Effects of pollinizer cultivars on berry growth and quality of Xiangmizao kiwifruit

Abstract: 【Objective】Kiwifruit, a dioecious plant, must be effectively pollinated to obtain fruit quality and optimal yield because their complete flowers produce unviable pollens and their male flowers have abortive pistils. Besides, pollinizer cultivars confer“xenia”effect on the fruit, which affects a range of important traits including fruit set, berry size, berry shape, color, developmental timing, metabolites composition, etc. In kiwifruit vineyard, the ratio of male and female plants is generally recommended as 1∶8. Compatible male cultivars for Actinidia arguta production are lacking and more studies are needed to select compatible pollinizer cultivars for any specific kiwiberry female cultivars. For selecting a compatible male cultivar, its flowering time, amount of pollen in each flower and pollen viability need to be considered. There have been few studies investigating the effects of pollen parents on berry quality in kiwiberry. Xiangmizao (A. arguta) is a new kiwiberry cultivar, which was selected from the wild kiwifruit population growing in Dawei Mountain, Hunan Province. To grow this new cultivar properly, compatible pollinizer cultivars are needed. To select the best pollinating male cultivars compatible for Xiangmizao, four pollinizer cultivars were used to investigate pollen parent effect on fruit set, berry growth, berry quality, and berry storability of this new female cultivar.【Methods】Pollens from a tetraploid A. arguta male named DWSR1 collected from Dawei mountain (T1- 22/23), a tetraploid A. arguta male named NER1 collected from Northeast China (T2-22/23), a tetraploid A. arguta male named R2px2 (T2-23), a tetraploid and a hexaploid A. deliciosa males (T3Ad-22) and a diploid A. hemsleyana male named AASAH1 (T4-23) were used to pollinate the female cultivar Xiangmizao. This study compared the differences in endogenous hormones in the ovaries at different developmental stages. Ovary samples were collected at different time points after pollination. Fruit set, berry growth, berry quality, and berry storability were recorded for the pollinated cultivar.【Results】T4-23 showed serious fruit drop after pollination and had the lowest fruit set rate in the end. Compared to T4-23, the other treatments had higher fruit set above 40%, with T1-22/23 and T2-22/23 having fruit set above 70%. Seventy-five hours after pollination, endogenous hormones and their ratios such as GA3, IAA, IAA/ABA, GA3/ABA and (GA3+ IAA)/ABA were all at relatively lower levels in the ovaries of T4-23, while its ABA content was the highest, compared to the other treatments. In contrast to T4-23, except for ABA content, endogenous hormones and their ratios in ovaries collected from T1-23 and T2-23 were higher than those from the other two treatments. In 2022, average berry weight of T1-22 (12.18 g) and T3Ad-22 (11.78 g) was significantly higher than that of T2-22 (10.26 g). Fruit shape index was above 1.2 in all treatments, with T1-22 having significantly higher fruit shape index (1.40) than T2-22 (1.30) and T3Ad-22 (1.28). Fruit firmness of berries from T1-22 (43.21 N) was significantly higher than from the other treatments. T1-22 also had a higher titratable acid content (1.13 %), which was 79.37% higher than T3Ad-22, which had the lowest titratable acid (TA) content. Berry firmness of T3Ad-22 declined more slowly than the other two treatments, T1-22 and T2-22, and their total soluble solids (TSS) were consistently lower than that of T1-22. In 2023, average berry weight of both T1-23 and T2-23 was significantly higher than that in T3Aa-23, with T2-23 being 2.37 times higher than T3Aa-23. There was no significant difference in berry shape index between the T1-23 and T2-23, which both reached 1.4 and significantly higher than that of T3Aa-23 (1.16). Fruit dry matter content was higher than 20% in all treatments at harvest, with T3Aa- 23 (21.03%) being the lowest. TSS contents of T1-23 (7.63%) and T2-23 (7.88%) were higher than that of T3Aa-23 (6.64%). Berry TA content of T1-23 (1.10%) was significantly (1.46 times) higher than that of T2-23 (0.75%) and also significantly (1.45 times) higher than that of T3Aa-23 (0.76%). Fruit firmness declined more rapidly in T1-23 and T2-23 than in T3Aa-23. Berry respiratory rate of T3Aa-23 was consistently higher than that of T1-23 and T2-23.【Conclusion】Results from this study suggest both DWSR1 and NER1 could be used as suitable pollinizers for Xiangmizao fruit production.