The Influence of Aging Conditions on the Short Term Growth of Green Pea Stem Segments

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Anaerobic conditions reversibly inhibit the elongstion of isolated green pea (Pisum sativum L. var Alaska) stem segments. Mlununation of segments maintaned under anoxia causes a resumption of growth. Po-laographic studies show pea stem segments are pbotosyntheticaly competent as determined by 02 evolution. Although 02 production is totaly inhibited by dichlorophenyldimethyluren (DCMU) and dinitrophenol (DNP) inhibits 02-dependent growth, neither DCMU nor DNP completely abolishes light-dependent growth, although both reduce the effect markedy. Phenazine methosulfate promotes the growth of anaero-bicaly maintained, Numinated, DCMU-treated segments. The data indicate that the princpal effect of light in inducing growth under anaerobic conditions is the photosynthetic provision of 02 for respiration. There is also some evidence that, at least in the absence of 02, a small amount of elongation is due to some other light-driven process, perhaps cyclic photophosphorylation. We have shown previously (12) that anaerobiosis reversibly inhibits the elongation of green pea stem segments. Under an-oxia, growth decreases rather abruptly to a low rate and thereafter decays slowly to zero. Shrinkage of the segments usually becomes evident after 3 to 4 hr. We have found, however, that this inhibition of growth and the subsequent shrinkage can be completely abolished by illumination of the segments. The light-dependent growth under O2-deprived conditions has been investigated to determine its mechanism and the possible implications on growth under more "nattral" conditions. Light-dependent growth under anoxia might be due to: (a) photosynthetic 02 evolution with restoration of aerobic conditions and respiration; and/or (b) contribution of some other growth-limiting intermediate by the chloroplasts. We shall show that at least the first proposal obtains, and provide indirect evidence for the second. MATERIALS AND METHODS Peas (Pisum sativum L. var Alaska) were grown in moist vermiculite in a glasshouse. One-cm segments were excised from the expanding second or third internode of 9-to 11-day-old plants. Segments were placed in 20 mm K-phosphate buffer (pH 6.2) and vacuum-infiltrated. If not used immediately, the stem segments were held at 3 C with vigorous aeration. Growth measurements were performed as previously described (12) with only slight modifications of the growth measur-' Current address: Department of Agronomy, University of Ne-braska, Lincoln, Neb. 68583. ing device (Fig. 1). All experiments were performed at 20 C within the water-jacketed growth chamber. Removal of 02 from the buffer for anaerobic treatments was difficult. Although the buffer was deoxygenated in the reservoir from which it was pumped, 02 was re-entering the system (primarily through the tubing of the peristaltic-type pump). The pump head was therefore enclosed with parafilm and flooded with N2. This reduced 02 levels in the growth chamber to less than 10 AM, a concentration which rapidly inhibited growth. In some experiments, buffer was not pumped past...

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confidence: 99%

Light-dependent Elongation of Anaerobically Maintained Green Pea Stem Segments and Its Implications

Parrish¹,

Davies²

1976

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“…(In our studies, additions of acid or base to shift the pH up to 2 units appeared to prompt a homeostatic response which returned the medium to a stabilized pH of about 5.) We suspect that the absence of an acidification response in some reports may stem from inadequate aeration: vigorous bubbling of the bathing medium with air or 02 is crucial in keeping submerged segments viable and responsive (21), and interruption of aeration reduced or reversed the pH drop in our studies. The possible significance of proper aeration and support for the cation/proton exchange mechanism for endogenous acidification comes from a report by Jacobson et al (15) that rapid bubbling of root tissue in a buffer medium promoted K+ influx more than slow bubbling.…”

Section: Resultsmentioning

confidence: 73%

“…5). Nonpeeled stems, in contrast, grow more slowly, remain viable longer, and are more responsive to IAA (21).…”

Section: Resultsmentioning

confidence: 96%

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On the Relationship between Extracellular pH and the Growth of Excised Pea Stem Segments

