Short-Term Fluctuations in the Transport of Assimilates to the Ear of Wheat Measured with Steady-State 11 C-CO 2 -Labelling of the Flag Leaf

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Abstract
In order to study the regulation of photosynthate import into the ear of wheat (Triticum aestivum L. cv. Anza), a system was developed in which the flag leaf, the major source of photosynthate for the ear, was provided with steady-state levels of 11C-CO2 for periods of several hours each time. 11C is a short-lived isotope (1½ c. 20·3 mm) whose breakdown products (following positron annihilation) include paired, high energy γ-rays (0·5 MeV). Consequently, the movement of radioactive photosynthate throughout the plant and into the ear could be studied in vivo during multiple labelling sessions over the development of the ear with detectors located at various positions around the plant. It was found that import into the ear was not constant in the light during periods of rapid ear filling, even though flag leaf photosynthesis and export were constant. In more than 50% of all cases, import of 11C-photosynthate into the ear took the form of large, regular oscillations with period lengths on the order of 70 mm. Furthermore, oscillations 180° out-of-phase with those in the ear were observed in portions of the stem below the point of flag leaf insertion, suggesting that regulation of oscillation may involve shifting the direction of transport between ear and lower plant parts. Rhythms in photosynthate import into the ear do not appear to be synchronized to immediate fluctuations in the environment, since oscillations detected simultaneously in neighbouring plants were not synchronized and had different period lengths.

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