Plant critical levels for the evaluation of boron toxicity in spring barley (Hordeum vulgare L.)

Summary Pot experiments on B toxicity in spring barley (Hordeum vulgare L., cv. Trumpf) using sandy soils indicated that there are significant relationships between B content of the leaves and of the shoots respectively in the toxic range and degree of damage of the leaves at stage 7–8 of the Feekes scale, which may be used to derive plant critical levels of B toxicity. Symptoms due to B excess begin to develop on the leaves (leaf tip necroses) relatively independently of the ontogenetical stage of development as soon as the B content of the leaf tissue reaches 60–80 mg/kg DM.The corresponding symptom-related toxic plant critical level of shoots (i.e. the B content of the whole shoot at which the first damage in leaves begins to occur) ranges from 30 mg/kg shoot DM (related to older leaves) to 80 mg/kg shoot DM (related to younger leaves). Grain yield is significantly reduced only when the B content of shoots at Feekes stages 7–8 exceeds the yield-related toxic plant critical level (yield reduction to 90% of the optimum yield) of 120–130 mg/kg shoot DM.B contents of the shoots at Feekes stages 7–8 from 80–120 mg/kg shoot DM define the range at which plants have marked toxicity symptoms, but at which there are no yield reductions.     

LIGHT EFFECTS ON LEAF MORPHOLOGY IN WATER HYACINTH (EICHHORNIA CRASSIPES)

Water hyacinth leaves in natural populations vary from being long and thin-petioled to being short with inflated petioles. A variety of factors has been used experimentally to alter water hyacinth leaf shape, but what controls the development of leaf morphology in the field has not been established. We measured photosynthetic photon flux density (PPFD) and spectral distribution of radiation in a natural water hyacinth population. PPFD in the center of the water hyacinth mat was reduced to 2.7% of full sunlight, and the red to far red (R:FR) ratio was reduced to 0.28. When shoot tips of plants were exposed to artificial light environments, only plants in the treatment with a R:FR ratio comparable to that in the natural population produced leaves with long, thin petioles. Shoot tips in full sun or covered with clear plastic bags or bags that reduced light quantity without greatly altering light quality produced shorter leaves with inflated petioles. We hypothesize that the altered light quality inside a mat is a major environmental control of water hyacinth leaf morphology.     

Effect of springtime solar ultraviolet-B radiation on growth of Colobanthus quitensis at Palmer Station, Antarctica

We examined the influence of solar ultraviolet‐B radiation (UV‐B; 280–315 nm) on the growth of Colobanthus quitensis plants by placing them under contrasting UV‐B filters at Palmer Station, along the Antarctic Peninsula. The filters reduced diurnal biologically effective UV‐B (UV‐B_BE) either by 83% (‘reduced UV‐B’) or by 12% (‘near‐ambient UV‐B’) over the 63 day experiment (7 November 1998–8 January 1999). Ozone column depletion averaged 17% during the experiment. Relative growth and net assimilation rates of plants exposed to near‐ambient UV‐B were 30 and 20% lower, respectively, than those of plants exposed to reduced UV‐B. The former plants produced 29% less total biomass, as a result of containing 54% less aboveground biomass. These reductions in aboveground biomass were mainly the result of a 45% reduction in shoot biomass, and a 31% reduction in reproductive biomass. Reductions in shoot biomass were owing to an 18% reduction in branch production by main shoots, while reductions in reproductive biomass were the result of a 19% reduction in individual capsule mass. Total plant leaf area was reduced by 19% under near‐ambient UV‐B, although total leaf biomass was unaffected because leaves had a greater specific leaf mass. The reduction in plant leaf area under near‐ambient UV‐B was attributable to: (1) production of 11% fewer leaves per main shoot system and plant, which resulted from an 18% reduction in branch production by main shoots. Leaf production per individual main shoot or branch was not affected; (2) shorter leaf longevity—main shoots contained 14% fewer green leaves at a given time; and (3) smaller individual leaves—leaf elongation rates were 14% slower and mature leaves were 13% shorter.     

