High Photosynthetic Photon Flux Research Articles

Differential effects of light quality, provided by different grass neighbours, on the growth and morphology of Trifolium repens L. (white clover)

The ability to respond in a specific manner to different light conditions imposed by different species of grass is a major factor contributing to white clover persistence in pastures. Gaps in a pasture provide light with a higher red:far-red ratio (R:FR) and higher photosynthetic photon flux density (PPFD) than the light filtered through neighbours. White clover (Trifolium repens L.) was grown under different light conditions in ways that tried to simulate as closely as possible some of the light conditions experienced in a natural field situation, being partially shaded and receiving light reflected from neighbouring grasses. The objective was to determine specifically if the mere presence of neighbouring grasses could influence the growth and morphology of white clover individuals without physically contacting them, and thereby send a signal of impending competition. In the first experiment, white clover was subjected to shading cast from three different grass species. There were differences in both the quantity and quality of light received under the various grass canopies. The canopies reduced overall growth and branching of clones, while increasing the length of and biomass allocation to petioles. Lolium perenne L. canopy shade had different effects compared to Holcus lanatus L. or Dactylis glomerata L., but between the latter two species, no differences were detected. In the second experiment, light reflected from grass neighbours was provided simultaneously with direct light. There was a strong increase in FR and a resulting decrease in the R:FR due to neighbouring D. glomerata, but few consistent effects on white clover growth and morphology; there was evidence of phototropic movement by the leaves. We show that plants must experience partial shading, and not just reflected light, in order to alter their morphology in response to the presence of different species of grass neighbours.

QUALITY BIOMASS PRODUCTION VIA PHOTOAUTOTROPHIC MICROPROPAGATION

The commercial use of conventionally micropropagated plantlets is often undermined by their poor physiological quality and growth. A substantial percentage of plantlets cannot survive or cannot grow fast and vigorously after transplantation in the ex vitro conditions. This limits the possible prospects of this unique propagation technique. Plantlets grown under conventional micropropagation system (with use of 20 - 30 g l -1 sucrose in the medium and airtight vessels) are often characterized by non-functional stomata and limited epicuticular wax deposition on the leaves. Thus, immediately after transplantation, rapid water loss due to uncontrolled transpiration occurs followed by wilting. These effects are probably due to the prolonged exposure of the plantlets to high relative humidity in vitro (> 95 %) and low CO 2 concentration and photosynthetic photon flux in the culture vessels during the photoperiod. The presence of sucrose in the medium and the accumulation of ethylene in the vessel may also play a role. The photoautotrophic micropropagation system (without any sucrose in the nutrient medium and with enriched CO 2 and high photosynthetic photon flux), especially under forced ventilation, has been proved to produce morphologically superior and physiologically normal quality transplants. Plantlets grown in vitro under photoautotrophic conditions show high net photosynthetic rate, enhanced shoot and root growth and shortened multiplication period and the leaves contain functional stomata and high wax content. These plantlets exhibit less water loss after transplantation, as a result leaves do not wilt and grow fast even without any special ex vitro acclimatization. In photoautotrophic micropropagation, humidity is controlled (< 90 %) and the CO 2 concentration is raised and ethylene can not accumulate simply due to increased air exchange rate of the culture vessel. Our recent research on the scaled-up vessel with forced ventilation system and on the aseptic culture room as an alternative for many large culture vessels for photoautotrophic micropropagatior. is introduced in this article.

