High Photosynthetic Photon Flux Research Articles

The impact of elevated carbon dioxide on the growth and gas exchange of three C4 species differing in CO2 leak rates

Recent work has suggested that the photosynthetic rate of certain C4 species can be stimulated by increasing CO2 concentration, [CO2], even under optimal water and nutrients. To determine the basis for the observed photosynthetic stimulation, we tested the hypothesis that the CO2 leak rate from the bundle sheath would be directly related to any observed stimulation in single leaf photosynthesis at double the current [CO2]. Three C4 species that differed in the reported degree of bundle sheath leakiness to CO2, Flaveria trinervia, Panicum miliaceum, and Panicum maximum, were grown for 31–48 days after sowing at a [CO2] of 350 μl l−1 (ambient) or 700 μl l−1 (elevated). Assimilation as a function of increasing [CO2] at high photosynthetic photon flux density (PPFD, 1 600 μmol m−2 s−1) indicated that leaf photosynthesis was not saturated under current ambient [CO2] for any of the three C4 species. Assimilation as a function of increasing PPFD also indicated that the response of leaf photosynthesis to elevated [CO2] was light dependent for all three C4 species. The stimulation of leaf photosynthesis at elevated [CO2] was not associated with previously published values of CO2 leak rates from the bundle sheath, changes in the ratio of activities of PEP‐carboxylase to RuBP carboxylase/oxgenase, or any improvement in daytime leaf water potential for the species tested in this experiment. In spite of the simulation of leaf photosynthesis, a significant increase in growth at elevated [CO2] was only observed for one species, F. trinervia. Results from this study indicate that leaf photosynthetic rates of certain C4 species can respond directly to increased [CO2] under optimal growth conditions, but that the stimulation of whole plant growth at elevated carbon dioxide cannot be predicted solely on the response of individual leaves.

Leaf Mass Partitioning as a Determinant of Dry Matter Accumulation in Zantedeschia

Dry matter accumulation and partitioning in plants of Zantedeschia Spreng. `Best Gold' aff. Z. pentlandii (Wats.) Wittm. (syn. Richardia pentlandii Wats.) were quantified under a range of temperature and photosynthetic photon flux (PPF) regimes using plant growth analysis. The relative rate of dry matter accumulation [relative growth rate (RGRM), g·g-1·d-1] was highly correlated with the partitioning of the daily increment of dry matter into leaf tissue [leaf matter partitioning (LMP), g·d-1 per g·d-1]. In contrast, a poor correlation existed between RGRM and net assimilation rate (NAR, g·m-2·d-1). Maximum values of RGRM increased linearly with increasing temperature (from 13 to 28 °C), with a base temperature of 2.1 ± 2.7 °C. The optimum temperature for growth was PPF dependent with maximum total plant dry mass occurring under high PPF (694 μmol·m-2·s-1) at 25 °C. However, as the plant responded to PPF by altering LMP, final total plant dry mass was actually greater under the low PPF regime (348 μmol·m-2·s-1) at temperatures <22 °C. The optimum temperature for dry matter accumulation was close to the average daily air temperature during the growing season for the natural habitat of the parent species. Similarly, the greater dry matter accumulation under the combination of either low PPF and cooler temperatures or high PPF and warmer temperatures was paralleled by the diversity of PPF habitats in the natural open grassland and forest margin the parent species occupies. It is therefore suggested that Zantedeschia `Best Gold' is well adapted to optimize growth under these environmental conditions.