Effects of Water and Nitrogen Supply on Water Use Efficiency and Carbon Isotope Discrimination in Edible Canna (Canna edulis Ker-Gawler)

Abstract: Transpirational water use efficiency (WUE) is affected by nutrient and water supply, but relatively little is known about the response of Andean root and tuber crops, such as edible canna ( Canna edulis Ker-Gawler). Two Canna edulis genotypes were studied in semi-controlled greenhouse experiments at the International Potato Centre (CIP) near Quito, Ecuador. Under conditions of low water supply, shoot dry matter (DM), leaf area and specific leaf nitrogen (SLN) all decreased, while WUE was higher and carbon isotope discrimination (Δ) decreased. The large water-storing hypodermal cells characteristic of this plant's leaf morphology shrank, concertina-like, the leaves became thinner, and the specific leaf area increased significantly (from 26 m^-2 kg^-1 to 43 m^-2 kg^-1). In a second experiment, plants were grown with three different levels of nitrogen supply: from low to high N supply, shoot DM increased significantly (from 16 to 37 g), along with leaf area and SLN. At the lowest level of nitrogen supply (N0), WUE was significantly lower and Δ increased. As expected, the linear correlation between Δ and WUE was negative, but variation in Δ could only explain 49 % of the variation in WUE. If, in addition to Δ, measured root fraction (RF) and estimated values of leaf-to-air vapour pressure deficit (Δe) and respiratory and unproductive carbon loss were used to calculate WUE, the correlation between measured and calculated WUE was substantially improved, except for the water stress treatment. It is considered that, for root and tuber crops, RF and Δe are the major variables when Δ of single leaves is to be used for up-scaling to plant WUE.     

Characterization of leaf boron injury in salt-stressed Eucalyptus by image analysis

Symptoms of boron toxicity (i.e., necrosis of leaf tips and margins) have been observed on eucalyptus trees in the San Joaquin Valley of California where the trees are being tested for their effectiveness at reducing the volume of agricultural drainage effluents. In a controlled, outdoor sand-tank study, Eucalyptus camaldulensis Dehn., Clone 4544 trees were grown and irrigated with combinations of salinity and B to determine their influence on tree growth and water use. Irrigation water quality treatments were prepared to simulate the Na-sulfate salinity, high B nature of these drainage effluents. Electrical conductivities (EC_iw) of the waters ranged from 2 to 28 dS m-1 and B concentrations ranging from 1 to 30 mg L-1. As an integral component of this study , we developed a method to quantify and correlate foliar damage with leaf B concentrations. By scanning both injured and uninjured leaves into computer files and processing with image analysis, we were able to simultaneously correlate salinity stress with its overall effect on leaf area as well as to quantify the relative fraction of leaf area affected by specific-ion (i.e., B) injury. Leaf area was unaffected by B stress but was reduced by salinity only in the younger leaves. Boron injury was correlated with increasing irrigation water B only in older leaves. The relative injured area (RIA) of the older leaves was related to the B concentrations of leaves from trees grown at various salinities . A regression equation was developed from injury data obtained from trees grown under boron and salinity stress for 223 days (r2=0.90). From this relationship, we were able to estimate leaf boron concentrations from injury symptoms in leaves selected at random from main trunk branches of trees grown for 333 days under the same stress conditions. The results suggest that this method may have potential as an effective tool for monitoring the response to toxic levels of boron in eucalyptus, once B toxicity has been established by analytical means. The RIA appears to be mitigated by increased salinity of the irrigation water and is consistent with the general reduction in leaf B by salinity. The interactive effects of boron and salinity on foliar injury depends on the physiological age of the leaf.     

