The rice plant-soil-water system

Crop and Environmental Sciences DivisionCrop and Environmental Sciences DivisionInternational Rice Research InstituteInternational Rice Research Institute

Los BaLos Bañños, Philippinesos, Philippines

Water transport soil-plant-atmosphereWater transport soil-plant-atmosphere

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 = +

-140

-120

-100

-80

-60

-40

-20

0

20

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

ponded water

muddy suspension

impermeable layer

subsoil

ground water table

Pressure head (cm)

Depth (cm)

Water potential in the flooded rice soil

The unsaturated soil“pulls” at the water andpotential is negative

When a paddy rice field falls dry, the soil water potential becomes negative and decreases

Positive water potential Negative water potential

Potential during the growing season in an aerobic soil(aerobic rice, Changping, China, 2002)

0

10

20

30

40

50

60

70

80

90

100

175 200 225 250 275 300Day number

Soil moisture tension (kPa)

Panicle initiation Flowering Harvest

Each soil type has a specific relationship between the content and the potential of water: the pF curve

0

1

2

3

4

5

6

7

0 0.1 0.2 0.3 0.4 0.5 0.6

Soil water tension (pF= log(h))

Soil water content (cm 3 cm-3)

Clay

Sand

Tension (pF)

Content (cm3 water cm-3 soil)

0.560.46Saturation

(pF = 0)

0.480.30Field capacity

(pF = 2)

0.340.03Wilting point

(pF = 4.2)

0.220.001Air dry

(pF = 7)

ClaySand

0.560.46Saturation

(pF = 0)

0.480.30Field capacity

(pF = 2)

0.340.03Wilting point

(pF = 4.2)

0.220.001Air dry

(pF = 7)

ClaySand

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 systemPotentials 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 potentialA dry clay soil pulls hard at water (difficult to take up by roots)

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

Drought stressDrought stress

Reduced transpiration as function of soil water tension (IR72)

leaf (Tact/Tpot)

Soil water tension

Link between transpiration and photosynthesis

0

0.2

0.4

0.6

0.8

1

1.2

1 10 100 1000 10000

Leaf rolling factor (-)

Soil water tension (kPa)

Rolled leaves => less canopy photosynthesis

Leaf rolling

Spikelet sterility

Turner (1986): relationship between leaf

rolling – increased canopy temperature

Spikelet sterility

Less grains

Less yield

Accelerated leaf death

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1 10 100 1000 10000

Drought-induced leaf death factor factor (-)

Soil water tension (kPa)

Dead leaves => less canopy photosynthesis

0

0.2

0.4

0.6

0.8

1

1.2

1 10 100 1000 10000

Reduction factor (-)

Soil water tension (kPa)

Leaf expansion,

Leaf death

Leaf rolling,Spikelet sterility

Leaf photosynthesis,transpiration

Summary effects of soil water tension; IR72

photosynthesis

Summary effects of drought

Less leaves

Reduced leaf expansion

Less canopy photosynthesis

Less biomass

Reduced partitioning to shoot

Reduced leaf photosynthesis, transpiration

Leaf rollingLess light interception

Spikelet sterility

Less grainsLess yield

Accelerated leaf death

Soil moisture tension

Less canopy transpiration

Effect of timing of drought: most sensitive at flowering

O’Toole, 1984

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