Transpiration Intensity in Cereal Crops under the Conditions of Osmotic Stress

Journal of Environmental Treatment Techniques

2019, Special Issue on Environment, Management and Economy, Pages: 1046-1049

J. Environ. Treat. Tech.

ISSN: 2309-1185

Journal web link: http://www.jett.dormaj.com

Transpiration Intensity in Cereal Crops under

the Conditions of Osmotic Stress

Vladimir N. Vorob’ev *, Evgeniy A. Alexandrov , Alexander L. Mikhailov

Institute of Fundamental Medicine and Biology, Kazan Federal University, Kazan, Russia

Kazan Institute of Biochemistry and Biophysics, FRC Kazan Scientific Center, Russian Academy of Sciences, Kazan, Russia

Received: 18/08/2019

Accepted: 17/11/2019

Published: 20/02/2020

Abstract

The study of leaf gas exchange among medium and highly drought tolerant spring varieties of Tríticum aestívum during

moderate osmotic stress modeling showed a significant difference in the dynamics of transpiration intensity (E) at the initial stage

of the experiment. The monotonic dynamics of E decrease correlated with the change in stomatal conductance of water (gw) and

CO2 assimilation rate decrease (A) in a highly drought tolerant variety, which is explained by stomata closure. The dynamics of

E among the varieties resistant to drought had the form of a complex curve with an extremum during the first 12 minutes of the

experiment, which is explained by the gas embolism of xylem vessels. Embolization of a part of xylem vessels has led to the

imbalance in gas exchange rates. 75% decrease of E during the experiment did not coincide with the change of gw, which

decreased only by 50%. In addition, under conditions of E decrease, a significant decrease of A was not observed. Transpiration

decrease during osmotic stress due to stomata closure led to 28% drop of A, and comparable decrease in transpiration due to

xylem vascular embolism reduced A by 6%. It has been shown that the tendency to xylem vascular embolization in the medium-

drought-tolerant variety of spring wheat Tríticum aestívum under the conditions of moderate water deficiency makes it possible

to maintain a high level of CO assimilation rate, which is comparable with a highly resistant variety that can ultimately lead to a

higher yield.

Keywords: Gas embolism, transpiration rate, stomatal water conductivity, CO assimilation rate

spring rye show a high degree of vascular segmentation in

the root-shoot transition zone. About 80% of these roots are

connected to the shoot system via tracheids. In spring

varieties of these plants, 60% of the root vessels are directly

connected to the shoot. Existing differences are

characteristic not only for different species, but also for

individual varieties of the same species (8, 25, 26). Despite

the obvious negative effect of embolism, most plant species

live with a certain share of non-functional xylem vessels

due to gas embolism [9]. Besides, embolism can also have

a positive value, since it protects the root system from

water loss during drought (10, 22-24). Based on the

foregoing, it can be assumed that the resistance to water

deficiency among different drought tolerant varieties is

Introduction

Loss of water during transpiration should be

equivalently supported by the rate of water uptake by the

root. The imbalance between absorption and transpiration

leads to water potential decrease in xylem vessels. Low

potentials of water induce the formation of air bubbles

within the xylem vessels (the so-called embolism or

cavitation) (1, 2). In general, the phenomenon of gas

embolism leads to water transport disruption in a plant,

which negatively affects its vital activity. The ability of

plants to restore the hydraulic conductivity of xylem (to

eliminate gas embolism) depends on metabolic activity (3,

), dormin level (5) and root pressure (for monocotyledons)

(

6).

associated, among other things, with

different

It is assumed that the likelihood of gas embolism in

predisposition to xylem vessels gas embolism. At the same

time, embolism of a part of vessels can reduce transpiration

loss of water without significant damage to photosynthesis.

To confirm this hypothesis, we analyzed the dynamics of

cereals depends on the hydraulic architecture of the roots.

The presence of a “safety zone” at the root-shoot transition

points is a characteristic of spring barley (7), wheat and rye

(

8). The central roots of winter wheat, winter barley and

transpiration intensities (E) and CO assimilation rates (A)

in medium and highly drought resistant varieties of spring

wheat Tríticum aestívum during osmotic stress modeling.

Corresponding author: Vladimir N. Vorob’ev, (a)

Kazan Federal University and (b) Kazan Institute of

Biochemistry and Biophysics, FRC Kazan Scientific

Center, Russian Academy of Sciences. E-mail:

almihailov@bk.ru. Tel.: 89274455173.

