STEM ELONGATION AND RUNNERING IN THE MUTANT STRAWBERRY, FRAGARIA VESCA L.

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1 Euphytica 22 (1973) : STEM ELONGATION AND RUNNERING IN THE MUTANT STRAWBERRY, FRAGARIA VESCA L. A R B O R EA STAUDT C. G. GUTTRIDGE Long Ashton Research Station, University of Bristol, England Received 27 October 1972 SUMMARY The long-stemmed strawberry mutant Fragaria vesca arborea STAUDT (arb/arb) was crossed with a non-runnering form (r/r) of everbearing strawberry F. vesca semperfiorens DUCH. and the four recessive everbearing segregants were identified (arb/arb, r/r), (arb/arb, +/+), (+, r/r) and (+, +). The longstemmed segregants runnered even when the dominant gene for runnering was absent. The morphological similarity between the mutant arb/arb and the short-stemmed (+) forms when the latter are sprayed with exogenous gibberellic acid suggests that the mutation removes a process which in the short-stemmed forms represses gibberellin synthesis in stem internodes. INTRODUCTION Genetical, morphological and physiological studies were made of hybrids between the long-stemmed mutant of the wild strawberry (2n = 14), Fragaria vesca L. arborea STAUDT (STAUDT, 1959) and a perpetual-fruiting (everbearing) non-runnering `Alpine' form, F. vesca semperflorens D UCH. cv. Baron Solemacher. The everbearing character of semperflorens derives from a single gene double recessive (BROWN & WAREING, 1965), seasonal flowering being dominant. Runnering is dominant, the monogenous recessive having no runners. The major genes for flowering and runnering segregate independently, and it is possible that the non-runnering character was introduced into the everbearer from F. vesca eflagellis (DUCH.) described by Duchesne in The arboreal character was found by Staudt in the progeny of plants raised from seed of F. vesca which had been gathered from the wild on the island of Madeira (STAUDT, 1959). Staudt showed that the arboreal mutant (internodes of which averaged 1.6 cm in length) was a monogenous recessive (arb/arb). He also noted that the plants produced only runners (stolons) and no branch crowns at axillary positions. Although stem growth is erect, elongated stems are not sufficiently strong to be selfsupporting when the stems become longer than about 10 cm. GENETICAL STUDIES Seed of F. vesca semperflorens cv. Baron Solemacher was purchased from a commercial source. Dr Staudt kindly provided seed of F. v. arborea. A clone of F. v. arborea was selected which proved to be shy flowering, but eventually three plants flowered and these were crossed with F. v. semperflorens, and by appropriate methods the following phenotypes were raised : 357

2 C. G. GUTTRIDGE 1. Everbearing (jji), arboreal (arb/arb), runnering 2. Everbearing (j/j), short stems runnering (+). 3. Everbearing (j/j), short stems non-runnering (r/r). The fourth expected phenotype - everbearing (j/j), arboreal (arb/arb), non-runnering (r/r) - did not occur, even in families in which the short-stemmed segregants were without runners. This observation suggested that the arboreal character might itself induce runners. To test this possibility, several arboreal segregants were back-crossed to a homozygous short-stemmed, non-runnering segregant. From these back-crosses the two homozygous arboreal parents were identified, the arboreal (arb/arb) runnering (+), and the arboreal (arb/arb) double recessive (r/r), lacking the dominant gene for runnering. The runners and runnering habits of these two parents were not different in any obvious ways, and it was concluded that the arboreal recessive (arb/arb) plants did not require the presence of the dominant gene in order to runner. Several morphological and physiological characters other than those described above segregated during this investigation, but their inheritances have not yet been analysed. Of most relevance to the present study is a shortened form of runner, which is the subject of further study. Other characters were large inflorescences, differentlybranched inflorescences, late flowering and a temperature dependence for runnering. MORPHOLOGY AND GROWTH HABIT BROWN & WAREING (1965) have described the growth habit of the June-bearing type, F. vesca, and of the everbearing, F. v. semperflorens cv. Baron Solemacher. The arboreal forms differ from their respective types in having elongated internodes and in having no branch crowns, only runners at axillary positions. Internode length in the arboreal form of F. vesca was observed to vary in the glasshouse with the time of year, being shortest in winter (5 to 10 mm) and,longest in early summer (10 to 20 mm) (Fig. 1). Internodes were noticeably longer in another experiment when the buds were shaded by dense canopies of foliage. Internodes close to the inflorescence were -much longer than internodes remote from the inflorescence (Fig. 1) or internodes on non-flowering plants. Apparently, inflorescences (which are themselves elongated) supply a growth factor which enhances growth of adjacent vegetative internodes.. Because of this effect of inflorescences on the elongation of adjacent internodes, the appearance of the everbearing arboreal form with its numerous inflorescences (Fig. 2) is remarkably different from that of the non-everbearing arboreal form which seldom flowers (Fig. 3). In one family, internodes that were adjoining the inflorescence were sometimes as much as 50 to 80 mm long and one was 205 mm, but others were shorter. When internodes were very long they resembled internodes on runners, both in length and in having bract-like leaves at the nodes. Usually, however, elongated stem internodes were much shorter than runner internodes, and it was not difficult to distinguish runners from axillary branch crowns with elongated stem internodes Euphytica 22 (1973)

