Nitrogen Remobilization and Proteolytic Enzymes in the Flag Leaves of Detached Wheat Shoots: Effects of Vascular Connections and Environmental Conditions

Senescence, nitrogen remobilization and proteolytic activities in the flag leaf lamina of detached wheat shoots were investigated in different environments. Source/sink relations were modified by cutting below the uppermost node ( +node/ +ear), above this node (- node) or below the node with simultaneous ear removal (+ node/-ear). After 144 h at low night and high day temperature senescence was most advanced in +node/+ ear, while the chlorophyll content remained highest in +node/-ear. At lower day temperatures chlorophyll was degraded most rapidly in -node, but total soluble proteins decreased in a similar manner in shoots with or without node. Amino acids accumulated to the 3-fold level in the - node lamina, while the contents decreased slightly in + node treatments. Endopeptidase activities increased to the highest level in - node leaves. The environmental conditions caused only minor differences in this respect. Carboxypeptidase activity decreased in a manner similar to that of total soluble proteins.
The results indicate that various types of interactions depending differently on environmental conditions may affect senescence and catabolic processes in leaves. The importance of the node and of the ear suggests that the export of solutes through the phloem could play a major role.