transportation in plant
植物体内的水分运输
（一）水分进入细胞的途径
（1）pathways that water go through into plant cells.
1、单个水分子通过膜脂双分子层的间隙进
a . A single water molecule go through the space between
lipid bilayer and go into plant cell.
入细胞
b . Water affluence go through the water channels
2、水集流通过质膜上水孔蛋白组成的水通
commposed
by aquaporins on plasma membrane and go into
plant cell.
道进入细胞
aquaporin：Aquaporins are membrane water channels
that
play critical roles in controlling the water contents of
水孔蛋白：是一类具有选择性地、高效转
cells.
运水分的膜通道蛋白。
Transportation of water in plant
植物水分的运输
soil water
root vessel
vein vessel
root hair
stem vessel
mesophyll cell
substomatic cavity
stoma
root cortex
pericycle
petiole vessel
intercellular space
atmosphere
Pathways with which water transport in plant body
水分在植物体内的运输途径
1）symplastic transport (共质体运输) 所有细胞的原生质体
通过胞间连丝联系形成从原生质体到原生质体的连续体系。
“活”的部分；阻力较大，短距离运输。
2）apoplastic transport (质外体运输) 指植物体内所有的细
胞壁、细胞间隙等没有原生质的部分所组成的一个连续整体。
该途径速度比共质体运输快，但慢于管分子通道运输。
Graphical Illustration of symplastic and apoplastic transport
3）vessel system transport (导管或管胞系统的运输)
由导管或管胞等死细胞组成的中空长管道，水分在其
中运输。管道中的水依靠水的内聚力可以形成一条连贯的
水柱。运输的动力来自上部的蒸腾拉力和下部的根压，上
拉下推使水分上升。
迅捷的长距离运输方式
2. Driving force for water acsend along vessel
水分沿导管或管胞上升的动力
水分上升的动力：根压和蒸腾拉力
水分上升的原因：内聚学说（cohesion theory），
蒸腾—内聚力—张力学说
（transpiration-cohesion-tension theory）
争论较多；
目前仍是唯一被认为足以给植物体内水分上
运提供一个合理解释的学说。
Transpiration-cohesion-tension theory
Plants’ roots,stem and leaves be regarded as a continuous watercolumn
(mesophyll cells-root cells), laminas absorb water from vessel because of
the transpirational dehydration. The tracheal watercolumn receives a
tractive power and also bounds by the force of gravity so that generates
a tension. Because of that water cohension is much bigger than tension of
watercolumn, so can maintain the continuity of water so that it can
acsend continuously.
1）tension of watercolumn (- 0.5 ~ - 3MPa)
2）biggish cohesion between water molecule (+20 ~ + 30MPa ),
cohension >> tension
3）water have great adhesion to vessel wall.
蒸腾－内聚力－张力学说。
植物的根茎叶导管内的溶液可看作一个连续的水柱
（叶肉细胞 —— 根细胞），叶片因蒸腾失水而向
导管或管胞吸水，导管或管胞的水柱受到蒸腾的拉
力向上牵引时，也必然受到重力的牵引，上拉下拽
使水柱产生张力，由于水分子内聚力大于水柱张力，
保证水柱的连续性而使水分子不断上升。
1）水柱有张力（- 0.5 ~ - 3MPa）
2）水分子间较大的内聚力（+20 ~ + 30MPa ），内聚力>＞张力
3）水分子对导管壁有很强的附着力
Water
transportation
for plant
Transpiration indexs
Transpiration rate：certain time, area of unit leaf ,quantity
of water dissipate. g-1m2 h
Transpiration ratio：the quantity of dry matter maked by
1kg of water dissipate , g-1kg
Transpiration coefficient：consumption of water that forms
1g dry matter, g-1 g
Section Ⅲ mineral nutrition
mineral nutrition ：the study of how plants obtain, transport
and assimilate mineral nutrients is called
mineral nutrition.
