And Cross-Pollination in Pollen Tube Growth, Early Ovule Development and Fruit Set of Camellia Grijsii

INTERNATIONAL JOURNAL OF AGRICULTURE & BIOLOGY ISSN Print: 1560–8530; ISSN Online: 1814–9596 18–1478/2019/21–4–819–826 DOI: 10.17957/IJAB/15.0960 http://www.fspublishers.org Full Length Article Comparison of Self- and Cross-Pollination in Pollen Tube Growth, Early Ovule Development and Fruit Set of Camellia grijsii Huan Xiong1, Feng Zou1*, Deyi Yuan1*, Xiaofeng Tan1, Jun Yuan1, Ting Liao1 and Genhua Niu2 1Key Laboratory of Cultivation and Protection for Non-Wood Forest Trees, Ministry of Education Central South University of Forestry and Technology, Changsha 410004, Hunan, China 2Texas AgriLife Research at El Paso, Texas A&M University System, 1380 A&M Circle, El Paso, TX 79927, USA *For correspondence: [email protected]; [email protected] Abstract Camellia grijsii Hance is one of the most important woody edible oil tree species in Southern China; however, it often has a low fruit set rate. To elucidate the causes of poor fruit set in C. grijsii, self-pollination (SP) with C. grijsii and cross-pollination (CP) of C. grijsii × C. villosa tests were conducted. Pollen germination and pollen tube growth into pistils, and early ovule development after SP and CP, were examined using a paraffin section and fluorescence microscopy. The fruit set percentage in SP and CP was also investigated. The results showed that pollen germinated normally on the stigma, and the pollen tubes both reached the style base after SP and CP, but the growth rates of pollen differed significantly between SP and CP, being faster for CP. The pollen tubes arrived at the style base 48 h after SP, but only 24 h after CP. No barriers to SP acted at the stigmatic or stylar level in C. grijsii; however, SP pollen tubes stopped at the upside of the ovary at 72 h due to the presence of ovarian self-incompatibility (OSI). There was also no callose deposition in the ovules at 84 h after SP. The inability of SP pollen tubes to penetrate the ovule and the absence of a mature embryo sac in the ovule were the critical factors that led to ovule abortion. Fruit set following SP (2.1%) was consistently and significantly lower than that obtained from CP (72.9%). Thus, we conclude that the presence of strong OSI gives rise to ovule abortion and is the main cause of the poor fruit set in C. grijsii. © 2019 Friends Science Publishers Keywords: Camellia grijsii; Ovule abortion; Fruit set; Ovarian self-incompatibility Introduction al., 2014). However, in many cases, the productivity is not satisfactory, and this is particularly true for C. grijsii, which Camellia grijsii Hance is one of the most important non- blooms profusely but has a relatively low fruit set (Zou et wood forest trees in the Theaceae family. It is native to al., 2013b). The low productivity has discouraged growers China and is widely distributed in the provinces of Hunan, from cultivating this species, impeding the development of Zhejiang, Fujian, Jiangxi, Guangdong, Guangxi, Hubei, the tea oil industry in China. Shanxi, Guizhou and Yunnan (Zhuang et al., 2012; Zou et The reproduction process of any plant has several al., 2013a). C. grijsii can be used not only as an evergreen important stages, namely, flowering, fruit development and ornamental shrub but also as an oilseed species. The oil seed maturation. Each stage is strongly related to fruit set. extracted from the seeds of C. grijsii can be used for Abnormal floral organs in plants, which are generally medicinal purposes and a range of other uses such as in formed after pollen abortion, play a key role in the fruit set cosmetics. The edible oil (called tea oil) is also known as (Ye et al., 2009). Although some histological studies that “oriental olive oil” and has high oleic acid content, typically have investigated sexual reproduction have considered exceeding 80% and low saturated fat content (Gao et al., different Camellia species (Cao, 1965; Chen et al., 2011; 2018). C. grijsii is a valuable germplasm species due to its Liao et al., 2014a; Gao et al., 2015a), only one study (Chen excellent characteristics of oil quality, high oil rate and thin et al., 2011) observed that pollen of irregular shape and shell (Zhuang et al., 2012). In addition, its fruit capsules are shrinkage was aborted in C. oleifera. Zou et al. (2013a) highly resistant to anthracnose disease (Colletotrichum reported that male gametes are fertile in C. grijsii, but Ye et camelliae Massee), which causes severe premature drop of al. (2009) reported embryo sac sterility, with few or no the fruit capsules of C. oleifera (Weng, 1997). ovules able to develop into fully mature seeds, resulting in a China is the largest producer of tea oil worldwide, poor fruit set. Although aborted ovules