The influence of branches and leaf area on rooting and development of Cotinus coggygria cv. Royal Purple cuttings

The influence of branches and mature leaves on the rooting and subsequent development of cuttings was examined, using Cotinus coggygria cv. Royal Purple. A model system was developed, whereby branched cuttings could be harvested from stock hedges and manipulated to alter leaf area, the number of actively‐growing, lateral branches and thus the source: sink ratio for photoassimilates. Highest percentage rooting ((80%) was promoted by retention of branches and a full leaf area. Reducing leaf area resulted in a lower rooting percentage (44%); however, greatest reductions in rooting were associated with the removal of lateral branches ((22%). Applying exogenous auxin (indole‐3‐butyric acid) at the excision point where branches had been removed significantly improved rooting potential, but did not fully substitute for the presence of branches with active shoot tips. Negative effects associated with removing a proportion of mature leaves appeared to relate to alterations in carbon balance rather than an influence on the supply of endogenous auxin to the potential rooting zone. The use of branched cuttings accelerated root and shoot development and resulted in a finished plant being produced more rapidly than is achieved from conventional, non‐branched cuttings. The results presented indicate a means for improving the efficiency of production of Cotinus coggygria, which may be applicable to a wider range of ornamental plants.     

Improvement of adventitious shoot formation from carnation leaf explants

Adventitious shoot formation from leaf explants of carnation (Dianthus caryophyllus L.) was investigated. The two leaves from one node of in vitro-grown plants showed different shoot-forming potential, depending on the order in which the leaves were removed from the stem. The leaf removed second formed more shoots and also had a large amount of adhering stem tissue. Explants with equal amounts of adhering stem tissue were obtained by making two incisions through the fused leaf bases, prior to their removal, resulting in an improved shoot formation. The procedure developed for leaf explants from in vitro-grown plants was also applied to leaf explants from greenhousegrown plants. Shoot formation from leaf explants taken from greenhouse-grown plants was further improved by cutting the leaf explant longitudinally into two parts.Abbreviations BA benzyladenine - NAA -naphthaleneacetic acid     

硝酸银对雌性黄瓜植株三种氧化酶同工酶的影响

雌性黄瓜植株经硝酸银处理后其茎尖和真叶过氧化物酶活性极显著地增加,茎尖24小时、真叶36小时酶活性达到最大值,分别增加了178.2%和284.6%,随后酶活性逐渐下降,但酶活性仍然较对照植株高。多酚氧化酶和超氧化物歧化酶的同工酶活性也增加。同时硝酸银能诱发黄瓜植株过氧化物酶、多酚氧化酶和超氧化物歧化酶产生新的同工酶, 用等电聚焦更能有效地观察新产生的同工酶。     

Functional Manganese Requirement and its use as a Critical Value for Diagnosis of Manganese Deficiency in Subterranean Clover (Trifolium subterraneum L. cv. Seaton Park)

The effects of manganese supply on plant growth and on photosynthesisand manganese concentrations in young leaves were examined inSeaton Park subterranean clover in three glasshouse water cultureexperiments. Plants werc grown initially with a copious supply of manganese,and transferred to solutions either with or without manganese.Sequential harvests were taken to determine the effects of developingmanganese deficiency on dry matter (DM) yield of whole plantsand selected characteristics [manganese, chlorophyll and photosyntheticoxygen evolution (POE)] of youngest open leaf blades (YOL).In addition, the deffect of leaf age and iron supply on POEwerc examined. Manganese concentrations and POE in YOL declined markedly andrapidly in plants transferred to solutions without manganese,while chlorophyll concentrations of YOL and plant DM yield respondedmore weakly and more slowly. As a result, a level of manganesedeficiency which depressed POE in young leaves by more than50 per cent had no effed on DM production. In youngleaves (YOL, YOL + 1, YOL–1), POE declined whentheir manganese concentrations were < 20 µg g^–1DM. Iron supply did not affect this rdationship. When learnwith < 20 µg Mn g^–1 DM were detached and incubatedfor 24 h in solutions containing high concentrations of manganese,their POE increased to normal rates; leaves with higher manganeseconcentrations did not respond. It is suggested that the valueof 20 µg Mn g^–1 DM is the functional manganese requirementfor POE in young subterranean clover leaves It is also suggestedthat this value may be used as a critical value for indicatingmanganese deficiency in subterranean clover. Functional nutrient requirements determined in this way by correlationof nutrient concentrations in young leaves with their biochemicalor physiological activities appear to offer more accurate andconsistent standards for use an critical values for diagnosisof plant nutrient status than do the critical values determinedin the usual way by correlation with plant dry weight. Trifolium subterraneum L. subterranean clover, manganese, functional requirements, deficiency diagnosis, nutrient requirements, critical values, photosynthetic oxygen evolution     