Interacting effects of photosynthetic photon flux density and temperature on canopy CO 2 exchange rate of spring wheat under different CO 2-concentrations

Summary The objective of the present study was to quantify the interaction between photosynthetic photon flux density (PPFD) and temperature on canopy CO 2 exchange race (CCER) of wheat at ambient and elevated CO 2 -concentrations. Spring wheat ( Triticum aestivum L. cv. Minaret) was grown from emergence to maturity in open-top chambers under ambient (360 ppm) and elevated CO 2 -concentrations (680 ppm). CCER was measured using a specifically designed open system consisting of two canopy chambers (ca. 1.26 m 3 each) and a monitoring unit. Air temperature and light intensity were measured directly above the canopy. CCER was measured from the first node stage until anthesis when leaf area index ranged between 2–3. PPFD ranged from 0 — ca. 1,100 μmolm −2 s −1 and temperature from 7.5°–39°C. CCER increased with increasing temperature and PPFD and was light-satured at 800 μmol m −2 s −1 under ambient CO 2 -concentration. CO 2 -enrichment stimulated CCER by up to 50% bui there was no complete light saturation. Under high PPFD conditions (> 600 μmolm −2 s −1 ) and at low temperatures ( 30 °C) and low PPFD conditions ( −2 s −1 ) the positive CO 2 -effect on CCER was reduced nearly to zero. Thus, temperature optima of the stimulation of CCER by elevated CO 2 -concentrations were found. The optimum temperature of CCER increased with increasing light intensity and was shifted by the CO 2 -enrichment to higher temperatures by ca. 0.5 °C (at low PPFD) — 2 °C (at high PPFD). The results were compared with the predicitions from recent mechanistic models of CCER.

Mass Propagation of Eucalyptus camaldulensis in a Scaled-up Vessel Under In Vitro Photoautotrophic Condition

A scaled-up culture vessel was designed for the large-scale photoautotrophic micropropagation of chlorophyllous plants. The culture vessel (volume 20l) contained a plug cell tray with 448 plantlets, and had a forced ventilation system to supply CO2-enriched air. A nutrient-reservoir was connected to the culture vessel from which nutrient solution was circulated to the culture vessel every 24h. Nodal leafy cuttings of Eucalyptus camaldulensis L. were cultured photoautotrophically in this system without sugar in the nutrient medium, but with an enriched CO2concentration and a high photosynthetic photon flux. The growth and the net photosynthetic rate of the in vitro grown plantlets and the survival percentage of the plantlets after transplanting to ex vitro conditions were compared with those of plantlets grown photoautotrophically under natural ventilation in conventional small culture vessels (Magenta-type vessels; volume 0.4l). Fresh and dry masses and net photosynthetic rate were significantly higher in plantlets grown in the scaled-up vessel compared to plantlets grown in the conventional small vessels (control). The environmental conditions created in this scaled-up vessel (with forced ventilation) also facilitated acclimatizationin vitro . Importantly, after transplanting to the ex vitro condition, plantlets grew well without any specializedex vitro acclimatization treatment.

Vertical gradients in photosynthetic light response within an old-growth Douglas-fir and western hemlock canopy.

We examined needle-level light response of photosynthesis across a vertical light gradient within 45-55-m-tall western hemlock (Tsuga heterophylla (Raf.) Sarg.) and Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) trees growing in a 400-500-year-old mixed species stand. We determined: (1) whether light-saturated photosynthetic rates, light compensation points, and respiration rates varied from the upper to the lower canopy, and (2) if light-saturated photosynthetic rates, light compensation points, and respiration rates varied between Douglas-fir and western hemlock. Over a 25-m gradient from the canopy top to the lower canopy, mean light-saturated photosynthetic rates, light compensation points, and respiration rates declined in overstory Douglas-fir and western hemlock needles, paralleling a 65% decline in the mean daily photosynthetic photon flux density (PPFD). At the canopy top, increasing light-saturated photosynthetic rates relative to lower canopy needles increased carbon uptake at high PPFD. In the lower canopy, reduced respiration rates relative to upper canopy needles increased carbon uptake at low PPFD by reducing the light compensation point. At all canopy positions, western hemlock had lower mean light-saturated photosynthetic rates, light compensation points and respiration rates than Douglas-fir. As a result, western hemlock had higher net photosynthetic rates at low PPFD, but lower net photosynthetic rates at high PPFD compared with Douglas-fir.

