LEAF DIMORPHISM IN THE AQUATIC ANGIOSPERM CALLITRICHE HETEROPHYLLA

Depending on the position of the shoot tip relative to the water surface, the aquatic angiosperm Callitriche heterophylla produces either ovate land-form or linear water-form leaves. This paper is concerned with the developmental basis for the leaf dimorphism of this species. Little significant difference is observed between the apical meristems of submerged vs. emergent shoots; moreover, land-form and water-form primordia undergo similar, if not identical, patterns of initial development until they attain a length of 350 to 400 μm. These findings are interpreted to mean that the divergent leaf forms result from the marked sensitivity of the primordia to their respective environments rather than from the mode of their inception. Subsequent growth of the young water-form leaf emphasizes longitudinal extension, while the immature land-form leaf continues balanced expansion in both longitudinal and lateral directions. The lateral growth of the land-form primordium is accomplished in part by a more persistent marginal meristem, but the morphological difference between the two leaf forms is mostly attributable to the difference in the predominant direction of intercalary expansion. In addition, certain anatomical features, such as vasculature, stomates, and cuticle, are much more prominent in mature land-form leaves than in water-form leaves. These anatomical differences seem to represent structural adaptations of each leaf form to the specific physiological requirements of its environment.     

ABSCISIC ACID INDUCES LAND FORM CHARACTERISTICS IN MARSILEA QUADRIFOLIA L.

In aseptically cultured rhizome segments of the aquatic fern Marsilea quadrifolia, the addition of abscisic acid (ABA) to the liquid medium induced development of morphological characteristics distinctive to the land form of the plant. The land-form characteristics induced by ABA included a change in leaf morphology, an increase in the surface area of leaflets, differentiation of stomata and trichomes, elongation of petioles and roots, development of lateral roots, shortening of rhizome, and reduction in the number of leaves and roots formed on each plant.     

In-vitro induction of aerial leaves and of precocious flowering in submerged shoots ofLimnophila indica by abscisic acid

Nodal explants of submerged shoots ofLimnophila indica (L.) Druce were cultured in Nitsch's liquid medium containing abscisic acid (ABA, 10^-9-10^-6 M). At 10^-7 and 10^-6 M, ABA induced typical aerial leaves (entire, ovate, opposite-decussately arranged) even under submerged conditions and completely suppressed the development of water leaves (pinnately dissected and whorled). Flowers that invariably arise from aerial shoots were induced precociously by ABA even on submerged nodes.Abbreviation ABA abscisic acid     

Effects of ethylene and abscisic acid upon heterophylly in<Emphasis Type="Italic"> Ludwigia arcuata</Emphasis> (Onagraceae)

In this study, we examined the effects of ethylene and abscisic acid (ABA) upon heterophyllous leaf formation of Ludwigia arcuata Walt. Treatment with ethylene gas resulted in the formation of submerged-type leaves on terrestrial shoots of L. arcuata, while treatments with ABA induced the formation of terrestrial-type leaves on submerged shoots. Measurement of the endogenous ethylene concentration of submerged shoots showed that it was higher than that of terrestrial ones. In contrast, the endogenous ABA concentration of terrestrial shoots was higher than that of submerged ones. To clarify interactions of ethylene and ABA, simultaneous additions of these two plant hormones were examined. When L. arcuata plants were treated with these two plant hormones, the effects of ABA dominated that of ethylene, resulting in the formation of terrestrial-type leaves. This suggests that ABA may be located downstream of ethylene in signal transduction chains for forming heterophyllous changes. Further, ethylene treatment induced the reduction of endogenous levels of ABA in tissues of L. arcuata, resulting in the formation of submerged-type leaves. Thus the effects of ethylene and ABA upon heterophyllous leaf formation are discussed in relationship to the cross-talk between signaling pathways of ethylene and ABA.Abbreviations ABA abscisic acid - ACC 1-aminocyclopropane-1-carboxylic acid - L/W ratio ratio of leaf length to width - LN leaf number - GAs gibberellins     

The Effect of Phytohormones on Chlorophyll(ide), Protochlorophyll(ide) and Carotenoid Formation in Greening Dark Grown Wheat Leaves

The influence of phytohormones on chlorophyll and carotenoid formation during the greening of irradiated dark grown wheat leaves (Triticum aestivum L. cv. Starke II Weibull) was studied. Leaves were floated on solutions of abscisic acid, gibberellic acid and kinetin for 24 h. The chlorophyll and carotenoid contents were determined during a subsequent period of 48 h of continuous irradiation. Leaves treated with abscisic acid showed a longer lag phase and a lower rate of accumulation of chlorophyll as compared to the control than did leaves treated with gibberellic acid and kinetin. The carotenoid content was low both in leaves treated with abscisic acid and in those treated with gibberellic acid. Treatment with abscisic acid lowered the protochlorophyllide regeneration after a saturating light flash while gibberellic acid as well as kinetin had no effect. The influence of ABA was partly dependent on an increase of the wounded part of the cut leaf segments. The accumulation of protochlorophyllide in leaves treated with δ-aminolevulinic acid was not affected by the different hormonal treatments. These results suggest that the main effect of abscisic acid is probably outside the chloroplast, i.e. on the formation or transport of δ-aminolevulinic acid.     

