Physiology of Eucalyptus plantlets grown photoautotrophically in a scaled-up vessel

Summary Nodal cuttings of Eucalyptus camaldulensis L. plantlets were cultured photoautotrophically (sugar-free nutrient medium and with enriched CO₂ and high photosynthetic photon flux) in a scaled-up vessel (volume 4.0 liters) under forced ventilation (SV-treatment). After 28 d of culture, physiological aspects of the plantlets were compared with plantlets grown photomixotrophically (20 g l⁻¹ sucrose in the medium) in a Magenta vessel (volume 0.4 liters) under natural ventilation (control). In the SV-treatment net photosynthetic rates were enhanced, normal stomatal closing and opening were observed, and the epicuticular leaf-wax content was significantly higher than the control. The anatomical study showed well-organized palisade and spongy mesophyll layers of SV leaves. The SV-treatment also allowed in vitro acclimatization, and after transplanting ex vitro, the transpiration rate and the percent water loss was lower than those of the control and thus the SV plantlets acclimatized easily ex vitro.     

Water Control and Survival of Ipomoea batatas Grown Photoautotrophically under Forced Ventilation and Photomixotrophically under Natural Ventilation

Water loss and its determining factors were compared in leavesof in vitro -grown sweet potato (Ipomoea batatas L. Lam, ‘Beniazuma’)plantlets cultured either photoautotrophically under forcedventilation (scaled-up vessel) or photomixotrophically undernatural ventilation. Functional stomata, a greater amount ofwax deposition on the leaves and smaller epidermal conductancewere observed in plantlets cultured photoautotrophically underforced ventilation. When transplanted directly into soil inthe greenhouse (without ex vitro hardening), plantlets grownphotoautotrophically under forced ventilation survived betterand grew faster. This was because they were better able to controltranspiration and thus lost less water and showed no signs ofwilting. In contrast, plantlets cultured under photomixotrophicconditions had open stomata which were not functional, a highertranspiration rate, and uncontrolled and rapid water loss immediatelyafter transfer ex vitro. Thus, wilting and severe desiccationwith irreversible tissue damage were observed in the leaves,which died within a few days. Copyright 2000 Annals of BotanyCompany Epicuticular wax, hardening, stomata, sweet potato, transpiration, water control     

Improved growth and acclimatization of somatic embryo-derived Oplopanax elatus plantlets by ventilated photoautotrophic culture

To improve large-scale in vitro production of Oplopanax elatus Nakai, we cultured somatic embryo-derived plantlets under a heterotrophic condition (semi-solid culture with sucrose), photoautotrophic condition (semi-solid culture without sucrose), or modified photoautotrophic condition (liquid culture with forced ventilation). The plantlets grown under the modified photoautotrophic condition had more leaves as well as higher chlorophyll content, and higher net photosynthetic rate than those grown under the conventional conditions. Further, the photoautotrophically grown plantlets acclimatized better and sooner upon ex vitro transplantation than did the conventionally cultured plantlets. Consequently, a photoautotrophic culture method with forced ventilation is effective for enhancing the growth and acclimatization of O. elatus.     

Development of a forced ventilation micropropagation system for large-scale photoautotrophic culture and its utilization in sweet potato

Summary A forced ventilation system has been developed for large-scale photoautotrophic micropropagation of chlorophyllous plants. The major goal of the system is to provide a uniform supply of CO₂ inside a large culture vessel (volume 3480 ml) to achieve uniform growth of the plantlets. The system has been designed such that sterile nutrient solution can be supplied throughout the culture period, which is essential for long-term culture. Sweet potato (Ipomoea batatas L. Lam., cv. ‘Beniazuma’) was used as a model plant for photoautotrophic culture with stagnant and nonstagnant nutrient solution in large vessels. Growth and net photosynthetic rates of the plantlets were compared with those of the plantlets grown in a small vessel under photoautotrophic conditions (with natural ventilation) and conventional photomixotrophic conditions. The results indicated that the large vessel with the forced ventilation system was effective for improving growth and uniformity of the plantlets and the rate of net photosynthesis. The stagnant nutrient solution condition under photoautotrophic forced ventilation treatment significantly increased the fresh mass of the plantlets; however, percent dry mass was highest in the treatment with nonstagnant nutrient solution condition. The results demonstrated that the conventional photomixotrophic culture system can cause seriously inhibited growth and development.     