The role of mineral nutrients in the increased growth response of tomato plants in solarized soil

Soil solarization is a non-chemical disinfestation technique that frequently promotes plant growth in the absence of known major pathogens, a phenomenon termed increased growth response (IGR). The effect of solarization on plant nutrients and their role in the IGR was studied with tomato plants grown in solarized or non-solarized (control) sandy soil, under controlled conditions. Solarization considerably increased the soil concentrations of water extractable N, K, Ca, Mg and Na at most sites, whereas Cl and DTPA extractable Mn, Zn, Fe and Cu were decreased by the treatment. Plant growth and specific leaf area were enhanced in solarized as well as in N-supplemented control soil. In tomato plants grown in solarized soil, concentrations of most nutrients in the xylem sap, including N, were increased compared to the control, whereas Cl and SO4 levels decreased. The most significant increase in leaf nutrient concentration caused by soil solarization was recorded for N. Furthermore, leaf N concentration was highly and positively correlated with shoot growth. The concentration of Cu increased in leaves from the solarization vs. the control treatment, whereas that of SO4 and Cl decreased, the latter presumably below the critical toxicity level. The correlation between shoot growth and leaf concentration was positive for Cu and inverse for Cl and SO4. In conclusion, we found that soil solarization significantly affects nutrient composition in tomato plants, and provided strong evidence that N, and eventually also Cl, play a major role in IGR.     

Endogenous ABA maintains shoot growth in tomato independently of effects on plant water balance: evidence for an interaction with ethylene

To examine whether the reduced shoot growth of abscisic acid (ABA)-deficient mutants of tomato is independent of effects on plant water balance, flacca and notabilis were grown under controlled-humidity conditions so that their leaf water potentials were equal to or higher than those of well-watered wild-type plants throughout development. Most parameters of shoot growth remained markedly impaired and root growth was also greatly reduced. Additional experiments with flacca showed that shoot growth substantially recovered when wild-type levels of ABA were restored by treatment with exogenous ABA, even though improvement in leaf water potential was prevented. The ability of applied ABA to increase growth was greatest for leaf expansion, which was restored by 75%. The ethylene evolution rate of growing leaves was doubled in flacca compared to the wild type and treatment with silver thiosulphate to inhibit ethylene action partially restored shoot growth. The results demonstrate that normal levels of endogenous ABA are required to maintain shoot development, particularly leaf expansion, in well-watered tomato plants, independently of effects on plant water balance. The impairment of shoot growth caused by ABA deficiency is at least partly attributable to ethylene.     

Smart pipes: The bundle sheath role as xylem-mesophyll barrier

Signs of abiotic toxicity often appear first at the margins of leaves and gradually spread toward the midrib. It has been suggested that the bundle sheath tissue surrounding the shoot vascular system acts as a solute transport-regulating barrier that prevents excessive quantities of toxic ions from entering the leaf and pushes them toward the hydathodes. We examined this hypothesis by examining the distribution of toxic boron (B) in mutant Arabidopsis leaves with flooded mesophyll and comparing it with that observed in control leaves that exuded guttation drops. As opposed to the control plants, which showed classical symptoms of B toxicity (necrosis starting at the leaf margins), in the mutants, necrosis was first observed inside the leaf. We will discuss this result and how it supports the hypothesis that the bundle sheath serves as a selective barrier filtering the xylem-to-leaf radial transport flow and pushing toxic solutes toward the hydathodes.     

