Jin-Hu Wu

ISHS VIII International Symposium on Kiwifruit DEVELOPMENT OF A METHOD FOR IN VITRO GERMINATION OF ACTINIDIA SEEDLINGS TO ENABLE PSA BIOASSAYS AND PLOIDY MANIPULATION

The effect of the ploidy level of the polleniser has not been considered during the characterisation and evaluation of colchicine-induced autotetraploids of Actinidia chinensis (kiwifruit). It is therefore possible that few induced autotetraploids reach their potential size. Five genotypes of tetraploid pollenisers were used for controlled pollination of red-fleshed autotetraploids. Fruit of colchicine-induced autotetraploids from two genotypes of female red-fleshed A. chinensis plants were, on average, 25–30% larger than fruit arising from open-pollination in an orchard containing only a diploid male as polleniser. Fruit dimensions increased more in cross-section than in length, so the fruit were more round. Seed set increased up to two-fold and the number of filled seeds also increased significantly (P ≤ 0.01). The average filled-seed weighed two-to-three times more than that of seed from open-pollination with pollen from a diploid male, and were heavier than those in fruit of their diploid progenitors under open-pollination. The distribution of the red-flesh pigmentation increased, although its intensity was not significantly improved. The total soluble solids content (TSSC) increased by 1–2% and the size of the core cavity at the stylar end was significantly reduced (P ≤ 0.01) and sometimes completely. All these effects on fruit attributes would benefit the commercial cultivation of red-fleshed kiwifruit. It is therefore recommended that, as for diploids, pollenisers of tetraploid red-fleshed kiwifruit should be of matching ploidy. Two abnormal fruit with soft red cores were found on a branch of a tetraploid regenerant during the evaluation of fruit generated by controlled pollination. The possible origin and application of this mutation for novel kiwifruit breeding are also discussed.

Non-preferential chromosome pairing was identified in tetraploid Actinidia chinensis and a higher mean multivalent frequency in pollen mother cells was found in colchine-induced tetraploids of A. chinensis compared with naturally occurring tetraploids. Diploid and tetraploid Actinidia chinensis are used for the development of kiwifruit cultivars. Diploid germplasm can be exploited in a tetraploid breeding programme via unreduced (2n) gametes and chemical-induced chromosome doubling of diploid cultivars and selections. Meiotic chromosome behaviour in diploid A. chinensis ‘Hort16A’ and colchicine-induced tetraploids from ‘Hort16A’ was analysed and compared with that in a diploid male and tetraploid males of A. chinensis raised from seeds sourced from the wild in China. Both naturally occurring and induced tetraploids formed multivalents, but colchicine-induced tetraploids showed a higher mean multivalent frequency in the pollen mother cells. Lagging chromosomes at anaphase I and II were observed at low frequencies in the colchicine-induced tetraploids. To investigate whether preferential or non-preferential chromosome pairing occurs in tetraploid A. chinensis, the inheritance of microsatellite alleles was analysed in the tetraploid progeny of crosses between A. chinensis (4x) and A. arguta (4x). The frequencies of inherited microsatellite allelic combinations in the hybrids suggested that non-preferential chromosome pairing had occurred in the tetraploid A. chinensis parent.

Fruit of colchicine-induced tetraploids of Actinidia chinensis were 50% to 60% larger than those of their diploid progenitors. In addition to fruit size, fruit quality is a key issue in any commercialization of these autotetraploids. We have made the first detailed study of the effects of chromosome doubling on fruit quality parameters other than size: these attributes include flesh firmness, color, soluble solid content (SSC), dry matter content (DM), vitamin C content, fruit skin thickness, and red pigmentation distribution in red-fleshed kiwifruit. Four selections from colchicine-induced tetraploids from the yellow-fleshed kiwifruit cultivar Hort16A were trialed for the stability of their fruit quality. Analysis of fruit at harvest over 3 years showed that fruit of the induced autotetraploids were significantly softer (lower flesh firmness), had lower DM, and had a less intense golden flesh color than fruit of their diploid progenitor. During development, SSC of fruit of the autotetraploid plants started to increase earlier than in the diploid ‘Hort16A’. This has been confirmed by replicated trials. No difference was found in vitamin C content between fruit of autotetraploids and diploids. Autotetraploids had significantly thicker skins than diploid ‘Hort16A’. Induced autotetraploids from three female genotypes of red-fleshed A. chinensis showed similar trends to autotetraploids of ‘Hort16A’ in fruit flesh firmness and outer pericarp flesh color, DM, SSC, and vitamin C. All the traits analyzed indicated that fruit of the autotetraploid plants matured earlier than those of their diploid progenitors. Furthermore, red pigmentation, one of the most important traits for red-fleshed kiwifruit breeding, showed a reduction in both intensity and distribution in the autotetraploids compared with their diploid progenitors. There was considerable variation among fruit of autotetraploid plants regenerated from each diploid progenitor. Therefore, selection among the regenerants may be required to achieve the best outcome after ploidy manipulation in kiwifruit breeding.

ISHS VII International Symposium on In Vitro Culture and Horticultural Breeding MANIPULATION OF PLOIDY FOR KIWIFRUIT BREEDING AND THE STUDY OF ACTINIDIA GENOMICS

Some otherwise promising selections of Actinidia chinensis (kiwifruit) have fruit that are too small for successful commercialization. We have therefore made the first detailed study in diploid kiwifruit of the effects of chromosome doubling induced by colchicine on fruit size, shape and crop loading. Flow cytometric analysis of young leaves and chromosome analysis of flower buds and root tips was used to confirm the stability of induced autotetraploids. Fruit weight, size and crop load were measured in the third year after planting in the field and for three consecutive years. DNA fingerprinting was used to confirm the origin of the material. There was a very significant increase in fruit size in induced autotetraploids of different genotypes of A. chinensis. With the commercially important diploid cultivar ‘Hort16A’, most regenerants, Type A plants, had fruit which were much the same shape as fruit of the diploid but, at the same fruit load, were much larger and heavier. Some regenerants, Type B plants, produced fruit similar to ‘fasciated’ fruit. Fruit of the autotetraploids induced from three female red-fleshed A. chinensis selections were also 50–60 % larger than fruit of their diploid progenitors. The main increase in fruit dimensions was in their diameters. These improved fruit characteristics were stable over several seasons. Chromosome doubling has been shown to increase significantly fruit size in autotetraploid A. chinensis, highlighting the considerable potential of this technique to produce new cultivars with fruit of adequate size. Other variants with differently shaped fruit were also produced but the genetic basis of this variation remains to be elucidated. Autoploids of other Actinidia species with commercial potential may also show improved fruit characteristics, opening up many new possibilities for commercial development.