Self-incompatibility in Citrus species is an important trait related to fruit set and seed formation. In particular, self-incompatible citrus varieties combined with sufficient parthenocarpy produce seedless fruits. The characteristics of self-incompatibility have been studied for many years, and essential traits, such as pollen tube elongation behavior and self-incompatibility genotypes, have been characterized. Recently, it has been shown that the genetic mechanism of self-incompatibility in citrus varieties is S-RNase-based gametophytic self-incompatibility. To date, 18 S-RNases (17 self-incompatible alleles and 1 self-compatible allele) have been identified. The DNA markers for S-RNases can enable the early identification of self-incompatibility/compatibility status. The expression of self-compatibility in Citrus species is ascribed to the presence of the self-compatibility S_m allele, which is a defective S-RNase, and to the suppression of S-RNase expression. Polyploidization induces self-compatibility in Citrus species: Citrus tamurana ‘Hyuganatsu’ is substantially self-incompatible; however, its bud mutation, ‘Nishiuchi Konatsu’, is self-compatible. ‘Nishiuchi Konatsu’ is diploid; however, it forms unreduced pollen, which causes the breakdown of self-incompatible reaction when self-pollinated because of a competitive interaction within the same individual. In addition, after fertilization by unreduced pollen, seed formation is also inhibited by triploid block caused by interploid hybridization between diploid pollen and haploid egg cells. Therefore, ‘Nishiuchi Konatsu’ shows self-compatibility regardless of the self-incompatibility haplotype and produces fruits with few seeds. The seedlessness trait could be beneficial for citrus breeding in the future; however, the genetic mechanisms involved in the expression of this trait remain unclear. This review focuses on the recent advances in the genetics of self-incompatibility in citrus plants, implicating the mechanisms involved in self-incompatibility and their applications for achieving the desired trait of seedlessness in citrus fruits.

The development of intergeneric hybrids for horticultural crops has been attempted to introduce new quality and resistance traits and to enlarge the gene pool. Interspecific and intergeneric hybridization are often hindered by incompatibility reactions occurring at various stages of hybridization, from early pollination to initial growth, and the reproductive stages of the progeny. In this study, we investigated intergeneric and interspecific cross-compatibility among six species in the tribe Maleae (Rosaceae), namely, Pyrus communis (European pear), P. pyrifolia (Japanese pear), Malus × domestica (apple), Eriobotrya japonica (loquat), Cydonia oblonga (quince), and Pseudocydonia sinensis (Chinese quince). In vivo pollen tube growth tests showed the presence of a postmating, prezygotic barrier in many cross-combinations, in which cross-compatibility was regulated by both genetic distance and crossing direction. Strong hybridization barriers were observed in pollen tube growth and fruit setting when intergeneric hybridization was performed with E. japonica, a species phylogenetically distant from the others studied. Different compatibility reactions in reciprocal crosses were observed in some intergeneric hybridizations; C. oblonga as a pollen donor was incompatible with P. sinensis, whereas the reciprocal cross was compatible, resulting in the development of hybrid seedlings. Furthermore, the pollen tube growth rate differed among Pyrus species when pollinated on the apple pistils, suggesting divergence of cross-compatibility response in a specific linage. Factors affecting intergeneric hybridization are discussed with reference to the genetic distance between species and morphological characteristics such as pistil length. Our comprehensive assessment of intergeneric cross-compatibility will help provide a way to overcome crossing barriers and develop new hybrid crops in the tribe Maleae.

The aim of this study was to select new pollinizers for Prunus spp. with high pollen germination rates at low temperatures and assess their effect on the fruit set of Japanese plum cultivars. In this study, we examined in vitro pollen germination in 17 plum cultivars and two Myrobalan plum lines (420-2-2 and 421-3-1) at eight temperatures (7.5°C, 10.0°C, 12.5°C, 15.0°C, 17.5°C, 20.5°C, 22.5°C, and 25.0°C). The extent of pollen germination was affected by the incubation temperature. The germination rates of most cultivars were highest between 20.0°C to 25.0°C and ≤ 20% between 7.5°C to 10.0°C. However, the two Myrobalan plum lines (420-2-2 and 421-3-1) showed higher germination rates than the other cultivars at 10.0°C with ≥ 25% germination. The high germination rate in Myrobalan 420-2-2 was further confirmed in experiments conducted by our group in 2020. Open field studies on the Japanese plum ‘Kiyo’ revealed that the fruit setting rate was 17.6% using Myrobalan 420-2-2 and only 9.9% in the control using ‘Hollywood’. The fruit setting rate of the Japanese plum ‘Taiyo’ was approximately 20% when pollinated with both the cultivars. However, both fruit setting rate and fruit quality did not differ significantly between ‘Kiyo’ and ‘Taiyo’ with either pollination treatment. The formation rates of perfect seeds in ‘Taiyo’ were 90% and 65% by pollination using Myrobalan 420-2-2 and ‘Hollywood’, respectively. However, pollination treatment using pollen from both cultivars did not show any variations in the early development of the ovary and ovule. S-genotyping in Myrobalan 420-2-2 was determined as S₇S₁₀; therefore, we assumed that Myrobalan could be cross-compatible with many other plum cultivars. In conclusion, we selected Myrobalan 420-2-2 as a new plum pollinizer as it can effectively pollinate Japanese plum and germinate at low temperatures with no adverse effect on fruit set and quality.