Effect of drought on sorbitol and sucrose metabolism in sinks and sources of peach

In peach (Prunus persica [L.] Batsch.), sorbitol and sucrose are the two main forms of photosynthetic and translocated carbon and may have different functions depending on the organ of utilization and its developmental stage. The role and interaction of sorbitol and sucrose metabolism was studied in mature leaves (source) and shoot tips (sinks) of ‘Nemaguard’ peach under drought stress. Plants were irrigated daily at rates of 100, 67, and 33% of evapotranspiration (ET). The relative elongation rate (RER) of growing shoots was measured daily. In mature leaves, water potential (Ψ_w), osmotic potential (Ψ_s), sorbitol‐6‐phosphate dehydrogenase (S6PDH, EC 1.1.1.200), and sucrose‐phosphate synthase (SPS, EC 2.4.1.14) activities were measured weekly. Measurements of Ψ_s, sorbitol dehydrogenase (SDH, 1.1.1.14), sucrose synthase (SS, EC 2.4.1.13), acid invertase (AI, EC 3.2.1.26), and neutral invertase (NI, EC 3.2.1.27) activities were taken weekly in shoot tips. Drought stress reduced RER and Ψ_w of plants in proportion to water supply. Osmotic adjustment was detected by the second week of treatment in mature leaves and by the third week in shoot tips. Both SDH and S6PDH activities were reduced by drought stress within 4 days of treatment and positively correlated with overall Ψ_w levels. However, only SDH activity was correlated with Ψ_s. Among the sucrose enzymes, only SS was affected by drought, being reduced after 3 weeks. Sorbitol accumulation in both mature leaves and shoot tips of stressed plants was observed starting from the second week of treatment and reached up to 80% of total solutes involved in osmotic adjustment. Sucrose content was up to 8‐fold lower than sorbitol content and accumulated only occasionally. We conclude that a loss of SDH activity in sinks leads to osmotic adjustment via sorbitol accumulation in peach. We propose an adaptive role of sorbitol metabolism versus a maintenance role of sucrose metabolism in peach under drought stress.     

Effect of Meloidogyne incognita and M. javanica on Leaf Water Potential and Water Use of Tobacco

Greenhouse lysimeter and field microplot tests were conducted to evaluate the effects of Meloidogyne incognita and M. javanica on plant water relations and growth performance of NC 2326 flue-cured tobacco. In the greenhouse, afternoon leaf water potential values at 8-11 weeks after transplanting were lower by as much as 0.22 MPa in plants infected with either nematode than in the control plants. From 11 to 22 weeks, leaf water potential values were similar in all treatments. Over the course of the 22-week experiment, all infected plants showed similar evapotranspiration patterns, and plants in these treatments used 87-88% of the water utilized by noninfected plants. Biomass production from nematode-infected plants, however, was only about 50% of the biomass of control plants. The field microplot study showed water use patterns similar to those in the lysimeter study.     

Production and nitrogen responses of the African dwarf shrub Indigofera spinosa to defoliation and water limitation

Summary The dwarf shrub Indigofera spinosa Forsk. (Papilionacea), a native forage species of arid Northwest Kenya, was propogated from seed, grown in a controlled environment, and subjected to three treatments of defoliation and watering frequencies in a factorial experimental design. Biomass production and nitrogen accumulation in tissue components were measured to determine defoliation responses in a water-limited environment. We hypothesized that plants would maintain biomass and nitrogen flows despite removal of aboveground meristems and tissues by defoliation. Principal experimental results included a slight reduction (11%; P=0.08) of total biomass production by clipping ca. 1/3 or 2/3 of new leaves and stems and all apical meristems every month. Total aboveground production was not affected by clipping, while final root biomass was reduced 17% by the 2/3 clipping. The least water stressed plants were affected most negatively by defoliation, and the unclipped plants responded more negatively to greater water limitation. Plants achieved partial biomass compensation through alterations in shoot activity and continued allocation of photosynthate to roots. A smaller fraction of leaf production was directed to litter in clipped plants although clipping only removed the youngest tissues, suggesting that clipping increased leaf longevity. In turn, each leaf probably contributed a greater total quantity of photosynthate. Photosynthetic rates were also likely to have been increased by clipping water-stressed plants. In contrast to biomass, plants overcompensated for nitrogen lost to defoliation. Total nitrogen uptake by individual plants was stimulated by defoliation, as there was more total nitrogen in leaves and stems. Increased nitrogen uptake was achieved by clipping stimulation of total uptake per unit of root rather than of total root mass.     