Integrative regulation of leaf morphogenesis by gibberellic and abscisic acids in the aquatic angiosperm proserpinaca palustris L

A two-hormone system regulating leaf development in the heterophyllous amphibious angiosperm Proserpinaca palustris L. is described. Aerial shoots develop expanded, lanceolate, serrate leaves under long-day photoperiods (LD, 16 h light: 8 h dark), whereas growth under short days (SD, 10 h light: 14 h dark) induces dissected leaf formation. The photoperiodic effect on leaf development of aerial shoots involves changes in endogenous gibberellins (GAs) since plants grown under SD in the presence of GA₃ develop expanded lanceolate serrate leaves. However, when submerged, shoots develop highly dissectedaquatic leaves regardless of photoperiod or GA₃ treatment. In the present study, submerged plants exposed to 1.0 or 5.0 μM abscisic acid (ABA) developed aerial-type leaves typical of the photoperiod under which they were cultured. Both exogenous ABA (5.0 μM) and GA₃ (10 μM) treatments were required for laminar expansion to occur on submerged shoots under SD. It is suggested that (1) leaf development in Proserpinaca is regulated by both endogenous GAs and ABA, and (2) the endogenous status of these phytohormones is modulated by different environmental stimuli of photoperiod and water stress, respectively. The adaptive significance of this mechanism is discussed.     

Osmotic stress, endogenous abscisic acid and the control of leaf morphology in Hippuris vulgaris L

Abstract. Previous reports indicate that heterophyllous aquatic plants can be induced to form aerial-type leaves on submerged shoots when they are grown in exogenous abscisic acid (ABA). This study reports on the relationship between osmotic stress (e.g. the situation encountered by a shoot tip when it grows above the water surface), endogenous ABA (as measured by gas chromatography-electron capture detector) and leaf morphology in the heterophyllous aquatic plant, Hippuris vulgaris. Free ABA could not be detected in submerged shoots of H. vulgaris but in aerial shoots ABA occurred at ca. 40ng (g fr wt)⁻¹. When submerged shoots were osmotically stressed ABA appeared at levels of 26 to 40ng (g fr wt)⁻¹. These and other data support two main conclusions: (1) Osmotically stressing a submerged shoot causes the appearance of delectable levels of ABA. (2) The rise of ABA in osmotically stressed submerged shoots in turn induces a change in leaf morphology from the submerged to the aerial form. This corroborates the hypothesis that, in the natural environment, ABA levels rise in response to the osmotic stress encountered when a submerged shoot grows up through the water/air interface and that the increased ABA leads to the production of aerial-type leaves.     

A simple bioassay for detecting “antitranspirant” activity of naturally occurring compounds such as abscisic acid

Summary Isolated epidermal strips of Commelina communis L. showed progressively smaller stomatal openings when incubated in abscisic acid solutions ranging in concentration from 10^-8 to 10^-4 M. The effects were reproducible and did not appear to be affected by the presence of auxin, gibberellic acid or kinetin. This specificity suggests that this method may prove valuable as a quick, sensitive bioassay for abscisic acid and other related compounds which might be used as antitranspirants on field crops. The fungal toxin fusicoccin, previously reported to cause increased stomatal opening on intact leaves, partially reversed the closure induced by abscisic acid.     

Endogenous Abscisic Acid Content Correlates with Photon Fluence Rate and Induced Leaf Morphology in Hippuris vulgaris

This research focused on studying how light and endogenous abscisic acid regulate leaf development in Hippuris vulgaris, a species of heterophyllic aquatic plant. Amounts of photosynthetically active radiation greater than 300 micromoles per square meter per second caused submerged H. vulgaris shoots to produce aerial-type leaves. Abscisic acid was not detected in shoots grown under noninducing light quantities (100 micromoles per square meter per second), but was present at 13.4 nanograms per gram fresh weight in shoot tips after plants were exposed to 1 photoperiod of inducing light (500 micromoles per square meter per second). This supports a role for abscisic acid in the high light-induced heterophylly in H. vulgaris, and provides additional support for the general hypothesis that abscisic acid regulates leaf development in heterophyllic aquatic plants. No relationship was observed here between postphotoperiodic light treatments of various red/far red ratios and heterophylly in H. vulgaris.     

Influences of Light Quality, Growth Regulators and Inhibitors of Protein Synthesis on Respiration of Protoplasts from Meristematic Cells in Barley Seedlings

The influences of light of different wavelengths and plant growthregulators on the respiration of protoplasts isolated from tissue0 to 5 mm above the basal intercalary meristem of barley (Hordeumvulgare L. cv. Patty) leaves were studied. Respiration was measuredusing oxygen electrodes and a Cartesian-diver technique. Red,far-red and blue light all stimulated respiration in the protoplastsbut not in mitochondria isolated from them. Gibberellic acid stimulated respiration in protoplasts but abscisicacid had the opposite effect. Physiological concentrations ofindole-3-acetic acid and kinetin had no influence in eitherdirection. Combinations of gibberellic acid with light of anywavelength always increased respiration. Red or far-red light treatments in the presence of abscisicacid decreased dark respiration and only blue light significantlyreversed the inhibitory effect of abscisic acid. Cycloheximidemarkedly increased dark respiratory activity; chloramphenicolwas without effect. These results indicate that mitochondrialactivity in the leaf basal intercalary meristem was partiallycontrolled through phytochrome and a blue light receptor, andby gibberellic and abscisic acids. Changes in cytosolic proteinsynthesis were important for the initiation of enhanced mitochondrialactivity in meristems. Hordeum vulgare L., barley, abscisic acid, Cartesian-diver microrespirometry, gibberellic acid, meristematic respiration, protoplasts     

Comparative Characterization of the Pigment Complex in Emergent, Floating, and Submerged Leaves of Hydrophytes

The content of chlorophylls (Chls) and carotenoids was studied in the leaves of 42 species of boreal aquatic plants with different degree of submergence (emergent, floating, and submerged) and isopalisade, dorsoventral, and homogenous types of mesophyll structure. Hydrophytes were shown to have a low Chl content (1–2 mg/g fr wt) and low Chls/carotenoids ratio (2.3–3.5) as compared to terrestrial plants. The pigment content per dry wt unit and unit leaf area was dependent on the type of mesophyll structure. It was a consequence of the changes in the parameters of leaf mesophyll structure characterizing the density of photosynthetic elements. In a sequence emergent floating submerged forms, the content of Chls and carotenoids decreased, and the photosynthetic capacity decreased due to a reduction in the chloroplast number per unit leaf area. Adaptation of submerged leaves to low illumination and slow CO₂ diffusion changed the functional properties of chloroplasts. An increase in the pigment content in the chloroplasts of submerged leaves (7 × 10^–9 mg Chl, 2 × 10^–9 mg carotenoids) as compared to emergent and floating leaves was accompanied by a decline in the photosynthetic capacity per Chl comprising 1.6 mg CO₂/(mg Chl h) versus 3.9 and 3.8 mg CO₂/(mg Chl h) in emergent and floating leaves, respectively.     

LONG-TERM DEVELOPMENTAL CHANGES IN XANTHIUM INDUCED BY GIBBERELLIC ACID

One application of gibberellic acid (GA₃) to Xanthium shoots resulted in an initial large stimulation, followed by inhibition, of internode elongation. After presumed translocation of the hormone from the locus of its application to the stem apex several morphological changes were observed. There was a significant increase in number of mitotic figures in the apical meristem and a twofold increase in volume of the apical dome. With time, the rate of leaf production was accelerated about 1.8 times. The phyllotaxis of leaf primordia initiated under the influence of GA:, changed from a (2, 3) contact parastichy pattern in control shoot to a (3, 5) pattern. Final petiole length was smaller than the control, and the absolute rate of lamina expansion decreased under prolonged treatment. Gibberellic acid had a pronounced effect on leaf morphology. GA_a induced the development of lanceolate leaves instead of typical deltoid leaves. The reduction in leaf area coincided with a 32% reduction in the average area of epidermal cells. Plastochron changes were correlated with anatomical and morphological changes during the course of leaf development.     

Stomatal functioning in relation to leaf age in in-vitro-grown plum shoots

The ability of stomata to close in response to abiotic factors was studied on different-aged leaves of in-vitro-grown clone Mr. S. 2/5 plum (Prunus cerasifera) shoots. Epidermal peels removed from the first, third and fifth leaf in basipetal sequence from the shoot tip were exposed to 50 mm mannitol, 10 mm coumarin or dark treatment. The control solution consisted of 1 mm KC1 in 0.5 mm 2-(N-morpholino)ethanesulphonic acid. The percentage of stomata that closed following such treatments became progressively lower with increasing leaf age. The effect of mannitol was greater than that of coumarin. With dark treatment, pronounced closure was observed in the apical leaf, while on the third and fifth leaf, values were lower but not statistically different. Finally, the kinetics of stomatal closure assessed during mannitol incubation indicated a progressively slower response from the first to the fifth leaf. The more effective stomatal functioning of the youngest leaf was confirmed by a more pronounced stomatal re-opening observed in epidermal peels first treated with mannitol and then incubated again in the control solution. Received: 20 June 1997 / Revision received: 17 July 1998 / Accepted: 25 August 1998     

REGULATORY ROLE OF INDOLE-3-ACETIC ACID AND GIBBERELLIC ACID IN VEGETATIVE DEVELOPMENT OF XANTHIUM PENNSYLVANICUM

A single application of gibberellic acid to young internodes significantly accelerated the rate of internode growth and the rate of leaf production in shoots of Xanthium pennsylvanicum Wallr. The average duration of one plastochron in treated plants was reduced to 43% of control levels. Gibberellic acid also had a pronounced morphogenetic effect on leaves so that the area and leaf length of treated plants were both significantly reduced. Depending upon concentration, auxin had both inhibitory and promotive effects on Xanthium shoots. Indole-3-acetic acid markedly altered the response of the gibberellic acid-treated internodes and those located above and below the site of application. In addition, high auxin concentrations induced the formation of adventitious roots in treated internodes. Auxin also brought about significant reductions in the length and area of leaves developed under the influence of this hormone.     

Effects of Growth Regulators on Ribonucleic Acid Metabolism of Barley Leaf Segments

Illumination or gibberellic acid treatment of etiolated barley leaf segments stimulates unrolling and results in an increased level of RNA. In contrast, segments treated with abscisic acid do not unroll and have a lower content of RNA. Gibberellic acid treatment enhanced the capacity of segments to incorporate radioactivity from ³²P-orthophosphate into all the RNA components detected by gel electrophoresis; abscisic acid greatly restricted the incorporation of precursors into all the RNA fractions. In conjunction with a changed capacity for RNA synthesis it was observed that abscisic acid-treated segments had a lowered soluble DNA-dependent RNA polymerase level in comparison to gibberellic acid-treated or illuminated segments. However, the influence of growth regulators on RNA polymerase content of the segments was associated with general effects on protein level rather than a specific effect on the synthesis of polymerase enzyme.     

The effect of light and dark periods on the production of ethylene from water-stressed wheat leaves

Light was found to inhibit substantially (i.e. up to 88%) the production of ethylene induced by water stress in excised wheat leaves and from the shoots of intact plants. The relatively small amounts of ethylene emanating fron non-stressed leaves were also inhibited by light but to a smaller degree (i.e. up to 61%). In water-stressed leaves the degree of light inhibition of ethylene production was shown to be related to the age of the leaves; the amounts of ethylene diffusing from young leaves (i.e. 6-days old) was inhibited 52% by light whereas in older leaves (i.e. 9-days old) it was inhibited by 85%. Previous studies [Wright (1979) Planta 144, 179–188 and (1980) Planta 148, 381–388] had shown that application of 6-benzyladenine (BA) to leaves a day before wilting, greatly increases the amount of ethylene diffusing from the leaves following wilting (e.g. 8-fold), and to smaller degrees do applications of indole-3-acetic acid (IAA) and gibberellic acid (GA₃). On the other hand abscisic acid (ABA) treatment reduces the amount of ethylene produced. In these earlier experiments the ethylene was collected from leaves held under dark or near-dark conditions, so in the present study the activities of these growth regulators (10^-4 mol l^-1 solutions) under dark and light conditions were compared. It was found that they maintained the same relative activities on ethylene emanation (i.e. BA>IAA>GA₃>water controls>ABA) under both light and dark conditions. However, because of the inhibitory effect of light, the absolute amounts of ethylene produced from all treatments were always much higher in the dark than in the light (usually about a 6-fold difference). An interesting effect of light treatment on ethylene biosynthesis was found when water-stressed leaves were kept in dark chambers for 41/2 h and then transferred to light. Quite unexpectedly, instead of the rate of ethylene production falling immediately, it continued to be produced at the dark rate (i.e. no light inhibition!) for over 2 h before the rate began to decline, and for a much longer period (i.e. in excess of 41/2 h) if the leaves had previously been sprayed with BA. Predictably, leaves placed in the light (i.e. in leaf chambers) and then transferred to darkness, immediately or very soon produced ethylene at the dark rate. One explanation of these results, which is discussed, would be that the biosynthesis of an ethylene precursor requires an obligatory dark stage. The possible implications of these studies to a survival role of ethylene in plants during periods of water stress is discussed.Abbreviations ABA abscisic acid - ACC 1-aminocyclopropane-1-carboxylic acid - BA 6-benzyladenine - GA₃ gibberellic acid - GLC gas-liquid chromatography - IAA indole-3-acetic acid - TLC thin-layer chromatography - _leaf leaf water potential     

