Root mass different between varieties

[Mike Edmonson]

Was transplanting trees that I grafted on eml111 rootstock in the spring noticed there is a marked difference between varieties root mass.  If I compared say Kingston black with Harrison, Harrison root mass was 2X the size of Kingston.  Does anybody know the science behind this. Does the scion tell the rootstock to grow bigger and supply more nutrients to the scion or is does the rootstock just respond to a more vigorously scion.  If the later, maybe it would make sense to graft two scion on the rootstock, a vigorous to get the rootmass and something like a Kingston, the apple you really want.  And maybe I have had too much cider this evening.

[CiderSupply.com]

Much is still a complete mystery with respect to just fruit trees on their own rootstock with no graft. Yes and no is the answer. We have hundreds cider cultivars grafted onto a full selection of rootstocks. And i must say that still it is almost impossible to draw any meaningful relationships, or cause and effects other than very basic above ground results. Much of what goes on underground is even more a mystery.

Once one starts swapping around cultivars and rootstocks, the variables that confound test results become a three dimensional array of problems that require control and measure.

What you ask is totally valid. Both the rootstock and scion communicate chemically. In the summer and fall the scion and its leaf mass drive rootgrowth, and in late winter and spring the balancing act switches where the rootstock drives the leaf-out. This is a very simplified look at the agreement, but can be helpful for pruning.

Provided both graft unions have an identical take, are grafted at the same location from the soil lines, both rootstocks were taken from the same mother at the same location, the soils are identical, an over vigerious rootstock on a slow growing scion typically will overgrow the graft union in a big ugly ball and send up suckers all over the place. A vigerious scion on a vigerious rootstock can often result in too much leaf and wood growth with very delayed fruiting and/or reduced fruiting.

And yet when science seems to get close to finding out how all of this really works, a vew varyable is found that brings the results back into question.

For all of us the cider apple growers, finding if a certain combination of rootstocks with one or more interstocks stacked ontop of one another can control biannualism would be the discovery on the Millennium.

It all starts with questions like yours.

Best regards
Chris Rylands

[Mike Edmonson]

Rule of thumb questions:

If you are trying to get as large of root mass as possible, prune in the spring and not in the summer since summer pruning will limit root mass.

My neighbors prematurely shake their pecan trees to reduce the nut load to avoid biannualism.  The explanation they give is a high nut load stresses the tree too much so the tree rests the next year.  I experienced it this year on my apple trees and assumed it was because of the heavy load of apples the year before.  Is that the real story?  If so, wouldn't a tree with a large root mass and a smaller top experience less biannualism since the root mass can supply the tree with more mystery stuff.  I use the word mystery since Sunday afternoons I like taking a nap in lawn chair in the orchard along side some bee hives and for some reason it feels very fulfilling.  My suspicion is more is going on between the earth and trees than just chemical transfer.     

[Cornelius Traas]

Hi Mike,

Just regarding the biennial bearing and heavy crops, apart from varietal differences, the following can make a difference (assuming no other interventions such as thinning):

Lack of summer moisture – increases it

More N fertiliser – decreases it

It appears to not only be related to the amount of “effort” the tree needs to put into the current years crop, while simultaneously creating the flower buds for the following year. There is good evidence that plant hormones play a big role, and for instance, a twenty ton per acre crop in a year when pollination is not so good (perhaps 2 or 3 seeds in each apple) as opposed to the same crop in a year when pollination is better (5 or 6 seeds per apple), will lead to less biennialism when there are less seeds in the fruits on the tree, as each seed is an active producer of hormones (which find their way into the mother tree), especially gibberellins, which can reduce return bloom.

This also explains the success of gibberellin inhibitors (such as prohexadione calcium) in increasing return bloom, and subsequent crop yields.

Of course there is still more going on there too, and to get back to Chris’s point, I have seen more combinations of varieties on more precosious rootstocks (such as MM106) produce apples in years when combinations on less precosious rootstocks (and indeed rootstock/interstem/variety combinations) did not, and again the science is there to back that up. Unfortunately though, published interstock/interstem research is limited, which is a pity, as I feel it is an area that “could yield fruitfully”.

 

Cornelius Traas

The Apple Farm, Cahir, Ireland. www.theapplefarm.com