In the citrus industry, juvenility and alternate bearing are serious problems that cause lengthening of the breeding cycle and instability of annual fruit production, respectively. Both phenomena are closely related to flowering behavior: juvenility is caused by suppression of flowering in young plants and alternate bearing mainly results from suppression of flowering by fruit production. Many researchers have conducted studies into citrus flowering in a quest to resolve these problems. In recent years, molecular and genetic approaches to studying citrus flowering have been performed on the basis of studies on flowering-related genes in Arabidopsis. In Arabidopsis, the protein encoding a flowering-related gene, FLOWERING LOCUS T (FT), plays an important role in the promotion of flowering. Similarly, a citrus orthologue of FT (CiFT) has been confirmed to have a function in the promotion of flowering in citrus. In studies of transgenic plants, a CiFT co-expression vector has been already used to shorten the juvenile phase of citrus. In addition, endogenous expression of CiFT is closely correlated with flowering under various conditions, suggesting that endogenous CiFT may regulate floral induction. Considering the accumulating data, the regulation of CiFT expression is hypothesized to be essential to understand the mechanism of citrus flowering and studies on CiFT are expected to contribute to the resolution of flowering-related problems in citrus.

View full abstract

Peach (Prunus persica) shoots were artificially inoculated with stone fruit bacterial spot bacteria (Xanthomonas arboricola pv. pruni) to evaluate varietal differences in peach genetic resources for their susceptibility to this disease. Current shoots of cultivars/selections were wounded, a bacterial suspension was injected by a syringe attached to multiple needles, and lesion length was measured a few months later. Inoculation was carried out in May, June and July with two concentrations of bacterial suspension: 10⁶ cfu·mL⁻¹ or 10⁸ cfu·mL⁻¹. Although the effect of inoculation time was not significant and the effect of inoculum concentration was significant, inoculation in June at a concentration of 10⁸ cfu·mL⁻¹ was the most suitable treatment. Among 69 cultivars/selections tested, there was no immune cultivar, however; there were varietal differences in susceptibility to bacterial spot. ‘Nishiki’ and ‘Mochizuki’, two cultivars for canning use, ‘Chimarrita’, a Brazilian cultivar, and ‘Tsukikagami’, a table peach cultivar, were relatively resistant and may be useful sources for breeding aimed at disease resistance.

View full abstract

Because bud mutation occurs in a specific part of a plant, the genomic backgrounds of mutant and wild-type branches are identical except for mutations. Therefore, bud mutants are ideal for identifying key genes governing important crop traits. We studied Giant La France (GLaF), a bud mutant setting large fruit, which appeared spontaneously in the European pear (Pyrus communis L.) ‘La France’. In GLaF, increased cell size and DNA reduplication occurred specifically in fruit flesh. With the goal of identifying genes expressed differentially between GLaF and ‘La France’, microarray analysis was performed with RNA extracted from the receptacle (fruit flesh) 1 week before the full bloom stage. The receptacle was isolated by laser microdissection. Genes encoding proteins localized in the nucleus and cytoskeleton were up-regulated in GLaF. Among these genes, several were homologous to genes previously described to be associated with DNA reduplication. These might be related to the molecular mechanism of GLaF fruit size mutation.

View full abstract

The growth and yield of ‘Budousanshou’ (Zanthoxylum piperitum (L.) DC. f. inerme Makino) trees were compared between grafts on ‘Fuyuzanshou’ (Z. alatum Roxb. var. planispinum Rehd. et Wils.) seedlings and ‘Karasuzanshou’ (Fagara ailanthoides Engl.) seedlings. Comparisons were made for 8 years after transplanting. The tree height for grafts on ‘Karasuzanshou’ rootstock was greater than for ‘Fuyuzanshou’ grafts throughout the experimental period after transplanting. For both graft treatments, tree height did not increase beyond 6–7 years after transplanting, suggesting that the trees had reached maturity. In addition, the canopy volume of grafts on ‘Karasuzanshou’ rootstock was greater than that on ‘Fuyuzanshou’ rootstock at 2 years after transplanting. The yield per tree and canopy volume were higher for grafts on ‘Karasuzanshou’ than grafts on ‘Fuyuzanshou’ rootstock by 7 years post-transplanting (i.e., the mature stage). Due to its greater canopy volume, the per tree yield of mature ‘Karasuzanshou’ grafts has the potential to be greater than that from ‘Fuyuzanshou’; therefore, ‘Karasuzanshou’ is a good candidate rootstock for the effective production of Japanese peppers.

