Breeding and Seed Production of Ornamental Crops (2+1)

Introduction

Use of Self-incompatibility

• This system takes the advantage of the natural breeding system. The self incompatibility systems are of two types, gametophytic and sporophytic system.
• Among ornamentals, the self-incompatibility system is present in Nicotiana and Petunia is gametophytic and can be utilised in cross pollination under open field conditions. However, in these two flowering plants one additional advantage in that there are enough seeds per pollination, which can adequately compensate the high cost of F₁ hybrid seeds.
• Sporophytic system of self-incompatibility is observed in Verbena, and dominance relationship between two self-incompatibility (SI) alleles influences the seed set.
• In such species/cultivars the degree of self-incompatibility can be assessed by examining flower styles with the ultra-violet fluorescent microscope, a few hours after the pollination.
• Pollen tubes which have penetrated the stigma and are thus able to grow down the style unimpeded, are able to be counted since they identify themselves by fluorescence. The lack of or a small number of pollen tubes throughout their length, indicate a strong incompatibility reaction.
• If the incompatibility reaction is weak, counting will prove impossible since numerous pollen tubes will be visible as a tangled mass within the stylar tissues. With the advent of tissue culture technology, maintenance of the female parent carrying SI alleles can be managed under protected conditions.
• In Sweet Williams the head of the flowers comprises individuals which mature at different times, usually starting with the centre flower of a cluster. Therefore, pollination, either by all but a single cluster, or by pollinating members of the head which have reached maturity followed by removal of all other buds will be effective.

Double Flower Condition

• In double type of flowers all the anthers are modified to form ray florets. This is a character of family compositae/ asteraceae where double form is achieved because of numerous petals but a central disc may be seen when petals are pulled aside and make a composite head such as in ageratum, aster, chrysanthemum, cornflower, dahlia, daisy, gaillardia, marigold, rudbeckia and sunflower.
• Sunflower illustrates most clearly the satisfactory example of pollination where each floret bears only a single ovary. In most cases, double flower results from a transformation of the anthers into petals. Therefore, the double flower character can be regarded as a form of the male-sterility.
• Further, this double flower character should be inherited as a dominant character to be manifested in F₁ hybrids. However, in the case of gerbera, where the pistils are hidden by petals, manual hand pollination is tedious for maintenance of the female parent.
• Reimann-Philip (1969) developed a breeding scheme using the double flower character in garden carnation (Dianthus caryophyllus) as a form of male sterility.
• There are also some floral abnormalities, like the 'cinderella' character in begonia and 'femina' in marigold and zinnia resulting in the male-sterility.

Triploidy

• The advantage of growing triploid varieties is that of continuous blooming period as there is little or no seed set. Among the flower species, in Tagetes, commercial triploid hybrid (2n = 36) results from a cross between diploid female Tagetes erecta (2n = 24) and tetraploid male Tagetes patula (2n = 48).
• The triploid hybrid 'Nugget' (Tagetes erecta x Tagetes patula) has the unique ability of holding the flowers on the plants for a longer period. However, in Begonia semperflorens, the loss of uniformity in triploids has been reported by Reimann-Philip (1983).

Pollen Sterility

• Wherever pollen sterility is governed by a single recessive gene, maintenance of the genetic stock is difficult as there will be continuous segregation of the fertile and sterile individuals in, 1:1 proportion. This phenomenon is present in Tagetes, Zinnia, Delphinium Antirrhinum, Calceolaria, Salvia and Impatiens.
• In Ageratum, however, both the male sterility and self incompatibility-systems are prevalent and a choice can be made on the basis of economy in the seed production.
• In Petunia, cytoplasmic male sterility has been observed but the use of this type of male sterility is not practically common because of breakdown of male-sterility in the maternal parent or malformation of flowers in F_l plants.
• Male sterility in Sunflower has been used to produce ornamental varieties like, 'Sunrich Orange' (Japan) and 'Orit' (Israel), which have no pollen grains and allergic effects like in other male fertile varieties grown for their seed.