Altered Carbohydrate Allocation Due to Soil Water Deficit Affects Summertime Flowering in Meiwa Kumquat Trees

Naoto Iwasaki,Asaki Tamura,Kyoka Hori

doi:10.3390/horticulturae6030049

Abstract

The summertime flowers of the ever-flowering Meiwa kumquat (Fortunella crassifolia Swingle) are the most useful for fruit production in Japan; however, summertime flowers bloom in three or four successive waves at approximately 10 day intervals, resulting in fruit of different maturity occurring on the same tree. Soil water deficit (SWD) treatment has been shown to reduce the flowering frequency and improve harvest efficiency; therefore, in this study, the effects of SWD treatment on the accumulation of soluble sugars in each tree organ above-ground were examined and it was discussed how SWD affects the whole-tree water relations and sugar accumulation by osmoregulation. The number of first-flush summertime flowers was higher in SWD-treated trees than non-treated control (CONT) trees (177.0 and 58.0 flowers, respectively), whereas the second- and third-flush flowers were only observed in CONT trees. The soluble sugar content was higher in SWD treated trees than CONT trees for all organs and tended to be higher in current-year organs than previous-year organs; however, when the sugar content of the current-year spring stems exceeded approximately 100 mg g−1 dry weight, the current-year leaf water potential decreased sharply and the rate of increase in the number of first-flush flowers also tended to decrease. SWD treatment significantly increased the total sugar content of the xylem tissue of the scaffold branches to three times the value in CONT trees (p = 0.001); however, the increase was observed even in sucrose, a disaccharide, similar to that in monosaccharides such as glucose and fructose. These results suggest that the increased sugar levels in the xylem tissue resulted from not only osmoregulation but also other factors as well; therefore, these sugars may affect whole-tree water relations as well as the development of flower buds.

Highlights

Meiwa kumquat (Fortunella crassifolia Swingle) is an ever-flowering tree that usually blooms in spring (April), summer (June to August), and autumn (September) in Japan
The summertime flowers appear on each axil of the current spring shoots that emerge around May and are the most useful for fruit production [1]; the flower bud differentiation period of the summertime flowers is regarded as approximately 1 month from the termination of spring shoot elongation to flowering [2]; the flowering of summertime flowers is divided into three to four separate time points, and when flowers do not bear fruitlets, new flowers appear at the same axil after 10–14 days
It was found that Soil water deficit (SWD) treatment reduced the frequency of sequential blooming of summertime flowers and increased the number of first-flush summertime flowers in Meiwa kumquat; summertime flowers and increased the number of first-flush summertime flowers in Meiwa the total number of flowers did not significantly change, indicating that SWD promoted flower kumquat; the total number of flowers did not significantly change, indicating that SWD

Summary

Introduction

Meiwa kumquat (Fortunella crassifolia Swingle) is an ever-flowering tree that usually blooms in spring (April), summer (June to August), and autumn (September) in Japan. The spring flowers appear on the previous year shoots, but are very few in number and not usually used for fruit production. The resulting flowers are known as the first-, second-, and third-flush flowers in Japanese production areas [1,3]. The fruits from first-flush flowers tend to grow well and Horticulturae 2020, 6, 49; doi:10.3390/horticulturae6030049 www.mdpi.com/journal/horticulturae. The fruit quality tends to be good in first-flush flowers. Since there are few first-flush flowers—the number of first-flush flowers varies annually—the second-flush flowers are used for fruit production in general [1]

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Conclusion