Seasonal Changes in the Dynamic State of Water for Excised Cherry Branches (Prunus lannesiana) Observed Using Dedicated Micro-Magnetic Resonance Imaging

Abstract

Seasonal changes in the dynamic state of water for cherry (Prunus lannesiana), a deciduous broad-leaved tree, were studied by combined k-space and q-space imaging using dedicated magnetic resonance imaging (MRI) with a 1.0-T permanent magnet. Water amounts, diffusion coefficients and transpiration were examined for excised branches cut from a tree with and without weak light (100–140 μmol m-2 s-1) throughout the year. The water amount in the cambium was large in spring and summer, decreased in autumn, and decreased further in winter. There were three components in the diffusion coefficient of the branch. The second component of the diffusion coefficient ascribed to the cambium did not fluctuate notably throughout the year, despite marked alternation in water amounts. However, diffusion coefficients in the secondary xylem, the primary component, were elevated in summer and decreased in winter. Upward water flow was restricted in the secondary xylem, and the positions where large flow was detected coincided with places exhibiting high diffusion coefficients and the arrangements of vessels. Total transpiration exhibited a tendency similar to that of the diffusion coefficients; however, total transpiration declined to zero when the plant had no leaf, whereas the diffusion coefficient decreased to 60 % of the maximum but did not decrease further. Light-enhanced transpiration related to potential photosynthetic activity increased in spring as the leaves sprouted and grew, considerably decreased in summer, decreased to one third of the maximum in autumn, and was not detected in winter. Measurement of the dynamic state of water for the excised branches will provide useful information for better understanding of the phenological changes of tree physiology.