Antioxidant responses of cucumber ( Cucumis sativus) to photoinhibition and oxidative stress induced by norflurazon under high and low PPFDs

Photooxidative damage is exacerbated by norflurazon (NF), which blocks carotenoid biosynthesis. This study examined the influence of photosynthetic photon flux density (PPFD) on the overall responses of both non-enzymatic and enzymatic antioxidants to NF-caused oxidative damage in leaves of cucumber ( Cucumis sativus). Seven-day-old cucumber plants were exposed to NF under either low PPFD (30 μmol m −2 s −1) or high PPFD (300 μmol m −2 s −1) for 3 days. The NF plants exposed at high PPFD had lower levels of F v/ F m ratio, quantum yield of electron transport, and 33-kDa protein of photosystem II as compared with the NF plants at low PPFD. In the NF plants, there was a reduction in total chlorophylls and carotenoids except newly formed zeaxanthin in either PPFD. The NF plants at high PPFD resulted in less level of photochemical quenching, q P, and Stern–Volmer quenching, NPQ, than those of the plants at low PPFD, whereas both plants had similar level of non-photochemical quenching coefficient, q N. However, the level of PPFD did not significantly affect the NF-caused induction of antioxidant enzymes including peroxidase, superoxide dismutase, glutathione reductase, and ascorbate peroxidase.

Daily Variations of Photosynthetic Efficiency of Greenhouse Tomato Plants during Winter and Spring

Daily and seasonal variations of photosynthetic activity, chlorophyll a (Chl-a) fluorescence and foliar carbohydrate content were studied in situ on greenhouse tomato (Lycopersicon esculentum Mill. `Trust') plants grown under CO2 enrichment and supplemental lighting. The objective of this study was to assess the effect of seasonal variation of the photosynthetic photon flux (PPF) on photosynthetic efficiency of tomato plants and to determine the presence or absence of photosynthetic down-regulation under greenhouse growing conditions prevailing in northern latitudes. During winter, the fifth and the tenth leaves of tomato plants showed low, constant daily photosynthetic activity suggesting a source limitation under low PPF. In winter, the ratio of variable to maximum Chl-a fluorescence in dark adapted state (Fv/Fm) remained constant during the day indicating no photoinhibition occurred. In February, an increase in photosynthetic activity was followed by a decline during March, April, and May accompanied by an increase in sucrose and daily starch concentrations and constant but high hexose level. This accumulation was a long-term response to high PPF and CO2 enrichment which would be caused by a sink limitation. Thus, in spring we observed an in situ downregulation of photosynthesis. The ratio Fv/Fm decreased in spring compared to winter in response to increasing PPF. The daily decline of Fv/Fm was observed particularly as a midday depression followed by a recovery towards the end of the day. This indicated that tomato leaves were subject to a reversible inhibition in spring. Fv/Fm was lower in March than in April and May even though PPF was higher in April and May than in March. These results suggest that tomato plants develop an adaptive and protective strategy as PPF increases in spring.

Number of air exchanges, sucrose concentration, photosynthetic photon flux, and differences in photoperiod and dark period temperatures affect growth of Rehmannia glutinosa plantlets in vitro.

Rehmannia glutinosa plantlets were cultured for 4 weeks under different culture conditions to determine the optimum environment for in vitro growth and ex vitro survival. Plantlet growth increased with an increasing number of air exchanges of the culture vessel, exhibiting greatest shoot weight, total fresh weight, leaf area, and chlorophyll content at 4.4 h−1 of air exchanges. High sucrose concentration (30 g l−1) increased root weight but reduced shoot growth. Net photosynthetic rates of the plantlets were greatest when sucrose was not added to the medium. On the other hand, ex vitro survival of the plantlets was not influenced by sucrose concentration. In the experiment on difference in photoperiod and dark period temperatures (DIF) and photosynthetic photon flux (PPF), plantlet growth increased as DIF and PPF levels increased. Particularly, increasing PPF level had a more distinctive effect on plantlet growth than increasing DIF level. The interaction of DIF × PPF was also significant, showing the greatest plantlet growth in positive DIF (+8 DIF) and a high PPF (210 μmol m−2 s−1). In conclusion, the results of this experiment suggest that increased number of air exchanges of the culture vessel, decreased sucrose concentration, and positive DIF in combination with high PPF level enhanced growth and acclimatization of Rehmannia glutinosa plantlets.