View full abstract

Light conditions are poor around the lower trusses of tomato plants in a low truss number, high plant density production system. We determined the effects of leaf rearrangements above the fruit trusses on fruit maturation and quality in tomato plants pinched above the third truss and cultivated under a high-density growing system. Integrated solar radiation at first and second fruit trusses and surface temperature of fruits at second fruit truss were increased in plants treated with leaf rearrangements above the trusses compared with those of the control, and the maturation of fruits at the third truss treated with leaf rearrangement was 4.6 days earlier than that of the control. The concentration of ascorbic acid (AsA) in fruits of plants treated with leaf rearrangement was higher than that of control fruits. However, leaf rearrangement had no effect on yield and Brix of the fruit. These results indicated that higher solar radiation together with leaf rearrangement promoted fruit maturation and increased AsA content in the fruit of lower trusses of tomato plants cultivated under a low truss number, high plant density growing system.

View full abstract

Low-cost technology is needed to alleviate high-temperature injury for high-yield greenhouse tomato production. To acquire information about the physiological and morphological effects of root-zone cooling, we grew young tomato plants for 2 weeks in nutrient solution held at about 25°C, considered to be the optimum temperature for tomato plants. We investigated plant growth, nutrient uptake, root activity (xylem exudation and root respiration rate), root indole-3-acetic acid (IAA) concentration, and internal root structure. The root-zone temperature was maintained at 24.7°C by cooling, while the air temperature and control temperatures were higher than optimum (30.8 and 33.7°C, respectively). Root-zone cooling increased the relative growth rate (RGR) of roots compared with the control, followed by shoot RGR. Root IAA was positively correlated with root RGR. Root-zone cooling increased Ca and Mg uptake as well as root xylem exudation and respiration. It also advanced the development of the internal structure of the xylem near the root tip. Thus, possibly by increasing root activity and root IAA, root-zone cooling promoted root growth and nutrient uptake mediated by the development of the root xylem, and thus shoot growth. These results suggest a physiological and morphological mechanism of growth enhancement by root-zone cooling under high air temperature conditions.

View full abstract

To analyze the relationship between flower morphology and organ-specific promotion of cytokinin biosynthesis within flower buds, we introduced Arabidopsis isopentenyltransferase 4 (AtIPT4) into torenia (Torenia fournieri L.) under the control of APETALA1 (AP1) or APETALA3 (AP3) promoter. AP1::AtIPT4 plants had an increased number of petals, whereas AP3::AtIPT4 plants had an expanded corolla, a paracorolla, and serrated petal margins along with an increased number of petals. In AP3::AtIPT4 plants, marked receptacle enlargement was observed when the flower buds were in the early corolla development stage in which the paracorolla primordia differentiate. As expected, AtIPT4 was expressed in the sepals and petals of AP1::AtIPT4 plants, and in the petals and stamens of AP3::AtIPT4 plants. Furthermore, the type-A response regulator (TfRR1) and cytokinin oxidase (TfCKX5) genes, which were used as indices of cytokinin signal, showed the same expression patterns as the transgene. These findings indicate that expansion of the corolla and development of the paracorolla and serrated petal margins after receptacle enlargement in AP3::AtIPT4 plants are induced by localized elevated cytokinin signal in the petals and stamens. In contrast, localized elevated cytokinin signal in the sepals and petals only induced an increase in the number of petals. Therefore, an elevated cytokinin signal in the stamen may be important for inducing corolla expansion and for developing a paracorolla and serrated petal margins.

