Rootstock Protocol

Citrus rootstock requirements and charac- teristics evaluated are quite different in most cases from those of scions, so the sources of genetic variation exploited and the screening methods used are likewise different in nature and scope (Fig. 2.3). Hybridization, both sexual and somatic, is a primary tool for rootstock improvement. The introduction of rootstocks from other industries or breeding programmes is also an important part of the overall programme to bring our industry the best possible cul- tivars to use, regardless of their source of

Sources of Genetic Variation

1. Germplasm introduction

2. Sexual hybrids

3. Somatic hybrids

4. Molecular genetics

Fig. 2.3. Protocol for citrus rootstock improvement.

origin; such legally acquired selections or cultivars are frequently included in repli- cated fi eld trials with our own advanced selections. As genetic engineering comes to play a greater role in the genetic improve- ment process over time, there will probably be greater exploitation of the technology to make important single trait improvements

in rootstock cultivars or selections with already proven value. As with scion improvement, the rootstock protocol is robust and plant material from any source or developed by any technique can be entered into the system at basically any point.

Seedlings from crosses or introduc-

tions, or equivalent material such as cut- tings of seedlings or somatic hybrids, receive their initial evaluations in the screening core facility or the fruiting-out orchard. The screening core facility is sup- ported by the programme to provide a loca- tion where screening can be carried out on a repeatable and routine basis; implicit in this activity is the interaction with experts in the respective areas of plant pathology and physical stress physiology. The traits selected for screening are at the discretion of the team member responsible for intro- duction or production of the materials. Selection within the fruiting-out orchard is generally based on polyembryonic seed production and tree performance, two criti- cal traits for rootstocks that are propagated by nucellar seeds. Primary candidates are those individuals that pass the screening core tests for citrus tristeza virus (CTV) resistance or tolerance, Phytophthora resistance and other tests as established by the investigator, such as tolerance of salin- ity or iron chlorosis due to high soil pH, abundant production of polyembryonic seeds, and so on. These candidates are entered into replicated trials designed pri- marily to conduct the fi rst fi eld evaluation involving soil, horticultural, pest and dis- ease factors. The emphasis is on representa- tive regional sites regarding soil conditions, scion cultivars and plant spacing. Following a fi rst round of fi eld trials, addi- tional trials may be appropriate to confi rm or otherwise expand the knowledge about a given rootstock candidate. Secondary can- didates are those that do not meet all of the screening criteria for a primary candidate, but in the judgement of the investigator they may merit fi eld evaluation none the less. These candidates are placed in smaller sized fi eld trials or opportunistic plantings where they may be subjected generally to a lower level of evaluation. Opportunistic plantings are not formal fi eld trials, but are plantings that may involve, for example, using new selections in reset situations, small groups of trees on one rootstock, plac- ing trees in a particular fi eld condition (heavy blight incidence; CaCO3 soil) or any

situations that are more commercial-like in size and planting arrangement; opportunis- tic plantings generally are used to assess trait performance for a limited number of site-specifi c problems for which the selec- tions tested may prove useful.

When suffi cient data are available from long-term fi eld trials, fi nancial analysis is used as a tool in rootstock evaluation, lead- ing to cultivar release. When evaluation is nearing the point when a release is antici- pated, it is critical to have seeds or vegeta- tive material available for distribution to support commercialization of the new releases, and appropriate measures are taken to ensure adequate supplies for esti- mated needs. Measures are taken to provide protection, as appropriate, for new cultivars and selections; again this protection includes Material Transfer Agreements with cooperating growers participating in the commercial evaluation, and proper patent protection, as needed. Release will generally occur when the preceding proto- col has been completed and a rootstock is judged as having suffi cient potential, either as an alternative or as a replacement for cur- rent commercial rootstocks. Germplasm with potential use as breeding lines, even though they have not met all criteria for new cultivar status, may be released from any stage of the protocol.