Assessing the drought tolerance variability in Mediterranean alfalfa (Medicago sativa L.) genotypes under arid conditions

This work investigated the variability in drought tolerance under arid conditions of Mediterranean alfalfa genotypes with the overall aim to assess the main criteria that are associated with the relative tolerance and to discover the most tolerant ecotypes. For this, 16 alfalfa genotypes originating from seven countries of the Mediterranean basin were tested in an experimental station in south of Tunisia. The trial was conducted under two irrigation treatments. The first was normally irrigated by providing an amount of water corresponding to the potential evapotranspiration of the crop, and in the second with water deficit which was applied by stopping the irrigation during 8 weeks in summer. A significant decrease was observed under water deficit for biomass production, leaf stem ratio and water use efficiency. The sensitive index, in stress treatment, varies between 13.8% and 46.2% for dry yield. Results showed that some genotypes exhibited high-forage yield potential even in the presence of stress, mainly Amerist, Sardi10 and Siriver. Proline accumulation in leaves was greater in water-stressed plants, while the K⁺ osmo-regulatory role was not definite. High biomass production, accumulation of proline and constancy of K⁺ in leaves are the most important criteria for tolerant alfalfa.     

Anaerobic conditions strongly promote extension by stems of overwintering tubers of Potamogeton pectinatus L

Elongation by dark-grown shoots of Potamogeton pectinatus tuberswas enhanced by the absence of oxygen. This promoting effectwas located in the stem and was stronger under water than ina gas phase and also stronger in unsparged water compared tosparged water. Anaerobic shoots elongated under water by almost13 cm in 5 d. This was the outcome of longer cells of the steminternodes and of some cell division coupled with leaf extensionwhich continued in the absence of oxygen, but at a slower rate.Continued attachment to a starch-filled tuber was required forsignificant anaerobic elongation, which could be sustained forat least 14 d. Switching intact, growing tubers from aerobicto anaerobic conditions stimulated stem extension within 24h. Conversely, stem elongation was slowed when tubers were transferredfrom an anaerobic to an aerobic environment. A marked gravitropicresponse occurred in anaerobic conditions, which involved bothstem and leaf tissue, and indicated that active internal growth-regulatingmechanisms continued to operate without oxygen. Shoot extension by tubers was also stimulated by hypo-aerobicconditions (5–8 kPa O₂) compared with fully aerated solutions(20.8 kPa O₂). This acceleration was smaller than that obtainedby the complete removal of oxygen, but still involved stem ratherthan leaf growth. Unlike elongation by apical shoots of tubers,that by shoot tips of rhizomes taken from mature light-grownplants was strongly inhibited by lack of oxygen, and in somecases shoot tips died within 5 d. All shoots and leaves werehighly aerenchymatous and the gas-filled lacunae were connectedby side-pores. Key words: Water plants, environmental stress, Potamogeton pectinatus, elongation, anaerobiosis, gravitropism     

Influence of plant nutrient concentration on growth rate: Use of a nutrient interruption technique to determine critical concentrations of N,P and K in young plants

