Rapid, bilateral changes in growth rate and curvature during gravitropism of cucumber hypocotyls: implications for mechanism of growth control

Abstract. The growth response of etiolated cucumber ( Cucumis sativus L.) hypocotyls to gravitropic stimulation was examined by means of time-lapse photography and high-resolution analysis of surface expansion and curvature. In comparison with video analysis, the technique described here has five- to 20-fold better resolution; moreover, the mathematical fitting method (cubic splines) allows direct estimation of local and integrated curvature. After switching seedlings from a vertical to horizontal position, both upper and lower surfaces of the stem reacted after a lag of about 11 min with a two- to three-fold increase in surface expansion rate on the lower side and a cessation of expansion, or slight compression, on the upper surface. This growth asymmetry was initiated simultancously along the length of the hypocotyl, on both upper and lower surfaces, and did not migrate basipetally from the apex. Later stages in the gravitropic response involved a complex reversal of the growth asymmetry, with the net result being a basipetal migration of the curved region. This secondary growth reversal may reflect oscillatory and or self-regulatory behaviour of growing cells. With some qualifications, the kinetics and pattern of growth response are consistent with a mechanism involving hormone redistribution, although they do not prove such a mechanism. The growth kinetics require a growth mechanism which can be stimulated by two-to three-fold or completely inhibited within a few minutes.     

Gravitropism of cucumber hypocotyls: biophysical mechanism of altered growth

Abstract. The biophysical basis for the changes in cell elongation rate during gravitropism was examined in aetiolated cucumber ( Cucumis sativus L.) hypocotyls. Bulk osmotic pressures on the two sides of the stem and in the epidermal cells were not altered during the early time course of gravitropism. By the pressureprobe technique, a small increase in turgor (0.3 bar, 30 kPa) was detected on the upper (inhibited) side, whereas there was a negligible decrease in turgor on the lower (stimulated) side. These small changes in turgor and water potential appeared to be indirect, passive consequences of the altered growth and the small resistance for water movement from the xylem, and indicated that the change in growth was principally due to changes in wall properties. The results indicate that the hydraulic conductance of the watertransport pathway was large (.25 h ¹ bar ¹) and the water potential difference supporting cell expansion was no greater than 0.3 bar (30 kPa). From pressureblock experiments, it appeared that upon gravitropic stimulation (1) the yield threshold of the lower half of the stem did not decrease and (2) the wall on the upper side of the stem was not made more rigid by a cross-linking process. Mechanical measurements of the stress/strain properties of the walls showed that the initial development of gravitropism did not involve an alteration of the mechanical behaviour of the isolated walls. Thus, gravitropism in cucumber hypocotyls occurs principally by an alteration of the wall relaxation process, without a necessary change in wall mechanical properties.     

Phototropism: mechanisms and ecological implications

Abstract. Phototropism in seed plants, either etiolated or de-etiolated, is mediated by unidentified photoreceptor(s) sensitive to blue and near-UV regions of the light spectrum. Green plants may have an additional phototropic system sensitive to red light. Fluence-response studies of the blue light-sensitive phototropism, initially made on oat coleoptiles, have indicated the occurrence of multiple response types. Of those, two are found to be general: the first pulse-induced positive phototropism (fPIPP), or the so-called first positive curvature, and the time-dependent phototropism (TDP) or the second positive curvature. The fPIPP, elicited by a pulse stimulus shorter than a few minutes, is characterized by a bell-shaped fluence-response curve and the validity of reciprocity. The TDP, elicited by prolonged irradiation, is characterized by its dependence on the exposure time and the invalidity of reciprocity. Studies made on these two response types have revealed the following: (1) plants acquire directional light information for phototropism by sensing internal light gradients created by light scattering and absorption; (2) phototropism results from redistribution of growth, i.e. inhibition on the irradiated side and compensating stimulation on the shaded side; (3) lateral movement of growth regulators, the principle of the Cholodny-Went theory, can account for the growth redistribution, and auxin is clearly the mediating regulator in maize coleoptiles. This review further describes some mechanistic implications of fPIPP. Experimental results indicate that (1) fPIPP is mediated by a single step of photoreaction, (2) the responsiveness, reflected in the height of the fluenceresponse curve, is reduced by pre-irradiation with blue light and recovers gradually afterward, and (3) the light sensitivity, reflected in the position of the fluence-response curve along the log fluence axis, is also reduced by the pre-irradiation and recovers gradually. Analyses of these results, based on kinetic models, suggest that the bell-shaped fluence-response curve is caused by the difference in the amounts of a photoproduct between irradiated and shaded sides, and that fPIPP represents a mechanism of TDP. It is also indicated that phytochrome in the red-absorbing form exerts two separate effects on phototropism: reduction of the light sensitivity and enhancement of the responsiveness. Along with the discussion of the mechanisms of phototropism, their ecological implications are considered.     

Cold tolerance in tomato. I. Seed germination and early seedling growth of Lycopersicon esculentum

Tomato seed germination times were evaluated foi three "cold germinating" Lycopersicon esculentum Mill, accessions, PI 120256, PI 174263 and PI 341988 and a control breeding line, T3, at temperatures of 6 to 20°C. Accelerated failure analysis indicated that although PI 120256, 174263 and 341988 germinated more rapidly than T3 from 20 to 9°C, the minimum temperatures for germination were similar, and germination times of PI 120256 and 341988 were relatively more inhibited by progressively lower temperatures than was T3. Rapid germination of these three Pls at 10°C may not be due to cold tolerance, but to seed characteristics that promote rapid germination. Hypocotyl and root elongation over time were described by a three-parameter logistic equation; the growth rate parameter for hypocotyl elongation of all four genotypes was greatly inhibited from 20 to 15 and 10°C. Multivariate and univariate analyses of hypocotyl growth parameters indicated significant differences among accessions, but no significant genotype by temperature interaction. Rapid emergence reported for these Pis at 10°C is attributable to early germination, rather than rapid hypocotyl growth.     

