Wednesday 23 October 2019

What's the buzz on pollination?

Busy bees and other pollinators are crucial to fruit crops. Gerry Daly explains why

The pollen of fruit trees is too heavy and sticky to blow on the wind and so it is carried by pollen-feeding insects such as bees. These need good weather, reasonably warm and not too windy, to be able to fly
The pollen of fruit trees is too heavy and sticky to blow on the wind and so it is carried by pollen-feeding insects such as bees. These need good weather, reasonably warm and not too windy, to be able to fly
The kiwifruit-related Kolomikta vine is grown for its leaf colour, not its fruit.
Fans of Monty Don - and who isn't? - will want a copy of Japanese Gardens, the companion to his BBC series documenting his travels through cherry blossom, Zen gardens, as well as the traditional skills of bonsai and ikebana. Published by Two Roads, €49, out now.

Gerry Daly

There has been a lot of attention given recently to the importance of pollinators in the production of food, and this is most obvious in fruit-growing. While there are some seedless grapes and bananas are also seedless, pollination is necessary to grow most fruits.

Pollination generally means cross-pollination, such as trees swapping pollen, and is followed by fertilisation when the seed is formed. Fertilisation of female ovules in the fruit is carried out by the male pollen grains of another flower. But some, such as 'Victoria' plum are self-fertile and do not need other pollen.

Many kinds of fruit trees, as well as others, reject their own pollen and wait for pollen from a suitable tree, apple for apple, for instance. Cross-pollination confers an evolutionary advantage on the plants that employ it. In each batch of hybrids there is the chance that a new and improved variety of hybrid will emerge.

A new kind may have the advantages of vigour, disease resistance, flavour, cold resistance or some other attribute. First-generation hybrids generally have about 10pc more vigour. Of course, this evolutionary process applies to wild plants, but cultivated plants have retained the feature.

Some apple varieties are sterile - they have extra chromosomes that confer vigour and yield, but make them infertile because the chromosomes cannot divide successfully. These varieties are called 'triploid varieties' -'Bramley's Seedling' is one example - they can't be used for pollination.

Like all apple trees, 'Bramley' must have a pollinator, but the pollinator itself must have its own pollinator because 'Bramley' doesn't return the favour. So if a Bramley is grown, there must be at least two other apple varieties within flying distance of pollinating insects. These apple trees could be wild crab-apple or ornamental crab-apple, or other apple trees in neighbouring gardens.

To produce fruit, trees must be healthy and capable of carrying flowers. If the soil is too wet, dry or poor, or if the trees are diseased, it will simply not have enough strength to carry flowers, and no flowers means no fruit. Even if a tree manages a few flowers, it may not have the strength to develop the fruits if they are pollinated.

The pollen of fruit trees is too heavy and sticky to blow on the wind and so it is carried only by pollen-feeding insects such as bees and flies. These need good weather, reasonably warm and not too windy, to be able to fly. An exposed garden doesn't encourage visits by pollinating insects but a sheltered one will.

When the pollen is transferred by the insect, the pollen tube germinates and begins to grow down the style - the central pointed part - of the receiving flower. So, even if pollination and pollen transfer has taken place, fruit setting still has to happen.

Each apple sets about 10 seeds, two seeds in each chamber, and each seed needs its own pollen grain. The developing seeds produce hormones that bring about fruit development. If only one of two seeds set, the fruit doesn't get enough hormone and falls off.

So if there isn't enough pollen or it's not delivered to the flower in sufficient quantities, or it doesn't grow down the pollen tube, not enough ovules will be fertilised and not enough seeds develop to produce the threshold level of hormone. The fruit won't have 'set'.

Frost can damage the fruit blossoms, shrivelling the male and female parts, or stop the pollen tubes growing down the style. Pollination is vitally important but is only part of the story of fruit set and development.

Easy does it

Ballymaloe Cookery School is running a one-day course for the avid gardener. No Dig for Healthy Abundance and Less Weeds, is on May 20th, led by Charles Dowding, the man behind the no-dig system. Price, €170; for more, visit cookingisfun.ie or phone (021) 464 6785.

In the pink

The kiwifruit-related Kolomikta vine is grown for its leaf colour, not its fruit. The new leaves open tinged a purplish-bronze colour and then turn green, some of them developing pink tips - the colouring takes up about a third of the leaf. Some of the leaves bleach out to white, especially if there is strong sunshine. But too much shade tends to reduce colouring. The Kolomikta vine is a climber but not overly.

Look east

Fans of Monty Don - and who isn't? - will want a copy of Japanese Gardens, the companion to his BBC series documenting his travels through cherry blossom, Zen gardens, as well as the traditional skills of bonsai and ikebana. Published by Two Roads, €49, out now.

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