Phenotypic Characterization and Simple Sequence Repeat Identification of Red-fleshed Kiwifruit Germplasm Accessions

Big fruit size and nice red pigmentation combined with good flavor should be the major target for red-fleshed kiwifruit (Actinidia spp.) breeding programs. Genetic diversity and plant characteristics were evaluated on a set of kiwifruit accessions with predominantly red flesh to identify the superior individuals for further breeding or study of commercial application. The leading phenotypic characters varied widely among the accessions. Accession R reached average fruit weight ’100 g, whereas it ranged from 43.15 to 84.71 g for the other accessions. Fruits of L and Q were flatter in shape than the others. The core volume accounted for fruit proportions ranging from 2.33% to 11.42%. ‘Chuhong’, ‘Honghua’, and K exhibited a round fruit apex, whereas most others showed a depressed apex. R, L, and Q had the highest a* values in the inner pericarp and also the most appealing visual coloration. Results revealed significantly higher soluble solid content (SSC), total sugar, and sugar/acid ratio in Q, R, and L. The 12 pairs of simple sequence repeat (SSR) markers were successfully used to characterize the genetic variability and confirm true-to-type identity for four accessions. However, the limited number of markers had no ability to discriminate among the other 11 accessions. Based on additional 28 SSRs, six of the indistinguishable accessions were confirmed to be genetically different, and three seemed to belong to the same clone vine. The results demonstrated that application of SSR data could improve the efficiency of identifying red-fleshed kiwifruit germplasm. Red flesh was first found in the fruit of A. chinensis from Hubei (Liang, 1982) and described by Cui (1993) again. In the late 1990s, the first red-fleshed kiwifruit cultivar in the world, ‘Hongyang’, was developed and firstly released from Cangxi County of Sichuan Province in China (Wang, 2003; Wu, 1992), which now has the largest cultivation acreage of red-fleshed kiwifruit in China. Red-fleshed kiwifruit are novel fruits that have unique and attractive appeal and make consumers willing to buy, although the greenand yellow-fleshed cultivars still represent the main body of the kiwifruit traded internationally (Ferguson and Seal, 2008). To date, ‘Hongyang’ has been introduced into at least eight Provinces in China. Other cultivars or selections with red flesh or red core include ‘Chuhong’, ‘Hongmei’, ‘Honghua’, ‘Yuanhong’, ‘Xiangjihong’, ‘Tianyuanhong’, and ‘Longshanhong’, which were later registered in China (Wang et al., 2005a, 2005b, 2006; Wu and Wu, 1998). However, their comprehensive characteristics (especially the red coloration, fruit size, sweetness, or flavor) did not meet ‘Hongyang’ standards. However, as the predominant cultivar in redfleshed kiwifruit production ( 5000 ha in China), besides its small fruit size, ‘Hongyang’ is susceptible to Pseudomonas syringae pv. Actinidiae (Psa), one of the most dangerous bacterial pathogens of kiwifruit plants worldwide, which causes devastating damage to phyllosphere organs (Balestra et al., 2009; Feng and Li, 2009; Rossetti and Balestra, 2008). Moreover, its fruit coloration is badly inhibited by high day temperatures, which results in an obvious reduction or almost disappearance in pigmentation of the inner pericarp (Zhong et al., 2007; unpublished data). Larger fruit, higher intensity of the red pigments, and their high stability under warmer climates combined with strong resistance to Psa and good flavor should be the major targets for breeding and improvement of red-fleshed cultivars. Kiwifruit cultivars on the world market are mostly selected from wild germplasm, budsports, seedlings, and controlled crosses (Chen et al., 2009; Ferguson and Huang, 2007). The kiwifruit industry in Cangxi of Sichuan Province began with a batch of mixed seeds of wild A. chinensis from the Funiu