What stressor has the greatest effect on crop yield?
Temperature. Frost/freeze can devastate yields. More than all other stressors combined.
Freezing water kills cells
How do plants mitigate freeze damage?
Antifreeze. Prevent ice from forming in places it would cause damage
Supercooling
Down to -40°C without forming solid ice
Why do plants need dormancy and what initiates acclimation?
Actively growing tissue cannot survive winter temps.
Shortening day lengths and decreasing temperatures
Factors affecting plant dormancy
Late season growth (too much fall fert/water)
Cultivar/rootstock
Managing crop load
Wind machines
3 types of dormancy
Endodormancy - hormones
Ectodormancy - external like temp and light. Environment is not conducive to growth
Paradormancy - apical dominance
Why do you need chill?
What is chill?
You need to accumulate enough chill to break Endodormancy. Once Endodormancy is broken, Ectodormancy will still prevent growth until environmental conditions are good.
Temperatures between 0-12°C
Effects of insufficient chill
Bad fruit
Delayed foliage development
What is DTA?
Differential thermo analysis
Looking at plant temperatures as the tissue freezes to measure the exotherms and see when the plant is dying.
DTA doesn’t work on apples because apple buds have vegetative and reproductive tissue.
How do we measure cold hardiness of apples?
DTA doesn’t work so you cut the stem and see how much the freeze has damaged the xylem and phloem causing electrolytic leakage which causes oxidation
Why is it important to know the cold hardiness of the trees you’re growing?
It informs growers as to what cultivars they should plant in their area given their winter low temps.
What’s a biofix?
The start of growing degree day accumulations. It’s usually a biological event when you start adding up the GDDs.
Example, you’d spray for moths 250 degree days after you catch 10 or more moths over a 2 day period
