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

Abstract

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.

Two Phloem Nitrate Transporters, NRT1.11 and NRT1.12, Are Important for Redistributing Xylem-Borne Nitrate to Enhance Plant Growth

“…9, B and C), indicating that xylem-to-phloem transfer mediated by NRT1.11 and NRT1.12 for nitrate redistribution in leaves is important for high-nitrate-enhanced shoot growth. The amino acid concentrations in the phloem sap can be 30 to 100 times higher than that of nitrate (Allen and Smith, 1986;Hayashi and Chino, 1986;Shelp, 1987;Peuke, 2010). Although nitrate is not the major nitrogen in the phloem, the growth phenotype of nrt1.11 nrt1.12 double mutants suggested that phloem-mediated nitrate redistribution is not dispensable, and compared with amino acid, nitrate may have additional effects on young leaf growth as a signaling molecule, osmoticum, and/or regulator of redox stasis.…”

Nitrogen Nutrition, Its Regulation and Biotechnological Approaches to Improve Crop Productivity

“…The nitrogen status of the plant regulates genes coding for nitrate transporters and N-assimilatory enzymes at transcriptional, translational and post-translational levels [276] [277]. Expression of nitrogen assimilatory genes nitrate reductase, nitrite reductase, glutamate synthase and carbon metabolism genes are induced by nitrate [136] [278]- [283]. Assimilation of one molecule of 3 NO − generates 0.67 molecule of OH − which is effluxes into the soil from the roots.…”

Interplay of Water and Nutrient Transport: A Whole-Plant Perspective

“…From these data, xylem and phloem fluxes as well as total net uptake by the root can be estimated for the whole lifetime of the plant [summarised, e.g. by Peuke (2010)]. A critical point is that the composition of vascular saps may be highly variable with time and data just represented a-somewhat arbitrary-snapshot, whereas the nutrient content of root and shoot tissue has evolved over the whole lifetime of that plant.…”