Mountains, Henan Province in 1978, from which ‘Hongyang’ was developed. Large variations in flesh color, size, shape, flavor compounds, storage life, and Table 1. Accessions origins used in this study. Cultivar name or code of accession Origins Cultivars Hongyang One seedling from mixed wild-collected seeds of A. chinensis var. chinensis and A. chinensis var. deliciosa from the Funiu Mountains (Wu, 1992) Chuhong One wild plant of A. chinensis var. chinensis from the mountainous region of Xupu County in the western part of Hunan Province (Wang et al., 2005b) Hongmei One seedling from wild-collected seeds of A. chinensis var. deliciosa in the northern mountains of the Sichuan Basin (Wang et al., 2005a) Honghua One hybrid of ‘Hongyang’ and A. chinensis var. deliciosa male plant (Wang et al., 2006) Accessions AB, AF, H, K, L, O, Q, R Selections from the seedlings of mixed wild-collected seeds of A. chinensis var. chinensis and A. chinensis var. deliciosa from the Funiu Mountains C, I, J Phenotypic variations from ‘Hongyang’ orchards Received for publication 27 Oct. 2011. Accepted for publication 26 Mar. 2012. This work was supported by the National Natural Science Foundation of China (Grant no. 30671433, 31171945), the cooperation project from the Government of Cangxi County in Sichuan Province, and the Knowledge Innovation Program (KSCX2YW-Z-052) of Chinese Academy of Sciences. We thank Miss Zhan Qu for her kind review in language and thank Shi-Song He and Shi-Quan Wu for their help during the sampling. To whom reprint requests should be addressed; e-mail [email protected]. 992 HORTSCIENCE VOL. 47(8) AUGUST 2012 other fruit characteristics have been increasingly discovered in the seedling population that was derived from the batch of wild seeds. Some of the fruiting individuals were wrongly introduced to ‘Hongyang’ orchards in Cangxi or other provinces. However, few organized investigations have been conducted to select for ‘Hongyang’ purification and cultivar improvement in the seedling population because the seeds were originally distributed among some local farmers. In the present study, some red-fleshed individual plants with great phenotypic variations from the historic seedling population were selected, and their vegetative and fruiting traits were investigated under the same conditions within the same garden. They were characterized to identify better parents for a breeding program of red-fleshed kiwifruit or further evaluate the commercial potential. In addition, novel variations can be derived from the budsports of ‘Hongyang’ and improper introduction. SSR markers were used to characterize genetic variability and confirm true-totype identity of the accessions. Materials and Methods Plant material. A large seedling population was produced for rootstocks in Cangxi County in 1982 using the wild kiwifruit seeds (A. chinensis var. chinensis and A. chinensis var. deliciosa) from the Funiu Mountains. Based on a great deal of investigations of the ungrafted seedlings, hundreds of superior individuals were collected for further identification for their use in the red-fleshed kiwifruit breeding program. Eight red-fleshed accessions (AB, AF, H, K, L, O, Q, and R) were selected from the superior individuals. Accessions C, I, and J were the three phenotypic variations in ‘Hongyang’ commercial orchards. Mature shoots of these accessions were grafted on seedling rootstocks in a repository at Cangxi in 1999. Each of the variants was propagated by grafting (i.e., three to five plants) and managed according to normal commercial practice. ‘Hongyang’, ‘Honghua’, ‘Hongmei’, and ‘Chuhong’ were grafted in parallel and managed together with other 11 accessions in the repository and used as controls for plant evaluation and microsatellite analysis. The origins of the materials are listed in Table 1. Evaluation of fruit characteristics. Fruit shape, hairs, and fruit apex shape were described quantitatively. From 2006 