Basipetal auxin versus acropetal cytokinin transport, and their interaction with NO3 fertilisation in cotyledon senescence and sink:source relationships in cucumber (Cucumis sativus L.)

The paramount role of cytokinins (CKs) 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 CKs 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 CKs 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 CKs 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 CKs in the leaves. Further research in this area may allow leaf senescence and plant yield to be more efficiently regulated by manipulating CKs and/or basipolar auxin transport instead of nitrate.

Correlations in concentrations, xylem and phloem flows, and partitioning of elements and ions in intact plants. A summary and statistical re-evaluation of modelling experiments in Ricinus communis

Within the last two decades, a series of papers have dealt with the effects of nutrition and nutrient deficiency, as well as salt stress, on the long-distance transport and partitioning of nutrients in castor bean. Flows in xylem and phloem were modelled according to an empirically-based modelling technique that permits additional quantification of the uptake and incorporation into plant organs. In the present paper these data were statistically re-evaluated, and new correlations are presented. Numerous relationships between different compartments and transport processes for single elements, but also between elements, were detected. These correlations revealed different selectivities for ions in bulk net transport. Generally, increasing chemical concentration gradients for mineral nutrients from the rhizosphere to the root and from the xylem to leaf tissue were observed, while such gradients decreased from root tissue to the xylem and from leaves to the phloem. These studies showed that, for the partitioning of nutrients within a plant, the correlated interactions of uptake, xylem and phloem flow, as well as loading and unloading of solutes from transport systems, are of central importance. For essential nutrients, tight correlations between uptake, xylem and phloem flow, and the resulting partitioning of elements, were observed, which allows the stating of general models. For non-essential ions like Na(+) or Cl(-), a statistically significant dependence of xylem transport on uptake was not detected. The central role of the phloem for adjusting, but also signalling, of nutrition status is discussed, since strong correlations between leaf nutrient concentrations and those in phloem saps were observed. In addition, negative correlations between phloem sap sugar concentration and net-photosynthesis, growth, and uptake of nutrients were demonstrated. The question remains whether this is only a consequence of an insufficient use of carbohydrates in plants or a ubiquitous signal for stress in plants. In general, high sugar concentrations in phloem saps indicate (nutritional) stress, and high nutrient concentrations in phloem saps indicate nutritional sufficiency of leaf tissues.

Influence of different nutrient regimes on the regulation of carbon metabolism in Norway spruce [Picea abies (L.) Karst.] seedlings

Phosphoenolpyruvate carboxylase (PEPC) activity, fructose 2, 6-bisphosphate (F26BP), starch and soluble sugar contents were determined m needles and roots of Norway spruce seedlings grown in a semi-hydroponic cultivation system under different nutrient regimes, tn needles, a surplus of nitrogen caused an increase in specific PEPC activity (up to six times control activity) and F26BP content (up to three times control level) while starch content was reduced. Sucrose contents were not affected. Basically, the responses in root samples were similar. Here, PEPC was highest at an imbalance in nutrition (+ N/ -P) F26BP, with root contents being 3- to 11 -times higher than those in needles, significantly exceeded control values at + N/+ P. The results show that alteration of nitrogen supply leads to marked changes in allocation of carbon between pathways, which is also influenced by P-nutrition. Pool sizes of F26BP and activity of PEPC are indicators for these changes in leaf as well as in root tissues of Norway spruce.