# The rice plant-soil-water system - Rice Knowledge Bank ...

The rice plant-soil-water system - Rice Knowledge Bank ...

The rice plant-soil-water system

Crop and Environmental Sciences Division

International Rice Research Institute

Los Baños, Philippines

Water transport soil-plant

plant-atmosphereatmosphere

Water moves from soil through roots, stems, leaves, to

atmosphere

Rate of water flow is f(potential difference, resistance)

Potential unit name

Corresponding value

Water height (cm)

1

10

100

1000

15850

pF (-)

0

1

2

3

4.2

Bar (bar)

0.001

0.01

0.1

1

15.85

Pascal (Pa)

100

1000

10000

10000

1585000

Kilo Pascal (kPa)

0.1

1

10

100

1585

Mega Pascal (MPa)

0.0001

0.001

0.01

0.1

1.585

Potential of water is positive in “free liquid water

Potentials in the soil-plant-atmosphere are negative

(in flooded rice soil, potential is positive)

Water moves from high potential (top of hill) to low

potential (bottom of hill)

Tension is –potential: water moves from low tension to

high tension

Potential = 0

Potential is +

Potential = -

Potential = 0

Potential = +

20

Depth (cm)

0

-20

ponded water

-60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60

muddy suspension

impermeable layer

-40

subsoil

-60

-80

The unsaturated soil

“pulls” at the water and

potential is negative

-100

-120

-140

ground water table

Water potential in the flooded rice soil

When a paddy rice field falls dry, the soil water potential

becomes negative and decreases

Positive water potential Negative water potential

100

Soil moisture tension (kPa)

90

80

70

60

50

40

30

20

10

0

Panicle

initiation Flowering Harvest

175 200 225 250 275 300

Day number

Potential during the growing season in an aerobic soil

(aerobic rice, Changping, China, 2002)

Each soil type has a specific relationship between the

content and the potential of water: the pF curve

Air dry

(pF = 7)

Sand

0.001

Clay

0.22

Tension Soil water (pF) tension (pF= log(h))

7

6

Clay

5

Wilting

point

(pF = 4.2)

0.03

0.34

4

3

Sand

Field

capacity

(pF = 2)

0.30

0.48

2

1

Saturation

(pF = 0)

0.46

0.56

0

0 0.1 0.2 0.3 0.4 0.5 0.6

Content Soil water (cmcontent 3 water (cm 3-3 cm soil) -3 )

A clay soil stores much water, but at a high tension, so

it is difficult for the roots to extract

A sandy soil holds little water, but at a low tension, so it

is easy for the roots to extract

A medium-textured, loamy soil, holds intermediate

levels of water at intermediate tensions, so there is

relatively much water for extraction by roots

No issue for flooded rice soil, but becomes an issue

when a soil falls dry during a dry spell

Leaf

Stem

Root

Example of potentials in soil-plant-atmosphere system

Potentials drop with each added resistance

Potential of water in the atmosphere (above leaves)

drives the potential transpiration rate, which is

f(radiation, wind speed, vapor pressure, temperature).

A hot sunny day => pulls hard at water from plant

Potential of water in the soil is determined by the soil

properties (texture, SOM,..) and water content:

• Clay soil pulls hard at water

• Sand soil pulls softly at water

• Much water: high potential

• Little water: low potential

A dry clay soil pulls hard at water (difficult to take up by

roots)

Drought stress

When the soil is too dry (high soil water tension), it

becomes too difficult for roots to take up water and

water flow in the plant gets reduced:

• Reduced transpiration

• Reduced photosynthesis

• Reduced leaf area expansion

• Leaf rolling

• Accelerated leaf death

• Spikelet sterility

Reduced transpiration as function of soil

water tension (IR72)

leaf (T act /T pot )

Soil water tension

Leaf rolling

1.2

Leaf rolling factor (-)

1

0.8

0.6

0.4

0.2

0

1 10 100 1000 10000

Soil water tension (kPa)

Rolled leaves => less canopy photosynthesis

Spikelet sterility

Turner (1986): relationship between leaf

rolling – increased canopy temperature

Spikelet sterility

Less grains

Less yield

Accelerated leaf death

1.20

Drought-induced leaf death factor factor (-)

1.00

0.80

0.60

0.40

0.20

0.00

1 10 100 1000 10000

Soil water tension (kPa)

Dead leaves => less canopy photosynthesis

Summary effects of soil water tension; IR72

1.2

Reduction factor (-)

1

0.8

0.6

0.4

Leaf expansion,

Leaf death

Leaf photosynthesis,

transpiration

photosynthesis

Leaf rolling,

Spikelet sterility

0.2

0

1 10 100 1000 10000

Soil water tension (kPa)

Soil moisture tension

Summary effects of drought

Less canopy

transpiration

Reduced leaf

expansion

Less

leaves

Less canopy

photosynthesis

Less

biomass

Less grains

Less yield

Reduced

partitioning to

shoot

Reduced leaf

photosynthesis,

transpiration

Leaf rolling

Less light

interception

Spikelet sterility

Accelerated leaf

death

O’Toole, 1984

Effect of timing of drought: most sensitive at flowering

Moderate drought in early growth stages

Leaf rolling in early growth stages

Severe drought in early growth stages

Severe drought in upper field near Roi Et, Oct. 2004

SMJ

RD15

Severe drought

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