Heredity 8(3): 421-429 (1954)
HORMONE-INDUCED PEAR-APPLE HYBRIDS
R. D. BROCK
John Innes Horticultural Institution, Hertford, Herts. and Division of Plant Industry, C.S.l.R.O., Canberra, Australia

I. INTRODUCTION

MANY intergeneric hybrids are known, and in the Pomoideae attempts to produce such hybrids by plant breeders have met with some success. A sexual hybrid between pear and apple was produced by Crane and Marks (1952) by using a growth regulating substance or hormone, β-naphthoxyacetic acid, on the pear ovary at the same time that it was pollinated with apple pollen.

Hormones have of recent years been used for two purposes. First there is the production of parthenocarpic fruit without pollination (Avery and Johnson, 1947; Zimmerman and Hitchcock, 1948). Osborne and Wain (1951) obtained parthenocarpic apples and pears in this way. Of the substances they tested only one, α-(2-naphthoxy) propionic acid, was effective in producing mature fruit on pears and then only on a limited number of varieties. This substance was ineffective when applied to apples. A different substance, however, α-phenoxypropionic acid, produced mature apples but again on only one of the three varieties tested. Thus the induction of parthenocarpic fruits in Pyrus seems to be a highly specific reaction.

Secondly, hormones have been used as an aid to crossing. Emsweller and Stuart (1948) found that for giving hybrids between species of Lilium, the most effective was naphthalene acetamide.

It is likely that the action of the hormones in assisting hybridisation is related to parthenocarpy. These studies were aimed at finding out the mode of action of β-naphthoxyacetic acid and also the reason for its rather limited effectiveness as an aid to crossing pears and apples.

3. FRUIT AND SEED SET

Table I summarises the results of the crossing experiments and the following points are apparent:

  1. Parthenocarpy was induced in four of the pear varieties but in none of the apples.
  2. Parthenocarpy requires hormone but does not require pollination.
  3. Pear-apple hybrids were only formed in varieties capable of parthenocarpy.
  4. The naturally parthenocarpic variety, Fertility, was the most favourable for hybridisation.
  5. Hormone-induced parthenocarpy and very slight seed formation occurred in self incompatible varieties. This seed setting may be due to the breakdown of the incompatibility system when the flowers are kept on the plant for longer than normal (East, 1923), or to parthenocarpy allowing recovery of single-seeded fruits which would have otherwise been lost (Lewis, 1948).

6. CHARACTERS OF THE HYBRIDS

Chromosome number.—Proof of the hybrid nature of the seed was given by cytological examination of the seedlings. Six seedlings from the cross diploid pear x tetraploid apple were all found to be triploid (2n = 51). Six seedlings from the cross diploid pear x diploid apple were all diploid (2n = 34).

Seed germination and seedling growth.—Both the diploid and the triploid seed from the pear-apple crosses had 82 per cent. germination as compared with 98 per cent. germination for the seed from the pear-pear cross. As with the seedlings grown by Crane and Marks (1952), the subsequent development was, however, unusual. Within a month some of the seedlings were wilting and lacking in vigour. The triploid seedlings were the most affected and after four months 85 cent. of the triploid seedlings and 39 per cent. of the diploid seedlings had died.

The seedlings appeared to have a defective root system and grafts were made to apple and pear stocks by Mr A. G. Brown. Preliminary results suggest that when grafted they grow much better. More success has been achieved with apple stocks but some grafts are growing well on pear stocks.

External characters.—As the hybrid seedlings weze only observed during their first season it was not possible to define their genetic characters with certainty. The hybrids developed intermediate characters and were distinct from either apple or pear seedlings. Crane and Lewis (1949) have listed various genetic characters for pear varieties and Lewis and Crane (1938) some for apple varieties. Table 3 is an attempted classification of the F1 pear-apple hybrids using these characters. The hairiness of the summer shoots and the serrate leaf margins resembled the apple parents and the dull green leaf colour was like that of apple seedlings. In the cross involving Cornice, however, the pale red colour of the stem and the relatively few glands on the leaf midrib resembled the pear parent. Many of the hybrid seedlings had occasional lobed leaves, a character which is commonly observed in pear seedlings, but rarely in apple seedlings.

8. SUMMARY

  1. Hybridisation and parthenocarpy were produced by applying b-naphthoxyacetic acid to the base of the style of the pear at the same time as the apple pollen was applied to the stigma.
  2. The hormone stimulated the growth of the apple pollen tubes and thus allowed fertilisation to occur, but this was later than pear-pear fertilisation.
  3. The success of the cross also depends on the effect of the hormone in inducing parthenocarpy and preventing the loss of the late developing hybrids.
  4. The growth of the pear-apple seedlings was abnormal but four months after germination 37 per cent. still survived. The seedlings were otherwise intermediate in character between pears and apples.

9. REFERENCES