Fatty Acid Concentration in Portulaca oleraceae L. is Altered by Photosynthetic Photon Flux

Purslane (Portulaca oleraceae L.) seedlings were grown under an instantaneous photosynthetic photon flux (PPF) of 299 or 455 μmol·m–2·s–1 for a daily duration of either 8, 12, 16, or 20 h. Thus plants were exposed to a daily PPF of 8.6, 12.9, 17.2, and 21.5 mol·m–2·d–1 in the low PPF treatment (299 μmol·m–2·s–1) and 13.1, 19.7, 26.2, and 32.8 mol·m–2·d–1 in the high PPF treatment (455 μmol·m–2·s–1). All treatments received a 20-h photoperiod using a PPF of ≈5 μmol·m–2·s–1. At low PPF, purslane grown under 16-h PPF duration produced the highest concentration of total fatty acid (TFA), linoleic acid (LA), and linolenic acid (LNA) per unit leaf dry mass (DM) and leaf area; but at high PPF, the concentration of these compounds was highest under 8- and 12-h PPF duration. Trend analysis indicated that maximum TFA, LA, and LNA concentrations occurred with a PPF of 14.1, 16.9 and 17.2 mol·m–2·d–1 respectively; and protein, chlorophyll, and LNA concentrations in thylakoid membranes were maximized at PPF of 21.8, 19.9, and 16.1 mol·m–2·d–1, respectively. LNA as percent of TFA was unaffected by treatment. DM increased with PPF up to the highest PPF exposure of 32.8 mol·m–2·d–1.

INORGANIC CARBON UTILIZATION AND GROWTH ABILITIES IN THE MARINE RED MACROALGA GELIDIOPSIS SP.

We studied the effects of NH4+, photosynthetic photon flux (PPF), and temperature on growth rates, together with inorganic carbon (Ci) utilization properties of Gelidiopsis sp. cultivated in tanks. At 25% sunlight, weekly growth rates and dry weight yields increased up to 6-fold with increasing NH4+(0–2 mM); however, at 5% or 100% sunlight the effects were much lower. Contents of photosynthetic pigments (chlorophyll a and phycoerythrin) increased in correlation with increases of NH4+. Maximal chlorophyll a concentrations occurred under high PPF, while phycoerythrin concentrations were higher at low PPF. Ash amounts in Gelidiopsis sp. did not vary significantly with different NH4+ or PPF levels. Optimal temperatures and PPFs for growth were 20–25 °C and 170–320 μmol m−2 s−1, respectively, correlating with short-term photosynthetic O2 evolution measurements. The pH of both thallus surface and bulk medium increased during photosynthesis, reaching average values of 8.75, and resulting in low rates of O2 evolution. Activities of carbonic anhydrase (CA) were detected inside and outside the cells and were apparently involved in the Ci uptake system of Gelidiopsis sp. since both acetazolamide (membrane-impermeable) and ethoxyzolamide (membrane-permeable) inhibited photosynthetic O2 evolution by 89% on average. Half-maximal rates of photosynthetic O2 evolution (K0.5) were reached at 17 μM CO2 at pH 5.0 and 2–3 mM Ci at pH 8.0, indicating high affinity for CO2 and close to saturated photosynthesis at Ci levels of seawater. Thus the Ci uptake system of Gelidiopsis sp. probably uses an extracellular CA catalyzed conversion of HCO3− to yield CO2, which could diffuse into the cells, and an intracellular CA catalyzed HCO3− ↔ CO2 interconversion which may assure CO2 for Rubisco. Direct uptake of HCO3− may also be required based on the K0.5 (CO2) estimated for Gelidiopsis sp. and the pH generated at the thallus surface at which CO2 concentrations would only be approximately 10 μM. Therefore, in addition to limitations of low NH4+ concentrations and high temperatures during the summer, growth of Gelidiopsis sp. from the Israeli Mediterranean may also be restricted by its limited Ci utilization system and the low CO2 concentrations prevailing in seawater.

Culture on sugar medium enhances photosynthetic capacity and high light resistance of plantlets grown in vitro