were responsible for which is a main source of income for many rural the low seed set of C. grijsii (Zou et al., 2013b), whether communities in the hilly areas (Zhuang et al., 2012; Zou et this is caused by poor pollination and fertilization or other To cite this paper: Xiong, H., F. Zou, D. Yuan, X. Tan, J. Yuan, T. Liao and G. Niu, 2019. Comparison of self- and cross-pollination in pollen tube growth, early ovule development and fruit set of Camellia grijsii. Intl. J. Agric. Biol., 21: 819‒826 Xiong et al. / Intl. J. Agric. Biol., Vol. 21, No. 4, 2019 factors is unknown. The processes of pollen germination, 25 pollen tube growth and ovule fertilization are regulated by 2009 2010 pollen–pistil interactions (Radunić et al., 2017). Pollen-pistil 20 compatibility interactions are essentially controlled by the genotype. Fluorescence microscopy has been used to study the compatibility of pollen tube growth into the pistil in 15 many Camellia species including C. oleifera (Liao et al., 2012; Gao et al., 2015b) and C. sinensis (Chen et al., 2012; 10 Zheng et al., 2014). Thus, information on pollen tube growth into the pistil can be useful for interpreting the (℃) Temperature variation in fruit set (Feijó et al., 1999; Jahed and Hirst, 5 2017). However, current information about pollen germination and pollen tube growth rates following self- 0 pollination (SP) and cross-pollination (CP) and their roles in C. grijsii fruit set are limited. 10 16 22 28 6 12 18 23 In this study, we investigated pollen germination on February March the stigma, pollen tube growth into the pistil and early ovule Fig. 1: Mean daily air temperatures during two flowering seasons development in C. grijsii following SP and CP. We also of C. grijsii at Zhuzhou. Data presented are from 10 February to investigated the effects of SP and CP on C. grijsii fruit set. 23 March Our aim was to determine the reason for poor fruit set in C. grijsii. Pollination Treatments Materials and Methods Flower buds were emasculated during the white bud stage and covered in parchment paper bags to prevent biotic Plant Materials pollination. To determine whether pollen grains were viable after in vitro germination before hand-pollination The plant of Camellia grijsii were grown at the Camellia experiments, anthers were collected from the Camellia Garden at the Central South University of Forestry and orchard at the balloon stage. The germination medium was Technology in Zhuzhou city, Hunan province (27°55′30″N, prepared by dissolving 2% agar, 10% sucrose and 0.01% 113°09′50″E; approximately 70 m above sea level) were boric acid in boiling water at pH 5.6 (Zou et al., 2009). The used in this study. This site is in a typical subtropical moist percentage of germinated pollens was determined using a climate with a mean annual precipitation of 1392 mm and a BX-51 microscope (Olympus, Tokyo, Japan) according to mean annual temperature of 17.5°C. Another Camellia Xiong et al. (2016). SP (C. grijsii × C. grijsii) and CP (C. species, C. villosa Chang was planted in the Camellia grijsii × C. villosa) were performed the next morning from Garden and used as the pollen source for CP. C. villosa is an 8:00 to 11:00 am, and the buds were re-bagged quickly after important species with large fruit and good oil quality, and pollination. The bags were removed in early April when all is distributed across the Hunan, Zhejiang, Guangxi, Guizhou of the flowers were withered. A total of 300 flowers were provinces (Wang et al., 1990). C. grijsii and C. villosa used in each treatment for histological observation. Other usually bloom from February to early April and fruiting 200 pollinated flowers with chosen randomly from three occurs in late October. To record the climate conditions in trees were performed to investigate fruit set after SP and CP. 2009 and 2010 during the full flowering phase, daily mean SP and CP of C. grijsii were conducted in the spring of both temperatures were collected by a weather station located 2009 and 2010. less than one mile from experimental plots. When more than 50% of flowers were open, this was considered the full Pollen Germination and Pollen Tube Growth in the flowering phase (Zhuang et al., 2012). The flowering period Pistil following SP and CP lasted for about 40 days for C. grijsii. Temperature differences among the years were mostly small during Ten pistils for each treatment were sampled at different flowering (Fig. 1) and thus the effects of temperature on the intervals (1, 4, 8, 12, 24, 36, 48, 60, 72, 84, 96, 120, 144, fruit set were assumed to be negligible. Experimental trees and 168 h) after pollination to observe pollen tube growth in the orchard had no diseases or pests and were planted into pistils or ovules.

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