Sheet-mulching cultivation promotes the number of floral buds via up-regulation of Citrus Flowering Locus T expression in two citrus cultivars, ‘Haraguchi-wase’ (Citrus unshiu Marcow.) and ‘Harehime’ (C. spp.)

Sheet-mulching cultivation during the fruit developmental stage is often carried out to produce high-quality Satsuma mandarin (Citrus unshiu Marc.) and ‘Harehime’ ((‘Kiyomi’ × ‘Osceola’) × ‘Miyagawa-wase’) fruits because they show high Brix% by exposure to drought stress conditions. In this study, we investigated the effect of drought stress on the number of floral buds in ‘Haraguchi-wase’ Satsuma mandarins and ‘Harehime’. To clarify the relationship between drought stress and the number of floral buds, we applied four different drying treatments to the fruit trees, (i) first-half drying (drying treatment during the first-half of the fruit development stage), (ii) second-half drying (drying treatment during the second-half of the fruit development stage), (iii) all-drying (drying during the full fruit development stage), and (iv) well-watered (non-drought stress during the fruit development stage). The drying treatment was applied to the fruits at ψ max -0.7 to -1.2 MPa at an intensity comparable to proper drought stress for high-quality fruit production according to our previous studies. In ‘Haraguchi-wase’, the all-drying treatment produced a higher number of floral buds in the following spring season compared with the well-watered treatment, in which the increase in the number of floral buds took place concomitant with the enhancement of CiFT expression in December. As expected, the fruits after all-drying were smaller with a significantly higher Brix% and a similar level of citric acid content, suggesting that drought stress of suitable intensity resulted not only in high-quality fruit production, but also in an increased number of floral buds in the following spring season. The results also demonstrated that the first-half drying caused higher CiFT expression and more inflorescences than in the well-watered plants, while there were no differences in CiFT expression or number of floral buds between the second-half drying and well-watered treatments. These tendencies observed in ‘Haraguchi-wase’ were the same as in ‘Harehime’. Therefore, the drying treatment during the first-half of fruit development could be an effective method to increase the number of floral buds.