A study on morphology, floral biology and fertility of interspecific hybrids of coffea arabica l.and coffea canephora pierre ex froehner. including backcrosses of the hybrid to c. arabica

Abstract

A breeding programme is in progress at the Coffee Research Station. Ruiru with a main objective being to develop Coffea arabica cultivars resistant to coffee berry disease caused by Colletotrichum coffeanum Noack and coffee rust Hemlleia vastatrix B. & Br. A number of C. arabica varieties have been used as progenitors of disease resistance. At the same time interspecific hybrids of £. arabica and tetraploid £. canephora called arabusta were also made to enable introgressian of disease resistance and vigour of £. canephora. into cultivars of C. arabica. This report summarises the results of a comparative study on the morphology. ~loral biology and fertility of arabusta its parent species and backcrosses of the hybrid to C. arabica. Arabusta plants could be classified into two phenotypic classes distinctive for leaf size and canopy shape. Notwithstanding. all arabusta plants were found to resemble other tetraploid plants in stomata density. length of the guard cells. pollen diameter and leaf length to width ratio. From observations on mitotic chromosomes in root tips and meiotic chromosomes in pollen mother cells. the indication from morphological characters that the two arabusta phenotypes were not the result of chromosome number differences was confirmed. The results from a number of growth components showed arabusta of phenotype FI R to have remarkable vigour. (iv) Fertility was estimated from pollen stainability, in vitro pollen germination, % used ovules and fruit set from crosses as well as open pollination. The results showed in each case that arabusta had a very low fertility; 6.0 ~ 0.9% on pollen germination compared to 72.0 ~ 2.6% in C. arabica cv. SL 28 both tested under the same conditions. A large range in fertility was observed within arabusta families indicating effectiveness of selection for fertility within this material. An important result with regard to introgression of disease resistance and vigaur of £. canephora into.£. arabica cultivars is that fertility is restored to almost normal levels. already in the second backcross of the arabusta to C. arabica. It was observed that arabusta produced normal flowers with very few star flowers unlike other coffee interspecific hybrids. The results also showed that artificial doubling of the number of chromosomes in £. canephora was of no consequence on self incompatibility in this species. It is suggested that the genetic Isystem .for self incompatibility in £. canephora may probably not be monofactorial gametophytic as previously reported. It was found that at Ruiru. meiosis in pollen mother cells could be studied in flower bud samples drawn between 42 - 48 hours after breaking bud dormancy. From an analysis of a number of meiotic components which included metaphase I chromosome associations. distribution of the chromosomes over the anaphase I poles and the frequency of microspores formed per tetrad, it was shown that the reduced fertility (v) of the arabusta is mostly an expression of their subnormal meiosis. More precisely. the results indicated that a poorly regulated meiosis could be responsible. Some practical implications of these results with regard to the use of arabusta in introgression of disease resistance and vigour. as well as its possible role in robusta coffee improvement are discussed.