My experience is in a greenhouse with stone fruits that ripened from April until September. So naturally I wanted high brix on all of those. And the trees were planted at densities from ~15 to 32 sqft per tree. The trees were very close together. Thus, there was no watering one tree different from another. They all got watered the same all year long. There was no increasing water deficit during the last three weeks before ripening.
What I don’t know is how severe the water deficit was in the trees before or during ripening. I think at least at times pretty severe. At the worst lower leaves started to drop and new growth nearly ceased. IME peaches that stop growing in summer are pretty dry, really dry. Leaf drop is clearly too dry. I aimed to slow growth down, partly to reduce tree size at those high planting densities but also to increase brix.
Water application averaged about 3 inches per month in summer. My guess is that’s about 1/2 of fully watered. The greenhouse environment is more mild than outside. I’d guess that water use inside my greenhouse is similar to water use outside in New York in summer. The greenhouse is a more mild environment than outside here which is pretty brutal, dry and sunny.
Results were mid to high 20s brix in stone fruits from May until September. Fruit size was large.
In cases where water application was excessive like by accident or ignorance before getting dialed in, brix was in the 15-18 range on nectarines.
My take is that early season water deficit, if not too severe, doesn’t decrease brix or fruit size by very much.
I also think that excess water isn’t just a dilution factor. I think it’s more than that. A well rooted tree like a nectarine does adjust to increasing water deficit. It has to or it could no longer pull water from an increasingly dry soil. So the plant dries out it’s tissues to maintain a water gradient into the tree. That dries out the fruit. Plant physiologist measure plant leaf water potential to determine how dry the tree is. That’s what I’d need to know how dry my trees are during the growing season. To measure leaf water potential a leaf is placed in a pressure container with only the leaf petiole exposed. Then pressure in the container is increased until water is expelled from the petiole. That pressure is a measure of how dry the tree is at that point.
Good fruit grower has some recent info about pressure chambers and their use in irrigation management. In fact on their current front page, they have a video about a handheld pressure chamber that you pump up by hand. I’ll try to link to that video.
The hand held one is still $1400-2000: Low-tech device can help measure water stress in orchards — Video - Good Fruit Grower
This except from the above says negative 16-20 bars is the right level of water stress to reduce growth and increase fruit brix.
“A reading of negative 16 to negative 20 bars of stem water potential in prune means moderate to high stress with shoot growth slowing or stopping, but also a boost in fruit sugar levels.”
I really think commercial growers are behind us on increasing fruit quality via water management. But they do have other highly legitimate concerns about high water stress like winter hardiness, return bloom, yield, and fruit size.
The permanent wilting point of soil is usually considered to be about -15 bars. That how dry the soil is when most plants wilt. The plant needs to be drier to continue to remove water from the soil. So minus 16 to 20 bars seems right. Some plants, like desert plants, can dry the soil more than that.