Structure and Function Reflect Adaptations to an Environment, Which Is a Product of Natural

Plant adaptations

Chapter 6

Studying adaptations

•Structure and function reflect adaptations to an environment, which is a product of natural selection

•Evolutionary “solutions” represent tradeoffs

•Fundamental constraint on life is acquisition of energy

•Allocation of energy (and matter) is an important trade-off

Photosynthesis

•Solar energy used to drive processes to fix CO2 into carbohydrates

•Photosynthesis is separated into two processes

C3 plants

Respiration

•Harvesting of energy from breakdown of sugars

•Both respiration and photosynthesis occur in plants

–Net photosynthesis is total photosynthesis minus respiration

•Measured typically as moles CO2 per unit leaf area

Light affects photosynthetic activity

Material exchange

•When stomata are open, CO2 and H2O move down concentration gradients

–Transpiration is water loss through stomata

–Water uptake must equal water loss for cells to remain turgid

–Very important photosynthetic constraint

–Transpiration rate increases with temperature

•Aquatic plants lack stomata

Water moves along water potential (ψ ) gradient

Closing of stomata

Water Use Efficiency

•Ratio of carbon fixed (photosynthesis) per unit water lost (transpiration)

•Varies from species to species

Photosynthesis and respiration respond directly to temperature

Leaf temperature

•Determined by difference between solar radiation received and solar radiation reflected and omitted

•Plants must lose heat to environment to have positive net photosynthesis

–Loss by evaporation and convection

•Rate of evaporation determined by rate of transpiration, which depends on humidity of air and soil moisture

Leaf shape influences convection

Carbon balance

•Allocation of net carbon gain influences growth, survival and reproduction

–Roots provide access to water and nutrients and anchor the plant

–Stems provide vertical support and conduction of materials

–Leaves photosynthesize

Shift in carbon allocation influences net carbon gain

Plants live in sun or shade

Davies research

•Studies of Macaranga

Change in leaf shape

Change in carbon allocation

Genotypic differences in response of plant species as well

•Shade-intolerant or sun-adapted species

•Shade-tolerant or shade-adapted species

Reich et al.

Seedling strategies for shade

Kitajima study

•Both species grown under varying light environments

•Shade-intolerant species responded to light availability earlier than shade-tolerant species (rapid return strategy)

–Died under low light levels

•Larger seed size does not lead to faster growth due to reserve strategy

After reserve stage, shade species still grow slowly

•Greater allocation to roots and lower photosynthetic surface in shade species in general

Moisture stress

C4 photosynthesis

•Different leaf anatomy

•Spatially separated photosynthesis

C4 trade-offs

•Benefits – Higher water use efficiency

•Costs – More energy expended to produce PEP and PEP carboxylase

•Most mostly in grasses and xeric shrubs

Distribution of C4 grass species

CAM Pathway

•Temporally separated photosynthesis

•Found mostly in succulents

Carbon allocation

Genetic constraints

•One species xeric, one species mesic

•Which is which?

Temperature Response Curves

TRC varies with photosynthesis type

Plasticity in TRC

Seasonal shifts

Freezing

•Ice crystals puncture cells

•Frost hardening is the ability to tolerate extreme cold

•Cells distribute substances that act as antifreeze

Plant Nutrients

•Availability of nutrients is important

•Macronutrients and micronutrients

Nitrogen influences photosynthesis

Nutrient concentration influences uptake

Reduced photosynthesis in nutrient poor environments

Leaf longevity is an adaptation to low nutrients