Monday, May 21, 2007

Breeding new plants

Last week I discussed the way that many new cultivars originate from eagle-eyed gardeners and naturalists who notice variations in plant growth. This accounts for many new introductions, but among the more intensively selected garden plans a completely different way of raising new varieties is followed.
They are deliberately cross-pollinated to give raise to new forms.
The first humans probably undertook this process unwittingly as they slowly selected grains and vegetables for their improved traits – disease resistance, improved flavour, greater yield, and ease of cultivation.
Although the genetic basis of the improvements was not guessed at until one hundred years ago, gardeners soon realised that if two similar plants were grown alongside each other they would sometimes cross, and the resultant plants would sometimes (not always by any means!) be an improvement on both parents.
Today we know the general rules that apply in the process, having an understanding of the roles of dominant and recessive genes play in the recombination of DNA between the two parents. This has enabled professional plant breeders to be able to predict the results of their hybridisation with greater certainty.
Plant breeding has become a major industry in some parts of the world. We have all noticed the wide array of varieties of both flowers and vegetables on the seed market – these varieties are the results of years of work by breeders, mainly in the United States, Japan and Europe.
Think of the humble petunia. The varieties that were top of the pops twenty years ago – the Magic and Satin series – have long gone and have been replaced by a bewildering range of different types. These new varieties have improved resistance to mildew, they are more weather resistant and they flower sooner and longer. Bedding marigolds have undergone a similar transformation and modern Impatiens bear no resemblance to the taller-growing older types, and have a much wider colour range.
Similar changes have happened among vegetables, although not all gardeners will agree that the changes have been for the better. The introduction of F1 hybrids with their improved vigour and increased uniformity has suited commercial growers very well, but the increased price of seed and the uniformity of growing times have not always pleased home gardeners, who prefer to have a staggered harvest for their crop.
Sometimes gardeners think the old vegetable varieties tasted better too. I often hear how old tomato strains were much tastier that the varieties we purchase in the supermarket, but I am not convinced. I have grown a number of heritage forms over the years and I have never found one that tasted as nice as the modern F1 hybrids I grow. I accept that glasshouse varieties are bred to have thicker skins, and probably don’t have such a strong taste, but the modern varieties I grow outside have better and earlier crops, they are more disease resistant, and they taste better than the old ones I have tries.
The presence of professional breeders in the market makes it harder for the amateur to raise noteworthy varieties. I correspond with iris breeders in the Unites States, a few of whom make their living introducing new varieties. Professionals generally carry out rose breeding too – they raise literally thousands of seedlings to select each one they release onto the market. The amateur probably has to work among less popular varieties, or to try and beat the professionals by making the sorts of wild crosses they would not consider- maybe crossing a dwarf rose with a rampant climber, to see what would happen.
The process of cross-pollinating is the same for all plants. For the Pacific Coast irises I breed, I remove the male sexual organs form the flower I want to use as the father, having first determined that the pollen is shedding. I then remove the petals from the flower I have selected to be the mother, the pod parent. I use a paintbrush to apply the pollen onto the style arms that lead to the female sexual organs.
Rose breeders work in a similar manner. The stamen are removed from the male parent, and stored for a day or so to allow the pollen to ripen and drop. A female flower is prepared by the removal of all the petals and the stamen, and then the pollen from the male is brushed over the female parts.
This technique will work for most plants that have large flowers – camellias, rhododendrons, orchids, cacti even, all are bred in this manner, and I have crossed some of my garden pinks using this method. It might been be fun to cross some of those old heritage tomatoes with modern hybrids to see what happened.
I have been crossing my red Lapageria with a white flowered form, using an identical technique. It is too early to tell how well it is going to work but I do have some white flowers flushed with pink so I am hopeful that it will prove successful.
Sometimes, though, the plants perform better if the two varieties selected for crossing are simplyallowed to grow alongside each other. Dahlias, for example, are difficult o hand pollinate and are grown in this manner. I suspect that many new hebe and manuka varieties are also bred this way.
Be prepared for some disappointments if you are thinking of taking up plant breeding. Not all of your wonderful crosses will result in seeds – sometimes that plants are too distantly related, sometimes the weather is not right, and sometimes they are just stubborn - and then, once the seed is harvested, not all will germinate. As if that was not enough to deter you, the majority of your plants will show no improvement on their parents and will need to be discarded.
But, and it is a huge but, when you do find something new it is worth all the trouble. Proud parents have nothing on the breeder of a successful new dahlia, rose, orchid or Pacific Coast iris.

No comments: