Effects of flooding on Eucalyptus camaldulensis and Eucalyptus globulus seedlings

Summary Flooding for up to 40 days induced morphological changes and reduced growth of 6-week-old Eucalyptus camaldulensis and Eucalyptus globulus seedlings. However, the specific responses to flooding varied markedly between these species and with duration of flooding. Both species produced abundant adventitious roots that originated near the tap root and original lateral roots, but only E. camaldulensis produced adventitious roots on submerged portions of the stem. Flooding induced leaf epinasty and reduced total dry weight increment of seedling of both species but growth of E. globulus was reduced more. In both species dry weight increment of shoots was reduced more than dry weight increment of roots, reflecting compensatory growth of adventitious roots. Adaptation to flooding appeared to be greater in E. camaldulensis than in E. globulus. the importance of formation of adventitious roots in flooding tolerance is emphasized.Research supported by the College of Agricultural and Life Sciences, University of Wisconsin, Madison; CEPEC (Cacao Research Center), Bahia, Brazil; and BMBRAPA (Brazilian Research Institute), Brasilia, Brazil     

Release of cotyledonary shoots of pea (<Emphasis Type="Italic">Pisum sativum</Emphasis>) seedlings from inhibition as related to endogenous zeatin and ethylene and exogenous IAA and benzyladenine

This paper deals with apical dominance using a dicotylar model obtained after decapitation of pea seedlings with two shoots — one dominant and the other inhibited. When the dominant shoot was decapitated the inhibited one is released from inhibition and after 24 to 72 h begins to grow. However, the levels of trans-zeatin and production of ethylene increase within 4 and 6 hours respectively after release from inhibition, and within an interval of 72 h the levels of both phytohormones begin gradually to decrease. This indicates that also in this model, the release from apical dominance is associated with an increase in the level of cytokinin zeatin and, thereafter, also with an increased production of ethylene. If indolyl-3-acetic acid (IAA) is applied on the decapitated main stem after decapitation of the dominant shoot, the growth of the initially inhibited one is very strongly retarded; if, however, IAA is applied on the decapitated dominant shoot, this inhibition is significantly weaker. This means that the inhibiting effect of IAA on the inhibited shoot originates to a greater degree from the main stem rather than from the dominant shoot. The effect of benzyladenine (BA) is transferred equally from the decapitated main stem and from the decapitated dominant shoot because the initially inhibited shoot begins to grow as well as also other shoots from serial cotyledonary buds.     

Adventitious shoot formation in decapitated dicotyledonous seedlings starts with regeneration of abnormal leaves from cells not located in a shoot apical meristem

Regeneration of new shoots in plant tissue culture is often associated with appearance of abnormally shaped leaves. We used the adventitious shoot regeneration response induced by decapitation (removal of all preformed shoot apical meristems, leaving a single cotyledon) of greenhouse-grown cotyledon-stage seedlings to test the hypothesis that such abnormal leaf formation is a normal regeneration progression following wounding and is not conditioned by tissue culture. To understand why shoot regeneration starts with defective organogenesis, the regeneration response was characterized by morphology and scanning electron and light microscopy in decapitated cotyledon-stage Cucurbita pepo seedlings. Several leaf primordia were observed to regenerate prior to differentiation of a de novo shoot apical meristem from dividing cells on the wound surface. Early regenerating primordia have a greatly distorted structure with dramatically altered dorsoventrality. Aberrant leaf morphogenesis in C. pepo gradually disappears as leaves eventually originate from a de novo adventitious shoot apical meristem, recovering normal phyllotaxis. Similarly, following comparable decapitation of seedlings from a number of families (Chenopodiaceae, Compositae, Convolvulaceae, Cucurbitaceae, Cruciferae, Fabaceae, Malvaceae, Papaveraceae, and Solanaceae) of several dicotyledonous clades (Ranunculales, Caryophyllales, Asterids, and Rosids), stems are regenerated bearing abnormal leaves; the normal leaf shape is gradually recovered. Some of the transient leaf developmental defects observed are similar to responses to mutations in leaf shape or shoot apical meristem function. Many species temporarily express this leaf development pathway, which is manifest in exceptional circumstances such as during recovery from excision of all preformed shoot meristems of a seedling.     

The roles and interactions of ethylene with gibberellins in the far-red enriched light-mediated growth of <Emphasis Type="Italic">Solanum lycopersicum</Emphasis> seedlings

Wild type (WT) tomato seedlings responded to a low red to far-red (R/FR) ratio with increased stem elongation, similar leaflet area expansion and lower shoot ethylene levels. The levels of endogenous growth-active GA₁ and its immediate precursor GA₂₀ were decreased by low R/FR ratio, whereas the levels of GA₁ catabolite, GA₈, increased. To examine the interaction of ethylene with GAs in regulating tomato shoot growth under low R/FR ratio, transgenic (T) seedlings bearing Le-ACS2 and Le-ACS4 antisense mRNA were utilized. Low R/FR ratio increased stem elongation and decreased ethylene levels in T tomato shoots, as it did in WT shoots. However, T stems were significantly taller than the WT stems under low R/FR ratio. Leaflet areas were significantly larger for T, than WT seedlings under both R/FR ratios. Low R/FR ratio did not decrease endogenous levels of GA₁ and GA₂₀ in T shoots, but did increase GA₈ levels, which were higher than in WT shoots. These results, and hormone/inhibitor application studies, showed that in tomato shoots subjected to low R/FR ratio, GAs play a growth-promotive role in stem elongation, whereas ethylene is growth-inhibitory. Further, these results may imply that decreasing ethylene production under low R/FR ratio causes increases in stem elongation and GA levels.     

Accumulation of 32P and [14C]sucrose in decapitated and intact shoots of the fern Davallia trichomanoides blume

The transport and accumulation of ³²P and [¹⁴C] sucrose in decapitated and intact shoot segments of the fern Davallia were studied. The apical buds of intact shoots and the expanding buds of shoots decapitated 4 weeks before application are major sinks for these nutrients. Decapitation results in a shift of ¹⁴C accumulation from the apex to the lateral buds within 36 h. This shift can be reversed in shoots decapitated for 12 h by replacing the apex. Increased ¹⁴C accumulation into the stump region occurs when decapitated shoot segments are treated with indole-3-acetic acid, and decreased label accumulation into the apical region results when intact shoots are treated with 2,3,5-triiodobenzoic acid.     

Effect of decapitation on absorption,translocation, and phytotoxicity of imazamethabenz in wild oat (Avena fatua L.)

The release of apical dominance by the physical destruction in situ of the apical meristem and associated leaf primordia (decapitation) promoted the growth of tillers in non-herbicide-treated wild oat plants, as indicated by increased tiller lengths and fresh weights. At 96 h after [¹⁴C] herbicide treatment following decapitation, the absorption of [¹⁴C]imazamethabenz and total translocation of radioactivity were respectively increased by 28% and 49%. By 96 h after [¹⁴C]imazamethabenz application, the radioactivity detected in the roots of decapitated plants was 45% higher than that in the roots of nondecapitated plants while the radioactivity in tillers of decapitated plants was 2.6-fold that in tillers of intact plants. Decapitation together with foliar spraying of imazamethabenz at 200 g ha^–1 further reduced tiller fresh weight, greatly decreased the total tiller number, and thereafter significantly increased overall phytotoxicity by 32% as measured by total shoot fresh weight. The results of this study support the hypothesis that main shoot apical dominance limits translocation of applied imazamethabenz to lateral shoots, rendering tillers less susceptible to growth inhibition by the herbicide.     

Effects of neighbouring vegetation on eucalypt seedlings at two sites subject to different levels of abiotic stress

Abstract In spite of numerous studies on the effect of nutrient levels and/or standing crop on the intensity of resource competition the debate has not been resolved. Field studies that have used natural productivity gradients have generally supported the argument that competitive intensity and resource availability are positively correlated, whereas studies that have used artificial resource gradients have generally refuted the same argument. Here we report the results from study in which both approaches were used within the same system. We studied two species of eucalypt that occupy contrasting parts of the same landscape: Eucalyptus camaldulensis, found mostly along creek lines and in valleys with deep alluvial soils, and Eucalyptus microcarpa, found on hillsides and ridges with shallow soils. We studied the response of seedlings of the two species to the combined effects of competition and manipulated nutrient levels in a glasshouse experiment, and also investigated their responses to removal of neighbouring plants in the field. Eucalyptus microcarpa was less responsive to increased resource availability, which is consistent with one of the principal assumptions of Grime’s C‐S‐R model. In the glasshouse experiment both species of eucalypt responded in a qualitatively similar fashion to the combined effects of resource availability and competition: release from competition resulted in increased growth, but only in pots that received additional resources. In the field we found that neighbouring vegetation could severely limit the establishment of E. camaldulensis but the removal of neighbouring vegetation did not affect the performance of E. microcarpa seedlings. Eucalyptus camaldulensis seedlings suffered high levels of damage from herbivores. Our results thus generally support the predications of the C‐S‐R model, however, they indicate that the effects of competition and herbivory may be heavily confounded.     

Effects of root-zone solutes on Eucalyptus camaldulensis and Eucalyptus bicostata seedlings: Responses to Na+, Mg2+ and Cl-

Specific-ion effects in salt-treated eucalypts were examined with two species known to differ in salt tolerance viz. E. camaldulensis (more tolerant) and E. bicostata (less tolerant). Sand-cultured plants were irrigated with different nutrient solutions designed to impose either osmotic stress (concentrated macronutrients with balanced cations and anions) or specific ion stress from either NaCl or MgCl₂, or from nutrient solutions rich in particular ions viz. Na⁺, Mg²⁺ and Cl^- (balancing counter ions were provided in all cases). Half-strength Hoagland nutrient solution served as control. All treatments were applied at osmotic pressures of approximately 0.52 MPa by appropriate concentrations of each solution. In general, salt-induced growth reductions were greater for E. camaldulensis than for E. bicostata, although E. camaldulensis showed strongest exclusion of Na⁺, Mg²⁺ and Cl^- from shoots. Application of NaCl and concentrated macronutrients resulted in similar growth reductions. E. bicostata seedlings exposed to high Cl^- concentrations in the presence of Mg²⁺ and concentrated cations suffered significantly more shoot and root reduction than those exposed to other salts. Treatment with solution rich in Cl^- resulted in extensive leaf damage, which suggested that Cl^- may have exerted a specific effect. No specific Na⁺ effect was observed for either species, even though shoot Na⁺ concentrations were considerably higher for E. bicostata than for E. camaldulensis. Root growth was considerably less for plants treated with Mg²⁺ salts and this effect was associated with low root Ca²⁺ concentrations.     

Differential Responses of Buds along the Shoot to Factors Involved in Apical Dominance

Intact and decapitated 6-node shoots of Hygrophila sp. weregrown aseptically immersed in liquid half-strength Knop's solutionwith microelements and 2% (w/v) sucrose (control medium), andin medium with 0.1 mg l^–1 benzyladenine (BA). In intactshoots grown in control medium apical dominance suppressed outgrowthof the lateral buds; in decapitated shoots buds grew out atseveral of the most apical nodes, increasing in size acropetally.There was a lag in outgrowth of the bud at the most apical node,attributable to its initially smaller size. Lateral shoots grewout first at basal nodes of intact shoots in BA medium, decreasingin size acropetally; in decapitated shoots in BA medium lateralshoots of approximately equal size grew out at all nodes. Differentialeffects of decapitation and cytokinin treatment on lateral shootoutgrowth along the shoot could be interpreted by postulatinga basipetally decreasing gradient of endogenous auxin concentrationin the intact shoot. Application of 20 mg l^–1 indoleaceticacid (IAA) in agar to decapitated shoots completely preventedbud outgrowth for at least 7 d in control medium, inhibitingit thereafter, and inhibited bud outgrowth in BA medium, thussupporting the hypothesis. Comparison of lateral shoot outgrowthin whole decapitated shoots and severed decapitated shoots (isolatednodes) lent no support to the alternative hypothesis that theremight be an acropetally decreasing concentration gradient ofa bud-promoting substance in the intact shoot, and demonstratedmuch greater lateral shoot growth in isolated nodes. The resultsemphasize important correlative relationships between the partsof a shoot with several nodes.     

Vegetative micro-cloning to sustain biodiversity of threatened Moringa species

Efficient vegetative cloning in vitro requires definition of plant growth regulator regimes for each genotype, and therefore formulation of a uniform culture protocol for a genetically heterogeneous wild or uncultivated plant population is often impossible. The likelihood of cloning a wide array of plant genotypes by avoiding the use of plant growth regulator(s) was explored with Moringa oleifera Lamk., Moringa stenopetala (Baker f.) Cufod, and Moringa peregrina Forssk. ex Fiori tree seedlings. Propagation was achieved by multiple shoot regeneration from the cotyledonary node of decapitated seedlings, followed by axillary shoot growth from single node shoot segments and rooting of excised shoots. All steps were accomplished on basal Murashige and Skoog medium without plant growth regulator supplements. The results revealed competence for generation of multiple shoots from cotyledonary node tissue, stimulated by repeated shoot harvest, in seedlings of all three tree species. Tens of plants per seedling were regenerated in about 4 mo from culture initiation. In a given species clone size was seedling-dependent, which presumably stems from genotypic variability among seedlings in regeneration ability in vitro. By this means the laborious search for a plant growth regulator regime suitable for organogenesis induction and adapted per genotype became redundant, and biodiversity of the seed germplasm could be maintained. The approach ideally suits establishment of clones of wild plants of endangered species, like those of the Moringaceae, species with high ability for producing supplementary shoots, and without the need to add plant growth regulators, including the rooting stage.     

Arbuscular and ectomycorrhizal colonization of two <Emphasis Type="Italic">Eucalyptus</Emphasis> species in semiarid Brazil

Our goal was to evaluate the mycorrhizal colonization, as well as the density of arbuscular mycorrhizal (AM) fungal spores, in Eucalyptus camaldulensis and E. grandis monocultures at 2 years in a semiarid part of Brazil. Soil and root samples were collected in 2 consecutive years. Eucalyptus camaldulensis showed varied AM colonization level according to season of sampling, and Glomus was dominant in spore numbers. Eucalyptus grandis showed dominant ectomycorrhizal (ECM) colonization and lower AM fungal spore density. Overall results suggest that E. camaldulensis has both AM and ECM dependencies, whereas E. grandis is solely ECM dependent in the monocultures.     

A simple plant regeneration-ability assay in a range of Lycopersicon species

Plant growth regulator-dependent (PGR-dependent) in vitro shoot organogenesis has been extensively studied in tomato (Lycopersicon esculentum), whereas PGR-independent adventitious shoot organogenesis received marginal attention in L. esculentum and no consideration at all in other Lycopersicon species. In the present study, induction of PGR-independent adventitious shoots was by decapitation of the apex and removal of preexisting shoot meristems of the seedling, and seedling culture on a medium with no PGR supplements. The existence of PGR-independent regeneration-ability was verified in L. esculentum genotypes (high pigment photomorphogenic mutants and wild-type counterparts) and was uncover amongst L. cheesmanii, L. chilense, L. chmielewskii, L. hirsutum, L. parviflorum, L.␣peruvianum and L. pimpinellifolium. Compared to species other than L. esculentum, high pigment photomorphogenic mutants displayed the weakest PGR-independent regeneration-ability. Our results imply that decapitated seedlings cultured on a medium without PGRs can serve as a convenient assay system for genotypic variation in self-controlled, PGR-independent, shoot regeneration-ability in a wide range of Lycopersicon species. Using transverse thin slices of the hypocotyl placed onto a medium supplemented with 0.2 μM zeatin reboside and 0.04 μM IAA, we assessed PGR-mediated shoot regeneration in L. esculentum genotypes. In a given genotype, more plants per seedling were established by PGR-mediated than by PGR-independent regeneration. However, with both modes of organogenesis, only a fraction of shoot buds eventually grew into normal plants, while others developed into abnormal regenerants having no stem. Percentage of stem-deficiency, in a given genotype, was higher in PGR-treated cultures, which indicates that PGRs amplify the formation frequency of imperfect adventitious apical shoot meristems. Unlike L. esculentum, adventitious shoot buds of other Lycopersicon species, induced by wounding seedlings that were not treated with PGRs, rarely formed regenerants lacking a stem.     

Changes in Phytohormone Status in Stems and Roots after Decapitation of Pea Seedlings

Experiments were performed on the first and second internodes and 4-cm-long apical segments of main roots of pea (Pisum sativum L.) seedlings, grown in the light and decapitated above the second node on the seventh day after seed germination. Endogenous phytohormones were measured by the enzyme-linked immunosorbent assay during three days after decapitation of seedlings. The IAA level in the internodes decreased 2–3 times on the second day after decapitation of seedlings while the cytokinin level increased 5–6 times for zeatin and zeatin riboside (Z and ZR) and 1.5–2 times for isopentenyl adenine and isopentenyl adenosine (IP and IPA). In contrast to internodes, the IP and IPA contents in the roots of decapitated seedlings did not change, but the levels of Z and ZR increased 1.5–2 times compared to intact plant roots. The IAA level in the apical region of root remained almost unchanged after the removal of shoot apex. It was concluded that the apical meristem of the main root is not the site of the cytokinin response to the auxin signal coming from the stem apex and that a slight accumulation of Z and ZR after decapitation is due to upper zones of the root. There was no difference in the content of gibberellin-like substances between the internodes of intact and decapitated seedlings. However, the content of gibberellins (GA) in the root tip decreased after decapitation of seedling, which suggests an essential role of apical bud in supplying the root with GA and/or intermediates for their biosynthesis.     

Growth-active gibberellins overcome the very slow shoot growth of Hancornia speciosa, an important fruit tree from the Brazilian “Cerrado”

Shoot elongation of Hancornia speciosa, an endangered tree from the Brazilian savannah “Cerrado”, is very slow, thus limiting nursery production of plants. Gibberellins (GAs) A₁, A₃, and A₅, and two inhibitors of GA biosynthesis, trinexapac-ethyl and ancymidol were applied to shoots of Hancornia seedlings. GA₁ and GA₃ significantly stimulated shoot elongation, while GA₅ had no significant effect. Trinexapac-ethyl and ancymidol, both at 100 μg per seedling, inhibited shoot elongation up to 45 days after treatment, though the effect was statistically significant only for ancymidol. Somewhat surprisingly, exogenous GA₃ more effectively stimulated shoot elongation in SD-grown plants, than in LD-grown plants. The results from exogenous application of GAs and inhibitors of GA biosynthesis imply that Hancornia shoot growth is controlled by GAs, and that level of endogenous growth-active GAs is likely to be the limiting factor for shoot elongation in Hancornia. Application of GAs thus offer a practical method for nursery production of Hancornia seedlings for outplanting into the field.     

Ethylene and the growth of rice seedlings

Etiolated whole rice seedlings enclosed in sealed vials produced ethylene at a rate of 0.9 picomole per hour per seedling. When 2-centimeter-long shoots were subdivided into 5-millimeter-long sections, the sections containing the tip of the shoot evolved 37% of the total ethylene with the remaining 63% being produced along a gradient decreasing to the base of the shoot. The tip of the coleoptile also had the highest level of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid and of the ethylene-forming enzyme activity. Ethylene is one of the factors controlling coleoptile elongation. Decapitation of the seedling reduced ethylene evolution to one-third its original level and inhibited coleoptile growth. In short-term experiments, the growth rate of decapitated seedlings was restored to almost that of intact seedlings by application of ethylene at a concentration of 10 microliters per liter. Apart from ethylene, O₂ also participates in the control of coleoptile growth. When rice seedlings were grown in a gas mixture of N₂ and O₂, the length of the coleoptiles reached a maximum at a concentration of 2.5% O₂. Lower and higher concentrations of O₂ reduced coleoptile growth. The effect of exogenous ethylene on coleoptile growth was also O₂ dependent.     

An improved and reliable chili pepper (Capsicum annuum L.) plant regeneration method

In this work we report a new method forin vitro chili pepper (Capsicum annuum L.) plant regeneration based on shoot formation from wounded hypocotyls. Chili pepper seeds were surface sterilized and germinated on agar (0.8%) at 25 ± 2°C in the dark. Five factors that may influence shoot regeneration were studied: age of seedlings, hypocotyl wounding site, time elapsed between wounding the hypocotyls and decapitation of seedlings, culture media and cultivars. In order to study the influence of the first three factors on shoot regeneration, the apical, middle or basal hypocotyl regions of seedlings of cv. Mulato Bajio at different stages of development (9, 15, 16, 21 and 28 d old) were wounded with a syringe needle, and the seedlings were cultured on MS semisolid medium without growth regulators at 25 ± 2°C under a 16/8 h light/dark photoperiod (daylight fluorescent lamps; 35 mol m-² s-^-1) until decapitation. The seedlings were decapitated (3 mm below the cotyledons) at different times after wounding (0, 2, 4, 10, 12 and 14 d), and each explant was evaluated for bud and shoot formation ( 5 mm in length) at the wounded site after 30 d of incubation. In general, seedlings at the stage of curved hypocotyl (9 d old) wounded in the apical region of hypocotyl were the best explants for shoot regeneration when inoculated on culture medium without growth regulators. Decapitation after wounding also influenced the shoot regeneration efficiency, with 10–14 d being the best period. Up to 90% shoot regeneration in cv. Mulato Bajio was obtained under these conditions. Statistically significant differences were observed for shoot formation among 21 cultivars tested. Regeneration of whole plants was achieved by rooting the shoots with indole-3-butyric acid pulses of 60 mg L^–1 for 3 h and then subculturing on MS medium without growth regulators.     

Relationship of Apical Dominance to the Nutrient Accumulation Pattern in Pisum sativum var. Alaska

Decapitation of the pea plant resulted in the growth of all the lateral shoots. The initial growth of all lateral buds was somewhat similar. The differential growth rates developed later on. The pattern of growth of lateral shoots varied with the age of the plant when decapitation was performed. The basal shoots dominated when the plants were decapitated at the 2-leaf stage. At 3-leaf stage decapitation resulted in the dominance of shoot 5. Decapitation at 4-or-more-leaf stage resulted in the eventual dominance of the suhterminal lateral shoot. As a rule P-32 moved to the most actively growing part of the plant, i.e. apex in intact vegetative plant, the growing lateral shoots in a decapitated plant, the elongating subapical parts of the stem and the roots. The various metabolic sinks seemed to compete actively for this nutrient, therefore P-32 accumulation in any particular growing region of the plant was taken as an indicator of nutrient utilization potential of that part. The stem apex of an intact plant seemed to loose its dominance with the increasing age of the plant. The loss of apical dominance was almost complete during the reproductive phase of the plant, during which the upper lateral shoots initiated growth. Their growth, however, was inhibited soon because of competition with the other developing sinks, viz., the flower and the fruit. The amount of soluble carbohydrates in various parts of the pea plant followed essentially the same pattern as did P-32 accumulation. These distribution patterns were apparently correlated with the growth of the plant.     

赤桉ACT1和ACT2基因的克隆与生物信息学分析

为了解赤桉(Eucalyptus camaldulensis)肌动蛋白(Actin)在生长发育过程中的功能,根据赤桉幼苗转录组数据库中的肌动蛋白基因序列,从赤桉嫩叶中克隆了2条Actin基因片段,并利用RACE技术获得Actin基因的全长cDNA,分别命名为ECACT1和EC-ACT2基因。生物信息学分析表明,这两条基因的全长cDNA分别为1533 bp和1387 bp,均含有1个编码377个氨基酸的开放阅读框。经比对分析,赤桉Actin蛋白的氨基酸序列与其他植物Actin蛋白的具有较高的相似性,并且具有Actin蛋白特有的保守序列和相关特征。因此推测这两条基因对桉树的生长发育具有一定的调控作用。     

Effects of stump and slash removal on growth and mycorrhization of Picea abies seedlings outplanted on a forest clear-cut

The objectives of this study were to investigate impact of stump and slash removal on growth and mycorrhization of Picea abies seedlings outplanted on a forest clear-cut. Four non-replicated site preparation treatments included: (1) mounding (M), (2) removal of stumps (K), (3) mounding and removal of logging slash (HM) and (4) removal of logging slash and stumps (HK). Results showed that height increment of the seedlings was highest in K and lowest in M after the third growing season, and similar pattern remained after the fourth season. Ectomycorrhizal (ECM) colonisation of seedling roots was highest in M (96.6%) and lowest in K (72.3%), and even in HK (76.0%) and HM (76.3%). Morphotyping and sequencing of internal transcribed spacer of fungal ribosomal DNA revealed a total of 13 ECM species. Among those, Thelephora terrestris and Cenococcum geophilum were the most common, found on 27.4% and 26.3% of roots, respectively. The rest of species colonised 26.6% of roots. Richness of ECM species was highest in M (10 species) and lowest in K (three species). Consequently, stump and slash removal from clear-felled sites had a positive effect on growth of outplanted spruce seedlings, but negative effect on their mycorrhization. This suggests that altered soil conditions due to site disturbance by stump and slash removal might be more favourable for tree growth than more abundant mycorrhization of their root systems in less disturbed soil.     

Nitrate reductase activity and growth response of forest species to ammonium and nitrate sources of nitrogen

Summary Three tree species,Eucalyptus regnans (F. Muell.),E. obliqua (L'Herit.),Pinus radiata (D. Don) were grown in sand culture with different proportions of nitrate and ammonium. Nitrate Reductase Activity (NRA) was induced in root tissue of all species and in leaf tissue of the eucalypts. An increasing proportion of nitrate resulted in increasing NRA in all species and hence NRA alone is no indication of N-preference. The highest NRA was found withE. regnans, a result which has also been obtained in the mature forest. The growth ofE. regnans was least with NH₄ ⁺ alone, whereas that ofE. obliqua was least with NO₃ ^– alone. The soils of matureE. regnans forest have a high potential for nitrification while those ofE. obliqua forest show little nitrification. Thus the preference for particular N sources shown by seedlings in culture is supported by related properties of mature forests. It is postulated however, that the inducibility of a high level of RNA in seedlings is more likely a result of a preference for NO₃ ^– than a cause.