Water relations and photosynthetic performance of fruticose lichens from the semiarid southeast of spain

The lichen species Teloschistes lacunosus (RUPR.) SAV. and Ramalina bourgeana NYL. are characteristic of the semiarid southeast Iberian Peninsula. These species showing different distribution patterns and ecological requirements, were collected at the coast ( R. bourgeana ) and in the inland ( T. lacunosus ). Laboratory experiments revealed that net photosynthesis (NP) and dark respiration (DR) responses to temperature differed between both species. Optimal temperatures of NP were higher in T. lacunosus than in R. bourgeana at optimal thallus water content at the various photosynthetic photon flux density (PPFD) levels. In the upper temperature range studied (20–25°C), R. bourgeana proved to be less efficient than T. lacunosus . In both species, lower and upper temperature compensation points were below 0°C and above 30°C, for moderate and high PPFD levels, respectively. At high temperatures and high thallus water content, T. lacunosus showed lower DR rates than R. bourgeana . NP response to PPFD also showed differences between both species. The light compensation points were high in both species for temperatures above 20°C (at 20°C around 50 μmol m −2 s −1 in T. lacunosus , and around 60 µmol m −2 s −1 in R. bourgeana ). Light saturation of NP was higher in T. lacunasus at all temperatures in the experiments. In T. lacunasus , the value was below 1200 µmol m −2 s −1 at 0-15°C, but at temperatures above 15°C, it was above 1200 µmol m −2 s −1 . In R. bourgeana the values were always below 1200 µmol m −2 s −1 . In both species, NP was strongly depressed at supraoptimal water content. The physiological differences observed between R. bourgeana and T. lacunasus , and their relation to the distribution patterns of these lichens are discussed.

Impact of Measurement Irradiance on Acclimation of Photosynthesis to Elevated CO&lt;sub&gt;2&lt;/sub&gt; Concentration in Several Plant Species

Controlled environment chamber and glasshouse studies were conducted on six herbaceous annual species grown at 350 (AC) and 700 (EC) &mu;mol(CO2) mol-1 to determine whether growth at EC resulted in acclimation of the apparent quantum yield of photosynthesis (QY) measured at limiting photosynthetic photon flux density (PPFD), or in acclimation of net photosynthetic rate (PN) measured at saturating PPFD. It was also determined whether acclimation in PN at limiting PPFD was correlated with acclimation of carboxylation efficiency or ribulose-1,5-bisphosphate (RuBP) regeneration rate measured at saturating PPFD. Growth at EC reduced both the QY and PN at limiting PPFD in three of the six species. The occurrence of photosynthetic acclimation measured at a rate limiting PPFD was independent of whether photosynthetic acclimation was apparent at saturating measurement PPFD. At saturating measurement PPFD, acclimation to EC in the apparent carboxylation efficiency and RuBP regeneration capacity also occurred independently. Thus at least three components of the photosynthetic system may adjust independently when leaves are grown at EC. Estimates of photosynthetic acclimation at both high and low PPFD are necessary to accurately predict photosynthesis at the whole plant or canopy level as [CO2] increases.

