PMC8861528_01
Locations of prefectures where Japanese pear is produced. Black markers indicate main production prefectures (>8,000 tons) [statistics from MAFF]1. White markers indicate northernmost and southernmost prefectures producing Japanese pear. Squares indicate locations of flowering disorder surveys.
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F1
In Fukuoka Prefecture (Figure 1 and Table 1), about 25% of 'Kosui' trees are grown in greenhouses [statistics from Fukuoka Prefecture]3, and flowering disorder occurs every year and has become a serious problem.
Typical symptoms of flowering disorder at blooming. (A) Healthy (normal) flower. Pear flower bud is a mixed flower bud, with one or two floral primordia (constituted of several florets) and sometimes leafy primordia. (B-D) Flowers with only one or two florets blooming. (E,F) Flowers with shortened peduncle. (G) Flowers with all florets aborted and only elongated leaf primordia. (H) Flower buds located at distal parts (on long or spur shoots) bloomed but those on basal parts were delayed or did not flower.
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F2
Because this disorder mainly occurs in reproductive organs (flowers) but not or rarely in vegetative organs (leaves) (see Figure 2G), we hereafter refer to this physiological disorder as "flowering disorder." However, because significant changes in floral organ formation are not the main symptoms of this disorder, it is still unclear whether it should be defined as a flowering disorder or a dormancy disorder. Conversely, in trees with a low flowering rate, delayed flower bud break, dwarf floral organs, decreased number of florets, and flower bud abortion occurred, like in field-grown trees (Figure 2). This may explain why disorders occur more frequently in floral growth than in vegetative growth (see Figure 2G).
Mean temperature and temperature differences over the last 60 years. (A) Changes in decadal mean temperature in main production areas of Japanese pear. (B) Seasonal increase in mean temperature from the 30-year period in 1960-1989 to the subsequent 30-year period (1990-2019). Temperature data were recorded at Japan Meteorological Agency observatories [statistics from JMA]a.
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F3
The changes in the decadal mean temperature in the main Japanese pear production areas did not change noticeably from the 1960s to the 1980s, but rose markedly in the 1990s and have continued to rise gradually ever since (Figure 3A). Figure 3B shows the seasonal increase in mean temperature from the 30-year period of 1960-1989 to the subsequent 30-year period (1990-2019).
Endodormancy breaking date of Japanese pear 'Kosui' in Tsukuba. Endodormancy breaking date estimated from the DVR model and observed hourly temperature. DOY, day of the year (number of days from January 1).
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F4
Typical appearance of flower buds of two cultivars after removing outer scale leaves. Trees were grown at five localities and observed in the 2015-2016 season. Rectangles indicate floret damage, ovals indicate floret abortion. (A) Healthy florets, (B,C) injured florets, (D,E) (distal) florets aborted in winter (modified from).
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F5
The floret damage/injuries observed during dormancy could be classified into two types: (ii) "floret injury," where the surface and/or the inside of the floret was partly or completely brown (e.g., Figures 5B,C); and (ii) "floret abortion," where the floret was shrunken, completely brown, or had dropped from the base (e.g., Figures 5D,E).
Proportion of injured florets out of total number of florets counted at blooming (observed in March 2016) (modified from).
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F6
The incidence of floret injury was similar in 'Kosui' and 'Niitaka' at the same latitude (Figure 6) and showed the highest rate at Kagoshima (lat. 31 N).
Seasonal changes in freezing tolerance of axillary flower buds of two cultivars grown at five or three localities (2015-16 season) (modified from).
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F7
The seasonal patterns of flower bud freezing tolerance (LT50) were quite similar between 'Kosui' and 'Niitaka' at the same locations (Figure 7).
Relationship between chill units and rate of distal floret abortion in 'Kosui' (open symbols) and 'Niitaka' (closed symbols) (modified from).
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F8
The rate of distal floret abortion (number of aborted florets/number of differentiated florets) was negatively correlated with CU up to approximately 1,900 in 'Kosui' and 2,500 in 'Niitaka' (Figure 8), suggesting that insufficient chilling may have caused distal floret abortion.
Annual trends in severe tree ratez in heated greenhouses (A-H greenhouses) (taken from with the permission by JSHS). zTrees with a flowering rate of lower than 30% were defined as "severe flowering disorder trees." Proportion of severe trees out of all trees in the greenhouse was defined as the "severe tree rate."
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F9
Trees with a flowering rate of lower than 30% were defined as "severe flowering disorder trees," and the proportion of these trees out of all trees in the greenhouse was defined as the "severe tree rate." A survey of eight greenhouses (greenhouses A-H) revealed that the "severe tree rate" varied widely from 0 to 58.7% (the maximum was in greenhouse C in the 2015-2016 season), and the rate differed among years and among greenhouses (Figure 9).
Changes in dormancy- and flowering-related gene expression in normal trees (NTs) and flowering disorder trees (FDTs) (taken from with the permission by JSHS). (A) Transcript levels of PpFT2a.
(B) Transcript levels of PpMADS13-3. Relative gene transcript levels were normalized to that of PpHistonH3. Vertical bar indicates standard error (n = 3-4 axillary flower buds). Different letters indicate significant differences at 5% significance level (Tukey-Kramer test).
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F10
Analyses of gene expression in the flower buds of dormant branches revealed lower transcript levels of PpFT2a, a flowering-related gene, in FDTs than in NTs just before flowering (February 27, 2017) (Figure 10A). Flower buds in NTs sampled on this date showed very low transcript levels of DAM (MADS13-3) (Figure 10), suggesting that exposure to temperatures over 20 C decreased the transcription of this gene.
Depth of dormancy in normal trees (NTs) and flowering disorder trees (FDTs) (taken from with the permission by JSHS). (A) Dormancy depth and (B) changes in dormancy depth. Vertical bar indicates standard error (n = 3).
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F11
The dormancy depths of NTs deepened on November 11 and February 3, which were considered to be the deepest stages of endo- and eco-dormancy, but that of FDTs remained shallow until February 3, before the trees were covered with plastic film, and the dormancy stage was unclear (Figure 11).