Indicator Candidate Traits for Autonomous Fruit Set Ability Under High Temperatures in Capsicum

Global climate change is causing a gradual increase in ambient temperatures. Climate models predict that by the end of the 21st century, average temperatures will continue to rise by 0.3–1.7 °C or even by 2.6–4.8 °C as estimated by Representative Concentration Pathways scenarios 2.6 and 8.5 (IPCC 2014), respectively. Tubiello et al. (2007) estimated that the gradual rise in temperature predicted for the first half of the 21st century could lead to an increase in crop yield in high-temperate and high-latitude regions and a decrease in yield in semiarid and tropical regions. The additional warming expected for the second half of the 21st century would adversely affect yields globally. In fact, the negative effects of climate change on the yields of some crop species in some areas have already been reported (Lobell & Asner 2003; Peng et al. 2004). Reproductive ability in plants is expected to decrease as temperature increases because of global warming (Hedhly et al. 2008; Driedonks et al. 2016; Mesihovic et al. 2016), resulting in decreased yields.

The Capsicum genus includes important crops such as chili, bell pepper, and ornamental forms. This genus belongs to the Solanaceae family and consists of several wild species and five domesticated species: C. annuum, C. chinense, C. frutescens, C. pubescens, and C. baccatum (Bosland & Votava 2000). C. annuum is the most widely cultivated species in the world and includes both pungent and nonpungent cultivars. Most cultivars of C. chinense and C. frutescens are highly aromatic and pungent (Bosland & Votava 2000) and are often produced in tropical and subtropical regions. Recently, these Capsicum species have also been cultivated in temperate regions such as Japan, occasionally in greenhouses.

Self-pollination that occurs when pollen falls to the stigma in the absence of a pollen vector is called “autonomous self-pollination”, “autonomous autogamy” (Richards 1986), or “autofertility” (Yashiro et al. 1999). In greenhouse cultivation, there are no pollinators or wind unless artificially introduced. “Autonomous fruit set” refers to self-pollination and fruit set without any promotive treatments. Tomatoes and chili peppers have the ability to set fruits without stimulation also in greenhouses. In particular, we considered that chili peppers have a strong ability to autonomous fruit set since they are actually cultivated without artificial pollination. However, under high temperatures, even chili peppers cannot maintain their ability to set fruits autonomously in greenhouses.

The fruit set and reproductive abilities decrease under high temperatures, which is a major problem for agricultural productivity in Solanaceae species, including Capsicum spp. The fruit set and pollen viability of C. annuum were shown to decrease under a mild heat treatment at 33 °C for 120 h (Erickson & Markhart 2002), whereas in C. chinense, such a decrease was observed at temperatures above 35 °C (Garruña-Hernández et al. 2012). There are numerous traits related to fruit set under high temperatures. For example, pollen germination and pollen viability, as well as flower morphology – both style, anther length, anther dehiscence, pollen dispersal, and ovule fertility – affect fruit set under high-temperatures conditions (Levy et al. 1978; Sato et al. 2006). Attempts have been made to search for high-temperature-tolerant genotypes having genes associated with this trait (Reddy & Kakani 2007; Gajanayake et al. 2011; Ayenan et al. 2021) or related genes (Xu et al. 2017; Sun et al. 2019) and to breed heat-tolerant cultivars by introgression of genes (Rani et al. 2021; Usman et al. 2018) in Solanaceae.

Previously, we found an F₁ hybrid of C. chinense with autonomous fruit set ability in high-temperature greenhouses, although both parent cultivars ‘Sy-2’ and ‘No. 3686’, which seldom set fruit autonomously under high temperatures (Yamazaki & Hosokawa 2019). However, little is still known about the genotype of Capsicum with the ability of autonomous fruit set in hot climate and in high-temperature greenhouses. Consequently, more Capsicum genotypes with autonomous fruit set ability in high greenhouse temperatures should be further explored. Thus, the first aim of this study is to identify cultivars that can set fruit autonomously under high-temperature stress. In addition, indicator traits are needed to screen more genotypes that have autonomous fruit set ability under high temperatures. According to Zhou et al. (2015), the chlorophyll fluorescence parameter Fv/Fm can be used to select tomato genotypes for their thermotolerance in autonomous fertility. The second aim is to identify indicator traits in order to screen and find more genotypes with autonomous fruit set abilities under high temperatures. Therefore, we investigated various traits related to the fruit sets and tested the correlation between each trait, and autonomous fertility.