Photoautotrophic culture of Coffea arabusta somatic embryos: development of a bioreactor for large-scale plantlet conversion from cotyledonary embryos

Somatic embryos were developed from in vitro-grown leaf discs of Coffea arabusta in modified Murashige and Skoog medium under 30 micromol m(-2) s(-1) photosynthetic photon flux (PPF). Cotyledonary stage embryos were selected from the 14-week-old cultures and were placed under a high (100 micromol m(-2) s(-1) PPF for 14 d. These pretreated embryos were grown photoautotrophically in three different types of culture systems: Magenta vessel; RITA-bioreactor (modified to improve air exchange); and a specially designed temporary root zone immersion bioreactor system (TRI-bioreactor) with forced ventilation. The aims of the study were to achieve large-scale embryo-to-plantlet conversion, and to optimize growth of plantlets under photoautotrophic conditions. The plantlet conversion percentage was highest (84 %) in the TRI-bioreactor and lowest in the modified RITA-bioreactor (20 %). Growth and survival of converted plantlets following 45 d of photoautotrophic culture in each of the three culture systems were studied. Fresh and dry masses of leaves and roots of plantlets developed in the TRI-bioreactor were significantly greater than those of plantlets developed in the modified RITA-bioreactor or Magenta vessel. The net photosynthetic rate, chlorophyll fluorescence and chlorophyll contents were also highest in plantlets grown in the TRI-bioreactor. Normal stomata were observed in leaves of plantlets grown in the TRI-bioreactor, whereas they could be abnormal in plantlets from the modified RITA-bioreactor. Survival of the plants after transfer from culture followed a similar pattern and was highest in the group grown in the TRI-bioreactor, followed by plants grown in the modified RITA-bioreactor and Magenta vessel. In addition, ex vitro growth of plants transferred from the TRI-bioreactor was faster than that of plants from the other culture systems.     

Developmental and Photosynthetic Characteristics of a Photoautotrophic Chrysanthemum Culture

Chrysanthemum plantlets were cultivated in vitro on media with 2.0, 0.3, or 0 % sucrose, or photoautotrophically without an organic carbon source but with supplementation of the culture vessel atmosphere with 2 % CO₂. The photoautotrophically cultivated plantlets showed a better growth and multiplication, higher contents of chlorophyll (Chl) and carotenoids, higher Chl a/b ratio, net photosynthetic rate and ribulose-1,5-bisphosphate carboxylase/oxygenase and phosphoenolpyruvate carboxylase activities than plantlets grown on the medium with sucrose. This revised version was published online in June 2006 with corrections to the Cover Date.     

Sugar metabolism, photosynthesis, and growth of in vitro plantlets of Doritaenopsis under controlled microenvironmental conditions

Suboptimal environmental conditions inside closed culture vessels can be detrimental to in vitro growth and survival of plantlets during the acclimatization process. In this study, the environmental factors that affected Doritaenopsis plantlet growth and the relationship between growth and sugar metabolism were investigated. Cultures were maintained under heterotrophic, photoautotrophic, or photomixotrophic conditions under different light intensities and CO₂ concentrations. Photoautotrophic growth of Doritaenopsis hybrid plantlets could be promoted significantly by increasing the light intensity and CO₂ concentration in the culture vessel. The concentration of different sugars in the leaves of in vitro-grown plantlets varied with different cultural treatments through a 10-wk culture period. Starch, reducing sugars, and nonreducing sugar contents were higher in plantlets grown under photoautotrophic and photomixotrophic conditions than in heterotrophically grown plantlets. Net photosynthesis rates were also higher in photoautotrophically and photomixotrophically grown plantlets. These results support the hypothesis that pyruvate, produced by the decarboxylation of malate, is required for optimal photoautotrophy under high photosynthetic photon flux density. Growth was greatest in plantlets grown under CO₂-enriched photoautotrophic and photomixotrophic conditions with high photosynthetic photon flux density. The physiological status of in vitro-grown Crassulacean acid metabolism (CAM)-type Doritaenopsis showed a transition from C3 to CAM prior to acclimatization.     