Parrish¹,

Davies²

1977

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Studies with stem segments of peas (Pisum sadvum L. var. Alaska) suggest that the pH of the medium bathing elongating tissue does not always reflect intramural (cell wall) conditions or that pH is not a controling fator in elongation. Peeled, green segments, and peeled or nonpeeled etioated segments appear to regulate the pH of their bathing medium causing it to become acidified with or without the addition of auxin. The growth rates of segments are greatest during a period before acidification is evident and slow during the time in which the medium becomes acidifed. We cannot reproduce the dramatic auxin-induced pH shifts reported in the literature because the control segments are becoming more acid also; but there is some evidence that acidification may occur in response to auxin treatments. K+ additions mimic the acidfying tendency of auxin but are without growth-promoting effect. Emergent growth (an extremely rapid burst of growth following anaerobic treatments) is not accompanied by a drop in pH of the bathing medium. Proper aeration of the bating medium in extracellular pH studies is crucial and may explain differences between our results and other published accounts. The datu suggest that the techniques used for most extracelluar pH studies may not very dosely approximate in vivo conditions or properly reflect intramural H+ concentration fuxes.In 1932, Strugger (29) reported that acidic media promoted the growth of sunflower roots. Bonner (2) found that oat coleoptiles were also responsive to lowered pH and suggested that "there might be some relation between growth substance [auxin] and the 'acid growth' of Strugger." Most of the early workers concluded, along with Went and Thimann (32), that acid growth was probably due to the dissociation of auxin from the salt to free IAA (which was presumed to be the growth-active form). In the next 30 years, a number of workers alluded to or reported without comment pH effects on growth and auxin action (see 19). In 1970, the topic was revived by at least three new reports of the effect of pH on growth (e.g. 26). These were followed by a lengthy and well supported paper by Hager et al. (11) who proposed that auxin acted by promoting extrusion of protons, acidifying the cell wall with consequent loosening and elongation. Several more reports appeared which documented the acid growth effect and lent indirect support to an active acidification mechanism (e.g. 7). Hager's proton pump model received direct support when several workers reported that auxin induced a pH drop in the solution's bathing treated tissues (3,16,33). Marre and co-workers proposed that the proton extrusion was partially electrogenic (17) and that K+ acted as a counterion (18). That

“…3). In 12 replicate experiments the average growth during the hr starting 30 min after the addition of auxin was 3.24 ± 0.81 mm for stimulated and 3.92 ± 0.57 mm for nonstimulated segments. The effect of stimulation on the length of the latent period of auxin-induced growth was also examined in these experiments but no significant changes were observed.…”

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confidence: 92%

Effects of Mechanical Stimulation on Avena Coleoptile Segment Elongation in a High Resolution, Continuous Growth-recording System

Haugland¹,

Cline²,

Evans³

1978

ABSTRACTtiles, resulting from inner epidermal stimulation, has apparently gained some degree of acceptance in the literature (1, 10, 12), the evidence for this hypothesis cannot be considered as conclusive. It seems possible that instances of enhanced growth previously attributed to mechanical effects were in fact induced by changes in auxin metabolism (4) or changes in auxin sensitivity in isolated coleoptile segments (14). In addition, the mechanical stimulation hypothesis does not take into account probable growth effects of residual auxin which must be presumed to remain in the tissue for at least a short time following excision.We have therefore reexamined the role of inner epidermal mechanical stimulation in the mediation of the IRG period as well as the effects ofmechanical stimulation on subsequent endogenous growth and on the response of excised segments to exogenously supplied IAA. MATERIALS AND METHODSPlant Material. A vena sativa L. var. Victory seeds were sown on moist paper towels in a humid container and incubated in the dark at 27 C for 60 to 72 hr at which time shoot emergence was evident. The seedlings were then given a 30-min red light treatment to suppress mesocotyl elongation. After 4 to 4.5 days coleoptiles which were 20 to 35 mm long and had outgrown their primary leaves by at least 3 mm were selected for harvesting. From each coleoptile a 10-mm segment was excised beginning 2 mm from the tip using a double-bladed cutting device.Growth Measurement. Columns of either 8 or 10 coleoptile segments were positioned in growth chambers constructed after the design of Dela Fuente and Leopold (2). The coleoptile segments were held in place in the growth chambers either by stringing on a wire as previously described (2)