Effect of Phloem Interruption on Senescence and Protein Remobilization in the Flag Leaf of Field-grown Wheat

The phloem represents a link between secescing leaves (sources) and maturing (major sinks) in plants characterised by monocarpic senescence such as cereals. The redistribution of nutrients and assimilates might be involved in the control of senescence on a whole plant level. Field-grown winter wheat was steam-girdled at the base of the flag leaf lamina or below the ear in order to interrupt the phloem at these positions and to identify effects on flag leaf senescence. A phloem interruption below the ear delayed senescence of the flag leaf lamina as judged by chlorophyll contents and activities of assimilatory enzymes (phosphoribulokinase, glutamine synthetase). The rate of net photosynthesis was lowered in these leaves during the first week after the treatment, recovered and remained afterwards higher than in those of untreated control plants. Steam-girdling at the base of the leaf initiated its rapid senescence. The rate of photosynthesis declined rapidly after this treatment and was completely lost within one week, while dark respiration was initially slightly stimulated and decreased later than photosynthesis. The good correlations between enzyme activity measurements and immunoblot analyses indicate that not only the activities, but also the quantities of assimilatory enzymes were reduced. The interruption of source/sink relations affected flag leaf senescence, but the effects strongly depended on the position of the treatment. Leaf constituents were rapidly degraded after steam-girdling at the leaf base, although the export of breakdown products became impossible.