Multiple shoot induction and leaf and flower bud abscission of Annonacultures as affected by types of ventilation

Nodal explants of Annona squamosa L. and Annona muricata L. were cultured in vitro under various types of ventilation: airtight vessel (sealed condition; number of air exchange 0.1 h^–1), natural ventilation (via a polypropylene membrane; number of air exchange 1.5 h^–1), and forced ventilation (5.0 cm³ min^–1 in a 60 cm³ vessel; number of air exchange 5.0 h^–1). In both species, numbers of leaves, leaf areas and numbers of nodes per shoot increased with improving standards of ventilation, while leaf abscissions were substantially reduced; all the leaves had abscised in the airtight vessels after 12–15 days, but none had done so with forced ventilation. Flower-bud abscission in A. muricatashowed a similar trend after 21 days. These effects were associated with reductions in the accumulation of ethylene within the culture vessels, produced by increasing the efficiency of ventilation; ethylene was not detected in those fitted with a forced ventilation system. CO₂ concentrations in culture headspaces and the net photosynthetic rates of the plantlets were also evaluated. CO₂ concentrations decreased well below the ambient in the natural and airtight vessels; however, under forced ventilation, CO₂ concentrations were significantly higher during the photoperiod, compared to those of the natural ventilation and airtight vessel treatments. In general, net photosynthetic rates per unit leaf area increased with increasing photosynthetic photon flux (PPF) and rates were highest in plantlets grown under forced ventilation, intermediate under natural ventilation and lowest in the airtight vessels.Eighteen different media were investigated for their effects on multiple shoot induction in both species. The best medium for multiple shoot induction and growth in A. squamosa was Murashige and Skoog medium (MS) + 6-benzylaminopurine (BA; 1.5 mg l^–1) + casein hydrolysate (1.0 g l^–1) and for A. muricata MS + BA (1.0 mg l^–1) + naphthaleneacetic acid (NAA; 0.1 mg l^–1).     

Growth and Photosynthetic Characteristics of Solanum tuberosum Plantlets Cultivated in Vitro in Different Conditions of Aeration, Sucrose Supply, and CO(2) Enrichment

Growth characteristics, oxygen exchange, and carbohydrate and chlorophyll contents were determined 30 days after subculturing of single node-derived plantlets of Solanum tuberosum cv Haig cultivated in vitro. Cultivation conditions were: (a) photomixotrophy in closed vessel, (b) photomixotrophy in closed vessel on medium supplemented with silver thiosulfate, (c) photomixotrophy in aerated vessel, (d) photoautotrophy in air, (e) photoautotrophy in CO₂-enriched air. In photomixotrophic conditions, aeration of the vessel enhanced sucrose utilization and had a positive effect on plantlet growth. In photoautotrophic conditions, growth of the plantlets was slow in air and was strongly enhanced by CO₂ enrichment of the atmosphere. Starch to sucrose ratios were higher in plants grown photoautotrophically than in plants grown with sucrose in the medium. Oxygen exchange characteristics on a chlorophyll basis were similar between the plantlets when measured under moderate light, and resembled those of greenhouse plant leaves. In high light, however, plantlets grown photoautotrophically in a CO₂-enriched atmosphere had higher oxygen exchange rates. We concluded from these results that potato plantlets in vitro in conditions (c), (d), and (e) developed C3-plant photosynthetic characteristics, which were in photoautotrophically grown plantlets comparable to those of field-grown plants.     

Photoautotrophic growth response of in vitro cultured coffee plantlets to ventilation methods and photosynthetic photon fluxes under carbon dioxide enriched condition

Effects of two ventilation methods (forced and natural) and two photosynthetic photon fluxes (PPF, 150 and 250 μmol m⁻² s⁻¹) on the photoautotrophic growth of in vitro cultured coffee (Coffea arabusta) plantlets were investigated. Number of air exchanges was 2.7, 5.9 and 3.9 h⁻¹ for forced low rate, forced high rate and natural ventilation, respectively. Single node cuttings of in vitro cultured coffee plantlets were cultured on Florialite, a mixture of vermiculite and cellulose fibers with high air porosity, emerged in liquid half strength basal MS medium, without sucrose, vitamins and plant growth regulators. The study included 40 days in the in vitro stage and 10 days in the ex vitro stage. Mean fresh and dry weights, leaf area, shoot and root lengths and net photosynthetic rate per plantlet were significantly greater in forced high rate treatments compared with those in natural and forced low rate treatments. PPF had a distinct effect on shoot length suppression and root elongation of coffee plantlets in forced high rate treatments. The control of carbon dioxide concentration inside the culture box according to the plant demand when growing was easy with the forced ventilation method in photoautotrophic micropropagation. This revised version was published online in June 2006 with corrections to the Cover Date.     