3 Length of stem (cm) 34 STEM ELONGATION AND RUNNERING IN STRAWBERRY 30 Summer internodes 20 Q Q f4 10 r Winter internodes Vegetative plants Flowering plants Fig. I. Successive internodes on main stems of 3 vegetative and 3 flowering plants of F. vesca arborea (drawn to scale). 0 = terminal inflorescence, vegetative growth continuing from the uppermost axillary bud. Fig. 2. A long-stemmed everbearing segregant of Fragaria vesca arborea x F. v. semperflorens, Baron Solemacher. Such plants are prostrate in habit. This plant was arranged on a flat surface, with the plant pot on its side, for photographing. The main stem (A) bears four inflorescences, the largest (B) lies on the right side of the photograph. A group of runners appears on the left. Euphytica 22 (1973) 359

4 C. G. GUTTRIDGE Fig. 3. A long-stemmed, non-everbearing segregant from the same cross as in Fig. 2. PHYSIOLOGY The arboreal mutant was found to form runners in both long days (summer in the glasshouse) and short days (10 h at 10-13'C in a growth room) and no branch crowns, either with or without elongated internodes, have been seen on arb/arb plants grown in various environments, although in conditions of poor lighting some axillary buds remained dormant. Evidently runner initiation is not daylength or environment dependent in either (arb/arb, +) or (arb/arb, r/r) genotypes as it is in the shortstemmed type (+, +). The morphology and growth habit of the arboreal mutant can be induced in short- -stemmed counterparts by the application of exogenous gibberellic acid (GA 3 ) (GUTTRIDGE & THOMPSON, 1964 ; GUTTRIDGE, 1968). In short-stemmed plants, sprays of ppm GA3 result in stem internode elongation and in runner formation both in the absence of the gene for runnering and when it is present, but runnering is suppressed by short day. The close similarity between the arb/arb mutant and the (+) type, when the latter is sprayed with GA 3, suggests that the mutation removes from the system a process which in the short-stemmed types blocks or represses the formation of endogenous gibberellin in main stem and axillary internodes. In axillary positions only, the block is presumably removed or an alternative pathway for gibberellin synthesis provided by the gene for runnering when it is present. There are, however, two differences in growth habit between the (arb/arb) mutant and the (+) type sprayed with GA 3. First, exogenous gibberellin inhibits inflorescence initiation whereas the mutant allele does not ; and second, the stem internodes in the mutant attain only about 1.5 cm length although near the inflorescences they attain lengths comparable with internodes induced by large doses of GA 3 (e.g. 100 ppm sprays) Euphytica 22 (1973)

5 STEM ELONGATION AND RUNNERING IN STRAWBERRY REFERENCES BROWN, T. & P. F. WAREING, The genetical control of the everbearing habit and three other characters in varieties of Fragaria vesca. Euphytica 14 : GUTTRIDGE, C. G., Hormone physiology of growth regulation in strawberry. In : Plant growth regulators. S.C.I. Monogr. no. 31. GUTTRIDGE, C. G. & P. A. THOMPSON, The effect of gibberellins on growth and flowering of Fragaria and Duchesnea. J. exp. Bot. 15 : STAUDT, G., Eine spontan aufgetretene Grossmutation bei Fragaria vesca L. Naturwissenschaften 46 : Euphytica 22 (1973) 3 6 1