Materials and Methods

For research, we used 7-10-day old seedlings of winter

wheat variety highly resistant to drought (Kazan 560),

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Journal of Environmental Treatment Techniques

2019, Special Issue on Environment, Management and Economy, Pages: 1046-1049

spring wheat variety medium resistant to drought

(

Simbirtsit) and spring highly resistant to drought wheat

variety (Omskaya-36) provided by the Tatar Agricultural

Research Institute (TatARI). Plants were germinated with

tap water at illumination of 150 μmol/m2 with

photoperiod of 16/8 at the temperature of 22 °C.

The transpiration intensity (E) and the assimilation rate

of CO (A) were determined by WALZ GFS 3000 gas

1,0

analyzer (Germany) with built-in software for the following

parameters: relative humidity in the measuring cell - 70%

(

at lower humidity, the transpiration intensity is not stable);

air flow rate through the cell - 750 μmol/s; photoactive

radiation (PAR) - 1000 μmol/m2; the temperature in the

0,4

cell is 22 °C; the concentration of CO

00 ppm.

The sample placed in the gas analyzer cell was kept at

in the cell makes

100

Time of exposure, min

Figure 1: Dynamics of transpiration under the conditions of

osmotic stress of spring and winter wheat Tríticum aestívum: a -

spring wheat cultivar Simbirtsit, b - winter wheat cultivar Kazan

these measurement parameters for 30 minutes (the time was

determined empirically and is sufficient to stabilize

transpiration under new conditions). A fixed transpiration

level (5-6 values with an interval of 2 minutes) was the

initial for subsequent measurements. Then a pause was

provided in the measurement program, during which the

growing solution quickly changed to the solution of

polyethylene glycol (PEG) 6000 of the corresponding

osmotic potential. Immediately after the solution

replacement, the measurement program was started with

the record of the main indicators of gas exchange (E, A,

stomatal conductivity of water - gw) for 60 minutes within

the interval of 2 minutes. To simulate osmotic stress, they

used 18% PEG solution with theoretically calculated

osmotic potential of -0.4 MPa. The experiments were

repeated 4-6 times. The graphs show the characteristic

dependences of transpiration intensity dynamics. The text

presents mean ± SD.

It is known that vascular embolization is accompanied

by a short-term increase of transpiration (11, 12). It is likely

that a sharp increase by 0.4 MPa of the gradient between

the air medium in the measuring cell of the gas analyzer

and the aqueous medium in the tube where the roots are

located leads to the formation of large cavitation bubbles in

the vascular system of seedlings of the spring medium-

drought-resistant wheat variety (Simbirtsit). For a winter

cultivar highly resistant to drought (Kazan 560), such a

pressure gradient is not enough for xylem vessel

embolization. Regardless of the differences in the dynamics

of transpiration intensity, stabilization of E is observed by

the 30-40th minute of the experiment. However, the

achieved level of spring variety transpiration intensity was

0.53 mmol m-2s-1, whereas it made 0.24 mmol m-2s-1 for

the winter variety.

Results and Discussion

The existing differences in the architecture of the root-

shoot transition zone are characteristic not only between

spring and winter crops, but also among spring varieties

The possibility of gas embolism formation in spring

and winter cereals during water deficiency is determined by

the differences in the architecture of the hydraulic system

of water transport from leaves to roots (7, 8). The central

roots of winter wheat are highly segmented in the root-

shoot transition zone, which can significantly reduce the

risk of gas embolism. The increase of the osmotic gradient

due to the placement of roots in the PEG solution leads to

the expected decrease of winter wheat leaf transpiration

rate of the variety Kazan 560 (Fig. 1, b), which is observed

from the 8th minute after the solution replacement. The

variety Simbirtsit, related to spring varieties, that is,

without tracheids in root-shoot transition zone,

demonstrated the complex dynamics of transpiration

intensity. The increase in transpiration intensity (E) was

observed already during the fourth minute after root

placement in PEG solution (-0.4 MPa) and lasts 8 minutes.

After reaching an extremum, E was sharply decreased

going to a plateau after 30-40 minutes of root exposure in

hypertonic PEG solution (Fig. 1, a).

