In many plant species, including grape cultivars, autogamy (i.e. self-fertilization) is all that is necessary to successfully reproduce. However, some cultivars require cross-pollination in order to reproduce and reach maturity. The level of successful pollinations that occur will determine the grape yield, thereby understanding pollination dynamics of the grapevine is very important.
Depending upon the species and cultivar, grapevines may be pollinated by wind or by insects. One study in particular found that the presence of honeybee hives did not increase the number of fruit, but it did decrease the number of seedless grapes, thereby indicating that even though the grape yield was not increased by the presence of bees, the number of viable fruits had increased due to more efficient pollination by the bees.
To date, most studies involving pollination dynamics of grapevines examined finding links between annual pollen concentrations and grape yield, and how different environmental factors affect pollen dispersal. According to the authors of the current study, there has been a resurgence in the study of grapevine pollination dynamics (Vitis vinifera, specifically), due to the increased presence of transgenic grapevines and the need to justify possible cross-pollination of these cultivars with other cultivars in the same area.
The goal of the study presented today was to further add to the scientific knowledge regarding pollination dynamics of Vitis vinifera, specifically anemophilous pollination (wind pollination), by examining airborne pollen concentrations.
The study vineyard was located in Zafra, in Badaioz in the southwest of Spain. Vines were planted in rows 3 meters apart with 1.5 meters between each vine. The main cultivar in the vineyard was Cabernet Sauvignon, though one small plot also contained Alphonse Lavallée, Cinsaut, Garnacha, Garnacha Tintorera, Mazuelo, Merlot, Morisca Real, Syrah, and Tempranillo. All cultivars were grafted onto American rootstock.
Grapevine pollen concentrations were measured for the entire day, and also for each hour. The average concentration per hour was calculated. Hourly meteorological data was also measured.
Pollen grain dispersal was measured during sunlight hours of 1600 and 1830. In order to take into account possible outside environmental influences, pollen concentrations of nearby species that were not grapevines were measured. The most abundant plant nearby the vineyard was the olive tree, which was found no closer than 500m from the vineyard.
In order to measure the influence of height on the dispersal of pollen grains, samples were taken at 4 different heights in the center of the Cabernet Sauvignon vineyard and between the vineyard rows. Pollen concentrations (both grapevine and olive tree) were measured at 0.5, 1m, and 2m above the ground. This sampling took place on May 30, 2001. For measuring horizontal dispersion of pollen, three locations 1.5m, 3m, and 4.5m from the vine were selected on the edges of the Cabernet Sauvignon vineyard. 14 samples were taken for 30 minutes on May 29th and 30th (6 samples at 1.5m, 4 samples at 3m, and 4 samples at 4.5m). Mean concentrations of pollen (both grapevine and olive tree) were measured and calculated during these samplings.
To determine the effects of distance on pollen concentration in the vineyard, pollen concentrations of all cultivars were measured on May 22nd when 4 of the cultivars had more than 10% inflorescences with some open flowers. Pollen concentration was measured by sampling each cultivar for 30 minutes.
- Max daily concentrations of grapevine pollen occurred between May 26th and June 1st.
- The pollination period for Cabernet Sauvignon was only 7 days.
- During the max pollination period for Cab Sauvignon, the mean concentration of pollen was 62.7grains/m3 with a high concentration of 87.5grains/m3 noted on May 29th.
o These concentrations are low for wind pollinated cultivars.
- Daily mean temperatures ranged from 15.1 to 28.5oC; relative humidity ranged from 29.3 to 79.4%; daily wind speed ranged from 2.8 to 10.4km/hr; wind direction ranged from 60o to 253o ENE to SWS; and no rain was recorded during the sampling period.
- Relative humidity significantly affected pollen negatively
o i.e. Higher relative humidity = lower pollen concentrations.
- Wind speed and wind direction had no significant effects on pollen concentrations.
o Hourly and daily meteorological data affected pollen concentrations in the exact same manner.
- Pollination levels of Cab Sauvignon were at a minimum between 2300 and 0700 hours, and at a maximum during midday.
- Concentrations of olive tree pollen were greatest up to 0.5m aboveground and gradually decreased with increasing height aboveground.
o This is possibly due to reduced wind speed at locations closer to the ground.
- Concentrations of Cab Sauvignon pollen were 200grains/m3 at 1m aboveground.
o At 2m, there was a 10x reduction in pollen concentrations.
§ This result demonstrates the limited vertical dispersion of Cabernet Sauvignon pollen.
- Olive tree pollen concentrations remained steady, but pollen concentrations of Cabernet Sauvignon and other vinifera cultivars decreased beyond 1.5m after the end of the row.
The results of this study indicate that the dispersal capability of Vitis vinifera is relatively low, particularly for a wind-pollinated (anemophilous) plant. This result is not surprising, considering the fact that Vitis vinifera species still retain some entomophilous (insect pollinated) characteristics from their ancestors, such as the production of sterile pollen by female flowers and the production of scented compounds which under true entomophilous species would attract insect pollinators. The fact that Vitis vinifera possesses characteristics of both insect and wind pollinated plants indicates that this is a species in transition, with human selection driving the species toward a fully anemophilous state.
Finally, due to the pollination of this species only being effective at a short distance from the pollen source, if pollination were to take place between cultivars, it would need to occur between plants within very close proximity to each other. Since wind speed and wind direction do not affect pollen concentrations, having a vineyard with transgenic vines neighboring a non-transgenic site would likely not result in the undesired cross-pollination between the two cultivars.
I’d love to hear what you all think about this topic! We rarely see studies that are so heavily based in Ecology, so it’d be great to hear from you plant and insect lovers out there! Please feel free to comment below (html tags will be deleted, so please do not post them).
Source: Muñoz-Rodríguez, A.F., Tormo, R., and Silva, M.I. 2011. Pollination Dynamics in Vitis vinifera L. American Journal of Enology and Viticulture 62(1): 113-117.
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