Effects of Temperature on Bud Dormancy and Flower Bud Differentiation in Satsuma Mandarin

Abstract

One-year-old potted satsuma trees (cv. Okitsu Wase) on trifoliate orange ootstock were used to examine the relation between dormancy and flower induction of axillary buds under different temperature conditions maintained in growth chambers.
1. A summer growth flush on the spring cycle shoots resulted after defoliation was observed half-monthly from mid-June in the field. Time required for a growth flush was 12 or 13 days when defoliated early, but decreased gradually to several days until mid-September. No growth flush occurred in the field from early October when the air temperature fell below 20°C. Trees with leaves brought forth summer cycle shoots from early July to early August. However, no trees sprouted flower buds.
2. After defoliating and transferring the trees from the field into a 25°C room in the growth chamber, the number of days required for a summer growth flush was determined half-monthly from mid-June. Time required for a growth flush was 10 days when treated until mid-July, whereas it decreased to 3 or 4 days when treated from mid-July to mid-September. Thereafter, days required for a growth flush increased toward the peak (12 days) in late October. Delayed growth flushes around late October seem to imply the dormancy of axillary buds in satsuma trees.
3. Trees in the field were first transferred into 15, 20 and 25°C rooms in mid-June, and then defoliated and transferred again into a 25°C room at half-month intervals. Number of days to a growth flush was recorded. Until mid-July, trees both in the field and in the 25°C room resumed growth about 10 days after defoliation; thereafter the mean number of days for a growth flush decreased to less than 5 days from late July to late August. Until late August, however, the trees kept in the 20 and 15°C rooms for 1.5 months and over required more than 10 days for a growth flush initiated by defoliation. It seems that the dormancy of axillary buds was induced by low temperature condiions. Therefore, flower buds sprouted on the trees defoliated from late July in the 15°C room and from late August in the 20°C room. Regarding 25°C as the threshold temperature for differentiation of flower buds, cumulative temperatures below 25°C necessary for producing 2 flowers per bearing shoot were 750°C, being applicable to fruit production in a heated plastic greenhouse.
4. Trees placed in the 15°C room from mid-June were transferred into a 25°C room at different times. The longer the periods of preliminary 15°C treatments, the fewer the number of days required for regrowth in the 25°C room. Trees with leaves placed in the 15°C room for 2.5 months or more commenced regrowth about 20 days after transferring them into the 25°C room. Flower bud sprouting at 25°C was observed in the pretreatment of 15°C for 2 months or more. In particular, more flowers were produced by the longer periods of 15°C pretreatment.
5. Trees were placed in the 15°C room for 3.5 months from mid-June, and defoliated or left intact with leaves. After defoliation, the trees with or without leaves were kept in the 15°C room, and then transferred to the 25°C room in late September. Defoliation from mid-June to mid-July resulted in sprouting of 2 or 3 summer cycle shoots within 30 days in the 15°C room. When the trees were defoliated after 1.5 months or later in the 15°C room, no summer cycle shoots sprouted. A few days after transferring trees into the 25°C room, vegetative shoots flushed. Flower buds sprouted only in those trees which were defoliated after mid-July. The longer the periods with leaves at 15°C, the larger the number of flowers produced. On the other hand, the shorter the periods with leaves at 15°C, the larger the number of vegetative shoots initiated. Percentage of flower buds to total buds sprouted was higher in the trees with leaves kept for longer periods at 15°C.