to 2007, eight to 10 randomly selected fruits of above females were harvested from each vine at 150 to 155 d after flowering, when the fruit was considered ripe for commercial picking, and used for evaluating shape, color, and hair characteristics. These are the traits typically assessed for commercial desirability. Fruit apex shape was divided into eight types according to Hu et al. (2006) and UPOV (2003) guidelines for Actinidia. Individual average fruit weight, longitudinal diameter (LD), maximum transverse diameter (TDmax), and minimum transverse diameter (TDmin) of fruit at the equator were measured at harvest stage. Fruit core maximum and minimum transverse diameter were also measured for calculating the core average transverse diameter (ATDC). Fruits were cut at the equatorial region for color measurements. The color of two exposed section planes of the inner pericarp flesh were assessed using a Minolta CR-300 chroma meter (Osaka, Japan) with a measuring head, diffuse illumination, and 0 viewing angle geometry. These parameters provided an 8-mm diameter measuring area for each specimen. Channel 00 of the chroma meter was first calibrated to a white calibration plate. The L*-a*-b* color space (CIELAB) (CIE, 1976) (Pointer, 1981) was chosen for the present study, where L* indicates lightness and -a*-b* are the chromaticity coordinates. L*,-a*, and -b* range over a scale from 0 for black to 100 for white, –50 for green to 50 for red, and –50 for blue to 50 for yellow, respectively. Measurements were conducted at six different sites on the two exposed section planes, and the average was calculated for each fruit. For each measurement, the center of the measurement head was put just at the Table 2. Fruit weight, size, shape, and core volume of Actinidia accessions. Accessions Wt (g) LD (mm) TDmax (mm) TDmin (mm) TI PCV (%) AB 63.66 43.5 44.89 42.5* 0.9468 8.12 AF 59.68 52.63 44.32 40.95 0.9240 6.45 C 43.15 41.34 40.06 38.58 0.9631 5.34 H 54.75 50.04 44 38.15 0.8670 7.25 I 57.49 54.21 43 38.43 0.8937 2.33 J 78.12* 58.52 46.53* 42.82 0.9203 6.71 K 84.59* 63.15* 47.85* 43.5 0.9091 7.8 L 82.97* 58.06 50.78* 41.6* 0.8192 11.42** O 84.71* 67.27* 46.5* 42.15* 0.9065 6.59 Q 70.08 57.25 45.79 38.7 0.8452 9.89* R 100.82** 66.26* 50.29* 43.19* 0.8588 9.1* Honghua 81.13* 65.97* 44.73 40.82 0.9126 4.65 Hongmei 61.68 55.1 41.33 39.03 0.9444 6.68 Hongyang 68.5 54.74 43.67 40.44 0.9260 9.09* The value is an average of 24 to 50 fruits per accession. *a < 0.05, **a < 0.01 (Student’s t test). LD = longitudinal diameter; TDmax = maximum transverse diameter; TDmin = minimum transverse diameter; TI = transverse diameter index (= TDmin/TDmax); PCV = percentage of core volume [= (ATDC / TDmean) · 100%]; ATDC = average transverse diameter of core. Table 3. Descriptions of fruit shape, hair character, and stylar end shape of Actinidia accessions. Accessions Fruit shape Hair Shape of stylar end AB Oblate short cylindrical Absent Depressed, then blunt protruding AF Oblate conical Absent Depressed, then blunt protruding C Oblate short cylindrical Sparse, velutinous Deeply depressed H Oblate short cylindrical Absent Depressed I Oblate long cylindrical Absent Depressed J Oblate cylindrical Sparse, velutinous Narrow, depressed K Oblate cylindrical Sparse, velutinous Rounded L Oblate cylindrical Absent Depressed, then blunt protruding O Oblate cylindrical Sparse, velutinous Rounded Q Oblate conical Sparse, velutinous Depressed, then blunt protruding R Oblate cylindrical Absent Depressed, then blunt protruding Honghua Oblate cylindrical Velutinous Rounded Hongmei Cylindrical Absent Depressed Hongyang Oblate short cylindrical Sparse, velutinous Deeply depressed Hongyang Oblate short cylindrical Sparse, velutinous Deeply depressed Fig. 1. Coloration in inner pericarp of some red-fleshed kiwifruit accessions. The fruits were harvested at 150 to 155 DAF and then stored at 4 C for 1 month before photographs were taken. DAF = days after