Based on hand pollination, 217 crabapple cultivars were examined to determine their self-fruitfulness potential. One hundred and six cultivars failed to self-pollinate for fruits set. More than 50% self-pollinated fruit were obtained from 29 cultivars. Especially, the fruit set of ‘Jiringo’, ‘Karafutozumi’, ‘Katherine Crab’, Malus hupehensis, M. scheideckeri, ‘Mary Porter Crab’, ‘Mokoto’, ‘Nepal Apple Collections No. 85-I-74-3 86090-3’, ‘Scheidecker Crab’, ‘Silver Moon Crab’, ‘Tachikaido’, and ‘Tea Crab 81105’ were more than 50% by self-pollination over two years. ‘Jiringo’, ‘Katherine Crab’, ‘Mary Porter Crab’, ‘Scheidecker Crab’, and ‘Tachikaido’ have the potential to be self-compatible cultivars because their self-pollinated fruits contain complete seeds. Non-pollinated fruits of M. scheideckeri, ‘Nepal Apple Collections No. 85-I-74-3 86090-3’, and ‘Tea Crab 81105’ may contain apomictic seeds. Moreover, self-pollinated fruits contained seeds and non-pollinated fruits contained no seeds, suggesting that ‘Karafutozumi’ and ‘Mokoto’ may be self-compatible and parthenocarpic. The study findings not only provide breeding material for self-compatible apple cultivars, but also have led to the discovery of new apomictic and parthenocarpic research materials.

Pepper yellow leaf curl disease (PepYLCD) caused by begomoviruses is one of the most devastating diseases affecting pepper (Capsicum spp.) production worldwide. In our previous study, a loss-of-function allele, pepy-1, encoding messenger RNA surveillance factor Pelota was identified as a begomovirus resistance gene from a C. annuum cultivar BaPep-5. In this study, to investigate the effectiveness of pepy-1 conferred resistance against begomovirus in the field, we conducted a three-year evaluation under natural field conditions in Indonesia. The lowest PepYLCD incidence and significantly higher fruit productivity were observed in BaPep-5 when compared to six other commonly cultivated pepper cultivars. The subsequent comparison between BaPep-5 and the susceptible BaPep-4 showed that pepy-1 slowed down the disease onset and progression, resulting in a higher fruit productivity trait in the field. Multiple comparison analyses using an F₂ population obtained by crossing BaPep-5 with BaPep-4 showed that the pepy-1 homozygous individuals had significantly higher fruit productivity, twice than those of the Pepy-1 homozygous or heterozygous individuals. In conclusion, the introgression of pepy-1 is effective to reduce the economic loss of pepper fruit production under natural field infection of begomoviruses.

The relationship between fruit Brix and the electrical conductivity (EC) of the nutrient solution was investigated under gradually increasing EC conditions to predict and control tomato fruit Brix in commercial greenhouses in Japan. Based on the three experiments, fruit Brix was significantly and highly correlated with the cumulative EC of the drainage during the period from anthesis to harvest (cECd). This relationship followed a linear regression function. We then modelled fruit Brix based on cECd and validated this model to predict and control fruit Brix in four other experiments in different growing seasons using two cultivars, slab substrates, and irrigation systems. Using this model, we calculated the target cECd (cECd_t) to achieve a target fruit Brix of 6% or higher and used cECd_t as an indicator to manipulate the EC of the nutrient solution. In the validation experiments, cECd was lower than cECd_t at the beginning of harvest in all experiments. cECd reached cECd_t at 72.3–214.0°C·day after the first harvest. When cECd was higher than cECd_t, more than 86.9% of the fruit had a higher than Brix 6%. In addition, the marketable yield was higher than 88.2%. RMSEs between the observed fruit Brix and predicted fruit Brix were 0.60–1.25. These results indicate that our model can predict and control fruit Brix.

In Taiwan, the early harvesting of young ginger is a cultivation technique for domestic ginger, which can be harvested early by forced sprouting. At present, ethylene is used as the sprouting agent. Despite its favorable sprouting effect, the technique can still be improved. Experimental results revealed that 1% citric acid, 300-ppm and 450-ppm ethephon treatments effectively facilitated the formation of 2.4, 2.4, and 2.6 large buds (diameter of ≥ 1 cm), respectively, in the rhizome. The stems and leaves of the ginger seed rhizomes that were subjected to forced sprouting emerged from the soil one month after planting, and emergence rates of 46.7% and 83.3% were achieved using citric acid and ethephon, respectively. Although the fresh weight of semi-matured ginger rhizomes obtained by forced sprouting treatment was lower, the results can still provide a reference for the early harvest of young ginger in Taiwan. Among the examined gingers, semi-matured ginger that was subjected to 50-ppm GA₃ forced sprouting treatment exhibited the most favorable growth, and the average weight of its rhizomes reached 1,567 g, which was not significantly different from the weight obtained by conventional cultivation, but significantly greater than that obtained by other treatments. The GA₃ forced sprouting treatment was not very effective, but it had an excellent effect on improving the yield of semi-matured ginger rhizomes. In the future, this treatment will be conducive for the production of semi-matured or matured ginger.