The significance of photosynthetic photon flux (PPF) and sugar feeding for the production of plants in vitro is only poorly understood. Nicotiana tabacum L. plantlets were grown photoautotrophically and photomixotrophically (3% sucrose) at two different PPFs (60 µmol m−2 s−1 and 200 µmol m−2 s−1) to investigate the effect of these culture parameters on photosynthetic performance and growth. Photomixotrophically‐grown plantlets showed an increase in carbohydrate content, mainly in glucose and fructose. Plant growth, dry matter accumulation and total leaf area were higher under photomixotrophic than photoautotrophic conditions. Not only biomass formation but also photosynthesis was positively affected by exogenous sucrose; the chlorophyll (Chl) content and the light‐saturated rate of photosynthetic oxygen evolution were higher in photomixotrophic plantlets. Photoinhibition occurred in plantlets that were grown photoautotrophically at the higher PPF. It became apparent as a loss in Chl content and photochemical efficiency. Photoinhibited plantlets showed a decrease in the D2/LHCII and CP47/LHCII ratios, suggesting a preferential loss of proteins from the photosystem II (PSII) core. The increased content of xanthophyll cycle pigments in photoinhibited plantlets indicated that also protective mechanisms were activated. Photomixotrophic growth of the plantlets prevented the occurrence of photoinhibitory symptoms. Therefore, we conclude that culture on sugar medium increases not only the photosynthetic potential but also the high light resistance of plantlets grown in vitro.

Photosynthetic capacity and dry mass partitioning in dwarf and semi-dwarf wheat ( Triticum aestivum L.)

Efficient use of space and high yields are critical for long-term food production aboard the International Space Station. The selection of a full dwarf wheat (less than 30 cm tall) with high photosynthetic and yield potential is a necessary prerequisite for growing wheat in the controlled, volume-limited environments available aboard long-term spaceflight missions. This study evaluated the photosynthetic capacity and carbon partitioning of a full-dwarf wheat cultivar, Super Dwarf, which is routinely used in spaceflight studies aboard U.S. space shuttle and NASA/Mir missions and made comparisons with other dwarf and semi-dwarf wheat cultivars utilized in other ground-based studies in plant space biology. Photosynthetic capacity of the flag leaf in two dwarf (Super Dwarf, BB-19), and three semi-dwarf (Veery-10, Yecora Rojo, IBWSN 199) wheat cultivars (Triticum aestivum L.) was assessed by measuring: net maximum photosynthetic rate, RuBP carboxylation efficiency, chlorophyll concentration and flag leaf area. Dry mass partitioning of carbohydrates to the leaves, sheaths, stems and ear was also assessed. Plants were grown under controlled environmental conditions in three replicate studies: slightly enriched CO2 (370 micromoles mol-1), high photosynthetic photon flux (1000 micromoles m-2 s-1; 58 mol m-2 d-1) for a 16 h photoperiod, 22/15 degrees C day/night temperatures, ample nutrients and water provided by one-half strength Hoagland's nutrient solution (Hoagland and Arnon, 1950). Photosynthetic capacity of the flag leaf was determined at anthesis using net CO2 exchange rate versus internal CO2 concentration curves measured under saturating light (2000 micromoles m-2 s-1) and CO2 (1000 micromoles mol-1). Dwarf wheat cultivars had greater photosynthetic capacities than the taller semi-dwarfs, they averaged 20% higher maximum net photosynthetic rates compared to the taller semi-dwarfs, but these higher rates occurred only at anthesis, had slightly greater carboxylation efficiencies and significantly increased chlorophyll concentrations per unit leaf area. The reduced-height wheat had significantly less dry mass fraction in the stem but greater dry mass partitioned to the ear than the taller semi-dwarfs (Yecora rojo, IBWSN-199). Studies with detached heads confirm that the head is a significant sink in the shorter wheat cultivars.

Interactive effects of photon fluence rates and drought on CAM-cycling in Delosperma tradescantioides (Mesembryanthemaceae).

