Basipetal auxin versus acropetal cytokinin transport, and their interaction with NO 3 fertilisation in cotyledon senescence and sink:source relationships in cucumber ( Cucumis sativus L.)

The paramount role of cytokinins ( CK s) in initiation, as well as prevention, of senescence is well established. In recent years, experimental methods have become available to raise and lower the CK concentration and experimentally manipulate senescence. Decapitating the apical shoot and adding the synthetic auxin naphthylacetic acid to the cut stem reduced endogenous CK s to low levels. Conversely, if no auxin was applied, xylem and leaf CK levels increased dramatically, indicating that basipolar auxin transport is a key determinant in the synthesis of CK s and is potentially more important than NO 3 . Manipulating the concentration of applied NO 3 caused considerable variation in leaf CK levels and concomitant changes in senescence. These and other results suggest that the frequently discussed decrease in nitrogen use efficiency ( NUE ) may be more highly regulated by CK s than by NO 3 . Analysis of the re‐metabolisation and re‐allocation of chlorophyll, proteins, amino acids and starch in three different cucumber cultivars indirectly showed that these metabolites were significantly affected by the concentration of CK s in the leaves. Further research in this area may allow leaf senescence and plant yield to be more efficiently regulated by manipulating CK s and/or basipolar auxin transport instead of nitrate.