Phloem transport in Ricinus: Concentration gradients between source and sink

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Abstract
Solute concentration gradients were studied in Ricinus communis L. stems using refractometry linked with cryoscopic measurements of osmotic pressure. Techniques were developed to study the local composition of sieve-tube sap, effectively isolating bark segments by ring-massage or massage-girdling. In intact plants concentration gradients in stems were found to be steep near leaves, as reported previously, and also near roots. However, when transport from these organs was prevented, the steep gradients disappeared. Apparently they are caused by sap influx from nearby organs into stems during sampling. Concentration gradients in stems proper were almost negligible under standard growing conditions. They became positive (more concentrated above) when photosynthesis was enhanced, and negative in darkness. During exudation, leaves could secrete abundant concentrated sap for long periods. Stems had a similar but more limited capacity to secrete and, to a still lesser extent, roots also. Secretion was triggered even from natural sinks by exudation from an incision. Release from turgor pressure seems responsible rather than dilution of the phloem sap, suggesting that in whole plants a sink induces secretion and pressure-flow from distant sources by regulating the pressure within sieve tubes. The rate of exudation decreased as sap concentration fell, indicating that a reduction in pressure corresponds to assimilate exhaustion. The hydraulic conductivity of bark was not high, but was greater when the bark was secreting solutes. A change in mass-flow conduction between sieve tubes and storage cells is proposed to explain these differences in water permeability. The common assumption that solute concentration gradients correspond to pressure gradients seems inapplicable to whole Ricinus plants. Solute transfer can exceed the water flux so that osmotic equilibrium may not be reached at a particular location. In intact plants, solute gradients probably reflect source and sink activity with the interconnecting sieve tubes behaving rather passively, like pipes with a solute exchange-capacity. Actual pressure differences in sieve tubes are best indicated by sap concentration differences between source and sink. In Ricinus plants 0.5–1 m tall these pressures can be 5–10 bar.