The ability of photosynthesis and CAM to acclimate to low (220 µmol m-2 s-1 ; LL) and relatively high (550 µmol m-2 s-1 ; HL) photosynthetic photon flux densities (PPFD) was investigated in the CAM-cycling species Delosperma tradescantioides by means of CO2 gas exchange and chlorophyll fluorescence analysis. Furthermore, the influence of short-term drought on malic acid accumulation and the activity of photosystem II (PSII) was studied to assess the possible interactions between drought and the prevailing PPFD in this species. HL plants showed features of sun versus shade acclimation relative to LL plants. Nocturnal malic acid accumulation (Δ-malate) and leaf water content also tended to be higher in HL plants. Irrespective of the PPFD during growth, the weak Δ-malate doubled within 3 days of drought. Despite largely restricted CO2 uptake, photosynthetic activity as estimated from fluorescence analysis declined only ca 5%. After 7 days of drought, when plants showed CAM-idling and Δ-malate had decreased again, potential carbon assimilation was still ca 84% of that in well-watered plants and remained relatively constant throughout the day. Decarboxylation of malic acid accounted for ca 23% of potential assimilation assuming total oxidation of a maximum portion of this organic acid. Drought did not affect predawn maximum photochemical efficiency (Fv /Fm ). Nonphotochemical quenching (qN) increased (24%) in response to desiccation and resulted in a more or less constant reduction state of PSII. This increase in qN resulted mainly from the change in its fast-relaxing component (qNF), while the slow component (qNS) was significant only at or above saturating PPFD in both HL and LL plants. The photon response characteristics of PSII, which differed between LL and HL plants, were unaffected by short-term drought. Photon harvesting and photon use were always adjusted to guarantee a low reduction state of PSII. Results suggest that in both LL and HL plants CAM-cycling may help to stabilize photosynthesis but to a large extent by other means than simply providing internally derived CO2 .

Seasonal variation of carbon dioxide, water vapor, and energy exchanges of a boreal black spruce forest

Measurements of the fluxes of latent heat λE, sensible heat H, and CO2 were made by eddy covariance in a boreal black spruce forest as part of the Boreal Ecosystem‐Atmosphere Study (BOREAS) for 120 days through the growing season in 1994. BOREAS is a multiscale study in which satellite, airborne, stand‐scale, and leaf‐scale observations were made in relation to the major vegetation types [Sellers et al., 1995]. The eddy covariance system comprised a sonic anemometer mounted 27 m above the forest, a system for transferring air rapidly and coherently to a closed path, infrared gas analyzer and a computer with the Edinburgh EdiSol software. Over the measurement period, closure of the energy balance on a 24 hour basis was good: (H + λE)/(Rn ‐ G ‐ B ‐ S) = 0.97. The midday Bowen ratio was typically in the range 1.0–2.5, with an average value of ∼1.9 in the first Intensive Field Campaign (IFCl) and 1.3–1.4 in IFC2 and IFC3. Daily ecosystem evapotranspiration from moss, understory, and trees followed daily net radiation. Mean half‐hourly net ecosystem flux followed photosynthetic photon flux density (PPFD) closely, reaching −9 μmol m−2 s−1 in June and August. The mean respiratory efflux on nights during which the atmosphere was well mixed (u*>0.4 m s−1) reached 6 μmol m−2s−1. The PPFD‐saturated biotic CO2 assimilation reached 20 μmol; m−2 s−1 and showed little response to air temperature or vapor pressure deficit (VPD). Storage of CO2 in the air column at night did not account adequately for respiration on stable nights, so nighttime efflux was modeled for periods when u*<0.4 m s−1. There was a net gain of CO2 on most of the 120 days, but on 31 days of high temperature or low PPFD there was a net carbon loss. High PPFD promoted influx of CO2 by the foliage, whereas high temperatures reduced net CO2 influx through high respiration rates by the roots and soil microorganisms, leading to lower net uptake at high PPFD. Over the 120 day period, 95 g m−2 of C were stored (an average of 0.8 g m−2 d−1), and 237 mm of water evaporated (an average of 2 mm d−1).

Branch and root formation in Trifolium repens is influenced by the light environment of unfolded leaves.