Supporting material affects the growth and development of in vitro sweet potato plantlets cultured photoautotrophically

A comparative study was conducted to optimize the vegetative growth of sweet potato (Ipomoea batatas L. (Lam), cv. Beniazuma) plantlets cultured in vitro in five different types of supporting materials: agar matrix (a seaweed derivative; Kanto Chemical Co. Inc., Tokyo), gellan gum (a Pseudomonas derivative; Kanto Chemical Co. Inc., Tokyo), vermiculite (a kind of hydrous silicates), a mixture of vermiculite and cellulose fiber (Florialite; Nisshinbo Industries, Inc., Tokyo) and cellulose plug (Sorbarod; Baumgartner Rapiers SA., Switzerland). Single nodal cuttings were cultured photoautotrophically (without any sugar in the medium and with enriched CO2 and high photosynthetic photon flux) for 21 d on MS basal medium. Plantlets exhibited the greatest growth when Florialite was used as supporting material. The leaf and root fresh and dry mass were 2.4× and 2.9×, and 2.2× and 2.8× greater, respectively, than those of the plantlets grown in the agar matrix (control). Plantlets cultured in Sorbarod supporting material exhibited the second greatest fresh and dry mass of leaves and roots followed by vermiculite and gellan gum supporting material. The most interesting feature was the development of a large number of fine lateral roots from the main adventitious root in the Florialite treatment. Among the treatments, the highest net photosynthetic rate was evident in the Florialite grown plantlets. The percent porosity of the supporting materials was highest in Sorbarod followed by Florialite and vermiculite. Plantlets transplanted from the Florialite supporting material exhibited the highest acclimatization percentage followed by that of the Sorbarod treatment.

Comparative Study of the C3/CAM Intermediate Species Clusia parviflora Saldanha et Engi. and the Obligate CAM Species Clusia hilariana Schlecht. Growing Sympatrically Exposed and Shaded in the Coastal Restinga of Brazil

Abstract: The C3/CAM intermediate species, C/usia parviflora Saldanha et EngI., and the obligate CAM species Clusia hilariana Schlecht., occur sympatrically in the coastal sand dune vegetation of the Restinga of Brazil. Their photosynthetic activity at an exposed and at a shaded site was compared by measuring gas exchange (porometry), chlorophyll a fluorescence parameters, organic acid levels (malic and citric) and carbon isotope ratios. At the shaded site, low photosynthetic photon flux densities (PPFD) strongly restricted photosynthetic activity. However, C parviflora could readily make use of light flecks. At the exposed site, C. parviflora was much less affected by photoinhibition than C. hilariana. The CAM species showed higher apparent rates of linear photosynthetic electron transport (ETR) and higher effective quantum yield of PSII (ΔF/F'm) than did C. parviflora during high insolation in the middle of the day, i.e., the time of Phase Ill of CAM. Nevertheless, it suffered much more severe acute photoinhibition that was not reversible after 20 min of darkening during this time, and even some chronic photoinhibition not reversible overnight. Comparative studies of sympatric physiotypes with different modes of photosynthesis of a given leaf morphotype, as available in the genus Cksia, challenge some CAM dogmas, e.g., CAM may not always be superior at exposed sites and may not always provide better photoprotection at high PPFD. However, the idea that C3/CAM plasticity allows occupation of a wider range of habitats is supported.

Importance of light and CO2 on the effects of endomycorrhizal colonization on growth and photosynthesis of potato plantlets (Solanum tuberosum) in an in vitro tripartite system

A factorial analysis was conducted to investigate the effects of different levels of photosynthetic photon flux (PPF) and CO2 concentration on the interactions between the vesicular–arbuscular endomycorrhizal fungus Glomus intraradices and potato plantlets (Solanum tuberosum) cultured in an in vitro tripartite system. We observed that CO2 enrichment from 350 to 10000 ppm stimulated root colonization by the fungus, and that this stimulation was more pronounced under high PPF (300 μmol m−2 s−1) than low PPF (60 μmol m−2 s−1). Consistent with these observations, the effects of G. intraradices on dry matter production in potato plantlets were strongly dependent on the CO2 and PPF levels during cultivation. There was no significant effect of the mycorrhizal fungus on dry matter production at 350 ppm of CO2. However, under the high CO2 concentration, mycorrhiza had opposite effects on dry matter production depending on the PPF: a decrease (−21%) and a stimulation (+25%) of dry matter production after 2 wk of growth under low and high PPF, respectively, were observed in presence of G. intraradices relative to plantlets grown in its absence. Furthermore, in mycorrhizal plantlets grown under high levels of both PPF and CO2, the chlorophyll and carotenoid contents as well as the quantum yields of photosynthetic electron transport and the photochemical quenching qP of the chlorophyll‐a fluorescence measured near the PPF during growth were all higher than in non‐infected plantlets. Our results therefore indicate that mycorrhizal G. intraradices can alleviate the down regulation of photosynthesis related to sink limitation, and its effect on dry matter production is strongly dependent on the levels of CO2 and PPF during growth which determine the balance between the photosynthetic carbon uptake by the plantlets and the carbon cost by the fungus.

