By Peter KevanApples, pears, and some cherries need cross-pollination to set a

crop. How does the pollen move from tree to tree so that cross-pollination takes place? Onthe bodies of insects, especially bees. For orchard pollination, honey bees make excellentpollinators. Managed orchard bees can be used but the technology is not well developed.Wild bees, including bumble bees, can be important.

Honeybees are excellent pollinators of apples. If they are excluded from the flowers, aswith bags (see Figure below), few flowers develop to give fruit. The few fruit that do devel-op may arise because of experimental contamination.

The issue is not so simple as to be solved by just adding bees to the orchard. For applesand pears, cross-pollination must take place between cultivars. For efficient orchard harvesting, block plantings would seem most practical, but for pollination as a crucialprocess, the size of blocks becomes a real issue. How far can pollen move on the bodies ofpollinating insects?

Research results taken from commercially run orchards with high density trees grown ondwarfing rootstock show that pollen does not move far! How can one give that answer? Itis next to impossible to determine how far a bee flies and how many different trees it visitson a foraging trip. Even if one could do that, the problem of how much cross-pollen the beemoves from tree to tree remains. Our team at the University of Guelph took a geneticapproach to solving the problem.

Flowers of apple bagged to prevent pollinators visiting and pollinating them. These flowers could then be used for various controlled pollinations by hand to investigate howimportant pollination and different pollinizers are in apple production.

First, the genetic fingerprints of the main cultivars were determined. Each of the studycultivars had distinctive genotypes. With that information, it is possible to determine thepaternity of the embryos in the apple seeds. Working within multiple rows of apple culti-vars, and divided rows with two cultivars, it was possible to determine how far away fromthe mother trees the nearest potential father (pollinizer) tree was. In other words, knowingthe genotype of the mother trees in the rows (producing apples of a given cultivar), thepaternity of the seeds, and the distance to the nearest potential father (pollinizer) tree (alsoa production cultivar), the paternity tests revealed how far cross-pollen could move.

So, how far does pollen move in an apple orchard? Only about four to five trees!To place those findings into the context of a map of an orchard means that for pollina-

tion to function at its best, cultivars need to be in close proximity to each other. Given therecommended tree densities for typical high-density orchards, most pollen travels onlyabout 20 meters, indicating that four or, at most, five rows of a given cultivar should begrown together to allow cross-pollination to take place for proper pollination. The rows atthe edges of orchards should be only two, or at most three trees wide of a single cultivar.

Without pollinizerinterplantings withinrows, or pollinizergrafting onto treeswithin rows, cross-pol-lination takes placeonly from a few treesaway on either side ofthe rows of any givencultivar. The resultsindicate that pollinizerinterplantings shouldbe such that every pro-duction cultivar tree iswithin two or threetrees of the pollinizer.In a cultivar production

block orchard (i.e. an orchard without differing groups of rows, but specializing in one cul-tivar) that means that within rows, pollinizers should be planted about every 8th to 12thtree, but about four - six trees out-of-step between rows. The exact geometry must be cal-culated depending on planting density, row spacing, and production vs. pollinizer cultivar.

Not all pollinizers are equal! How to choose a pollinizer? The pollinizer must bloomsynchronously, and produce viable pollen that is compatible, with the production trees it isthere to service. Some orchards use crab apples as pollinizers, other use grafts of other cul-tivars or crabs on the production cultivar. Research is needed on the strategies that best fitthe various ways of growing pome fruit and planning orchard design. Just from our limitedstudies on the interactions between pollen and fruit production, the results presented sur-

prises. The pollen of some cultivars is better at fertilizing flowers than isthat of others. Fuji and Granny Smith produce “stud” pollen. We dis-covered this by hand pollinating flowers with known mixtures of pollenthat we had prepared. Moreover, some cultivars, such as Mutsu and Jon-agold, produce little or no viable pollen and are in essence eunuchs, use-less as pollinizers.

Pollen from Fuji and Granny Smith were much more virile in fertilizingflowers of Empire, Golden Delicious, and McIntosh than were pollenfrom Ida Red, Northern Spy, and Vista Bella. The reasons for the differences are not understood, but “stud” pollinizers should be used incombination with production cultivars. Tests to determine which is thebest “stud” for each production cultivar are needed because it cannotbe assumed that the stud for one is also a stud for another.

