Root:shoot balance controls flush phenology and carbohydrate translocation dynamics in citrus (Citrus x sinensis)...

Flush shoot growth presents a fluctuation pattern alternating with root growth. The cyclic pattern determines the balance of root:shoot and can affect the direction and speed of carbohydrate translocation during the vegetative growth period. In this study, we used water deficit to limit corresponding growth in sweet orange (Citrus x sinensis) "OLL 4" grafted on "US-942" rootstock, and then observed the changes of translocation dynamics between two flush statuses. Our first hypothesis was that water deficit would reduce root growth and extend the root growth phase during the growth cycle, delaying the following flush. We then tested the related second hypothesis that shoot flushes would switch the direction and slow the speed of carbohydrate transport due to fluctuation between single and dual sinks. After recovery from a severe deficit, the flush was synchronized and emerged within 2 weeks. Mild and moderate water-deficit plants showed a delayed new flush. Next, we used a ¹⁴ C-labeling method to test whether translocation was affected by the presence of new flush. Basipetal translocation was dominant, but the new flush increased the likelihood of acropetal translocation. Translocation speeds were not different in both directions regardless of flushing status, though speed estimates were highly variable, even though ¹⁴ C export from the source leaf increased when new flush was present. The results suggest that flush timing across an environmental gradient is governed by source-sink dynamics. The presence of new flush altered the direction of photoassimilate translocation and rate of leaf export, but stem transport speeds were not distinguishably different.