Collections of Genetic Material

To some people, plant breeding may simply mean crossing two flowers in the hope of producing a new cultivar. In some cases, this may in fact give the desired result. Genetic improvement of plants is a key activity of many research institutes and a priority for most of our fruit industries. This is the first in a series of three articles which aim to give growers a better understanding of what is involved in plant breeding programmes.

Introduction
The term 'plant breeding' implies conscious human effort to improve old and to develop new, alternative crops. The beginnings of plant breeding, which relate to the domestication of wild plants and the beginnings of agriculture, date back some 10,000 years. Initially, it was only through selecting the best fruits and seeds, which were then cultivated, that more food could be provided than by merely gathering it from the wild.

Plant breeding in the sense of deliberately creating something new, something that could not be found in natural populations, is a much more recent activity. The fact that plants reproduce sexually was only accepted after the Swedish botanist Linnaeus published his studies in 1760. The work of Darwin, published in his famous book "On the Origin of Species by Means of Natural Selection" (1859), gave strong impetus to plant breeding. In 1865, Mendel made important discoveries by crossing pea varieties which possessed obviously different characteristics. However, Mendel's discoveries were not confirmed immediately and their importance to the genetic improvement of plants was not fully realized until the 1900s.

In this century, plant breeding based on methods of hybridising different cultivars (cultivated varieties) within the same species, together with special methods of selection in subsequent generations, has achieved extraordinary results which have considerably enhanced agricultural production.

The science of plant breeding covers a wide range of different techniques. A previous article (1) explained why acquiring overseas germplasm and cultivars is an essential part of all breeding programmes, and how it potentially gives New Zealand growers a competitive advantage. This article explains what is involved in developing and maintaining a germplasm collection. The second article in this series will describe some of the techniques used by plant breeders in New Zealand to generate breeding populations which have increased ability to produce superior new cultivars.

A third article will discuss some of the methods and selection criteria used in the evaluation and selection of new cultivars. The techniques of molecular or cellular biology (genetic engineering), leading also to improved cultivars, are complementary to traditional plant breeding methods. These newer techniques have the potential to greatly accelerate breeding, and offer exciting new opportunities. These techniques and their potential were the subject of an earlier article on plant improvement (2).

Germplasm Collections
The most important foundation for a breeding programme is an extensive collection of genetically diverse plant material (or germplasm). These collections contain existing and old fashioned fruit cultivars, selections from our breeding programmes, cultivars and breeding selections imported from overseas breeding programmes, and species collected from the wild. This ensures the collections incorporate a wide variety of different genes. They can also contain rare and unique material which could be irreplaceable, as overseas sources may no longer exist.

HortResearch maintains the only comprehensive fruit germplasm collections in New Zealand: approximately 200 introductions of Actinidia (including kiwifruit), 40 cherimoya, 150 citrus, 40 Cyphomandra (including tamarillo), 640 grapes, 50 persimmon, 114 strawberry, 50 hops, 55 macadamia, 500 apple, 80 avocado, 400 stonefruit, 200 pear, 130 berries, 100 pepino and smaller numbers of other fruit species. These collections take up just over 10 ha of HortResearch's Research Orchard Network. Each collection is held in climatically suitable regions, where most of the crops are commercially grown.

The germplasm collections have been identified as a national resource, a safeguard for our future, and will continue to receive government support even if funding for breeding new cultivars fluctuates. Germplasm collections provide a valuable source of agronomic traits that may not be important now but may be highly valuable in the future. Consumer requirements may change, for example, the current requirement for "chemical free" food. Many older cultivars carry the only known resistance to many pests and diseases, and are therefore useful in plant improvement programmes. This flexibility to meet the changes in consumer requirements would not be possible without keeping collections of diverse genetic material.

Acquisition of germplasm is a lengthy, time-consuming and expensive process. It involves locating suitable material, importing it (often requiring at least two years in quarantine) and then establishing the plant material in New Zealand. Seeds must be raised and grown and scionwood budded onto compatible rootstocks. Existing material in the collections must also be continuously maintained as any disruption could lead to permanent loss of valuable and rare or unique plant material. Although most fruit plants are long-lived, they are not everlasting and there has to be a programme of continual renewing and propagation, especially of cultivar collections held on rootstocks.

Preserving fruit crop germplasm is a much more costly and involved process than for crops such as cereals. In cereals, the seeds are bred to come up true-to-type and remain viable for long periods of time, so seed storage is a very cost-effective way of preserving the germplasm. With fruit trees, seeds will not develop to be identical to the parent and therefore the best way of preserving desirable combinations of characteristics is through vegetative propagation. Unfortunately, this is considerably more expensive in terms of both maintenance costs and storage space required.

Quarantine
Quarantine fees alone represent a major cost. A permit to import material costs $50. Most fruit tree material has to be kept in closed quarantine and the fees for each individual introduction range from $375 for kiwifruit, persimmons and tamarillos, to $750 for most fruits including pipfruit, stonefruit, and berryfruits, to $1000 for citrus or plums imported from certain countries. These fees mean that to replace our existing germplasm collections, even if this were possible, would cost several million dollars in quarantine fees alone.

Virus-free collections
Virus-free collections of fruit trees represent a special case. Many commercial cultivars, especially of apples and citrus, are infected with viruses which can have major deleterious effects on tree growth, fruit production and quality. After many years work, most important pipfruit cultivars have been freed of virus and citrus are currently being freed of virus. Collections of virus-freed trees are held under appropriate precautions as part of our germplasm collections. For example, the citrus virus-free collection, currently comprising 30 cultivars, is housed in an aphid-proof glasshouse to prevent infection with citrus tristeza virus. Such virus-free collections provide material for plant improvement programmes, for cultivar testing and, ultimately, are a source of budwood to be released to growers through industry organisations.

Databases
Recording all the details of the plant material in the collections is of vital importance to its subsequent use in plant improvement programmes. Comprehensive database systems using application programmes on computers have been developed for some plant collections. These systems contain information on individual introductions: from whom it was obtained, where it was obtained from, date of introduction, quarantine status, classification (species etc), geographic location in the collection etc. Information on and representative specimens of cultivars that have Plant Variety Rights are also maintained in the collections.

HortResearch is the International Registrar of Kiwifruit Cultivars. This role involves assembling information on all new kiwifruit developments, ensuring correct classification and naming, and collating and translating existing literature on present named kiwifruit selections. The apple germplasm collection is particularly good and a comprehensive database is being extended and updated. Most of the apples have been "fingerprinted" using enzyme techniques and only the evaluation of disease resistance has to be completed. Information is being collated for a register of New Zealand apple cultivars: the register will include photographs, descriptions of agronomic characteristics and evaluations.

Conclusion
The germplasm collections provide essential research input for the plant improvement programmes undertaken by HortResearch. These programmes aim to extend further the range of crops that can be grown in New Zealand, and to make our existing fruit industries more competitive through the use of better cultivars. Articles in the next two issues of the Orchardist will explain in more detail some of the methods and techniques used in these breeding programmes.

References
(1) Muggleston S. and Scott D. 1994.
Retaining competitive advantage for New Zealand growers. The Orchardist of NZ. 67(8): 56-57.
(2) Muggleston S. 1994.
Biotechnology aids plant breeding. The Orchardist of NZ. 67(4) : 40-44.

Source:
The Orchardist, October 1995, Vol: 68, Number: (9):49