Plant Adaptations Outline Photosynthesis and respiration Environmental controls

Plant Adaptations Outline: • Photosynthesis and respiration • Environmental controls on photosynthesis • Plant adaptations to: –High and low light –Water limitation –Nutrient availability Readings: Chapter 6

Conditions and Resources • Conditions are physical / chemical features of the environment – E. g. Temperature, humidity, p. H, etc. Ø Not consumed by living organisms (but may still be important to them) • Resources are consumed – Once used, they are unavailable to other organisms – Plants: sunlight, water, mineral nutrients, … – Animals: prey organisms, nesting sites, …

Plant Resources • Plants are autotrophs - make their own organic carbon form inorganic nutrients – Need light, ions, inorganic molecules • Plants are sessile – Grow towards nutrients

PHOTOSYNTHESIS Conversion of carbon dioxide into simple sugars LIGHT 6 CO 2 + 12 H 2 O C 6 H 12 O 6 + 6 O 2 + 6 H 2 O

Light reactions

Dark reactions carboxylation

Photosynthetically Active Radiation, PAR

RESPIRATION C 6 H 12 O 6 + 6 O 2 6 CO 2 + 6 H 2 O + ATP

Net photosynthesis = Photosynthesis - Respiration

Photosynthesis involves gas exchange

Controls on photosynthesis • Light • Water • Nutrients • Temperature

1. Light

PAR

Tradeoff • Shade plants grow better in the sun than in the shade, • but sun plants grow faster than shade plants in direct sun Shade plant Sun plant

Tradeoff • Shade plants survive well in either sun or shade • Sun plants cannot tolerate shade Shade plant Sun plant

• 9 tree species of Macaranga from Borneo, Malaysia

Phenotypic plasticity • Most plants have the ability to alter their morphology (within limits) in response to light conditions

Phenotypic plasticity • Sun and shade leaves can exist within the same tree More deeply lobed -> More rapid heat loss

Sun leaf • thicker • more cell layers • more chloroplasts Shade leaf • flat • thin • larger surface area / unit weight

Sun leaves Shade leaves • Leaves at many angles • High saturation point • High compensation point • Produce more RUBISCO • Horizontal leaves, single layer • Low saturation point • Low compensation point • Produce less RUBISCO • High respiration • Less chlorophyll • RUBISCO availability limits photosynthesis rate • Low respiration • More chlorophyll • Light availability limits photosynthesis rate

2. Water Transpiration

For transpiration to occur atmosphere < leaf < root < soil

Water potential w = p + + m p= = hydrostatic pressure = = osmotic pressure m= = matric pressure

Stomata • Reduction in soil --> stomata close • Species differ in tolerance to drying soils

Strategies for drought i. Avoiders • • Short lifespan Wet season Seeds survive drought Drought deciduous species – Leaves shed in dry season

Strategies for drought ii. Tolerators • • • Leaves transpire slowly Change orientation of leaves Sunken stomata – • E. g. pines More efficient photosynthesis • • E. g. C 4 --> reduces photorespiration E. g. CAM --> stomata open at night

C 4 photosynthesis CAM photosynthesis

C 4

CAM

CAM

% of grasses that are C 4

Water absorption • Root hairs increase surface area

• Structure of the root system varies between species, depending on the amt. of soil moisture in their env’t • Individual species show phenotypic plasticity • wet soil --> shallow roots near surface (greater oxygen availability) • dry soil --> deep roots

3. Nutrients • Macronutrients – needed in large amounts (e. g. C, H, O, … N, P, K, Ca, Mg, S) • Micronutrients – trace elements (e. g. Fe, Mn, B) • Micro/macro refer to the quantity needed

Table 6 -1

Nutrient uptake rates • Reach plateau with increasing nutrient concentration

Maximum growth rate of a plant reflects N availability in its natural habitat. A. stolonifera occurs on more nitrogen-rich soils than A. canina.

Evergreen leaves • Plants adapted to nutrient-poor conditions tend to have evergreen leaves

4. Effects of temperature = Condition • Increase temperature --> increase biochemical reaction rate • At high temperature, enzymes denature --> death

• Gross photosynthetic rate increases up to a point with increasing temperature • Respiration rate also increases with temperature. • Net photosynthesis is maximal at a point slightly below that at which gross photosynthesis is maximal

Leaf temperature • • > 95% of sunlight absorbed by a leaf becomes heat Cooling of leaves: 1. Transpiration 2. Convection (movement of cool air around a leaf)

C 4 plants • Have higher temperature optima than C 3

Phenotypic plasticity • Individual species can modify their Topt according to the changing seasons = acclimatization

Response to cold Chilling injury Freezing - near, > 0 o. C - cell membranes rupture - < 0 o. C - ice inside cells = death - ice outside cells = dehydration (may survive) -may kill juveniles only

Saguaro cacti (S. W. United States) store large amounts of water; they can tolerate short periods of freezing temperatures

CLOSER TO HOME • Freeze-tolerant plants: frost hardening • When T decreases – plants synthesize sugars, amino acids, other molecules to act as antifreeze. • Winter – deciduous plants • Lose leaves in autumn • Leaves very efficient in summer – high photosynthesis rate • Leaves can’t survive freezing • Costly in energy, nutrients to rebuild leaves • Chilling breaks seed dormancy for temperate/boreal spp. • Germinates only in spring

Plants are phenotypically plastic