表油菜素内酯对黄化绿豆下胚轴切段MACC、ACC和乙烯生成的影响

1979年Grove等从油菜花粉分离得到一种新的植物生长调节物质——油菜素内酯,它具有生长素、赤霉素类似活性,并与生长素有加合作用(Yopp等 1979,1981;Takeno和Pharis 1982)。诱导乙烯产生是生长素一个显著的生物效应;Yopp等(1979)和Arteca等(1983)分别报道过油菜素内脂对绿豆等生长素诱导乙烯增加的加合作用,促进乙烯前体1-氨基环丙烷基-1-羧酸的合成(Schlagnhaufer等1984)。本文主要研究了BR及其与IAA共同对ACC和乙烯生成的影响的特征及其对MACC形成的促进作用。     

The Effects of Photoperiod, Glucose and Gibberellic Acid on Growth In Vitro and Flowering of Chenopodium Murale

In vitro culture of long-day plant Chenopodium murale L was established. The effects of photoperiod, glucose and gibberellic acid (GA₃) on flowering and growth in vitro were investigated. Oscillatory changes of photoperiodic sensitivity were noticeable with regard to plant age. The plants induced at the phase of the 1^st and the 3^rd pair of leaves flowered to higher degree than those induced at the phase of 2^nd pair. Plants induced at the phase of the 1^st pair of leaves flowered to 17 % on 5 % glucose-containing medium and the addition of 5 mg dm^-3 GA₃ resulted in maximum flowering (43 %). Neither glucose nor GA₃ were able to compensate for photoperiodic requirements for flowering. Hypocotyl growth was decreased and the 1^st internode elongation and development of leaves were increased due to inductive photoperiodic conditions, as compared to non-inductive ones.     

Gas chromatographic and GC-MS evidence for the occurrence of 3-indolylpropionic acid and 3-indolylacetic acid in seedlings of Cucurbita pepo

Gas chromatographic and GC-MS evidence is presented for the occurrence of two auxin substances, 3-indolylacetic acid and 3-indolylpropionic acid, in hypocotyls of Cucurbita pepo cv. Golden Hubbard seedlings grown in darkness or under blue-light illumination. The data provide the first unequivocal evidence for the presence of 3-indolylpropionic acid in a higher plant.     

Effects of hypergravity on the elongation growth in radish and cucumber hypocotyls

The elongation growth of the hypocotyls of radish and cucumber seedlings was examined under hypergravity in a newly developed centrifuge (Kasaharaet al. 1995). The effects of hypergravity on elongation growth differed between the two species. The rate of elongation of radish hypocotyls was reduced under basipetal hypergravity (H+20g) but not under acropetal hypergravity (H-13g), as compared to growth under the control conditions (C+1g and C-1g). In cucumber hypocotyls, elongation growth was inhibited not only by basipetal but also by acropetal hypergravity. Under these conditions, the reduction in the elongation growth of both radish and cucumber hypocotyls was accompanied by an increase in their thickness. Although no distinct differences in relative composition of neutral sugars were found, the amounts of cell-wall components (pectic substances, hemicelluloses and cellulose) per unit length of hypocotyls were increased by exposure to hypergravity.     

In vitro culture and plant regeneration of large flowered purslane

Culture conditions were established for callus induction from a range of Portulaca grandiflora Hook tissues. Rapidly growing calli were obtained on Murashige and Skoog medium with stem-, leaf- and sepal-derived explants. Plant regeneration via organogenesis was explant-origin dependent with hypocotyl tissues giving the highest shooting frequency. Light conditions, pH and carbon source had a pronounced effect on the percentage of explants regenerating buds and the number of buds formed. It was possible to establish stable regenerated plants in the glasshouse.Abbreviations BA 6-benzyladenine - 2,4-D 2,4-dichlorophenoxyacetic acid - NAA 1-naphthaleneacetic acid - IAA indoleacetic acid - MS Murashige and Skoog (1962) medium     

The effect of red and far-red light in the high irradiance reaction of phytochrome (hypocotyl growth in dark-grown Sinapis alba L.)

Abstract. Fluence-rate response curves were determined for the inhibition of hypocotyl growth in 54-h-old dark-grown Sinapis alba L. seedlings by continuous or hourly 5 min far-red light irradiation (24 h). Just as in red light ( Heim & Schäfer, 1982 ), a fluence-rate dependence was observed for both kinds of irradiations, even if only 35% of the continuous light effect could be substituted for by hourly far-red pulses. The same total fluence was used for the two different light regimes. Measurements of Pfr and Pfr/Ptot showed a strong fluence-rate dependence under continuous light which only partially paralleled the fluence-rate response curves for the inhibition of the hypocotyl growth. It was concluded that neither spectrophotometrically determined levels of Pfr nor Pfr/Ptot can be the only light-dependent factor controlling hypocotyl lengthening under continuous irradiation.