Plant Environment: Temperature

1. Plant Environment: Temperature • Biological activities of most plants occur within a range of temperatures • Temperature • 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 • Plant growth rate • Maturation • Temperature influences most plant activities: • Fruit ripening • Seed germination • Crop quality

3. 0 15 30 50 temperature Plant Environment: Temperature • Plant growth results from enzymatic reactions and is influenced by temperature • Temperature and plant growth Plant growth rate • Optimum temperature for most plants 20-30oC (68-86oF); grower can adjust growth rate by adjusting temperature

4. Plant Environment: Temperature • daytime and nighttime temperature influence growth rates of plants • Temperature and plant growth • 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 • time to harvest also influenced by temperature • Growers calculate degree days to schedule harvests • Temperature and plant maturation • Degreesdays = 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 • Sugar concentration in crops influenced by temperature • Temperature and quality of crops • 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 • Influences seed germination, root growth, water uptake • Soil temperature • 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 • Light energy that passes through greenhouse converted to heat and then trapped • Cooling greenhouses • 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 • Low temperature effects • Chillinginjury: plants damaged by low temperature but ice crystals did not form • Temperature stress • Tropical plants particularly sensitive to cool temperatures • Symptoms include: lesions, discoloration, defoliation, wilting, poor keeping quality (in fruit like bananas) • Freezinginjury: 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 • Low temperature effects • Freezinginjury: • Temperature stress Sunscald Frost crack

11. Plant Environment: Temperature • Winter hardiness • Winter hardiness refers to plants ability to tolerate or avoid freezing damage • Temperature stress • 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 • Winter hardiness • Winter hardiness of species determines where plants can grow • Temperature stress • 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 • High-temperature effects • Southwest trunk of thin-barked trees such as apples can overheat and kill vascular cambium; resembles sunscald • Temperature stress • 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 • Hardening off plants • Actively growing plant more susceptible to temperature stress than dormant plants • Temperature stress • 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