Plant PhysiologyHORT 301

Robert JolyHORT 209

41306joly@purdue.edu

Plant response to the environment

How do plants sense, respond and adapt to environmental change? Special emphasis on response to stress.

Objectives:

• understanding of processes, mechanisms• be able to explain sequence of events

change in environment final response of plant

Years before present (billions)

4 3 2 1 0

900 myamarine algae

450 myanon-vascular plants colonize land

425 myavascular plants (microfossils of xylem tracheids)

Plants among the earliest organisms to appear in fossil record.

What problems were faced & solved during transition from watery env’t dry land?

need roots, vascular system to obtain H2O, carry to some height

need cuticle, epidermis, stomata to conserve H2O

need embryos capable of withstanding dry conditions

Plant-Water Relations

• How does H2O get into and out of plant cells?

• How .. enter roots move through the plant?

• How do plants regulate this to avoid dehydration?

read Taiz & Zeiger chapter 3

Plants and water

1. Water is essential for life

structural integrity of biological molecules (hydration sphere)

all biochemical-enzymatic rxns occur in an aqueous environment

vital role as a solvent• mineral nutrients• products of photosynthesis

Plants and water

2. Liquid continuity:

soil water

Liquid-gas interface at evaporating surfaces in leaf

unbroken continuity(SPAC)

Plants and water

3. H20 constitutes 80-95% of the mass of growing tissues (>50% for woody tissues)

e.g., corn at tasseling~ 800 g~ 700 g water

40,000-50,00 g water have passed through

Plants and water

4. Virtually every aspect of plant physiology is affected by water content. Many processes impaired by water deficit.

• growth• photosynthesis• cell division• protein synthesis• cell wall synthesis• hormone levels

water deficitdirect, physical

changes in gene expression

Plants and water

5. Productive agriculture is absolutely dependent upon supplies of freshwater.

water under increasing demand from farming, industrial, human uses

even wealthy industrialized societies are not immune from such pressure

slight (-) charge

slight (+) charge

polar molecule, net charge = 0

Water – physical properties:

Water – physical properties:

1. solvent:

water will dissolve more substances than any other common liquid

especially effective for electrolytes

H2O molecules form “cage” around ions, shielding their electrical charge increase solubility

H+ H+

- O

H+

H+

- O

H+H+

- O

K+

H+H+

- O

H+ H+

- O

Cl-

H2O as solvent:

Water – physical properties:

2. H-bonding: H+H+

- O

H+H+

- O

H+

H+

- O

(+) side of one molecule attracted to ( - ) side of another

thermal, cohesive, adhesive properties

H+

H+

- O

H+H+

- O

H-bonding among H2O molecules

high specific heat

= energy required to raise the temp. of a substance by a specific amount. For water:

1 cal to raise 1 g H2O 1 °C

Water – thermal properties

high specific heat: H2O molecules vibrate faster at high temperature but great deal of energy is required to break H-bonds.

i.e., H2O molecules absorb large quantities of energy without much temperature increase

What consequences?

Consequences of high specific heat of H2O:

buffers plant tissue (which is mainly H2O) from temperature fluctuations

provides temperature stability (even when gaining or losing heat energy)

Water – physical properties

cohesion: mutual attraction between H2O molecules (due to H-bonding)

adhesion: attraction of H2O to the solid phase (e.g., cell walls, glass surface, etc.)

Water molecules are more strongly attracted to their neighbors in the liquid than to those in the vapor. (H-bonded)H2O (liquid)

H2O (vapor).. . .

...

..

.. What consequence?

see Figure 4.8 Taiz and Zeiger (2010) p. 94

A meniscus forms, and the air-water interface assumes minimum surface area.

This creates a surface tension

Surface tensions and development of negative pressure:

surface tension of water an important contributor to pressure inside xylem elements

origin: sites of evaporation in the stomatal cavity

water adheres to cell walls – and coheres to each other – and that force (tension) is transmitted through rest of the fluid

Consider a single cylindrical pore:

- 2 Ts cos r

P =radius ( -P (bar)

5 -0.3

0.5 -3.0

0.05 -30

0.01 -150

0.005 -300

P = hydrostatic pressureTs = surface tension of H2Or = radius=contact angle

pore in a cell wall

Figure 4.10 Taiz and Zeiger (2010) p. 97

Water transport in plants:

1. diffusion: within a cell or tightly localized

2. bulk flow (mass flow): long distance; no membranes crossed

3. osmosis: cell to cell, crossing membranes

1. Diffusion

Fick (1855) discovered that the rate of solute transport is directly proportional to the concentration gradient and inversely proportional to distance traveled.

Fick’s Law describes passive movement of molecules down a concentration gradient. Substances move from high [ ] to low [ ].

Diffusion:

Cs

X- Ds Js =

difference in concentration

distance

diffusion coefficient

flux of a solute in solution

=

(mass/surface area/time)