In plagiotropic plants, axillary buds on the stolon can be exposed to low red:far-red (R:FR) ratios, while the leaves may be positioned in the uppermost layer of the sward where they are exposed to a high R:FR ratio. We tested whether the light environment of unfolded leaves influences outgrowth of the axillary buds and the formation of nodal roots of Trifolium repens. Single plants were grown in a growth cabinet with high photosynthetic photon flux rate (PPFR) and a high R:FR ratio (FHRH, control), low PPFR and high R:FR (FLRH) or low PPFR and low R:FR (FLRL). In an additional treatment (SS), only stolons were shaded so that developing leaves grew into light conditions similar to the control treatment. Neutral shading (FLRH) had a minor effect on branching and did not influence root formation. A reduction in the R:FR ratio (FLRL) significantly delayed the outgrowth of axillary buds so that, compared to the control plants, the percentage of branched phytomers was reduced by 43% on the parent axis and by 75% on primary branches. Furthermore, the number of nodal roots per plant was reduced by about 30%. When only the stolons were shaded (SS), the percentage of branched and rooted phytomers was similar to that of the control plants. Extension of petioles and leaves was very variable, increasing the values in the FLRL treatment at least 2.5-fold compared with the control plants. It was concluded that the light environment of the unfolded leaves had a significant influence on the regulation of the outgrowth of axillary buds and that the high plasticity in petiole growth allows the positioning of the leaves in a light environment conducive to the stimulation of branch outgrowth.

Supplemental Light before Harvest Increases Phenethyl Isothiocyanate in Watercress under 8-hour Photoperiod

Watercress (Nasturtium officinale R.Br.) plants were grown in growth chambers at 15 °C or 25 °C and either 8- or 12-h photoperiod (PP). The photosynthetic photon flux (PPF) was 265 μmol·m-2s-1 in all chambers, but beginning 1 week before harvest, half of the plants in each chamber were subjected to a higher PPF (435 μmol·m-2·s-1). At harvest, watercress leaves and stems were analyzed for phenethyl isothiocyanate (PEITC) concentration. Without supplemental PPF, watercress grown at 25 °C and 12-h PP produced higher PEITC concentration in leaves and stems than plants grown at 15 °C and 12-h PP, or plants grown at 8-h PP and either temperature. With one week of supplemental PPF before harvest, plants grown at 15 or 25 °C and the 8-h PP produced PEITC concentrations as high as plants exposed to 12-h PP and similar temperatures. However, a week of supplemental PPF did not alter PEITC concentrations in plants grown at the 12-h PP, regardless of temperature. At 25 °C, plants grown under the low PPF and the 12-h PP produced 62% greater dry mass than plants exposed to a week of high PPF and the 8-h PP, but did not differ in PEITC content. Thus, the effect of one week of high PPF on PEITC concentration depended on photoperiod.

Contribution of the adaxial and abaxial surfaces of olive leaves to photosynthesis

Leaves of olive cultivars Frantoio and Maurino were irradiated with different irradiances from above, from below, or simultaneously from both directions to determine the contribution of the abaxial and adaxial leaf surfaces to photosynthesis. In both cultivars, irradiation of both sides of the leaf caused increases in net photosynthetic rate (P N) and apparent quantum yield compared to irradiating only one surface with the equal photosynthetic photon flux density (PPFD), but the PPFD needed to saturate P N decreased. At high and medium PPFD the P N determined at irradiating both leaf surfaces was less than the sum obtained at irradiation of only the upper or the lower surface with the same PPFD. At PPFD higher than 1000 µmol m-2 s-1 in cv. Frantoio and 1200 µmol m-2 s-1 in cv. Maurino, P N did not vary. At low PPFD (<200 µmol m-2 s-1), P N at irradiating the adaxial and abaxial leaf surfaces simultaneously was about the sum of the values obtained by irradiating the upper and lower surfaces separately. Consequently the compensation irradiance was reduced from about 50 µmol m-2 s-1 to about 30 µmol m-2 s-1 when irradiating both leaf surfaces. The natural leaf orientation of the olive cultivar influenced the utilization of radiant energy by the abaxial surface.

