Articles
CONSTRAINTS ON THE SELECTION OF SUPERIOR GENOTYPES IN INITIAL TRIALS OF CHRYSANTHEMUM PROGENY
In practice this usually means growing just one plant of each genotype at normal commercial spacing.
However, this could introduce serious constraints on the efficient detection of superior genotypes.
The value of replication (achieved by using cuttings) in increasing broad-sense heritability and, therefore, selection efficiency is demonstrated.
The consequential reduction in the number of genotypes which can be tested may, however, preclude this approach.
With or without replication, efforts should be made to minimise all sources of environmental variation and so raise heritability.
Cuttings have advantages over seedlings as planting material for such trials.
When each plant is of a different genotype, the effects of the advantageous environment at the edge of the cropping area can outweigh any superior genotypic potential of a plant away from the edge.
This could lead to mistakes in selection.
The use of ‘guard’ plants eliminates edge-effects but reduces the number of test genotypes by up to 20 %. Placing shade netting around the cropping area could be a useful alternative to reduce edge-plant advantage.
Chrysanthemum genotypes can have different competitive abilities but strong competitors are not necessarily better than weak ones in commercial cropping.
In progeny trials, competition between neighbours of different genotype can influence leaf production and dry weights of flowers, leaves and stems.
Selection, therefore, could favour strong competitors over weak ones.
Wide spacing reduces between-plant competition but it is not yet known whether this leads to increased selection efficiency.
Initial selection at commercial plant spacing should be concentrated on characters with high broad-sense heritabilities such as flower type, speed of flowering and height.
Characters with lower heritabilities such as flower number and stem diameter are best ignored at this stage.