Ilya11 had a related post on another thread a week or two ago. It got me to thinking, and I've spent hours studying research papers on cold hardiness of apples and plums. Similar papers came up on pears, peaches, and apricots when I searched on google.
So there is an amazing amount of information that might matter. Much of it rather discouraging to me, as in breeding hardier citrus, I need to be able to sort out those in each generation that are a degree or two more hardy.
First the good news. My idea of testing cold hardiness of scion wood works with apples. Good.
Now the bad news. For such a test to work, the trees the cuttings are taken from must be from the same location. And we all know that microclimates can vary even in small areas. Fertility must be good in the ground where every tree was grown. Rootstocks need to be the same. If the cuttings were taken from own-root plants, then root differences may effect twig cuttings.
One study treated freeze damage and cold damage seperately. Freeze damage is damage from sap freezing in the xylem. Cold damage is damage done though the sap in the xylem hasn't frozen. Spring and fall damage is often cold damage, not freeze damage.
I would like studies to be made of cuttings, scionwood-like cuttings, of different varieties and different species of citrus from different locations, tested different times during spring, winter, and fall. I think the resulting data would be quite useful. A little dorm-type refridgerater would be enough to test twigs of several trees at once. I think a couple of days might be enough to test freeze damage at a given time and place.
Testing for tolerance to quick drops in temperature, then the temperature going right back up, like single overnight freezes, will be harder to sort out.