Problems with Touriga Nacional Fruit Set? Poor Pollen Quality May Be To Blame: A Study on Pollen Quality in Vitis Vinifera Grapes

Pollen quality is a crucial factor in plant reproducibility, as without high quality pollen, failed fertilization often occurs, resulting in a failure to reproduce and in some cases, significant negative effects on the animals and insects that rely on said pollen as a primary food source.  With the case of the domesticated Vitis vinifera grapevine, the plant is hermaphroditic, meaning that it produces both male and female flowers on the same plant, allowing it to self-pollinate and not rely on pollination by bees or other insects (though note: wild grapes DO get

Pollination of wild grapes (NOT Vitis vinifera). Photo courtesy Flickr user Melissa Bridgman

pollinated by bees). So, while pollen quality in Vitis vinifera grapevines isn’t a concern to the bees, it is still very much a concern for the plant itself, since bad pollen likely won’t be able to be fertilized, germinate, and eventually produce the grapes.

There are many factors that can influence pollen quality, one of which is inbreeding, which can lead to a reduction in pollen viability (i.e. viable pollen = can reproduce.  Inviable pollen = can’t reproduce).  Once a pollen grain lands on a stigma, the grain must be able to germinate and grow a pollen tube toward an ovule. This pollen tube allows the sperm from the pollen grain to travel down to a waiting egg inside the ovule of the female plant. This entire process may break down at various stages, from the formation of the pollen in the male flower, to pollen death prior to pollen tube growth, to problems or failures with pollen tube elongation once on the female plant.  Poor quality pollen can often lead to these types of breakdowns, resulting in failed reproduction in the plant – or in grapespeak, resulting in poor fruit set.

Understanding pollen quality in a plant like Vitis vinifera is important not only from the physiological standpoint of the plant, but also an economic one as well, since a plant with poor pollen quality isn’t likely to produce the fruit set winemakers need for producing top quality wines. Pollen quality can also be studied to determine specific pollen grains that can be used for controlled pollination, as well as clonal selection, breeding trials, and in evaluating different viticultural techniques that might improve fruit set in the domesticated grapevine.

A new study, available online now and to be published in January 2018 in the journal Scientia Horticulturae, aimed to evaluate the pollen quality of 15 Vitis vinifera cultivars, to gain a better understanding of pollen quality in the domesticated grapevine.

Brief Methods

The pollen used in this study originated from a collection at the Instituto Nacional de Investigação Agrária e Veterinária in Portugal.

15 grape cultivars were studied, including:

White cultivars: Fernão Pires, Loureiro, Colombard, Sauvignon Blanc, Sémillon, and Ugni Blanc.

Red cultivars: Castelão, Touriga Franca, Touriga Nacional, Cárménère, Cabernet Franc, Cabernet Sauvignon, Malbec, Merlot, and Petit Verdot.

The following characteristics were analyzed for the pollen of the aforementioned cultivars: aperature count (i.e. the point of germination), grain count, viability, germination rate.

Selected Results

  • Pollen aperatures:
    • All 15 cultivars had at least 95% of their pollen as possessing 3 aperatures, while 100% of Touriga Nacional’s had 3 aperatures.
    • 2% of Cabernet Sauvignon and Loureiro’s pollen had only one aperature.
    • Loureiro had the highest percentage of pollen grains with 4 aperatures (3.8%).
    • Merlot had the highest percentage of pollen grains with 2 aperatures (4.4%).
  • Pollen viability/germination:
    • Pollen viability was significantly different between cultivars.
    • The lowest viability rate was Touriga Nacional with 19.3%.
    • The highest viability rate was Colombard with 99.3%.
    • The remaining cultivars had viability rates of at least 50%, with 8 of them being over 75% viable.
    • Pollen germination rates were significantly different between cultivars.
    • The lowest germination rates were found in Touriga Nacional (less than 7%), Cabernet Franc (less than 4%), and Cabernet Sauvignon (less than 12%).
    • The highest germination rates were found in Castelão, Loureiro, Malbec, and Petit Verdot (all greater than 40%).

Conclusions

Overall, this study simply compared some pollen quality characteristics of 15 different Vitis vinifera cultivars, in order to gain a better understanding of this aspect of the domesticated grapevine’s reproductive cycle and thus ultimately fruit set potential.

It has been reported that the cultivars Castelão, Petit Verdot, Touriga Franca, and Touriga Nacional have had problems with fruit set, so it was particularly interesting to see the results from those specific cultivars compared to the rest.  Interestingly, the only cultivar in this “problem” list that actually showed poor pollen quality by way of pollen viability and germination was the Touriga Nacional variety.  It is very possible that the very low pollen viability (19.3%) and low germination rates (less than 7%) strongly influence the poor fruit set that often plagues this cultivar.  When only 19.3% of the pollen the plant actually has the potential to fertilize and become a grape, and when less than 7% of the pollen actually germinates, it spells bad new bears for the cultivar having a wildly successful fruit set.

As for the other cultivars that have been reported to have problems with fruit set, there may be other mechanisms involved, as pollen viability and germination rates were at least 40% in these cultivars.  Touriga Franca did have the second worst pollen viability of the group at just over 40% (and germination wasn’t too hot), though pollen viability for Castelão and Petit Verdot were both over 80%, so certainly other mechanisms are involved for fruit set problems with those cultivars.

These results show that there are significant differences in pollen quality between Vitis vinifera grape cultivars, which could be potentially useful in breeding programs, and clonal

Photo courtesy Pixabay

selection in the future (or at the very least provide some answers as to why a grower might be having problems with a particular cultivar).

An aside: Even though this was a short and sweet study, I was still super stoked about reading it since it brought me back to my graduate school days, since pollen quality played a starring role in my research and Master’s thesis.  For anyone interested, I’ve included a couple of links to some of my past pollen quality research below in the “sources” section (note:  I didn’t study pollen quality related to the grapevine, but thought I’d use the opportunity to share my work since we’re talking pollen.).

Sources:

Pereira, M.R., Ribeiro, H., Cunha, M., and Abreu, I. (2018) Comparison of pollen quality in Vitis vinifera L. cultivars. Scientia Horticulturae 227(3): 112-116.

Yeamans, R. L. (2011) Ecological and evolutionary shifts in pollen chemistry and their implications for pollinators. 

Yeamans, R.L., Roulston, T.H., and Carr, D.E. (2014) Pollen quality for pollinators tracks pollen quality for plants in Mimulus guttatus. Ecosphere 5(7): 1-8.

 

4 comments for “Problems with Touriga Nacional Fruit Set? Poor Pollen Quality May Be To Blame: A Study on Pollen Quality in Vitis Vinifera Grapes

  1. Peter Cousins
    November 30, 2017 at 8:13 am

    Very interesting post! Thanks the alert on the upcoming article! Note that the image of the bee pollinating flowers does not show Vitis (grapevine); it’s probably an Ampelopsis species.

    • Becca
      November 30, 2017 at 8:15 am

      Thanks Peter! You’re correct–the image is not of Vitis—when I found it, it said it was the wild grapevine, not Vitis vinifera. I indicated it as such in the photo caption. I thought it’d be fun to include the image anyway, as it was a beautiful shot 🙂 Thanks for reading! Cheers!

  2. jean houle
    November 30, 2017 at 1:33 pm

    you will find a composite image of Vitis vinifera & Apis mellifera here

    http://www.florabeilles.org/serie/vitis-vinifera-subsp-vinifera_apis-mellifera-0

    • Becca
      November 30, 2017 at 2:51 pm

      Beautiful shots! Thank you for sharing!

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