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

Temperature Relations of Plants

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

Leaf Energy Budget

Qabs = Qrad + Qconv + Qtrans

Abs = energy absorbedRad = energy lost by radiationConv = energy lost by convectionTrans = 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

Q10 = rate at temperature ‘T’ + 10 C/ rate at temperature ‘T’If <2, then physical limitation; 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 ‘Tm’ (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” 1 - 28 C, 2h

2 - 45 C, 2h3 - 40 C 15’45 C, 2h4 - 40 C 30’45 C, 2h5 - 40 C 1 h45 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 CTemperate 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