1. Leaf (or cell) expansion

lower leaf area less Pn

Effects of water deficit:

Volume time = m (P – Y)

cell wall extensibility

yield threshhold ofcell wall

Growth rate =

Reduction in cell expansion:

Both m and Y are determined by chemical-mechanical properties of the cell wall

m (P – Y)

(6 – 4) growth

(4 – 4) no growth

when (P – Y) approaches zero no growth

Even if P is restored, growth may still be inhibited.

Why?

Both m and Y can be altered by metabolism, enzymatic activity leading to softening or stiffening of wall

Drought

m decreased

Y increased

(i.e., stiffercell walls)

… and the change is slow to be reversed

This slows or stops growth …

Effect of water deficit on maize shoot development

H2O content: 100% 16% 4% 2%

w (bars) -0.3 -2.0 -8.1 -16

What do you conclude?

@ same , leaf/stem growth inhibited to a greater degree than root growth

What do you hypothesize?

V/t = m(P – Y)

roots in dry soil have: higher m lower Y

What’s your next hypothesis?

Some (unknown) biochemical-enzymatic factor acts to cleave bonds in the cell wall, loosening wall structure and permitting growth.

Sharp, Silk & Hsiao:

found XET (xyloglucanendotransglycosylase) induced by ABA

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divides again when cell regains ~ original volume

no stress

stress

At any given point in time, cells are smaller, but DW growth is virtually unchanged.

time

volume growth slower

In non-stress conditions, rate of in growth is faster.

Therefore:

acquire & allocate C much faster

i.e., re-invest “capital” or not

Growth depends on growth that has already occurred (compound interest analogy).

Effects of water deficit:

2. Reduction in photosynthesis

Stomatal closure – impedes influx of CO2

Effects on biochemical capacity for photosyn.

• electron transport• rubisco activity

Effects of Water Deficit

3. Phloem translocation

phloem transport

photosynthesis

sink activity

governed by

Stress effects?decrease photosynthesis less exporteddecrease sink activity leaf expansion hard hit

0

5

10

15

20

25

30

0

2

4

6

8

0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.020

30

40

50

60

70

80

90

100

Reflects sink activity(impaired very early)

% 14C remainingin the source leafafter 24 h

Net CO2

assimilationmol m-2 s-1

14C translocationvelocity (cm h-1)

Leaf (- MPa)

Reflects phloem loadingand export

Not strongly inhibited until leaf falls lower than ~ -12 bars

escape desiccation desiccation postponement tolerance (maintain high ) (tolerate low )

H20 savers H2O spenders

osmotic adjustment

protoplasmicresistance

4. Osmotic adjustment (a mechanism of tolerance of low )

An increase in the solute concentration of cell sap resulting from more solute molecules per cell rather than from a lower cell volume.

Effects of water deficit:

a process by which can be decreased without an accompanying decrease in turgor

net increase in solute content per cell

do not confuse with increase in solute [ ] that occurs as a result of dehydration and shrinkage

Osmotic adjustment

leaf cell = - 8 bars

xylem = - 6 bars

soil = - 1 bar

w = + p

-8 = -12 + 4

water flows in

cell = - 15 bars (or water will flow out)

xylem = - 14 bars

soil = - 2 bar

w = + p

-15 = -12 + -3

consequence?

p decreases

(P – Y) zero

growth stops

If you were a bio-mechanical engineer:

design this plant so that it cope with this situation.

what would you modify or enhance?