Photosynthesis and stomatal conductance of mature canopy Oak (Quercus robur) and Sycamore (Acer pseudoplatanus) trees throughout the growing season

1. Gas exchange is strongly seasonal in temperate deciduous forests; however, virtually no studies have described measurements made at the leaf scale through the whole growing season. This level of detail is important to understanding processes as substantial differences may occur within the canopy between species and between sun and shade leaves. 2. Gas exchange was measured weekly throughout the growing season in leaves of Oak (Quercus robur) and Sycamore (Acer pseudoplatanus) trees at Wytham Woods, UK. In Q. robur sun, shade and intermediate leaves were sampled on each occasion. 3. Acer pseudoplatanus came into leaf before Q. robur and commenced net assimilation (A) earlier. Maximum irradiance saturated photosynthetic rates (Amax) in Q. robur were reached approximately 50 days after budburst and were maintained until approximately 20 days before abscission. Acer pseudoplatanus lost its leaves before Q. robur in the autumn and showed no seasonal trend in Amax. The lag between budburst and attainment of maximum photosynthetic rate in Q. robur was substantial and represented a quarter of the potential growing season. 4. A max of A. pseudoplatanus (3·5 μmol m–2 s–1) sun leaves was substantially lower than that of Q. robur (10·4 μmol m–2 s–1) despite A. pseudoplatanus being a fast‐growing species. The earlier flushing of A. pseudoplatanus leaves in the spring could not completely compensate for this. It is likely that other variables, such as total leaf area, are responsible for the high growth rate of A. pseudoplatanus, rather than high photosynthetic rates. 5. Shade leaves of Q. robur had lower Amax and mean A than sun leaves; their quantum efficiency was also lower. Consistent with this, sun leaves had a higher nitrogen content per unit area, resulting from a lower specific leaf area (SLA). 6. Week to week variations in solar radiation influenced A; this could be described by a non‐rectangular hyperbola. Stomatal conductance (gs) decreased in conditions of high vapour pressure deficit (VPD). However, this did not decrease A as high VPD was associated with high solar radiation and the stimulation of photosynthesis caused by high photosynthetic photon flux density (PPFD) more than compensated for the reduction by VPD.

Diurnal changes in leaf gas exchange and chlorophyll fluorescence in tropical tree species with contrasting light requirements

Interspecific ecophysiological differences in response to different light environments are important to consider in regeneration behavior and forest dynamics. The diurnal changes in leaf gas exchange and chlorophyll fluorescence of two dipterocarps, Shorea leprosula (a high light‐requiring) and Neobalanocarpus heimii (a low light‐requiring), and a pioneer tree species (Macaranga gigantea) growing in open and gap sites were examined. In the open site, the maximum net photosynthetic rate (Pn), photosystem II (PSII) quantum yield (Δ; F/Fm′), and relative electron transport rate (r‐ETR) through PSII at a given photosynthetic photon flux density (PPFD) was higher in S. leprosula and M. gigantea than in N. heimii, while non‐photochemical quenching (NPQ) at a given PPFD was higher in N. heimii. The maximum values of net photosynthetic rate (Pn) in M. gigantea and S. leprosula was higher in the open site (8–11 μmol m−2 s−1) than in the gap site (5 μmol m−2 s−1), whereas that in N. heimii was lower in the open site (2 μmol m−2 s−1) than in the gap site (4 μmol m−2 s−1), indicating that N. heimii was less favorable to the open site. These data provide evidence to support the hypothesis that ecophysiological characteristics link with plant’s regeneration behavior and successional status. Although Pn and stomatal conductance decreased at midday in M. gigantea and S. leprosula in the open site, both r‐ETR and leaf temperature remained unchanged. This indicates that stomatal closure rather than reduced photochemical capacity limited Pn in the daytime. Conversely, there was reduced r‐ETR under high PPFD conditions in N. heimii in the open site, indicating reduced photochemical capacity. In the gap site, Pn increased in all leaves in the morning before exposure to direct sunlight, suggesting a relatively high use of diffuse light in the morning.