培養器外直接発根法における光強度および培養液濃度がイチゴ小植物体の生長に及ぼす影響

The effects of the photosynthetic photon flux density (PPFD) and concentration of culture solution on the growth of strawberry (Fragaria×ananassa Duch. cv. Houko-uwase) plantlets rooted by the direct ex-vitro rooting method were examined. In the direct ex-vitro rooting method, the shoot elongation, rooting, and acclimatization of the plantlets are simultaneously achieved in the controlled environment. In this study, explants of micropropagated shoots were directly inserted into the rockwool plugs and cultured inside the environmental control unit by hydroponics using a culture solution at three different concentrations (0.5U : 1/2 strength of Enshi (Hori, 1966) component of culture solution for hydroponics, 0.2U : 1/5 strength of Enshi component, 0/0.2U : 0 (0-10th days) and 1/5 (11-30th days) strength of Enshi component) under high and low PPFDs (high PPFD : 180μmol m-2 s-1, low PPFD : 70μmol m-2 s-1).After 30 days in culture, dry and fresh weights, leaf area, number of leaves, and dry weight ratio of the plantlets were measured. All these parameters were greater in a high PPFD than in a low PPFD. The shoot/root ratio was larger in a low PPFD than in a high PPFD. In a high PPFD, growth (dry and fresh weights, etc.) of plantlets was in the order of 0.5U>0.2U>0/0.2U. In a low PPFD, the effect of the concentration of culture solution on the growth of plantlets was not as obvious as that in a high PPFD.

Light availability and photosynthesis of Pseudotsuga menziesii seedlings grown in the open and in the forest understory.

The light environment, photosynthetic dynamics and steady-state net photosynthetic rates of lateral branch shoots of Pseudotsuga menziesii var. glauca (Beissn.) Franco seedlings growing in the open and in the forest understory were investigated in situ. Mean incident photosynthetic photon flux density (PPFD) was 702.5 micro mol m(-2) s(-1) on open-grown branches and 52.0 micro mol m(-2) s(-1) on understory-grown branches. Mean daily durations of PPFD greater than 500, 200, and 50 micro mol m(-2) s(-1) were 8.5, 31.5, and 270.3 min, respectively, on understory-grown branches, and 559.1, 700.7, and 803.3 min, respectively, on open-grown branches. Sunflecks accounted for 32.4% of total daily photosynthetically active radiation incident on understory branches. Following 10 min at a PPFD of 50 micro mol m(-2) s(-1), the induction time required for net photosysnthesis to reach 50 and 90% of steady-state rates was shorter at a PPFD of 200 than at a PPFD of 500 micro mol m(-2) s(-1) and shorter in understory-grown branches than in open-grown branches. On a leaf area basis, dark respiration rates of understory-grown branches were lower and net photosynthetic rates were higher than those of open-grown branches exposed to low PPFD. However, at high PPFDs, understory-grown branches had lower photosynthetic rates than open-grown branches. When measurements were expressed on a leaf dry mass basis, there was no difference in dark respiration rates between understory branches and open-grown branches, but net photosynthetic rates of understory branches were equal to or higher than those of open-grown branches at all PPFDs.

カリフラワー組織培養小植物体の成長，光合成および順化に及ぼす自然通気培養ならびに光強度の影響

Growth and photosynthesis of the photomixotrophically grown cauliflower plantlets (Brassica oleracea L.) were investigated during a 1-week in vitro culture under naturally ventilated conditions followed by a 2-week ex vitro acclimating period. The in vitro generated leaves in a ventilated vessel had higher rates of photosynthesis than those in an airtight vessel. The enhanced rates of photosynthesis in those leaves, which contained both increased amounts of chlorophyll and total soluble protein, contributed to promote the growth in vitro and the acclimation of plantlets to ex vitro. A nearly half concentration of ambient CO2 was kept in a naturally ventilated vessel, in which the plantlets grew well by positive photosynthesis during the photoperiods. Both the leaf expansion and the shoot and root dry weight increased under higher photosynthetic photon flux density (PPFD, 50 vs. 100μmol m-2 s-1) during the ventilated in vitro culture period. In the first two weeks after transplanting the cauliflower plantlets, the high photosynthetic capacity of in vitro generated leaves had an important contribution to the unfolding of acclimating leaves and the successful acclimation of the whole plantlets.