Even given the fact that some cultivars produce “stud” pollen, anoth-er surprise was in store. Multiple paternity helps apple set and quality.As part of studies on the effect of pollinizer pollen, we found that ifpollen from several pollinizers reached the stigmas of the productioncultivar, then the seediness of the apples and the size of the applesincreased.

The combination of pollinizer proximity, pollinizer diversity, and pol-lination translates into greater production.

Continued on next page

THE GROWER

PAGE 20 –– APRIL 2010

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Using pollination research to boost orchard yields

A solid block of any given cultivar is not served by cross-pollination and so production is low. Cross-pollination andfruit set takes place where different rows of cultivars areneighbours, but if there is a diverse mix of rows of cultivars,then fruit set is even higher and apple quality that much better.

Pollen movement and diversityis important to pome fruit produc-tion. Apple studies exemplify thecomplexity of the issues and howthey are relevant to orcharddesign, management, and upkeep.But what of the honeybees andtheir activities.

The general recommendationsfor deploying honeybees in appleorchards mostly pertain to standard trees. Studies on highdensity and trellis orchards need tobe done with the considerationspresented above in mind. Not allcultivars flower with the sameintensities, and it may be that pol-linator management for cultivarsthat produce many, many flowersis different from cultivars withsparser flowering habits.

The structures of the flowersalso influence bee behaviour.Flowers of cultivars such as Jon-agold have the filaments of the sta-mens as a tight palisade around thestyle, Granny Smith and Mutsuhave loosely arranged filaments.When bees visit the flowers to for-age for nectar, they must probebetween the styles and the fila-ments in cultivars such as Jon-agold, but can reach in betweenthe filaments in Granny Smith.Hence, on cultivars with looselyarranged filaments, the bees canstand on the petals and “side-work” the flowers without pickingup pollen on their bodies, or touch-ing the stigmas. On cultivars withtight palisades of filaments, thebees must work from above andbecome dusted with pollen andtouch the stigmas. How these sub-tle differences in floral form influ-ence the deployment and numbersof bees need for pollination has yetto be investigated.

Although bee behaviour onflowers in important, it turns outfrom research done on Nova Scot-ian apples, that not every stigmaneeds to be pollinated for an appleflower to be fully pollinated.From experiments in which stig-mas were systematically removed,it was shown that as long as two ofthe five stigmas received pollen,the apples that were producedwere fully seedy and top quality.

It is expected that those find-

ings apply to all apple cultivars and is good news becausethey indicate that some damage to apple flowers (as from amild frost) would not adversely affect yields. Botanically,the findings were unexpected because conventional ideaswere that each stigma and style was connected to a particu-lar locule in the developing fruit, whereby each stigmawould have to receive pollen for full development of anapple. Anatomical research showed that the apple flower hasa zone, called the compitum, in the area where the stylesunite where pollen tubes can cross over from one section ofthe flower to another.

One would think that there has been so much researchdone on apple production that there is little new to learn.Canadian research has shown the way to considerations onhow orchards should be planned and planted to maximize themovement of pollen for cross-pollination, shown that not allcultivars have equal capacities for cross-pollination, thatsome have “wimp” pollen and others have “stud” pollen, thatcultivar (or at least pollinizer) diversity has beneficial effectsthrough increasing multiple paternity in the seeds of the fruit,seediness translates to higher quality apples and greateryields, and that how the genes from pollen move within theflower influences seed-set and fruit quality. These ideashave excited interest abroad where apple producers areapplying our findings.

The research describedin this arcticle was done inOntario by B. Husband, P. Kevan,P. Kron, and S. Belaoussoff, and inNova Scotia by C. Sheffield, R. F.Smith, S. M. Westby and P. Kevan. The team at CANPOLIN(the Canadian Pollination Initiative www.uoguelph.ca/can-polin), through its five year, $5 million grant from NSERCis ready to direct its research on pome fruits to broadeningthe base of the findings described above to orchard produc-tion in Canada.

THE GROWER

APRIL 2010 –– PAGE 21

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Using pollination research