Lecture 10 Environmental Adaptation

Xeric environment

A hot, dry, water-limited environment that plants must adapt to survive in

Avoidance strategy (drought)

Completing the entire life cycle during the rainy season and remaining dormant the rest of the time (e.g., desert ephemerals)

Succulence

Water-storage adaptation using parenchyma cells (water reservoir) and mucilage cells (thick, gluey, aid water retention) in leaves/stems

Stomata crypt

A sunken pit in the leaf surface that creates a humid microenvironment around stomata, reducing airflow and cutting transpiration by about 15%

Rubisco

The enzyme that fixes CO2 in the Calvin cycle; evolved when there was no atmospheric O2, so it can also mistakenly bind O2

Photorespiration

A wasteful process where rubisco fixes O2 instead of CO2, oxidizing one carbon to CO2 instead of fixing it, costing ATP and NADPH to reverse

Conditions that increase photorespiration

High temperature, bright light, high O2/low CO2, and limited water (closed stomata)

C3 photosynthesis

The ancestral photosynthetic pathway where the first product of CO2 fixation is a 3-carbon compound, directly via rubisco with no CO2-concentrating mechanism

C4 photosynthesis

A pathway where the first CO2-fixation product is a 4-carbon compound made by an enzyme that doesn't react with O2, spatially separating initial fixation from the Calvin cycle

Kranz anatomy

The leaf structure in C4 plants that isolates rubisco in the bundle sheath cells, away from atmospheric O2

PEP (phosphoenolpyruvate)

The 3-carbon molecule that combines with CO2 in C4 mesophyll cells to form malate

Malate (in C4 plants)

The 4-carbon compound formed in mesophyll cells that transports CO2 to the bundle sheath, where it releases CO2 near rubisco

CAM photosynthesis (Crassulacean Acid Metabolism)

A pathway that separates CO2 fixation from the Calvin cycle by time: stomata open at night to fix CO2 into malate, then close by day while malate releases CO2 for the Calvin cycle

Why C4 evolved

To concentrate CO2 around rubisco via structural (spatial) separation, minimizing photorespiration in hot, sunny environments

Why CAM evolved

To concentrate CO2 via temporal separation AND minimize water loss by closing stomata during the day, suited to hot, sunny, and dry environments

Water-use efficiency: C3 vs C4

C3 plants lose about 600g of water per gram of CO2 fixed; C4 plants lose only about 300g, about twice as efficient

Carnivorous plant nutrition

Carnivory is a strategy for acquiring nitrogen and other nutrients in waterlogged, sunny, nutrient-poor soils — not a defense mechanism

Pitcher trap

A carnivorous plant trap formed from a modified leaf containing digestive enzymes or bacteria (e.g., Nepenthes, Cephalotus)

Flypaper trap

A carnivorous plant trap that uses sticky mucilage to catch prey

Snap trap

A carnivorous plant trap that uses rapid leaf movement to catch prey

Bladder trap (lobster-pot trap)

A carnivorous plant trap, as in Utricularia, that sucks in and traps prey such as protozoa in water

Mycoheterotroph

A plant that obtains carbon through a three-way association: nutrients flow from a photosynthetic plant's roots to a mycorrhizal fungus to the mycoheterotrophic plant

Fungal dependence gradient

A spectrum of reliance on mycorrhizal fungi: Vaccinium depends on fungi only at germination, Pyrola depends on fungi its entire life, and Monotropa depends on fungi its entire life and has also lost its plastids

Parasitic plant

A plant that gains some or all of its carbon, nutrients, and water directly from a host plant (e.g., Dodder/Cuscuta, Rafflesia)

Haustorium

The specialized organ parasitic plants use to physically connect to and draw resources from a host plant

Horizontal gene transfer (HGT)

The transfer of DNA/RNA between host and parasite through the haustorium, potentially integrating host DNA into the parasite's genome

Detecting HGT (vertically inherited gene)

On a phylogenetic tree, a vertically inherited gene causes the parasite's gene to cluster with its own free-living close relatives

Detecting HGT (horizontally transferred gene)

On a phylogenetic tree, a horizontally transferred gene causes the parasite's gene to cluster with the host's lineage instead of its own relatives

Rafflesia-Tetrastigma case study

A parasite (Rafflesia) and host (Tetrastigma) that diverged about 115 million years ago, making false HGT signals from gene tree estimation error unlikely