View full abstract

Palatinose (isomaltulose) is an analog of sucrose and was regarded as non-metabolizable in plant tissues until recently. In the present study, we found that crude extracts from carnation petals had activity to hydrolyze palatinose. Preliminary characterization of this activity using a crude enzyme extract from ‘Lillian’ carnation petals revealed that hydrolyzing activity was exhibited by α-glucosidase, which uses isomaltose and palatinose, both being α-1,6-glucosides, as substrates. Exogenous application of palatinose stimulated flower opening of carnation cultivars (Dianthus caryophyllus ‘Lillian’, ‘Pure Red’, and ‘Light Pink Barbara’), but suppressed it in D. barbatus ‘Shin-higuruma’. Palatinose-hydrolyzing activity was much higher in the extract from carnation than that from D. barbatus. These observations suggested that palatinose stimulated flower opening in carnation by supplying glucose and fructose, but suppressed it in D. barbatus, probably through the inhibition of general metabolism, similar to the action of α-glucosidase, caused by its excess accumulation.

View full abstract

Floral scents of tulip (Tulipa L.) cultivars are highly diverse, ranging from citrus, honey, or grassy to medicinal. To clarify the diversity of the quality of tulip floral scents, we analyzed the scent compounds of 51 tulip cultivars with characteristic scents. The major scent compounds were five monoterpenoids (eucalyptol, linalool, d-limonene, trans-β-ocimene, and α-pinene), four sesquiterpenoids (caryophyllene, α-farnesene, geranyl acetone, and β-ionone), six benzenoids (acetophenone, benzaldehyde, benzyl alcohol, 3,5-dimethoxytoluene (DMT), methyl salicylate, and 2-phenylethanol), and five fatty acid derivatives (decanal, 2-hexenal, cis-3-hexenol, cis-3-hexenyl acetate, and octanal). Tulip cultivars were classified into nine groups according to the composition of major scent components and sensory assessment of a living flower: group 1, anise; group 2, citrus; group 3, fruity; group 4, green; group 5, herbal; group 6, herbal-honey; group 7, rosy; group 8, spicy; and group 9, woody.

View full abstract

This study was conducted to determine whether the optimal harvest time of pitaya (Hylocereus undatus) could be identified more accurately using the elasticity index as determined by a nondestructive resonant-vibration method. Seven-year-old pitaya grown in beds filled with sand in an unheated greenhouse were used, and pollinated fruit in July (Jul.-PF) and September (Sep.-PF) were harvested at intervals of 4 days during days 20–36 or 20–40 days, respectively, after anthesis. The second resonant frequency, the elasticity index, and flesh firmness decreased with ripening; however, values for Jul.-PF decreased faster than those for Sep.-PF. During the final harvest time, both groups of pollinated fruits had similar values for second resonant frequency (330 Hz), the elasticity index (60×10⁵), and flesh firmness (7 N·cm⁻²). The a-value of peel color for Jul.-PF and Sep.-PF increased rapidly during days 24–32 and 28–36, respectively, after anthesis and thereafter showed almost constant values. Total sugar content in Jul.-PF increased rapidly until 28 days after anthesis and then increased slightly. Total sugar content in Sep.-PF increased later than in Jul.-PF. Organic acid in Jul.-PF and Sep.-PF decreased rapidly until 28 and 36 days, respectively, after anthesis and thereafter decreased slowly. In Sep.-PF that were harvested at the optimal time, the second resonant frequency declined as fruit size increased, but the elasticity index was not affected by fruit size. High positive correlations were found among the second resonant frequency, the elasticity index, and flesh firmness in both groups of pollinated fruit. Considering a comprehensive suite of parameters including pulp rate, peel color, sugar, organic acid, flesh firmness, and occurrence of cracking, the optimal harvest time was estimated to be 36 and 40 days after anthesis for Jul.-PF and Sep.-PF, respectively, and the elasticity index of fruit at optimal harvest time was 62 × 10⁵ and 73 × 10⁵, respectively. This study showed that the optimal harvest time of pitaya could be determined more accurately by measuring the elasticity index of on-tree fruit.

View full abstract