Effect of Abscisic Acid on the Activities of Photosynthetic Enzymes and 14CO2 Fixation Products in Leaves of Pennisetum typhoides Seedlings

Abscisic acid inhibited the rate of ¹⁴CO₂ fixation in leaves of Pennisetum typhoides (Burm. f.) Stapf & Hubbard seedlings, but increased the activities of phosphoenol-pyruvate-carboxylase and malic enzyme. The leaves of the seedlings grown in the presence of abscisic acid incorporated, in comparison to the control, more radioactivity in the fraction of organic acids, but less radioactivity was recorded in the amino acid fraction. On the other hand, gibberellic acid which also inhibits photosynthetic ¹⁴CO₂ assimilation and decreases the activities of photosynthetic enzymes, favours greater incorporation in alanine, and reduces that in malate. It is deduced that bio-regulants can greatly influence the flow of ¹⁴C into individual photosynthetic products. As in growth, abscisic and gibberellic acids in combination tended to antagonize each other in their effects on enzyme activity as well as in incorporation of ¹⁴CO₂ into photosynthetic products.     

Transpiration does not control growth and nutrient supply in the amphibious plant Mentha aquatica

Mentha aquatica L. was grown at different nutrient availabilities in water and in air at 60% RH. The plants were kept at 600 mmol m^?3 free CO₂ dissolved in water (40 times air equilibrium) and at 30 mmol m^?3 CO₂ in air to ensure CO₂ saturation of growth in both environments. We quantified the transpiration-independent water transport from root to shoot in submerged plants relative to the transpiration stream in emergent plants and tested the importance of transpiration in sustaining nutrient flux and shoot growth. The acropetal water flow was substantial in submerged Mentha aquatica, reaching 14% of the transpiration stream in emergent plants. The transpiration-independent mass flow of water from the roots, measured by means of tritiated water, was diverted to leaves and adventitious shoots in active growth. The plants grew well and at the same rates in water and air, but nutrient fluxes to the shoot were greater in plants grown in air than in those that were submerged when they were rooted in fertile sediments. Restricted O₂ supply to the roots of submerged plants may account for the smaller nutrient concentrations, though these exceeded the levels required to saturate growth. In hydroponics, the root medium was aerated and circulated between submerged and emergent plants to minimize differences in medium chemistry, and here the two growth forms behaved similarly and could fully exploit nutrient enrichment. It is concluded that the lack of transpiration from leaf surfaces in a vapour-saturated atmosphere, or under water, is not likely to constrain the transfer of nutrients from root to shoot in herbaceous plants. Nutrient deficiency under these environmental conditions is more likely to derive from restricted development and function of the roots in waterlogged anoxic soils or from low porewater concentrations of nutrients.     

Fruit-set of unpollinated ovaries of Pisum sativum L.

The influence of removing the apical shoot and different leaves above and below the flower on the fruit-set of unpollinated pea ovaries (Pisum sativum L. cv. Alaska) has been studied. Unpollinated ovaries were induced to set and develop either by topping or by removing certain developing leaves of the shoot. Topping had a maximum effect when carried out before or on the day of anthesis, and up to four consecutive ovaries were induced to set in the same plant. The inhibition of fruit-set was due to the developing leaves and not to the apex. The third leaf above the first flower, which had a simultaneous development to the ovary, had the stronger inhibitory effect on parthenocarpic fruit-set. The application of different plant-growth regulators (indoleacetic acid, naphthylacetic acid, 2,4-dichlorophenoxyacetic acid, gibberellic acid, benzyladenine and abscisic acid) did not mimic the negative effect of the shoot.Abbreviations CCC (2-chloroethyl)trimethylammonium chloride - MH maleic hydrazide - IAA indole-3-acetic acid - NAA 1-naphthaleneacetic acid - 2,4-D 2,4-dichlorophenoxyacetic acid - GA₃ gibberellic acid - 6-BAP benzyladenine - ABA abscisic acid