DESIGN ISSUES FOR MICROPROPAGATION LIGHTING SYSTEMS

Issues related to design of micropropagation lighting systems are investigated. Average photosynthetic photonflux (PPF) and mean relative deviation (MRD) are calculated using a previously developed model. A comparison of aisleversus aisle-free benching shows that the aisle-free configuration can deliver 60% higher PPF with the same energy inputand without increasing the MRD. When compared to the presence of only one vessel, the presence of a full complement ofmicropropagation vessels on a shelf is shown to reduce PPF at the vessel lid level by 5.1% and at the plant level by25.0%. Vessels were also found to attenuate radiation preferentially in the 300 to 400 nm range. Measurements of spectralpower distribution along the lengths of fluorescent lamps show radiation output increases above 700 nm near the ends ofthe lamps. The implication that one can model and design for PPF, uniformity, and spectral quality is discussed.

Cytokinin and Light Intensity Regulate Flowering of Easter Lily

Lilium longiflorum Thunb. `Nellie White' plants were selected when their first flower buds reached 2 or 5 cm in length, sprayed with 2 mL of PBA at 0 or 500 mg·L–1, and then placed under 1440 or 60 μmol·m–2·s–1 photosynthetic photon flux (PPF) during flowering. PBA resulted in delayed anthesis and increased dry matter accumulation in flowers under the high PPF but had no effect under the low PPF. PBA did not decrease the severity of flower bud abortion under the low PPF. Application of PBA induced the formation of numerous bulbils in the leaf axils. Regardless of PPF, PBA-treated plants had less dry weight in the main bulbs than the control plants. Chemical name used: N-(phenylmethyl)-9-(tetra-hydro-2H-pyran-2-yl)-9H-purin-6-amine (PBA).

The Effect of Light Intensity before Harvest on Phenethyl Isothiocyanate Concentration in Watercress (Nasturtium officinale R.Br.) Depends on Photoperiod

Watercress plants were grown in growth chambers at 15°C or 25°C and either an 8- or 12-hour photoperiod (PP). The photosynthetic photon flux (PPF) was 265 μmol·m–2·s–1 in all chambers, but beginning 1 week before harvest, half of the plants in each chamber were subjected to a higher PPF (434 μmol·m–2·s–1). At harvest, watercress leaves and stems were analyzed for phenethyl isothiocyanate (PEITC) content. Watercress grown at 25°C, the 12-hour PP, and 1 week of high PPF produced the highest PEITC concentration in leaves and stems, and plants grown at 15°C, the 8-hour PP, and the low PPF until harvest produced the lowest PEITC concentration. Plants grown at the 8-hour PP, then exposed to 1 week of high PPF, produced 57.3% and 45.9% greater PEITC at 25 and 15°C, respectively, then plants exposed to the low PPF until harvest. However, plants grown at the 12-hour PP and subjected to 1 week of high PPF produced PEITC levels similar to plants grown under the low PPF at 25 and 15°C. At 25°C, plants grown under the low PPF and the 12-hour PP produced 62% greater dry mass than plants exposed to 1 week of high PPF and the 8-hour PP, but did not differ in PEITC content. Thus, the effect of 1 week of high PPF on PEITC concentration depended on photoperiod.

Dynamic Optimization of Photosynthetic Photon Flux for Efficient Photosynthesis and Growth of Leaf Lettuce Canopies

