1976 Volume 45 Issue 3 Pages 261-266
From late autumn to early summer, seasonal changes in leaf water potential, leaf diffusion resistance and the growth of two current shoot and trunk were examined in satsuma mandarin (Citrus unshiu MARC.) trees in well watered soil.
1. Leaf water potential before sunrise (ψmax) became lower than -3 bars from mid-November when the soil temperature fell below 12°C and leaf diffusion resistance increased sharply. This indicates that the water metabolism of satsuma mandarin trees became increasingly inactive from mid-November. ψmax was lowest between January and February, and reached around -3 bars in mid-April when the soil temperature surpassed 11°C. Thereafter, leaf water potential during the daytime (ψmin) remained stable in the range of -4 to -5 bars until early in June (during the summer, ψmax and ψmin show about -3 bars and -10 to -13 bars, respectively, under wet soil conditions).
2. Leaf diffusion resistance began to decrease sharply from late March, but average resistance was approximately 30sec/cm in mid-April when ψmax reached about -3 bars. Near recovery of leaf diffusion resistance to summer level (4 to 8 sec/cm) did not occur until June. On the other hand, diffusion resistance of juvenile leaves whose length attained 3 to 5cm showed around 15sec/cm and reached values similar to the summer level in eary June.
3. Sprouting was observed in early April when ψmax reached about -4 bars. During the period of sprouting and blossom opening, the diameter of shoot and trunk decreased slightly. This suggests that energies for sprouting and flowering depend mainly upon the reserve nutrients in a tree.