Sucrose transport into the phloem of Ricinus communis L. seedlings as measured by the analysis of sieve-tube sap

Abstract

Careful cutting of the hypocotyl of Ricinus communis L. seedlings led to the exudation of pure sieve-tube sap for 2-3 h. This offered the possibility of testing the phloem-loading system qualitatively and quantitatively by incubating the cotyledons with different solutes of various concentrations to determine whether or not these solutes were loaded into the sieve tubes. The concentration which was achieved by loading and the time course could also be documented. This study concentrated on the loading of sucrose because it is the major naturally translocated sieve-tube compound. The sucrose concentration of sieve-tube sap was approx. 300 mM when the cotyledons were buried in the endosperm. When the cotyledons were excised from the endosperm and incubated in buffer, the sucrose concentration decreased gradually to 80-100 mM. This sucrose level was maintained for several hours by starch breakdown. Incubation of the excised cotyledons in sucrose caused the sucrose concentration in the sieve tubes to rise from 80 to 400 mM, depending on the sucrose concentration in the medium. Thus the sucrose concentration in the sieve tubes could be manipulated over a wide range. The transfer of labelled sucrose to the sieve-tube sap took 10 min; full isotope equilibration was finally reached after 2 h. An increase of K(+) in the medium or in the sieve tubes did not change the sucrose concentration in the sievetube sap. Similarly the experimentally induced change of sucrose concentration in the sieve tubes did not affect the K(+) concentration in the exudate. High concentrations of K(+), however, strongly reduced the flow rate of exudation. Similar results were obtained with Na(+) (data not shown). The minimum translocation speed in the sieve tubes in vivo was calculated from the growth increment of the seedling to be 1.03 m·h(-1), a value, which on average was also obtained for the exudation system with the endosperm attached. This comparison of the in-vivo rate of phloem transport and the exudation rate from cut hypocotyls indicates that sink control of phloem transport in the seedlings of that particular age was small, if there was any at all, and that the results from the experimental exudation system were probably not falsified by removal of the sink tissues.

Gradients of Turgor, Osmotic Pressure, and Water Potential in the Cortex of the Hypocotyl of Growing Ricinus Seedlings

“…It appeared that removing the endosperm improved the water status of the hypocotyl by reducing the hydraulic resistance ofthe cotyledons. It is well known from other transport studies that the cuticle ofyoung cotyledons ofRicinus is fairly permeable for water and hydrophilic solutes such as sucrose (13,15). No significant differences of turgor of the inner cortex cell layers were found along the axes of the seedlings, i.e.…”

“…As discussed, the change should have been mainly due to a dilution of the wall solution. The experiment allows an estimate of the apoplasmic solute concentration as it was in the cortex before pressurization: 0.7 MPa correspond to a 0.28 M solute concentration, which is close to the concentration of sieve tube sap of seedlings (13). This high apoplasmic concentration only builds up after root excision.…”

“…For Ricinus, it has been shown that the flow of solutes from the cotyledons via phloem to the growing tissue of the hypocotyl is considerable. The plant has been used to study phloem loading and unloading processes (13). In relation to growth, the question is: how does the supply of solutes from the phloem interact with growth processes?…”

An aphid's Odyssey – the cortical quest for the vascular bundle

“…Ricinus communis (L. cv. Sanguineus; Benary Samenzucht GmbH, Hannoversch Münden, Germany) seedlings were cultivated under the above-mentioned conditions except that water-soaked seeds were planted in wet vermiculite in a humid atmosphere at 27°C (Kallarackal et al, 1989).…”

“…Kallarackal et al, 1989) containing 300mmoll -1 sucrose, 500mmoll -1 glucose or 1000mmoll -1 glucose. After incubation times of various length (2h, 4h, 8h, 20h and 24h), the seedlings were cut at the hypocotyl hook and the spontaneously exuding sap was collected by disposable 1l and 5l capillaries (Hirschmann Laborgeräte, Eberstadt, Germany).…”