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

Abstract

To evaluate the possible role of solute transport during extension growth, water and solute relations of cortex cells of the growing hypocotyl of 5-day-old castor bean seedlings (Ricinus communis L.) were determined using the cell pressure probe.Because the osmotic pressure of individual cells (ir') was also determined, the water potential (4/) could be evaluated as well at the cell level. In the rapidly growing part of the hypocotyl of wellwatered plants, turgor increased from 0.37 megapascal in the outer to 1.04 megapascal in the inner cortex. Thus, there were steep gradients of turgor of up to 0.7 megapascal (7 bar) over a distance of only 470 micrometer. In the more basal and rather mature region, gradients were less pronounced. Because cell turgor = wr and #~0 across the cortex, there were also no gradients of i' across the tissue. Gradients of cell turgor and iri increased when the endosperm was removed from the cotyledons, allowing for a better water supply. They were reduced by increasing the osmotic pressure of the root medium or by cufting off the cotyledons or the entire hook. If the root was excised to interrupt the main source for water, effects became more pronounced. Gradients completely disappeared and turgor fell to 0.3 megapascal in all layers within 1.5 hours. When excised hypocotyls were infiltrated with 0.5 millimolar CaCI2 solution under pressure via the cut surface, gradients in turgor could be restored or even increased. When turgor was measured in individual cortical cells while pressurizing the xylem, rapid responses were recorded and changes of turgor exceeded that of applied pressure. Gradients could also be reestablished in excised hypocotyls by abrading the cuticle, allowing for a water supply from the wet environment. The steep gradients of turgor and osmotic pressure suggest a considerable supply of osmotic solutes from the phloem to the growing tissue. On the basis of a new theoretical approach, the data are discussed in terms of a coupling between water and solute flows and of a compartmentation of water and solutes, both of which affect water status and extension growth.

Direct Demonstration of a Growth-Induced Water Potential Gradient

“…This conclusion differs from the recent one of Meshcheryakov et al (1992), who observed 9, close to zero in the cortex of the elongating tissues of castor bean seedlings using a pressure probe. Because water clearly was being absorbed by the tissue, Meshcheryakov et al (1992) concluded that gradients probably were present but were too small to detect.…”

“…Because water clearly was being absorbed by the tissue, Meshcheryakov et al (1992) concluded that gradients probably were present but were too small to detect. We cannot explain the small magnitude of the gradients in castor bean.…”

“…gradients between enlarging cells in soybean stems (Nonami and Boyer, 1989b) was not confirmed in castor bean (Meshcheryakov et al, 1992) or pea (Malone and Tomos, 1992) when measurements were made with a pressure probe. Therefore, to determine whether the gradients exist, we used a pressure probe to make detailed measurements of the gradients and checked them with a thermocouple psychrometer.…”

Mannitol inhibits growth of intact cucumber but not pea seedlings by mechanically collapsing the root pressure

“…Although Fx has repeatedly been suggested as a controlling factor in stem and leaf growth (e.g. Okamoto et al 1984 Nonami & Boyer 1990;Heydt & Steudle 1991;Meshcheryakov et al 1992, Pardossi et al 1994Chazen et al 1995), no consensus has been reached on whether a Fĉ hange is able to cause a lasting regulatory growth adaptation or only a transient, and hence inconsequential, response of both turgor pressure and growth (e.g. Termaat etal.…”

Is negative turgor fallacious?

“…It seems likely that the vastly different values of apoplastic solute concentrations given in the literature depend on the ohject and the method of determination (Meshcheryakov et al 1992). Canny (1995) argued that the reliability of sampling of apoplastic spaces is questionable, because the term apoplast has lost its precise meaning and begins to mean different spaces (with quite different properties) to different readers.…”