Micropropagation of Potato: Evaluation of Closed, Diffusive and Forced Ventilation on Growth and Tuberization

Different types of ventilation of the culture vessel headspace,each with and without the ethylene inhibitor AgNO₃(3.0 µM)or the ethylene precursor 1-aminocyclopropane-1-carboxylic acid(ACC) (2.0 µM) in the culture medium, were investigatedin terms of their effects on the growth of potato cuttings (Solanumtuberosum L. ‘cara’). Concentrations of CO₂ , O₂andethylene in the culture vessel headspaces were monitored duringthe study. Growth was substantially enhanced and vitrification(stunting and epinasty of leaves and hooking of stem apices)was reduced by increasing the efficiency of ventilation, theeffects being greatest with forced ventilation. In the conventionaldiffusive treatment (via a polypropylene membrane), leaf epinastyoccurred but the leaves were not stunted unless ACC had beenadded. AgNO₃prevented vitrification in the latter case and reducedit in the sealed treatment. On the other hand, with all forcedventilation treatments, even with the addition of ACC, the plantletsgrew well and some of the growth parameters exceeded those inthe diffusive + AgNO₃treatment. Ethylene removal was clearlyan important factor contributing to the better growth foundwith diffusive and especially with the forced ventilation treatment;with the latter, ethylene concentrations in the culture vesselswere virtually zero. In addition, enhanced CO₂supply probablycontributed to the better performance under forced ventilationcompared to diffusive ventilation. Callus developed on the stembases in all sealed (airtight) and diffusive treatments exceptwhere AgNO₃was used. No callus was observed in any treatmentwhere forced ventilation was applied and in vitro tuberization(tuber size) was considerably improved by this treatment. Copyright2001 Annals of Botany Company Callus, ethylene, potato, tuberization, vitrification     

Developing a Photoautotrophic Micropropagation System for Woody Plants

in vitro including cotyledonary stage somatic embryos have the ability to grow photoautotrophically (without sugar in the culture medium), and that the low or negative net photosynthetic rate of plants in vitro is due not to poor photosynthetic ability, but to the low CO₂ concentration in the air-tight culture vessel during the photoperiod. Furthermore, we have shown that the photoautotrophic growth of several woody plants in vitro can be significantly promoted by increasing the CO₂ concentration and light intensity in the vessel, by decreasing the relative humidity in the vessel, and by using a fibrous or porous supporting material with high air porosity instead of gelling agents such as agar. In this paper, the advantages of photoautotrophic micropropagation in a conventional, small culture vessel with a microporous gas filter for enhancing natural ventilation and in a large culture vessel with a forced ventilation unit are described for woody plants such as acacia (Acacia mangium), coffee (Coffea arabusta), eucalyptus (Eucalyptus camaldlensis), mangosteen (Garcinia mangostana), neem (Azadirachta indica), paulownia (Paulownia fortunei), and pine (Pinus radiata). Received 30 August 2001/ Accepted in revised form 27 September 2001     

Photoautotrophic Culture of Undifferentiated Cells and Shoot-Forming Cultures of Digitalis purpurea L.

Undifferentiated cells and shoot-forming cultures of Digitalispurpurea L. were grown photoautotrophically under 1% CO₂. During3 weeks of culture, the undifferentiated cells multiplied 3-foldand the shoot-forming cultures 2-fold on a fresh weight basis.The chlorophyll content, ribulose 1,5-bisphosphate carboxylaseactivity, Hill reaction activity of the isolated chloroplastsand photosynthetic O₂ evolution of the photoautotrophicallygrown cultures were somewhat higher than the values of the correspondingphotomixotrophic cultures. The digitoxin contents, however,were not improved by photoautotrophic culture. (Received November 9, 1983; Accepted June 11, 1984)     

Acclimation of tobacco plantlets to ex vitro conditions as affected by application of abscisic acid

Plantlets of Nicotiana tabacum L. cv. Petit Havana SR1 were grown in vitro on Murashige and Skoog medium containing 2% saccharose, and then transplanted ex vitro into pots with coarse sand and Hewitt nutrient solution. In the first day after transplantation, the anti-transpirant abscisic acid (ABA; 0.01, 0.05 or 0.10 mM) was added to the substrate. Leaf stomatal conductance (gs), which was high in plants during the first days after transplantation similarly as in plantlets grown in vitro, was considerably decreased by ABA-treatment. However, in the further days gs decreased more quickly in control than in ABA-treated plants, and after 2 or 3 weeks gs was significantly lower than that of plantlets grown in vitro but similar in control and ABA-treated plants. Two weeks after transplantation, net photosynthetic rate, chlorophyll a + b content, maximal photochemical efficiency, and actual quantum yield of photosystem II in plant leaves were higher in comparison with those in plantlets grown in vitro. ABA-treatment had slight positive or insignificant effect on photosynthetic parameters and enhanced plant growth. Thus ABA application can alleviate 'transplant shock' and speed up acclimation of plantlets to ex vitro conditions.     

