Temperature Relations of Plants Plants and endothermic homeothermic animals differ in how they regulate their body temperature

Leaf Energy Budget Q abs = Q rad + Q conv + Q trans Abs = energy absorbed Rad = energy lost by radiation Conv = energy lost by convection Trans = energy lost by transpiration Environmental variables: light, air temperature, humidity Plant characteristics: leaf color, leaf shape, leaf angle, stomatal responses, height above soil surface

Patterns of Plant Responses to Temperature Q 10 = rate at temperature ‘T’ + 10 C/ rate at temperature ‘T’ If 2, then process under metabolic control

Plant responses to temperature show phenotypic plasticity Atriplex confertifolia (Salt Bush) - cold desert plant Atriplex vesicaria - warm desert plant

Plant responses to temperature reflect genetic differences and geographical distributions

Responses to Low Temperature – Tropical/Subtropical Plants Lowered metabolic rate, slower growth, altered development Chilling injury: injury when temperature drops below a critical temperature ‘T m ’ (not freezing) Cellular membranes go from fluid to solid and do not function Result: death of plant

How does ice crystal formation kill a cell? Ice crystal formation inside a cell disrupts internal membranes and other structures Ice crystal formation outside a cell causes internal dehydration and damage to sensitive proteins Temperature and drought stress are very similar!

Responses to Low Temperature – Temperate Plants Lowered metabolic rate, slower growth, altered development Induction of specific genes results in specific avoidance mechanisms: ↑carbohydrates and other solutes; leads to lowering of freezing point (sound familiar?) ↑degree of unsaturation of membrane lipids: membrane more fluid at lower temperatures ↑super cooling of tissue water: ice crystals do not form without nucleation sites until -37 C

Responses of plants to high temperatures Heat dissipation through emission of long wave radiation, convection and transpiration* Drought stress causes stomates to close, leading to increase in leaf temperature; if temperature rises to 45 – 55 C, (for most plants) thermal injury or death results Hah! We can survive at 65 to 70 C! 

Responses of plants to high temperatures - photosynthesis

Responses of plants to high temperatures – heat shock proteins HSP (heat shock proteins) – synthesized in response to exposure to elevated temperatures -act as molecular chaperones to protect proteins from heat denaturation -related to “acquired thermotolerance” C, 2h C, 2h C 15’  45 C, 2h C 30’  45 C, 2h C 1 h  45 C, 2h

Fire – Ultimate Temperature Stress Natural feature of ecological zones with dry season or during dry years Heat in fire depends on quantity and quality of available combustible material “Cold” fire: trees survive, nutrients released, seeds in soil break dormancy “Hot” fire: living vegetation including trees are killed; longer ecosystem recovery time; related to build-up of brush and other fire suppression strategies

Effect of temperature on plant development Thermoperiod – temperature alternation between day and night related to developmental events: Tropical plants ~3 C Temperate plant 5 – 10 C -germination -vegetative development -flowering -fruit and seed development -senescence (death) & dormancy

Characteristics of Leaf Senescence ↓growth and metabolism ↑ABA, ethylene ↓chlorophyll (carotenoids ‘appear’) ↑respiration ↑anthocyanins ↑nutrient recovery and transport to mother plant ↑leaf abscission