A method is described for determining the way in which growth rate varies with plant nutrient concentration using a simple nutrient interruption technique incorporating only 2 treatments. The method involves measuring the changes in growth and nutrient composition of otherwise well-nourished plants after the supply of one particular nutrient has been withheld. Critical concentrations are estimated from the relationship between the growth rate (expressed as a fraction of that for control plants of the same size which remained well-nourished throughout) and the concentration of the growth-limiting nutrient in the plants as deficiency developed. Trials of the method using young lettuce plants showed that shoot growth rate was directly proportional to total N (nitrate plus organic N) concentration, and linearly or near-linearly related to K and P concentration over a wide range; the corresponding relationship for nitrate was strongly curvi-linear. Critical concentrations (corresponding to a 10% reduction in growth rate) determined from these results were similar to critical values calculated from models derived from field data, but were generally higher than published estimates of critical concentration (based on reductions in shoot weight) for plants of a similar size. Reasons for these discrepancies are discussed. Nitrate, phosphate or potassium concentrations in sap from individual leaf petioles were highly sensitive to changes in shoot growth rate as deficiency developed, with the slope of the relationships varying with leaf position, due to differences both in their initial concentration and in the rates at which they were utilized in individual leaves. Each nutrient was always depleted more quickly in younger leaves than in older ones, providing earlier evidence of deficiency for diagnostic purposes. Although the plants were capable of accumulating nitrate, phosphate and potassium well in excess of that needed for optimum dry matter production during periods of adequate supply, the rate of mobilization of these reserves was insufficient to prevent reductions in growth rate as the plants became deficient. This brings into question the validity of the conventional concept that luxury consumption provides a store of nutrients which are freely available for use in times of shortage. The implications of these results for the use of plant analysis for assessing plant nutrient status are discussed.     

Shoot growth in willow (Salix viminalis) in relation to abscisic acid, plant water status and photoperiod

The decline in growth rate of field-grown willow trees in Aberystwyth, U.K., began in mid-summer and was followed by the senescence and abortion of shoot tips. These events were not triggered by a decline in the length of the natural photoperiod but were coincident with low leaf water potentials that developed in summer. Transient increases in the abscisic acid (ABA) content of shoot tips were observed during the period of declining water potential. These increases were roughly coincident with the onset of growth decline and preceded abortion and senescence of shoot tips. Under controlled conditions growth of both rooted cuttings and potted plants was arrested by short days (8 h) without any increase in tip ABA levels. Growth of rooted cuttings under long days (16 h) was inhibited by exogenous ABA; this inhibition could be relieved by addition of gibberellic acid (GA3) to the nutrient solution. Growth of aseptically cultured apices was also inhibited by ABA; this inhibition was relieved by joint application of GA9 and zeatin riboside.     

The periodic wetting of leaves enhances water relations and growth of the long‐lived conifer Araucaria angustifolia

The importance of foliar absorption of water and atmospheric solutes in conifers was recognised in the 1970s, and the importance of fog as a water source in forest environments has been recently demonstrated. Araucaria angustifolia (Araucariaceae) is an emergent tree species that grows in montane forests of southern Brazil, where rainfall and fog are frequent events, leading to frequent wetting of the leaves. Despite anatomical evidence in favour of leaf water absorption, there is no information on the existence and physiological significance of a such process. In this study, we test the hypothesis that the use of atmospheric water by leaves takes place and is physiologically relevant for the species, by comparing growth, water relations and nutritional status between plants grown under two conditions of soil water (well‐watered and water‐stressed plants) and three types of leaf spraying (none, water and nutrient solution spray). Leaf spraying had a greater effect in improving plant water relations when plants were under water stress. Plant growth was more responsive to water available to the leaves than to the roots, and was equally increased by both types of leaf spraying, with no interaction with soil water status. Spraying leaves with nutrient solution increased shoot ramification and raised the concentrations of N, P, K, Zn, Cu and Fe in the roots. Our results provide strong indications that water and nutrients are indeed absorbed by leaves of A. angustifolia, and that this process might be as important as water uptake by its roots.