A Humidity-induced Convective Throughflow Ventilation System Benefits Annona squamosa L. Explants and Coconut Calloid

A simple apparatus is described for generating pressurized throughflowventilation in plant tissue culture vessels. No pumps or gas-cylindersare required and the flow is driven by humidity-induced diffusionacross microporous membranes. In the experiments described,pressurized flows of sterile humidified air were supplied atrates of up to 1 ml min^-1and these had beneficial effects onleaf survival and production inAnnona cuttings and on calloidform in coconut. Ethylene (ethene) was removed more quicklyfrom the pressure-flow ventilated culture vessels (t ₅₀, 0.4–0.7h) than from those aerated by diffusion through conventionalpolypropylene membranes (t ₅₀, 1.6–2.4 h). InAnnona cuttings leaf production was greatly increased andethylene-induced leaf fall considerably delayed when culturedwith the forced as opposed to diffusion-based ventilation ofthe vessels. With throughflow ventilation, coconut calloid was more convolutedthan under wholly diffusive aeration and had a smooth distinctepidermal surface and clearly defined sub-epidermal meristematicnodules. It resembled freshly initiated calloid from which regenerationof plantlets via somatic embryogenesis can be obtained. Underwholly diffusive aeration, calloid developed a rough, relativelyundifferentiated surface, more haustorial (i.e. cotyledonary)in appearance, and characteristic of cultures where regenerationpotential has been lost. It is suggested that other benefits of the pressurized throughflowventilation may be the removal of volatiles such as ethanoland acetaldehyde, the removal of excess carbon dioxide at nightand its improved supply during the day, and sustained oxygenconcentrations at levels close to atmospheric both night andday. Abscission; callus; ethylene; explants; tissue-culture; growth; ventilation     

Internal air current patterns depend on the ventilation method in a scaled-up culture vessel for micropropagation

Air current patterns were visualized inside a scaled-up culture vessel under natural or forced ventilation. Metaldehyde particles were used as tracers, and their patterns were recorded as video images by a high-resolution-and-contrast camera. Under natural conditions, the air currents were mainly influenced by natural convection that developed due to the lighting scheme, which caused differences in temperature among various articles in the chamber, including a sweet potato plantlet, supporting material, a multi-cell tray, and the culture vessel. Under forced ventilation, the air current pattern and air speed were affected by ventilation rates and by air-supply methods that were either parallel downward or circular upward. Uniformity of air movement could be achieved with air distribution pipes inside a modified vessel. Under forced ventilation, growth, photosynthetic rate, and transpiration of the micropropagated plantlets were enhanced around the air outlet as well as the inlet in the large-scale vessel. Those plant responses were probably induced by uniform spatial distribution of air current and gas concentrations.     

High photosynthetic photon flux and high CO2 concentration under increased number of air exchanges promote growth and photosynthesis of four kinds of orchid plantlets in vitro

Summary In vitro plantlets of Phalaenopsis ‘Happy Valentine’, Neofinetia falcate Hu, Cymbidium kanran Makino, and Cymbidium goeringii Reichb. f. were grown under photoautotrophic [high photosynthetic photon flux (PPF), high CO₂ concentration, and increased number of air exchanges] and heterotrophic (low PPF, low CO₂ concentration, no air exchanges) culture conditions. After 40 d of culture, a significant difference in plantlet growth was observed between the two cultures. Total fresh and dry mass were on average 1.5 times greater in photoautotrophic culture than in heterotrophic culture. Higher net photosynthetic rates were also observed for Phalaenopsis in photoautotrophic culture. In photoautotrophic culture, little difference was observed in air temperature between the inside and outside of the culture vessel, whereas in heterotrophic culture, air temperature inside the culture vessel was 1–2°C higher than that outside the culture vessel. Relative humidity inside the culture vessel was remarkably different between the two cultures: 83–85% in photoautotrophic culture and 97–99% in heterotrophic culture. These results indicated that growth and net photosynthetic rate of in vitro orchid plantlets were susceptible to the culture environments such as PPF, CO₂ concentration, relative humidity (RH), and the number of air exchanges, which would allow a more efficient micropropagation system for these orchid plants.     

