Lecturer: Olalekan Sak ariyaw o PhD
Course: PCP 301
Department: PP& CP
Water Requirements in Plant
Learni ng Ex pectati ons:
1.
2.
3.
4.
5.
6.
Function s of water in crop plant
Physical and chem ical properties of wate r
Thermodynamic de scription of wa ter
Driving force s of water in S PA C
Soil-Plant-Atmo sphe re Continuum (S PAC)
Water deficit, wate r use strate gy and crop yield
Func ti ons of w ater i n Crops :
1. Cell Enlargement: The gro wth pro ce ss in plant i s directly related to the upta ke and
tran sport ation of wate r into the cell. Presence of water deficit would greatly
com promise g ro wth p roce ss
2. Stru ctu ral suppo rt
3. Evaporative cooling
4. Sub strate fo r biochemical pro cess in crop s
5. Tran spo rt of solute s in the crop plant
Physic al a nd c hemical properties of w ater
1. Bipolarity: The angular arrangement of o xyge n and h yd rogen in water m olecule leads
to the emergence of bipolarity. The covalent bon d resulting from thi s bipolarity
results in hydrogen bond when two wate r molecules are found togethe r in a m edium.
All the propertie s the physi cal and chemical propertie s of wate r are as a re sult of thi s
hydrogen bond bet ween wate r molecule s.
2. Liquid at physiological tem perature: B ecause of the st rength of thi s hydrogen bond,
wate r remains a liquid at physiological temperatu re, de spite thi s comparative smaller
molecular weight with respect t o othe r molecules.
3. Incompressibility: A s a liquid, water is incompressible, observing all the laws of
hydraulics.
4. High Latent heat of Evaporation: The am ount of heat needed to transfo rm 1 gram of
wate r into vapour is high, owing to the st rong hydrogen bond greate r than Van der
Waals force. Thi s pa rticular propert y i s very im portant mo st especially during
tran spiration of water vapour leading to e vapo rative cooling.
5. Cohe sion and adhesion: Attraction of similar molecules lead s to cohe sion. This
property wa s p re sumed to e xplain the up ward m ovement of water in the xylem.
Adhesion i s the attraction of di ssimilar m olecules bet ween wate r and other polymers.
This wetne ss p ropert y ha s important property has im portant implications in water
relation s.
Thermodynamic descripti on of water
To better de scribe water quantitatively, it wa s ob se rved that therm odyn am ic concept s could
be u sed. In thi s ca se the property of wat er wa s de scribed with re spect to it s poten tial energy,
which is it s capability to do work. Pu re wate r was con ventionally adopted a s the stand ard
wate r potential, above which it is impossible to obtain higher m agnitude of value. The value
for pure water i s zero. The unit for exp ressing wat er p otential is Mega Pascal.
The com ponent s of water pot entials a re a s follows:
1. Solute potential
2. Turgor pressure potential
3. Matric potential
4. Gra vitational potential
Solute wate r potential is dete rmined by the concent ration of the solute pre sent. It decrea ses
with increa se in solute con cent ration, thu s it s negative value. Turgor pre ssu re p otential value
could be po sitive o r n egative. In a flaccid cell, whe re there i s a net out ward movem ent of
wate r m olecule, the value for turgor pre ssu re potential is negative, creating tension;
conversely with net in ward movem ent of water into the cell, leading to turgid cell, the value
become s po sitive or po sitive h yd rostatic pre ssure. The balance between negative value of
solute potential and po sitive value of Turg or P re ssure potential creates a balance, leading to
negative water pot ential, since it i s a rarity to ha ve pu re water in a cell. Matric Potential is as
a re sult of the adhe sion prope rty of wate r, it is mo st prom inent during the m ovement of wa ter
in the soil. Gravitational potential increa ses wh en wate r i s rai sed abo ve a height above a
refe rence point. Wate r flows down g ravitational potential gradient, all things been equal. At
the microscopic level of the plant va scular ti ssu e one m ay omit the role of gravitational and
matric potential components of water p otential, though their relevance increase s with the
increase in organiza tional level of the plant.