Mineral nutrients
mineral nutrients in plant
庄稼一
枝花，
全靠肥
当家
elemental composition
macroelement 10 -2
C、H、O、
Mg、Ca、K、
S、 P、 N
microelement 10 -3-- -5
Cu、 Fe、 B、
Zn、 Mo、 Mn、
Na、Cl ……
收多
收少
在于
肥
Root system uptakes mineral elements from soil inorganic salt,
In natural condition，all the soil inorganic salt is airslaked
from mineral elements.
according to the
physiological functions
of mineral elements
according to the
requirement for plant
essential elements
constituents：participate in the composing of
Plant cells physical structure
regulatory components：regulate life activities
of plant cells.
macroelement： Its content is more than 0.01%
（10-2 %） of dry matter in plant.
microelement: Amounting 10-5- -3 % of dry
matter in plant ，
Essential elements for plant growth
Essential elements:
C、H 、 O 、 S 、 P 、 N 、 K 、 Ca 、 Mg 、 Fe 、Cl 、
Mn 、 Zn 、 Cu 、 B 、 Mo 、Ni
 An essential element is defined as one whose absence
prevents a plant from completing its life cycle or one that
has a clear physiological role.
 These elements can synthesize compounds which plant
needs or participate in plant metablism.
The fuction and symptoms of specific nutritional deficiencies
main function for some common minerals in plant cells
When any of the essential elements are deficient in the plant,
characteristic deficiency symptoms appear.
作物营养元素缺乏症状检索表
病
症
缺乏元素
A．病症在老叶 （老叶先发病）
B．病症常遍及整株，基部叶片干焦
C．植株浅绿，基部叶片发黄，干燥时呈褐色，茎短而细 ……………………………………氮
C．植株深绿，常呈红或紫色，基部叶片发黄，干燥时暗绿，茎短而细 ……………………磷
B．病症常限于局部，基部叶片不干焦但杂色或缺绿，叶缘杯状卷起或皱缩
C．叶杂色或缺绿，有时呈红色，有坏死斑点，茎细 …………………………………………镁
C．叶杂色或缺绿，有坏死的大斑点或小斑点
D．坏死斑点小，常在叶脉间，叶缘最显著，茎细 …………………………………………钾
D．坏死斑点大，普遍出现在叶脉间，最后出现于叶脉，叶厚，茎的节间短 ……………锌
A．病症在嫩叶（幼叶先发病）
B．顶芽死亡，嫩叶变形和坏死
C．嫩叶初呈钩状，后从叶尖和叶缘向内死亡 …………………………………………………钙
C．嫩叶基部浅绿色，从叶基起枯死，叶卷曲 …………………………………………………硼
B．顶芽存活但缺绿或萎蔫
C．嫩叶萎蔫，常有斑点或缺绿发黄，茎尖柔弱 ………………………………………………铜
C．嫩叶不萎蔫，具有缺绿症
D．坏死斑点小且散布全叶、叶脉仍绿 …………………………………………………… 锰
D．无坏死斑点
E．叶脉仍绿 ……………………………………………………………………………… 铁
E．叶脉仍绿 ……………………………………………………………………………… 硫
3. Absorption for mineral elements in plants
(1). Passive absorption：
Absorb mineral elements by physical process such as
diffusion action, one kind of energy-free metabolism.
Substance moving from a region of high concentration to
a region of lower concentration by simple diffusion. The
main reason for passive absorption is the intracellular or
extracellular concentration difference.
(2). Active absorption：Active absorption is the expenditure
of energy by a cell that results in molecules or ions entering or
leaving the cell against a diffusion gradient.
Mechanism of active absorption：
ion carrier theory
ion pump theory
ion channel theory
Root system also can absorb some mineral elements by ion
exchange process. For example, CO2 produced by root breath
dissolves into the soil solution and becomes H+ and HCO3－,
H+ exchange with K+ adsorbed on the surface of soil colloidal
particles, so that K+ goes into the soil solution and can be
absorbed.
it also can absorb mineral elements by contact exchange
process, namely, H＋ on root cells directly exchange with K＋
on soil particles.
3、absorption features：
1）Associated with moisture absorption and also relatively
independent ;
2）The selective absorption for ion
3）toxic action of single salt
plants, cultured in a single salt solution despite an essential lowconcentration salt, absorb more than a certain amount of mineral
element, and then will be poisoned and die. This principle is
known as toxic action of single salt.
4）ion antagonism
if adding some other metal-ion salt into a single salt solution, it
will reduct or eliminate the toxic action of single salt. This kinds of
interactions between ions is called rivalry or antagonism.
4.Transportation of mineral elements in plant cells
The same to water transportation:
symplastic transport, apoplastic transport
and vessel system transport
Different in trace：
water is transported to plant leaves, while
mineral elements are transported to a growth
center of a time.
1. endoderm 2. casparian strip 3. pericycle 4. root hair
5. epidermis 6. cortex 7. xylem 8. phloem
Transportion of minerals in plant cell