Influence of in vitro growth conditions on in vitro and ex vitro photosynthetic rates of easy- and difficult-to-acclimatize sea oats (Uniola paniculata L.) genotypes

Net photosynthetic rates (P _n) of easy (EK 16-3) and difficult-to-acclimatize (EK 11-1) sea oats genotypes were examined under the following culture conditions: (1) photoautotrophic [sugar-free medium, high photosynthetic photon flux (PPF), high vessel ventilation rates and CO₂ enrichment, (PA)]; (2) modified photomixotrophic [sugar-containing medium diluted with sugar-free medium over time, high PPF, and high vessel ventilation rates (PM)]; (3) modified photomixotrophic enriched [same as PM with CO₂ enrichment, (PME)]; or (4) conventional photomixotrophic [sugar-containing medium, low PPF, and low vessel ventilation rates (control)]. Regardless of genotype, plantlets cultured under PA conditions died within 2 wk, whereas under PM and PME conditions, plantlets increased their P _n. After 6 wk, P _n per gram dry weight was 1.7 times greater in EK 16-3 than EK 11-1 plantlets cultured under PME conditions. In vitro-produced leaves of EK 16-3 plantlets were elongated with expanded blades, whereas EK 11-1 produced short leaves without expanded blades, especially under control conditions. After in vitro culture, EK 16-3 PME plantlets exhibited the highest dry weights among treatments. EK 16-3 PME and EK 16-3 PM had similarly high survivability, shoot and root dry weights and leaf lengths ex vitro compared to EK 16-3 control and EK 11-1 PM and PME plantlets. Ex vitro growth, survivability and P _n per leaf area of either genotype were not affected by CO₂ enrichment under modified photomixotrophic conditions. These results suggest that growth and survivability of sea oats genotypes with different acclimatization capacities can be enhanced by optimizing culture conditions.     

Effects of nitrogen nutrition on salt-stressed Nicotiana tabacum var. Samsun in vitro plantlets

Tobacco shoots were grown in vitro for 35 d, in MS culture mediummodified to include various sources (nitrate-N, ammonium-N ora mixture) and levels (0–120 mM) of N, and in the presenceof 0–180 mM NaCI or iso-osmotic concentrations of mannitol.Growth of control plantlets was significantly inhibited whenNH₄⁺-N was the sole N source, and at high (120 mM) NO^–₃-N supply. Under conditions of salt stress (90 and 180 mM NaCI)growth was repressed, with roots being more severely affectedthan shoots. Salinity also inhibited root emergence in vitro.The only alleviation of the salt stress by nitrate nutritionobserved in this study was on shoot growth parameters of plantletsgrown on 60 mM NO^–₃-N and 90 mM NaCI. Although both weresignificantly inhibited by NaCI, nitrate reduc-tase activitywas more severely affected than nitrate uptake. When mannitolreplaced NaCI in the culture medium, similar Inhibition of growth,nutrient uptake and enzyme activity were recorded. These observations,together with the relatively low recorded values for Na⁺ andCI^– uptake, indicate that under in vitro salt stress conditionsthe negative effects of NaCI are primarily osmotic. Key words: Growth, nitrogen metabolism, osmotic stress, salinity     

Manipulation of the culture environment on <Emphasis Type="Italic">in vitro</Emphasis> air movement and its impact on plantlets photosynthesis

Two-dimensional air current speeds in the culture vessel were measured using a tracer-based visualization technique and the effect of the air movement in the culture vessel on the photosynthesis of in vitro potato plantlets was assessed under a photoautotrophic culture condition. The air current speeds inside the vessel were varied by controlling free convection induced by spatial variations of temperatures in the culture vessel. For all conditions examined, upward air currents were observed around the plantlets in the central part of the culture vessel and downward air currents were observed near inside walls in the culture vessel. The upward and downward air currents were restricted by the presence of the plantlet. The upward air current speeds were affected by plantlet size inside the vessel and it was 24, 8 and 4 mm s⁻¹ in culture vessels with no plantlets, a 10-mm-tall plantlet and a 60-mm-tall plantlet cultured inside the vessel, respectively. The upward air current speed was increased by 2 times by increasing wind velocity above the culture vessel from 0.1 to 1.0 m s⁻¹. Placing the black plate on the medium also increased the air current speeds by 1.5 times. The net photosynthetic rates of the plantlets increased from 2.0 to 2.5 μmol m⁻² s⁻¹ as the upward air current speed in the culture vessel increased from 2.4 to 8.0 mm s⁻¹. The air current speeds in the culture vessel were significantly slow. Enhancement of the air movement in the culture vessel is important to promote photosynthesis of the in vitro plantlets.