Duration of the grain-filling period in maize is not affected by photoperiod and incident PPFD during the vegetative phase

Total number of initiated leaves and duration from sowing to silking increases when photoperiod is increased during the photoperiod-sensitive phase in maize ( Zea mays L.). Little is known, however, about possible other effects of photoperiod and incident photosynthetic photon flux density (PPFD) on rate of development and duration of life cycle. A study was undertaken to quantify effects of photoperiod and incident PPFD from sowing to the 15-leaf stage on rate of leaf appearance and duration of the grain-filling period. The short-season maize hybrid Pioneer 3902 was grown in growth cabinets from sowing to the 15-leaf stage with either (i) a 10 h photoperiod at high PPFD (650 μmol m −2 s −1), (ii) a 20 h photoperiod consisting of 10 h of high PPFD followed by 10 h of low PPFD (5–50 μmol m −2 s −1), or (iii) a 20 h photoperiod of high PPFD. From the 15-leaf stage to maturity the plants were placed under a 16 h photoperiod in a growth room. Increasing photoperiod from 10 to 20 h increased final number of initiated leaves and delayed silking but did not affect rate of leaf appearance. Doubling incident PPFD to a value similar to that under Ontario field conditions during the summer resulted in a 16% increase in rate of leaf appearance and in a significant increase in total number of initiated leaves. Differences in final number of initiated leaves and in rate of leaf appearance from sowing to the 15-leaf stage among treatments resulted in a 4-day difference in silking date between the 10 h photoperiod treatment and the two 20 h photoperiod treatments. Duration of the grain-filling period did not differ among the three treatments.

Some characteristics of reduced leaf photosynthesis at midday in maize growing in the field

Maize was grown in the high-radiation arid summer environment of Davis, California, and its leaf photosynthetic rate was measured over diurnal courses on cloudless days with the leaf held perpendicular to the sunlight. On days of high atmospheric vapor pressure deficit (VPD), leaf photosynthesis reached a maximum in the late morning and then decreased gradually as the day progressed, though the soil was well irrigated. When CO 2 concentration in the measurement chamber was raised to about 1000 μmol mol −1, photosynthesis was enhanced, but more in the afternoon than in the morning. As a result, rates measured at high CO 2 in the morning and afternoon were essentially the same. There was also no difference in the curves of photosynthetic rate ( A) versus intercellular CO 2 concentration ( C i) for the morning and afternoon. Hence, photosynthetic capacity was similar for the two periods and there was no evidence of photoinhibition by the high photosynthetic photon flux density at noon. Further, C i and photosynthetic rates A measured over a range of photon flux density were lower in the afternoon than in the morning. These results indicate that A at noon and early afternoon was more limited than in the morning by epidermal conductance (mostly stomatal). On a day of low VPD, however, midday depression in A and epidermal conductance were not evident for the well-irrigated plants. Without irrigation and with leaves at a lower midday water potential, midday reduction in conductance and A was much more marked, beginning late in the morning. Epidermal conductance of maize grown in the field in Davis is are not sensitive to VPD. Therefore, the midday reduction in conductance and A was more likely the result of low leaf water potential caused by high transpiration rates.