Floral induction by grafting without vernalization treatment (NV grafting method) has potential to shorten breeding times and to diversify the seed production of cabbage, an important leafy vegetable with a long and absolute low temperature exposure requirement for its floral induction. However, it is unknown whether the NV grafting method can be actually used for cabbage breeding and seed production. This is because the NV grafting method’s effect on the field performance of obtained progenies has not been investigated, as opposed to the conventional floral induction method by vernalization treatment. Therefore, in this study we compared the effects of two different floral induction methods on the agricultural traits of the obtained progenies. Two clonal lines of ‘Watanabe-seiko No.1’ cabbage were used for the experiment. In the two-year field experiment, we observed a consistent effect of clonal lines on vegetative growth; however, almost no effects of the floral induction methods on either vegetative or reproductive growth were observed. This was further supported by similar expression levels of FLOWERING LOCUS C homologs in the progenies at the young seedling stage. Pollen production and seed formation of the progenies were confirmed regardless of the floral induction method. In conclusion, cabbage seeds obtained by the NV grafting method are likely to show the same traits as those obtained by the conventional vernalization method. This indicates the direct applicability of this method to cabbage breeding and seed production.

The demand for broccoli (Brassica oleracea L. var. italica) is increasing for use as fresh produce and for use in the processing industry. Therefore, growth prediction technology is required for its stable production. In this study, several cultivations of experiments were conducted to clarify the critical characteristics of the parameters needed to predict the growth of broccoli in a dry matter production model. The extinction coefficient was determined based on the leaf area index and intercepted solar radiation rate. Radiation-use efficiency was demonstrated using a linear function of the accumulated solar radiation intercepted amount and the total dry weight above-ground. The distribution of dry matter to the head and stem was indicated by the sigmoid function of the accumulated average temperature. The dry matter percentage of the leaf, dry matter percentage of the head and stem, and the ratio of leaf area to the fresh weight of the leaf were represented by a power function with dry weight above-ground. The fresh weight above-ground (R² = 0.92) and the fresh weight of the head and stem (R² = 0.98) were highly correlated with the estimated and observed values. Verification was performed using the developed growth model. As a result, the error between the harvest date and the predicted harvest date could be forecasted to within 4 days and the error in the fresh weight of the head could be predicted with an accuracy of −0.3 ~ +7.7 g·plant⁻¹.

We investigated the effect of ultraviolet-B (UV-B) irradiation on the development of black spot disease caused by Diplocarpon rosae Wolf., which is a major problematic disease in rose (Rosa × hybrida) production. The growth of D. rosae colonies was suppressed on potato dextrose agar (PDA) medium under UV-B irradiation (peak wavelength: 310 nm; full width at half maximum: 30 nm) at an intensity of 15 μW·cm⁻² with 1 h daily treatment. In addition, black spot conidia were inoculated to the rose ‘Danjiri Bayashi’ leaves and the effective growth suppression of black spot symptoms was observed on the leaves under UV-B irradiation. Next, various rose cultivars were planted in two greenhouses: one for supplemental UV-B irradiation treatment and one as a control without the treatment. In the UV-B irradiation greenhouse, the roses were irradiated at an intensity of 3–5 μW·cm⁻² every day from 23:00–23:30 and 0:00–0:30 (total: 1 h). No chemical pesticides other than a starch agent for aphid control were used throughout the experiment. With the exception of the data for ‘Papa Meilland’ in 2019, UV-B irradiation significantly reduced the number of leaves infected with black spot disease. In September 2019, the non-UV-B irradiated ‘Danjiri Bayashi’ and ‘Papa Meilland’ had severe black spot symptoms on over 20 leaves. The number of plants with black spot symptoms increased in July 2020 compared to 2019. On the other hand, in UV-B irradiated plants, fewer black spot symptoms were observed than in non-UV-B irradiated plants. Although some visible damage was observed in the UV-B irradiated plants, the chlorophyll and carotenoid contents in the leaves decreased, indicating that UV-B irradiation had a certain negative effect on the photosynthetic apparatus. Over a five-month period, the cumulative number of flowers in the UV-B irradiation greenhouse did not decrease, and actually increased, depending on the cultivar, compared to the control treatments. Our results suggest that supplemental UV-B irradiation is effective at suppressing black spot disease in roses and can contribute to the production of pesticide-free edible rose production.