`Waldmann's Green' leaf lettuce (Lactuca sativa L.) is being used as a model leafy vegetable crop to develop a protocol for variable control of photosynthetic photon flux (PPF) during crop production. Feedback from real-time photosynthetic gas exchange rates by lettuce canopies is used to modulate electronic dimming ballasts of lamp banks. Algorithms within process-control software are being fine tuned to maximize increments of photosynthetic output relative to increments of photon input. Dynamic optimization of PPF was 21% more efficient than constant high PPF saturating photosynthesis with respect to biomass accumulated per photons absorbed. Dynamic optimization also is being combined with principles of phasic control, in which environmental resources such as photosynthetically active radiation (PAR) and carbon dioxide (CO2) are deliberatively limited in input during specific phases of crop development when plants are less sensitive to inputs (e.g., lag, plateau, and senescence phases) but optimized for the responsive exponential phase. Preliminary results indicate that leaf lettuce growth benefits from optimizing environments for no more than 4 or 5 days during a 20-day production cycle. Dynamic optimization of CO2 level is achieved by controlling the injection of CO2 into the inlet air stream of Minitron II crop canopy cuvette/growth chambers. Algorithms are being modified to simultaneously vary PPF and CO2 for optimum photosynthesis.

Interacting Effects of Photoperiod and Photosynthetic Photon Flux on Net Carbon Assimilation and Starch Accumulation in Potato Leaves

The effect of photoperiod (PP) on net carbon assimilation rate (Anet) and starch accumulation in newly mature canopy leaves of 'Norland' potato (Solanum tuberosum L.) was determined under high (412 varies as mol m-2s-1) and low (263 varies as mol m-2s-1) photosynthetic photon flux (PPF) conditions. The Anet decreased from 13.9 to 11.6 and 9.3 micromoles m-2s-1, and leaf starch increased from 70 to 129 and 118 mg g-1 drymass (DM) as photoperiod (PP) was increased from 12/12 to 18/6, and 24/0, respectively. Longer PP had a greater effect with high PPF conditions than with low PPF treatments, with high PPF showing greater decline in Anet. Photoperiod did not affect either the CO2 compensation point (50 micromoles mol-1) or CO2 saturation point (1100-1200 micromoles mol-1) for Anet. These results show an apparent limit to the amount of starch that can be stored (approximately 15% DM) in potato leaves. An apparent feedback mechanism exists for regulating Anet under high PPF, high CO2, and long PP, but there was no correlation between Anet and starch concentration in individual leaves. This suggests that maximum Anet cannot be sustained with elevated CO2 conditions under long PP (> or = 12 hours) and high PPF conditions. If a physiological limit exists for the fixation and transport of carbon,then increasing photoperiod and light intensity under high CO2 conditions is not the most appropriate means to maximize the yield of potatoes.

Drought Stress Influences Gas-exchange Responses of Papaya Leaves to Rapid Changes in Irradiance

Field-grown `Red Lady' papaya (Carica papaya L.) plants were used to measure foliar gas-exchange responses to rapid changes in irradiance levels to determine if papaya stomata are able to track simulated sun-to-cloud cover transitions. Natural sunlight and neutral shade cloth placed over the leaf were used to provide high photosynthetic photon flux (PPF) of about 2000 μmol·m-2·s-1 until leaves reached steady state within the cuvette, followed by three minutes with low PPF of about 325 μmol·m-2·s-1, and a return to PPF of about 2000 μmol·m-2·s-1. Net CO2 assimilation (A) declined from an initial 20 μmol·m-2·s-1 to about 9 μmol·m-2·s-1 within 20 seconds of initiating low PPF, and remained fairly stable for the duration of the three minutes of low PPF. Stomatal conductance (gs) declined within 60 seconds of initiating low PPF, from 385 to about 340 μmol·m-2·s-1 during the three minutes duration of low PPF. Following the return to high PPF, A rapidly increased to about 18 μmol·m-2·s-1, then gradually increased to the original value. After a lag of about 1 minute following the return to high PPF, gs began to increase and returned to the original value after three minutes. Container-grown `Tainung #1' papaya plants were used in a second study to determine the influence of mild drought stress on gas-exchange responses to rapid irradiance transitions. For drought-stressed plants, gs declined to a greater magnitude following the high-to-low PPF transition, and gs and A recovered more slowly following the transition from low-to-high PPF than for well-watered plants. Water use efficiency declined to a minimum immediately following the high-to-low PPF transition for both sets of plants, but recovered more rapidly for drought-stressed plants. These results indicate that papaya stomata are able to track rapid changes in irradiance, and mild drought stress enhances the tracking response.