Plant Physiology HORT 301 Robert Joly HORT

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) mya marine algae 450 mya non-vascular plants colonize land 425 mya vascular 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 H 2 O, carry to some height  need cuticle, epidermis, stomata to conserve H 2 O  need embryos capable of withstanding dry conditions

Plant-Water Relations How does H 2 O 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.H 2 0 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 deficit direct, 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:

1. solvent:  water will dissolve more substances than any other common liquid  especially effective for electrolytes  H 2 O molecules form “cage” around ions, shielding their electrical charge  increase solubility

H+ - O - O - O K+K+ - O - O Cl - H 2 O as solvent:

Water – physical properties: 2. H-bonding: H+ - O - O - O (+) side of one molecule attracted to ( - ) side of another thermal, cohesive, adhesive properties

H+ - O - O H-bonding among H 2 O 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 H 2 O 1 °C

Water – thermal properties high specific heat: H 2 O molecules vibrate faster at high temperature but great deal of energy is required to break H-bonds. i.e., H 2 O molecules absorb large quantities of energy without much temperature increase What consequences?

Consequences of high specific heat of H 2 O:  buffers plant tissue (which is mainly H 2 O) from temperature fluctuations  provides temperature stability (even when gaining or losing heat energy)

Water – physical properties cohesion:mutual attraction between H 2 O molecules (due to H-bonding) adhesion: attraction of H 2 O 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) H 2 O (liquid) H 2 O (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) P = hydrostatic pressure Ts = surface tension of H2O r = 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:  C s  X - D s J s = difference in concentration distance diffusion coefficient flux of a solute in solution = (mass/surface area/time)