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内容記述 |
1. INTRODUCTION Calcium (Ca) is one of the essential macronutrients in plants. Ca has roles in stabilizing cell walls and membranes, and as a secondary messenger in signal transduction. Ca transport into plant is driven by transpiration. Therefore, Ca tends to accumulate in high-transpiration tissues (such as older leaves), while low-transpiration tissues (such as young leaves and fruits) often develop Ca deficiency symptoms, even when soil Ca is sufficient. One of the most serious Ca deficiency disorders in agriculture is blossom-end rot (BER) in tomato, which causes necrosis in the distal part of fruit and significantly reduces its commercial values. It has been known that the nutrient delivery to fruit largely depends on phloem rather than xylem. However, it remains unclear whether Ca delivery to fruits is directly via xylem as same as leaves or is translocated via phloem from neighboring leaves. To elucidate Ca transport dynamics in fruit, the real-time and non-destructive analysis is essential. So far, few studies are performed in big-fruit tomato cultivars. 2. MATERIALS AND METHODS To monitor Ca transport dynamics in big-fruit tomato (House momotaro, Takii seeds) in real time manner, we conducted tracer experiments using the radioactive isotope Ca-45 and a compact multi-point detection system under both light and dark conditions (in general, the transpiration is high in light and low in dark) [1] (Fig. 1). The plant was pulse-labeled with Ca-45 at two points on the main stem below the fruit truss, either during the day or at night. Immediately after the pulse, Ca-45 were chased with non radioactive Ca solution for 2–3 days. During the chase period, radioactivity was monitored over time along the several petioles, a peduncle, fruits and main stem (at several points) above the injection site using compact detectors. In addition, BAS imaging (autoradiograph) was also conducted on plant sections at detection points. 3. RESULTS AND DISCUSSION We obtained the time-course data with sufficient signal intensity to construct time-activity curves (TACs), successfully recording Ca dynamics reaching the fruit. In our preliminary data, no substantial difference in TAC initiation timing were observed between the peduncle (adjacent to fruit truss) and the neighboring petiole (a leaf), and both showed comparable radioactivity levels in BAS images. These results suggest that, the transport rate and amount of Ca in peduncle and petiole were similar, implying that lower Ca level in fruits compared to leaves is not caused by reduced transport to the peduncle but rather by another factor/structure between the peduncle to the fruits. Further analysis will focus on the slope and inflection points of TACs to determine transport rates and arrival times for each site and to examine the differences between day- night transport. Through these analyses, we expect to obtain new speculation about the dynamics of Ca in fruits which may provide new evidence for preventing BER in future. In addition, from an economic and practical perspective, we would like to emphasize the usefulness of a compact multi-point detection system for real time monitoring of radio isotope in large plant body, rather than increasing the detector size. This type of detection system is useful to capture the dynamics of radio isotope in complex form plants (including tomato), and we believe there is considerable potential demand for such systems in the field of plant science, especially crop and horticultural research. |
