Citrus trees alternate between rich and poor crops and are known to be alternate-bearing crops. Alternate bearing results from suppression of flowering due to bearing of fruits. To understand the molecular mechanism how fruit bearing affects flowering, we investigated the relationship between fruits and a flowering-related gene, citrus FLOWERING LOCUS T (CiFT). On trees with different amounts of fruits, the fruit weight/leaf area ratio at harvest was negatively and highly correlated with CiFT expression in the vegetative shoots during fall and winter, which is the period of floral induction. In addition, CiFT expression levels during fall and winter were positively and highly correlated with the flower number the following spring. These results indicate that fruit growth suppresses CiFT expression and decreases the flower number the next spring. In another experiment conducted to determine the effect of the period of fruit bearing on CiFT expression, trees having 3 primary scaffold branches were analyzed. From each branch in 1 tree, all the flowers or fruits were harvested at different times. In November, CiFT was expressed at different levels in each branch, with a tendency to be low in the stem of vegetative shoots from branches that bore fruits for longer periods. This result indicates that a long fruit-bearing period suppresses CiFT expression in vegetative shoots. CiFT expression was detected at much higher levels in fruit-bearing shoots than in vegetative shoots in September. In January, the high levels of CiFT expression in bearing shoots decreased to levels lower than those found in vegetative shoots. Thus, in fruit-bearing shoots, the CiFT expression of an unknown relationship to floral induction is observed during the period before floral induction and the seasonal change of CiFT expression in fruit-bearing shoots is different from that in vegetative shoots.

In citrus trees, a technique for predicting the number of flowers is needed to prevent inconsistent fruit production over time. In this study, we investigated whether a flowering-related gene, citrus FLOWERING LOCUS T (CiFT), can be used to predict floral production. We quantified CiFT expression levels in stems of Satsuma mandarin (Citrus unshiu Marc.) trees planted at two remote locations. We collected samples over 3 years, and determined that the CiFT mRNA levels in November were strongly correlated with the number of flowers produced the following spring. The correlation between CiFT expression and floral production exhibited almost no differences among years or between sample collection sites. Thus, the CiFT expression level in November can be used to predict the number of flowers the following spring, and this prediction method can be used at different locations and over several years. To verify the practical utility of this method, Satsuma mandarin shoots were transported in November from eight locations across Japan to our laboratory under cool conditions. Transport times were approximately 4-72 h, and samples were immediately frozen upon arrival. The CiFT expression levels in transported stems correlated with the number of flowers in the following spring. Additionally, those correlations were similar in transported samples and samples that were frozen immediately after collection. Our results suggest that the number of flowers can be predicted even with the CiFT expression levels quantified in transported samples.

In citrus, fruit bearing affects floral induction and the nutritional condition of the tree. For this reason, bearing too many or too few fruit causes fluctuation in flower number the following spring. This in turn leads to annual alternation between rich and poor crops, known as alternate bearing. To identify the metabolites related to alternate bearing, quantitative metabolomics analysis was conducted with stem tissues of vegetative shoots collected in November. Twelve Satsuma mandarin trees bearing different amounts of fruit were used in this study. Fruit weight per leaf area of these trees was significantly and negatively correlated with the expression of a flowering-related gene, citrus FLOWERING LOCUS T, in the stem in November and the number of flower buds the following spring. In metabolomics analysis, adenosine triphosphate was detected at high concentrations in lightly fruiting trees. Other coenzymes such as uridine triphosphate, nicotinamide adenine dinucleotide phosphate, and ascorbic acid were also more abundant in the off-crop trees. In addition, the off-crop trees accumulated sugar phosphates such as fructose 6-phosphate, glucose 6-phosphate, and ribulose 1,5-diphosphate. Furthermore, heavily fruiting trees accumulated more amino acids. These results indicate that fruit bearing affects the metabolism of coenzymes, sugars, and amino acids in the stem of vegetative shoots.

We studied the influence of a 2°C increase in air temperature on physiological fruit drop in Citrus using a growth chamber for two months after full bloom. One room of the growth chamber was maintained at the standard temperature (control), while the other was maintained at 2°C higher than the control (+2°C treatment). The average temperature was changed every ten days. In 'Okitsu wase', 'Ishiji', 'Shirakawa' Satsuma mandarin (Citrus unshiu Marcow.), and 'Shiranui' [(Citrus unshiu Marcow. × C. sinensis Osbeck) × C. reticulate Blanco], physiological fruit drop occurred more intensively in +2°C treatment than in the control after 10-20 days of full bloom. At the end of the experimental period, the physiological fruit drop ratio in plants receiving +2°C treatment was higher (5%) than that in controls for 'Okitsu wase' and 'Ishiji'. In 'Miyagawa wase' and 'Sasebo unshiu' Satsuma mandarin, in which number of fruit was controlled by artificial fruit thinning, physiological fruit drop also occurred more intensively in plants receiving +2°C treatment than in controls after 10-20 days of full bloom. Especially in 'Sasebo unshiu' at the end of the experimental period, the difference in the physiological fruit drop ratio between plants receiving +2°C treatment and controls was larger (26%) than that in 'Miyagawa wase'. In 'Miyagawa wase' and 'Sasebo unshiu' fruit diameter of plants receiving +2°C treatment was larger than that of controls. These findings suggested that with acceleration of fruit enlargement, and physiolosical fruit drop were both promoted by a 2°C increase in air temperature after full bloom.

Assessment of the tree vigor of grafted rootstocks in citrus requires intense labor, a long period, and large fields because tree vigor is judged through cultivation tests with adult trees. We have developed a method to predict tree vigor at an early stage based on the stem hydraulic conductance of 5-month-old seedlings. We used 15 cultivars (strains), including 11 crossed strains, 3 cultivars of commonly used trifoliate orange (common type, 'Pomeroy', and 'USDA'), and 'Swingle citrumelo'. The growth characteristics of 7-year-old satsuma mandarin trees grafted onto those rootstocks were compared with those of 5-month-old rootstock seedlings. The results revealed a relatively high correlation (r 2 = 0.633) between the trunk circumference of 7-year-old trees and the stem hydraulic conductance of seedlings, and furthermore, control cultivars existed approximately on that regression line. Therefore, it seems possible to predict tree vigor from the stem hydraulic conductance of seedlings in a very short period without using a large field and intense labor.