The vertical growth responses of corn seedlings (Zea mays L. Mo17 × B73) were determined over an 8-hour period. When seedlings were decapitated 3 millimeters from the coleoptile's tip and supplied with indole-3-acetic acid (IAA) in 1.5% agar blocks, the response was dependent both on time and IAA concentration. The dose-response curves changed in shape and magnitude depending on the total time of IAA application. High concentrations (>3.2 × 10−6 molar) initially produced high relative growth rates that decreased back to the intact rate (0.03 millimeter per hour per millimeter) after 3 hours. Low concentrations (<1.0 × 10−6 molar), or agar blocks without IAA, resulted in a rapid decrease from the intact rate to a level that stabilized at 0.01 millimeter per hour per millimeter until the growth rate began to recover after 3 to 4 hours. Intermediate concentrations produced responses similar to that of the intact organ, though some features of these responses were unique.
Gravitropism of vascular plants has been assumed to require a single gravity receptor mechanism. However, based on the evidence in Part I of this study, we propose that maize roots require two. The first mechanism is without a directional effect and, by itself, cannot give rise to tropism. Its role is quantitative facilitation of the second mechanism, which is directional like the gravitational force itself and provides the impetus for tropic curvature. How closely coupled the two mechanisms may be is, as yet, unclear.The evidence for dual receptors supports a general model for roots. When readiness for gravifacilitation, or gravifacilitation itself, is constitutive, orthogravitropic curvature can go to completion. If not constitutively enabled, gravifacilitation can be weak in the absence of light and water deficit or strong in the presence of light and water deficit. In either case, it can decay and permit roots to assume reproducible non-vertical orientations (plagiogravitropic or plagiotropic orientations) without using non-vertical setpoints. In this way roots are deployed in a large volume of soil.Gravitropic behaviours in shoots are more diverse than in roots, utilising oblique and horizontal as well as vertical setpoints. As a guide to future experiments, we assess how constitutive v. non-constitutive modes of gravifacilitation might contribute to behaviours based on each kind of setpoint.
Attempts were made to obtain bacteria‐free plants of Psychotria punctata from tissue cultures. Stem explants and callus derived from them were induced to form roots but failed to form buds on Linsmaier and Skoog medium and 96 chemical modifications of it, including most of those known to induce bud formation in other species. Roots formed with ample IAA (2 mg/liter or more) and a low kinetin concentration (0.25 or 0.50 mg/liter). Adenine inhibited root formation in these media, but tyrosine did not. Tyrosine did lower the percentage of calluses commencing growth. When enzyme‐hydrolyzed lactalbumin (1.3 g/liter), kinetin (0.5 mg/liter) and IAA (5 mg/liter) were added to Linsmaier and Skoog medium modified by decreasing inorganic nitrogen and increasing inorganic phosphate, callus grew at the fastest rate observed (increasing threefold in fresh weight in three weeks) and formed numerous roots. This was adopted as the stock callus medium. Casein hydrolysates also stimulated growth but less so than lactalbumin hydrolysate. When lactalbumin hydrolysate or a casein hydrolysate lacking tryptophan was supplied, growth occurred without added auxin if sufficient cytokinin was added. Cytokinin was required at unusually high concentration and was tolerated at still higher concentration. Formation, elongation, and branching of roots persisted on a saturated solution of BA which inhibited callus growth about 70 % and delayed callus senescence. Light caused earlier callus senescence after growth had ceased but did not affect callus growth or root formation. Light‐induced senescence was prevented by a high cytokinin concentration.
Plant organs may respond to gravity by vertical (orthogravitropic), oblique (plagiogravitropic) or horizontal (diagravitropic) growth. Primary roots of maize (Zea mays L.) provide a good system for studying such behaviours because they are reportedly capable of displaying all three responses. In current work using maize seedlings of the Silver Queen cultivar, stabilisation of growth at an oblique orientation was commonplace. Hypothetically, plagiogravitropism may be accomplished either by a process we call graded orthogravitropism or by hunting about a sensed non-vertical setpoint. In graded orthotropism primary bending is unidirectional and depends on facilitative stimuli that determine its extent. The hallmark of the setpoint mechanism is restorative curvature of either sign following a displacement; both diagravitropism and orthogravitropism are based on setpoints.Roots settled in a plagiogravitropic orientation were tested with various illumination and displacement protocols designed to distinguish between these two hypotheses. The tests refuted the setpoint hypothesis and supported that of graded orthotropism. No evidence of diagravitropism could be found, thus, earlier claims were likely based on inadequately controlled observations of graded orthotropism.We propose that orthotropism is graded by the sequential action of dual gravity receptors: induction of a vectorial gravitropic response requires gravitational induction of a separate facilitative response, whose decay in the absence of fresh stimuli can brake gravitropism at plagiotropic angles.
Attempts were made to obtain bacteria-free plants of Psychotria punctata from tissue cultures. Stem explants and callus derived from them were induced to form roots but failed to form buds on Linsmaier and Skoog medium and 96 chemical modifications of it, including most of those known to induce bud formation in other species. Roots formed with ample IAA (2 mg/liter or more) and a low kinetin concentration (0.25 or 0.50 mg/liter). Adenine inhibited root formation in these media, but tyrosine did not. Tyrosine did lower the percentage of calluses commencing growth. When enzyme-hydrolyzed lactalbumin (1.3 g/liter), kinetin (0.5 mg/liter) and IAA (5 mg/liter) were added to Linsmaier and Skoog medium modified by decreasing inorganic nitrogen and increasing inorganic phosphate, callus grew at the fastest rate observed (increasing threefold in fresh weight in three weeks) and formed numerous roots. This was adopted as the stock callus medium. Casein hydrolysates also stimulated growth but less so than lactalbumin hydrolysate. When lactalbumin hydrolysate or a casein hydrolysate lacking tryptophan was supplied, growth occurred without added auxin if sufficient cytokinin was added. Cytokinin was required at unusually high concentration and was tolerated at still higher concentration. Formation, elongation, and branching of roots persisted on a saturated solution of BA which inhibited callus growth about 70 % and delayed callus senescence. Light caused earlier callus senescence after growth had ceased but did not affect callus growth or root formation. Light-induced senescence was prevented by a high cytokinin concentration.
Phloem regeneration in Coleus internodes, earlier wounded so that one or more phloem bundles were severed, is estimated quantitatively by microscopic examination of permanent slides prepared in the following way: The wounded internode is removed from the plant after a given regeneration period, is fixed in Craf III for 24 hr and is transferred to 85% lactic acid for 12-24 hr. While still in lactic acid, a "strip", which is composed of the phloem and all tissues peripheral to it in the internode, is peeled from the internode, leaving only the xylem-pith cylinder. The strip is stained for 6-12 hr in 0.1% aniline blue in 85% lactic acid, then is transferred to 60% alcohol containing 0.5% HCI. While in the latter solution, the epidermis, scar tissue, and most of the cortical tissue is carefully dissected from the strip while it is observed in a dissecting microscope. The strip is restained for an hour or more and is passed through two 5-10 min changes each of acidified 60% alcohol, absolute alcohol, and xylene, and is then mounted on a glass slide in damar-xylene. Counts of regenerated, interfascicular phloem strands, governed by a counting convention, which were shown to bear a fairly constant relationship to the actual number of regenerated sieve tube members, are made while examining under low and high power magnifications. This method is presently being used to study the physiology of phloem differentiation and its regulation in Coleus.
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