1 Atmosphere = 760mmHg @ Se a level, 450 latitude
= 1.013 bar
= 0.1013MPa
= 1.013 x 105 Pa
Water potential com ponent s:
Φw = φs + φt +φm +φg
Where:
Φw – Wa ter potential
Φs – Solute potential
Φt – Tu rgor potential
Φm – M atrix pot ential
Φg – Gra vitational potential
Tab.1 Driv ing forces of w ater i n SPAC
Process
Diffusi on
Driving forces
Concentration gra dient
Fick’s Law
Js = -Δ s Δ c/ Δ x
Where; Js – Rate of solute diffu sion
Δs – Diffusion coefficient, m easure s ea se of sub stance
movem ent via a m edium
Δc/Δx – Concent ration gradient
Δc – Difference in concentration
Δx – Difference in di stan ce
Bulk
Flow
Pre ssu re g radient
Volum e = πr4 Δ P/Δ x/8
Where:
Volum e - Flow rate
r – Radiu s
π – Vi sco sity of liquid
ΔP – Differen ce in pre ssure
Δx – Difference in di stan ce
ΔP/Δx – P ressure gradient
Osmosis
Compo site forces (Concent ration and p ressure gradient )
Tab.2 Transport of water in plant (Soil -Plant-Atmosphere Conti nuum)
M edium/Interface
Process
Driving force
Pathway
Soil
Water
movem ent in
the soil/Bulk
Flow
Pre ssu re
gradient
Soil Particle s
Soil-Plant
Interfa ce
Water upta ke
Compo site
force
φw =φs +φp
Plant
Long di stance
tran sport
(Cohe sion ten sion)
Pre ssu re
gradient
Plant-Atm osphe re
Transpirational
pull of water
Gradient of
water vapour
concen tration
(Diffusion)
o
Apopla st
o
Sym plast
o
Tran smembrane
Xylem
o
Stomata
o
Cuticle
o
Lenticle
M odels of w ater uptake in pla nts
Cohesion-tensi on Model
This model propo ses that tran spiration of water from the plant leads to the em ergence of
cohe sion among similar wate r m olecule s, leading to th e build up of negative h yd rostatic
pressure or ten sion. The emergence of tension increase ten sile st reng th, which i s the ability
of wate r molecule to resi st pulling force and b y ca pillary action, wat er i s being pulled up
along the xylem . Where ga s bubble are trapped in the wate r colum n, with an indefinite
expansion of thi s bubble, a collap se of ten sion in the liquid phase i s been observed, thus
leading to cavitations. Thi s phenom enon brea ks the wate r column, re sulting in reduced
wate r uptake b y plant.
Che ck this URL for animated version of t hi s m odel:
http://academic. kellogg.edu/he rbrand sonc/bio111/ animations/0031. swf
Othe r re sou rce s:
http://www.m m.helsinki.fi/mmeko/ kurssit/M E325/ kuljetusp ro sessitke rtau s. pdf
http://www. uoguelph.ca/ plant/cou rses/pbio-3 110/
www.mm.helsink i.fi/mmeko/kurssit/.../k uljetusprosessitk ertaus. pdf
Root Press ure Model
An alternative model for the uptake and t ransport ation of wate r in the plant is the root
pressure m odel. The m echanism i s a s follows; a bso rption of solute lead s to a reduction of
solute potential in the plant cell, by concent ration gradient wate r i s being t ran spo rted along
the xylem tissue creating increa se in po sitive hyd rostatic pre ssu re o r root pre ssure, thus
facilitating water uptake. E xce ssi ve upta ke of wate r could lead to guttation, a phe nomenon
whe reb y liquid droplet s are formed at the edges of the leaf m ost esp ecially in the m orning.