Stomatal characteristics and leaf anatomy of potato plantlets cultured in vitro under photoautotrophic and photomixotrophic conditions

Potato plantlets (Solanum tuberosum L. cv. Benimaru) were cultured under photoautotrophic (without any sucrose in the nutrient medium and with enriched CO2 and high photosynthetic photon flux) and photomixotrophic conditions (20 g 1−1 sucrose in the medium). Leaf anatomy and stomatal characteristics of the leaves were studied in relation to stomatal size and density. Leaf diffusive resistance, transpiration rate, and wax content of the leaves were also investigated. In the photoautotrophic treatment, stomata behaved normally by closing in the dark and opening in the light. The stomatal density increased twofold compared to that of the photomixotrophic treatment. Relatively thick leaves and an organized palisade layer were observed and the epicuticulal wax content was remarkably higher in this treatment, i.e., seven times greater than that of photomixotrophic treatment. In general, higher diffusive resistance of the leaves was observed than under photomixotrophic conditions; also the resistance increased in darkness and decreased in the light. All these characteristics led the plantlets to have a normal and controlled transpiration rate, which was exceptionally high in the photomixotrophic treatment throughout the light and the dark period.

Leaf gas exchange and cholorphyll fluorescence in relation to leaf angle, azimuth, and canopy position in the tropical pioneer tree, Macaranga conifera

We tested the hypothesis that, in tropical pioneer tree species, vertical leaf angle contributes to high carbon gain because it minimizes damage caused by high irradiances. Diurnal changes in leaf gas exchange and chlorophyll fluorescence were measured in east-facing (EL), west-facing (WL) leaves, and in leaves artificially held horizontal (HL) in the uppermost canopy of Macaranga conifera (Zoll.) Muell. Arg. Maximum values of net photosynthetic rate (P(n)) for EL and HL reached 12 &mgr;mol m(-2) s(-1), whereas maximum P(n) for WL was only 6 &mgr;mol m(-2) s(-1). Midday depressions of P(n) and stomatal conductance occurred at high photosynthetic photon flux densities (PPFD), especially for HL. Photosystem II quantum yield (DeltaF/F(m)') of HL for a given PPFD at the leaf surface was lower in the afternoon than in the morning. Values of DeltaF/F(m)' for HL measured at dusk were lower than those measured just before dawn, suggesting that HL suffered from high light and heat load. Variations in the morphology and physiology of the canopy leaves were associated with different light environments, and there was circumstantial evidence of a transitional point at a PPFD of about 20-30% of full sunlight. Maximum P(n) and nitrogen (N) content were higher in upper canopy leaves than in lower canopy leaves, and the differences were mainly associated with differences in lamina thickness. We conclude that the vertical leaf angle and thick lamina of the top canopy leaves contributed to enhance total carbon gain of the whole plant.

The physiology of Cymbidium plantlets cultured in vitro under conditions of high carbon dioxide and low photosynthetic photon flux density

SummaryCymbidium plantlets were grown in vitro under conditions of high CO2 and low photosynthetic photon flux density using the Miracle Packt culture system. Shoots and roots of plantlets showed differential growth characteristics. Shoot growth was not different in plantlets cultured under CO2-enriched (CDE) and non-enriched (NCDE) conditions. Root growth was promoted in plantlets cultured under CDE in the presence or absence of 2% sucrose (S) with rockwool (R) as the supporting material. Growth was poor in plantlets cultured in 1% agar. Root growth was best in plantlets cultured under CDE R+S. Sucrose is still an important component for root growth under CDE conditions even though CO2 can be used as an alternative carbon source. Photosynthetic measurements (CO2 uptake and total Rubisco activity) showed the presence of active and operational photosynthetic machinery in plantlets cultured under CDE and NCDE conditions. The apparent lack of photoautotrophy (as evident from the lack of starch grains in chloroplasts) in plantlets cultured under NCDE conditions is not the result of a lesser potential for photoautotrophy; rather it is a consequence of sub-optimal CO2 concentrations within the culture vessels.