Temperature

1
Plant Environment Temperature

• Temperature

Biological activities of most plants occur within
a range of temperatures
Temperature classification
Cool-season plants (e.g. peas apples spruce
trees)
Intermediate-season plants (e.g. tomato peach
Rhododendron)
Warm-season plants (e.g. melons citrus Norfolk
Island Pine)
2
Plant Environment Temperature

• Temperature influences most plant activities

Plant growth rate
Maturation
Fruit ripening
Seed germination
Crop quality
3
Plant Environment Temperature

• Temperature and plant growth

Plant growth results from enzymatic reactions and
is influenced by temperature
Plant growth rate
Optimum temperature for most plants 20-30oC
(68-86oF) grower can adjust growth rate by
adjusting temperature
4
Plant Environment Temperature

• Temperature and plant growth

daytime and nighttime temperature influence
growth rates of plants
Some plants grow taller if day temperature is
greater than night temperature
Shorter compact plants are produced if night
temperature exceeds day temperature
Most plants grow better if temperature changes
during the day than if temperature constant
5
Plant Environment Temperature

• Temperature and plant maturation

time to harvest also influenced by temperature
Growers calculate degree days to schedule harvests
Degrees days average of daily minimum and
maximum temperatures minus base temperature (in
oF)
Base temperature depends on location crop
moisture soil and cultivar
Assume base temperature of 40oF and a given day
with high of 75o and low 45oF. Degree days for
that day 20.
Harvest scheduled when degree days approaches
optimum for the crop (e.g. peas 120-170 after
bloom apples 1400-2800 after bloom)
6
Plant Environment Temperature

• Temperature and quality of crops

Sugar concentration in crops influenced by
temperature
Under warm conditions sugar in plants converted
to starch
Under cool conditions starch is converted to
sugar
Crops harvested under cool temperature sweeter
Brussel sprouts should be planted in summer for
fall harvest
harvest and store sweet corn in cool temperatures
to maintain sweetness (day-old corn not as sweet)
Cooler temperature produce brighter red flowers
and fruits because anthocyanins formed from sugars
7
Plant Environment Temperature

• Soil temperature

Influences seed germination root growth water
uptake
Cool-season plant will not germinate if soil
temperature too cold or too warm (60-70oF ideal)
Cold soil may slow germination and encourage
rotting called pre or post-emergence damping off
Roots resist water uptake in low soil
temperatures plants in cold soils may wilt
because not getting enough water
Cuttings root faster if rooting medium is heated
(70-80oF ideal for many species)
8
Plant Environment Temperature

• Cooling greenhouses

Light energy that passes through greenhouse
converted to heat and then trapped
Cooling greenhouse important even on sunny day in
winter
Passive vents at peak allow hot air to rise (air
convection) and escape greenhouse side vents
allow cool air to enter
Active exhausted with fans often pull air
through wet pad to cool (through evaporation)
outside air
Radiation reduced Spring-Fall by shading with
black clothes or white washing glass greenhouses
9
Plant Environment Temperature

• Temperature stress

Low temperature effects
Chilling injury plants damaged by low
temperature but ice crystals did not form
Tropical plants particularly sensitive to cool
temperatures
Symptoms include lesions discoloration
defoliation wilting poor keeping quality (in
fruit like bananas)
Freezing injury damage caused by freezing of
water inside plant
Common problem for young apple trees (sunscald)
In winter sun can warm southwest side of trunk
thawing tissue after sunset tissue freezes and
causes cell death
Dead bark dries in Spring and peels off exposing
wood
Orchards paint trunks white to reflect winter
sunlight
10
Plant Environment Temperature

• Temperature stress

Low temperature effects
Freezing injury
Sunscald
Frost crack
11
Plant Environment Temperature

• Temperature stress

Winter hardiness
Winter hardiness refers to plants ability to
tolerate or avoid freezing damage
Freeze-tolerant plants allow water to move
outside cells and freeze in extracellular spaces
require gradual exposure to cold (acclimation)
Freeze avoidance achieved by allowing some water
to leave cells which increases solute
concentration in cells requires lower
temperature to cause cellular freezing
Freeze avoidance also achieved by remaining low
to ground so covered by insulating snow
12
Plant Environment Temperature

• Temperature stress

Winter hardiness
Winter hardiness of species determines where
plants can grow
USDA publishes winter hardiness maps based on
average annual minimum temperatures zone 1
coldest zone 11 warmest
Provenance geographic origin of plant
Plants from warmer zones not as cold hardy as
those from colder zones
13
Plant Environment Temperature

• Temperature stress

High-temperature effects
Southwest trunk of thin-barked trees such as
apples can overheat and kill vascular cambium
resembles sunscald
Fruits and leaves can also become sunscalded
producing yellowish areas or dead tissue
Many desert plants (e.g. cacti) have adaptation
to prevent heat stress such as whitish hairs to
reflect sunlight and provide shade to stems
14
Plant Environment Temperature

• Temperature stress

Hardening off plants
Actively growing plant more susceptible to
temperature stress than dormant plants
Plants can become hardened or acclimated to
temperature changes (naturally use photoperiod
and temperature cues)
Important to slowly decrease temperature (and
water) of seedlings before planting outdoors