Seed remnants in seedless grapes

We grow Vanessa, a seedless table grape. Some years, this plant produces berries with huge seed remnants. Other years, the same plant produces practically seedless grapes. Does anyone know what causes this?

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I have noticed some varieties of seedless fruit does this. Cara Cara oranges and the seedless mandarins from the store are supposed to be seedless as well. Of course that is not always the case with those either. I read that with citrus cross pollination can cause seeds or more seeds. Maybe it is the same with grapes.

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Some seedless grapes will form larger seed remnants or even fully hard seeds in response to hih heat, drought or other stress. I have a Canadice and a Glenora that both had some hard seeds but they spent most of the summer in a container before I got them planted which is stressful plus it was a very hot summer here. I expect them to be seedless now that they are in the ground.

Seedless mandarins, at least the Clemantines are only seedless if they are not pollinated by another citrus so in California the Clemantine groves need to be completely isolated from all other citrus or they will have seeds.

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I had forgotten about this posting, until now. This is an excerpt from the following site which seems to explain what I’m observing with my Vanessa: years with a prolonged, cold spring tend to lead to more noticeable seed remnants due to the seeds having more time to develop.

"Seed trace development can also be variable
within varieties; for example, in ‘Flame Seedless’ they
are detectable in some years’ crops but not in others.
Many factors influence the detectability of seed trac-
es in stenospermocarpic varieties, including the size
of the traces, their degree of development, the tim-
ing of embryo abortion, and the number of fertilized
ovules. Embryo abortion generally occurs during the
early stages of fruit growth in varieties with small seed
traces, while the process may occur later in varieties
with large seed traces…

Unusually cool temperatures during the early stages of
fruit growth are thought to delay embryo abortion and
may increase the number of noticeable seed traces.

Several other factors influence seed trace devel-
opment, including the vine’s rootstock and year.
Vine age may also play a role, as
mature vines (older than 8 years) reportedly produce
berries with fewer seed traces than young vines. "

I didn’t observe this with my Vanessa in 2021, where we had an early spring which was quickly followed by a long, hot summer with drought like conditions. My lawn was burnt to a crisp, and my grape vine suffered but the fruits were very sweet without any seed remnants.

For some seedless grapes, like my Vanessa, it’s normal for it to form fruits after pollination, just like a seeded grape. What makes it seedless, is that the seeds gets aborted before they can completely form. The above article describe this as stenospermocarpy.

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Thanks Daren that information is very useful. It makes sense that my two seedless grapes last year that had fully developed seeds (Glenora) or very large seed traces (Canadice) were in their first year in the ground. Interestingly, my Vanessa, which was also in it’s first yer in the ground hd only very small seed traces. My old (15 years) Lakemont has never had anything but very small seed traces that I remember.