The ability of autonomous fruit set decreases under high temperatures in Solanaceae species, including Capsicum. Most Capsicum species are sensitive to temperatures above 35 °C and reduce fruit set under such conditions (Erickson & Markhart 2002; Garruña-Hernández et al. 2012). Our previous study also suggested that Capsicum plants have low fruit sets when the maximum temperature is 35 °C or higher (Yamazaki & Hosokawa 2019). Thus, cultivars setting fruits autonomously in these conditions are required to ensure fruit vegetable yields, especially in cultivation under covers and in the context of global warming. In this study, the ‘Takanotsume’ grown for spices and ‘Goshiki Kyokko’ of ornamental value were found to potentially have a high fruit set and pollen germination ability during growing in high temperature (Table 2). The fruit set abilities of these cultivars were consistent with our past observations (Yamazaki & Hosokawa 2019). In particular, the ability of autonomous fruit set under high stable temperatures in ‘Takanotsume’ was reconfirmed (Table 3). In addition, ‘Takanotsume’ was confirmed to have a high pollen germination rate even under high temperatures (Figure 4). We previously found that the F₁ hybrid of ‘Sy-2’ and ‘No. 3686’ also set fruits autonomously under high temperatures. The autonomous fruit set ability under high temperatures of ‘Takanotsume’ and ‘Goshiki Kyokko’ were much higher than that of the F₁ hybrid.

‘Takanotsume’ is phylogenetically classified in the Chili group, which is closely related to the ‘Hontaka’ and ‘Yatsubusa’ cultivars (Erwin 1929). In addition, this cultivar has been classified as similar to ‘Shishi Togarashi’, which is closely related to ‘Yatsubusa’ (Konisho et al. 2005), using EST-SSR markers (Shirasawa et al. 2013). Conversely, ‘Goshiki Kyokko’ is classified into the Goshiki group in the Japanese cultivar classification or into the Celestial group based on Erwin's classification and is genetically different from ‘Takanotsume’ (Kumazawa et al. 1954). In this study, both ‘Takanotsume’ and ‘Goshiki Kyokko’ showed a similarly high fruit set and pollen germination compared to other cultivars during the high-temperature period despite their different origins. Obtaining cultivars capable of setting fruit autonomously under high temperatures in different phylogenetic groups is useful because it enhances the possibility of breeding high-temperature-tolerant cultivars. Most cultivars other than C. annuum had low autonomous fruit set ability under high temperatures in this study. Thus, more genotypes capable of autonomous fruit set under high temperatures should be found for breeding a wide variety of Capsicum cultivars.

The most reliable plant research results can be obtained under cultivation conditions. However, conducting such experiments is laborious and costly. Therefore, it is especially valuable to find marker traits related to the trait sought by the breeder. The Fv/Fm parameter related to type II photosynthesis is used, inter ales in tomatoes, as an indicator of photoinhibition (Zhou et al. 2015). However, the Fv/Fm of ‘Takanotsume’ was 0.64, which was lower than in some other cultivars without autonomous fruit set ability under high temperatures, although Fv/Fm varies between 0.7 and 0.8 when there is no abiotic stress in Capsicum (Gisbert-Mullor et al. 2021). A correlation between fruit set and Fv/Fm was not significant in the Capsicum cultivars in the present study (Fig. 2), suggesting that Fv/Fm cannot be used as an indicator trait correlated with autonomous fruit set ability under high temperatures in Capsicum.

The highest positive correlation was found between pollen germination and fruit set in some C. annuum cultivars. It has been previously reported that pollen germination is important for the fruit set rate at high temperatures in Capsicum (Aloni et al. 2001) and other fruit vegetables like tomato (Sato et al. 2000). According to Hirose (1957), the pollen germination of ‘Takanotsume’ was 30.4% in an open field, and in the present study, in a greenhouse during the high-temperature period, it was 6.5 ± 4.2% (Table 2). The point is that ‘Takanotsume’ had lower pollen germination under a high temperature in the greenhouse and in the incubator than in a field, but this parameter was still higher than that of other cultivars. Both the two cultivars, ‘Takanotsume’ and ‘Goshiki Kyokko’, which had the ability of autonomous fruit set had a high percentage of pollen germination under high temperatures. Thus, pollen germination was considered to be an indicator trait for screening genotypes with autonomous fruit set ability under high temperatures.

The pollen germination in ‘Shima Togarashi’ was very high, but the fruit set was low, which was possibly due to a difficult dehisce under high temperatures. Anthers dehiscence was easy in all C. annuum cultivars but was not in other species. Therefore, the fact that anther dehiscence does not occur under high temperatures may cause poor fruit set in Capsicum spp. other than C. annuum. It is reasonable that a decrease in the anthesis stage reduces the autonomous fruit set. Thus, the anthesis stage was also considered to be an important trait for autonomous fruit set ability under high temperatures in Capsicum species other than C. annuum.

‘Takanotsume’ and ‘Goshiki Kyokko’ had the ability to autonomous fruit sets under high temperatures. Thus, they are useful cultivars for breeding programs focusing on autonomous fruit set under high-temperature conditions. A positive correlation between fruit set and pollen germination was significant among some cultivars of C. annuum. In addition, the correlation between fruit set and anthesis stage was positively correlated among all species. Thus, pollen germination in C. annuum is a convincing candidate and an indicator trait for the selection of individuals able to autonomous fruit set under high temperatures.