Wate r defici t, w ater use s tra tegy and crop yi eld
Disequilibrium experienced between water supply and demand creat e s water deficit in
plants. Alternatively, the co ncept could b e envi saged a s a situation when water content in
the cell/ tissue is less than highest water content exhibited at hydrated state. In fields,
drought conditions leads to wate r deficit accompanied with high temperatu re. Thi s is a
clim ati c condition.
The re spon se of plant to wate r stress, which is when wat er is limiting, is varied and
physiological re sponse s are ob served at different level s of o rgani sa tion of the crop.
Strategically, plant could avoid or tolerate water deficit. In avoidance, the plant could
syn chronise hi s phenology with the gro wing sea son in othe r t o optim ise th e available
reso urce s for p roper growth and development. With tolerance the re mu st be spe cific
mechanism to ensu re a vailability of water and wate r u se efficiency. Toleran ce o r re si stant
strat egy involve s:
1. Desi ccation tolerance at high wat er po tential
a. Water sa ver, u se wate r con servatively; e xam ple succulent
b. Water spender, ag gressive con sumption of wate r; e xam ple Ephemeral s
2. Desi ccation tolerance a t low wate r po tential, possess the ability to function while dehyd rated;
xeromorphic plant s/ non-succulent. There are two st rategy for de si ccation tolerance at
redu ced water deficit:
a. Acclimation, which is transi ent and ph enotypic in natu re
b. Adaptation, which i s constitutive and genotypic in nature
Dim ension of a cclim ation are a s follows:
I. Osm oregulation – the pro ce ss of accum ulation of solute s in cell s independ ent of
cellular volume change. The implication is redu ced wat er potential, osmotic potential
and through wate r uptake increa sed cellular turgor. The solute s accumulated could
be:
a. Com patible –
i. Nitroge n containing, e.g. Proline, glycine betaine
ii. Non -Nitrogen containing, e.g. suga r alcohol (Sorbitol, m annitol)
b. Non -com patible, e.g. Inorganic ion s
II. Redu ced g ro wth
III.
Phenological variability or phenotypic plasticity (Det erm inate and indeterminate
gro wth )
IV. Energy dissipation through
a. Redu ced g ro wth of leaf
b. Change s in leaf orientation (Pa ra and diaheliotropism )
c. Leaf modification
i. Wilting
ii. Rolling
iii. Pube scence
Dim ensions of adaptation:
1. Cra ssulacean Acid Metabolism (CAM )
2. Metabolic changes via gene exp ression; synthesi s of ne w protein type s su ch as
aquaporin, Ubiquitin, Late Embryonic Abundant protein.
From the cellular level, emergence of water deficit re sult s in the decrea se in the cellular
wate r content, leading to sh rin kage of cell and the relaxation of cell wall. Decrease in volume
leads to increa se in solute concentration, fa vou ring reduced tu rgo r p ressure. Experimental
result s indicated tha t there is a synth esi s of endogenou s g rowth inhibitors (AB A and C2 H2),
change s in pH value and inorganic ion dist ribution. The conseque nce of the se change s i s
the redu ction in the expan sion of leaf or leaf growth as exp ressed in the num ber of leafs or
other g ro wth pa ram eters of the shoot. In the ca se of seve re wate r deficit, re duction in total
leaf area, increa se senescence and leaf abscission accompany water deficit. If wate r deficit
is m ild the plant experiences red uction in transpiration rat e via st om ata closure increa sed
heat di ssipation and increa sing re sist ance to liquid phase water flo w.
At the crop level, reduced crop g ro wth through stomatal regulation, as a result of wa ter
stre ss i s reflected in reduced Leaf Area I ndex, thu s comprom ising the ra diant ene rgy
abso rption capacity and it s utilization efficiency (Ra diant Energ y Utilisation Efficiency). What
is e ventually expe rienced i s redu ced inte rnal concentration of Carbon Dio xide and reduced
Tran spiration rate thro ugh the stomata. With reduce d internal concen tration of C02 , carbon
a ssimilation is equally affected reflecting in reduced Harvest Index and ultimately yield.