Growth Rate and Turgor Pressure

The water transport properties of etiolated pea (Pisum sativum L.) internodes were studied using both dynamic and steady-state methods to determine (a) whether water transport through the growing tissue limits the rate of cell enlargement, and (b) whether auxin stimulates growth In part by increasing the hydraulic conductance of the growing tissue.Measurements using the pressure probe technique showed that the From the results of these dynamic and steady-state experiments, we conclude that the internal gradient in water potential (from the xylem to the epidermis) needed to sustain cell enlargement is small (no greater than 0.5 bar). Thus, the hydraulic conductance of the tissue is sufficiently large that it does not control or limit the rate of cell enlargement.From a physical standpoint, the enlargement of plant cells during growth requires two fundamental processes: transport of water into the expanding cells (inasmuch as enlarging plant cells are >90%o water) and irreversible yielding of the cell wall which surrounds the protoplast to accommodate the inflowing water (16,23). Although it is often assumed that water transport through plant tissues is so rapid that growth is controlled entirely by the yielding properties ofthe plant cell wall, in principle either process may limit the rate of cell enlargement.In plant tissues where the resistance to water movement is sufficiently large to impede the rate of cell expansion, a large gradient in water potential along the transport pathway would

Section: Resultsmentioning

confidence: 99%

Osmotic Properties of Pea Internodes in Relation to Growth and Auxin Action

Cosgrove

Cleland²

1983

“…2). Thus, as pointed out in principle by Green and Cummins (13), elaboration of the relationship between HII and water uptake (= growth) rate under the influence of growth-controlling agents can theoretically provide information on whether these agents act on the growth coefficient (determined by cell wall extensibility and tissue water conductance) or on the growth potential (determined by the osmotic pressure and the threshold pressure for irreversible deformation of the cell). This point is illustrated in Figure 2.…”

mentioning

confidence: 99%

Control of Seed Germination by Abscisic Acid

Schöpfer¹,

Plachy²

1985

144

The physical mechanism of seed germination and its inhibition by abscisic acid (ABA) in Brassica napus L. was investigated, using volumetric growth (= water uptake) rate (dV/dt), water conductance (L), cell wall extensibility coefficient (m), osmotic pressure (IIi), water potential (*i), turgor pressure (P), and minimum turgor for cell expansion (Y) of the intact embryo as experimental parameters. dV/dt, Hi, and *i were measured directly, while m, P, and Y were derived by calculation. Based on the general equation of hydraulic cell growth IdV/dt = Lm/(L + m) (Al -Y), where All = Hi -II of the external medium; the terms (L m/(L + m) and II -Y were defined as growth coefficient (kG) and growth potential (GP), respectively. Both kG and GP were estimated from curves relating dV/dt (steady state) to I of osmotic test solutions (polyethylene glycol 6000).During the imbibition phase (0-12 hours after sowing), kG remains very small while GP approaches a stable level of about 10 bar. During the subsequent growth phase of the embryo, kG increases about 10-fold. ABA, added before the onset of the growth phase, prevents the rise of kG and lowers GP. These effects are rapidly abolished when germination is induced by removal of ABA. Neither L (as judged from the kinetics of osmotic water efflux) nor the amount of extractable solutes are affected by these changes. Ili and *i remain at a high level in the ABA-treated seed but drop upon induction of germination, and this adds up to a large decrease of P, indicating that water uptake of the germinating embryo is controlled by cell wall loosening rather than by changes of II; or L. ABA inhibits water uptake by preventing cell wall loosening. By calculating Y and m from the growth equation, it is further shown that cell wall loosening during germination comprises both a decrease of Y from about 10 to 0 bar and an at least 10-fold increase of m. ABA-mediated embryo dormancy is caused by a reversible inhibition of both of these changes in cell wall stability.In a previous paper (29), we have provided evidence based on kinetics of water uptake and a factorial analysis that exogenous ABA controls germination of rape seeds by limiting water uptake of the embryo rather than by inhibiting energy* metabolism, protein synthesis, or related processes. It was concluded that ABA and osmotic stress interact at a common point controlling the water relations of the embryo specifically during the growth phase of germination. This phase commences about 12 h after sowing and is characterized by a resumption of rapid water uptake due to active embryo enlargement. In contrast, water uptake during the preceding imbibition phase (O to ,umol 1-' (29). However, a 10-fold higher concentration is needed to maintain the dormant state for longer periods of time.In the present paper, we attempt to localize the site(s) of action of ABA within the physical parameters governing the water relations of the germinating seed. The inhibitory effect of ABA on water uptake has been shown to be both rapidly ind...

“…After four or five readings (about 15 min), the indicated pH had usually become steady (pH 5.7 to 6 for maize, pH 6 Phosphate-citrate buffer (1 mM), pH 6, was again added so that the lower surface of the segment, one cut end, and the lower half of the other cut end were covered by buffer. The dish was then mounted on a compound microscope stage with the light beam coming up from below past the free end of the segment.…”

mentioning

confidence: 99%

Rapid Auxin-induced Decrease in Free Space pH and Its Relationship to Auxin-induced Growth in Maize and Pea

Jacobs

Ray²

1976

146

A pH microelectrode has been used to investigate the auxin effect on free space pH and its correbtion with auxin-stimulated elongation in segments of pea (Pisum sativum) stem According to the currently popular acid secretion hypothesis of auxin action (2, 7), auxins stimulate growth by causing acidification of the cell walls, which at low pH undergo an increase in extensibility that leads to rapid cell enlargement. Several lines of evidence are consistent with this hypothesis (7,9,19,24), but published measurements of auxin-induced release of acid from tissue into an external bathing medium (3,10,11,18) have not shown that auxin stimulates H+ secretion quickly enough to account for the rapid response (17) of elongation to auxin. If the acid secretion theory is correct it must be demonstrable that following exposure to an auxin the pH in the cell wall space falls to an elongation-stimulating value by the time that observable elongation in response to auxin actually commences, i.e. within the latent period for auxin action on elongation. A pH microelectrode that permits this type of measurement (13) Arbor, Mich.) of exposed tip length of 5 Aim and tip diameter of 2 ,um. The reference electrode was a segment of glass tubing (80 x 1 mm) the end of which was drawn into a capillary with a tip diameter of a few ,um. This tube was filled with 3 M KCI, in which was immersed an AgCI-coated silver wire that connected to the reference terminal of the pH meter. The electrode was read using the mv mode of operation of the meter and was calibrated against phosphate-citrate buffers (50 mm in K-phosphate and Na-citrate) of pH 4 and 6 before and after each experiment.The relation between pH of a buffer solution and mv reading by the pH microelectrodes was checked for several microelectrodes with a series of solutions varying in pH from 4 to 7. Because this relation was always found to be linear over the pH range, mv readings from experiments were converted to pH values by linear interpolation using the mv values measured for the calibration buffers of pH 4 and 6.The sensitivity of a pH microelectrode varied, for different experiments, from 50 to 58 mv/pH unit, but it was always constant through a given experiment. Between the beginning and the end of a typical experiment, however, the absolute mv values recorded by the microelectrode for the two calibration buffers would often change by a few mv. This drift in value was always of the same magnitude and in the same direction for both calibration buffers. Thus, the sensitivity of the microelectrode did not drift. Although calibration of the microelectrode against buffers of known pH before and after each experiment allowed us to monitor microelectrode drift, we used only the results of the postexperiment calibration for conversion of microelectrode mv readings to pH values.