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