(

8). Given this circumstance, they suggested that the

dynamics of highly drought-resistant spring variety

Omskaya 36 transpiration will be similar to the dynamics of

the winter highly resistant variety Kazan 560 transpiration

during replacement the solution of the root habitat with a

PEG solution. Figure 2 shows the dynamics of plant

transpiration of Omskaya 36 varieties highly resistant to

drought (Fig. 2, b), which differs little from the dynamics

of the winter varieties highly resistant to drought (Kazan

560) (Fig. 2, a).

It can be assumed that the change in the transpiration of

both varieties is based on the similarity of the stomatal

apparatus regulation rate and the similar structure of the

root-shoot transition zone, which determines the dynamics

of E. The decrease of transpiration during osmotic stress

may be due to water permeability decrease of the roots

and/or the closure of stomata. In general, stomatal

conductivity in both varieties decreases by 85 ± 5%.

Comparing the stomatal conductivity of medium and

drought resistant spring varieties, it should be noted that

with the same osmotic stress, the stomatal conductivity of

the Simbirtsit variety decreases only by 50 ± 4%. Under the

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Journal of Environmental Treatment Techniques

2019, Special Issue on Environment, Management and Economy, Pages: 1046-1049

conditions of the proposed model of osmotic stress of the

drought-resistant spring variety Omskaya 36, the dynamics

of transpiration intensity is practically repeated by the

dynamics of CO2 assimilation (Fig. 3). When E decreases

by 80 ± 6%, A decreases by 28 ± 3%.

1,4

1,2

1,0

0,8

0,6

0,4

0,2

Figure 4: Comparison of the dynamics of transpiration intensity - E

(empty squares) and the assimilation rate of CO2 - A (filled

squares) in the drought-resistant variety of spring wheat cultivar

Simbirtsit

100

Time of exposure, min

If at the beginning of the experiment the dynamics of A

with some lag repeats the dynamics of E, then from about

the twentieth minute there is a significant difference in the

recorded indicators. Thus, E decreases by 75 ± 5%, and A

by 6 ± 2% only. If we compare the variation of gw and E

for a given variety, then their imbalance becomes apparent.

With the decrease of E by 75 ± 5%, gw decreased only by

50 ± 4%. Such a mismatch may be due to embolism of

some vessels. Then the decrease of E is caused not so much

by the closure of the stomata as by the decrease of xylem

vessel number through which water is transported and this

is probably the reason for a slight decrease. It is known that

when moderate water deficiency occurs, anisohydric plants

lose water during transpiration in exchange for CO2 intake.

Such a strategy proves to be advantageous, and anisohydric

plants can outperform isohydric ones in growth and yield

Figure 2: Dynamics of transpiration under the conditions of

osmotic stress of spring and winter wheat Tríticum aestívum: а -

winter wheat cultivar Kazan 560; b - spring wheat of Omskaya 36

2,2

2,0

1,8

1,6

1,4

1,2

1,0

0,8

0,6

0,4

0,2

100

(

13-16). Comparison of the maximum yield of the used

Time of exposure, min

spring varieties shows the advantage of medium drought

tolerant variety, the yield of which can reach 6.58 t/ha,

while the yield of the drought tolerant variety does not

exceed 4.86 t/ha (17-21).

Figure 3: Comparison of the dynamics of transpiration intensity - E

empty squares) and the assimilation rate of CO2 - A (filled

squares) in the drought-resistant spring wheat variety Omskaya 36

(

It is likely that during the breeding of a variety highly

resistant to drought, selection was aimed at the

manifestation of isohydric properties, which are expressed

in a stable maintaining of water potential of cells due to the

closure of stomata. This thesis is confirmed by comparison

of gw and E. Among the varieties highly resistant to

drought under osmotic stress, gw decreases by 85 ± 5%. E

is also decreased by the same level approximately. Under

the same conditions, the drought-tolerant variety Simbirtsit

Summary

We have shown that the tendency to xylem vascular

embolization in a medium-resistant variety of spring wheat

Tríticum aestívum under the conditions of moderate water

deficiency allows to maintain a high level of CO2

assimilation rate with

transpiration intensity, which ultimately leads to higher

productivity as compared to a highly resistant variety.

comparable decrease of

(

Fig. 4) demonstrates an alternative change of the observed

processes.

Acknowledgements

The work is performed according to the Russian

Government Program of Competitive Growth of Kazan

Federal University.

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