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Usha Palaniswamy, Richard McAvoy, and Bernard Bible

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.

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Kara Senger Lewallen and Richard P. Marini

The influence of photosynthetic photon flux (PPF) on peach [Prunus persica (L.) Batsch] fruit quality and the relationship between ground color and flesh firmness was studied by performing three experiments. Fruit with varying ground colors were sampled from different canopy positions with varying PPF. Fruit skin color was measured with a tristimulus colorimeter and values for L* (lightness), chroma (brightness), and hue angle (numerical values for color) were calculated for each fruit. Fruit from the canopy exterior generally were larger, had more surface area colored red, had higher soluble solids concentrations, and were darker, duller, and redder than fruit harvested from interior positions. In all three experiments, the relationship between hue angle and fruit firmness was affected by PPF, but the nature of the relationship (linear vs. curvilinear) and the influence of position was not consistent. When fruit were covered with aluminum foil or a section of the fruit surface was covered with duct tape to prevent light-induced red coloration of the skin, the relationship between hue angle and fruit firmness was similar for different canopy positions. Therefore, the relationship between ground color and fruit firmness is influenced by the light environment in which a fruit develops, and not by canopy position. Ground color does not seem to be a good indicator of fruit firmness because fruit with the same hue angle had greatly differing firmnesses.

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James E. Faust and Royal D. Heins

The effects of temperature and daily-integrated photosynthetic photon flux (PPFDI) on African violet (Saintpaulia ionantha Wendl.) flower initiation and development were quantified to provide the basis for an inflorescence development model. The percentage of leaf axils in which an inflorescence initiated and continued development increased as the PPFDI increased from 1 to 4 mol·m-2·day-1, while the rate of inflorescence development was a function of the average daily temperature (ADT). The appearance of a visible flower bud (VB) in a leaf axil was related to the growth of the subtending leaf blade. A polynomial model based on ADT and PPFDI was used to describe leaf blade length at visible bud (LBLVB). A nonlinear model was used to describe the influence of ADT on leaf expansion rate (LER). Inflorescence appearance in the leaf axil was predicted by measuring LBL and estimating the time for the leaf blade to develop to the length required for VB. A phasic-development scale was developed to quantify inflorescence development. Days required for an inflorescence to develop from VB to first open flower was described as a function of ADT and either inflorescence height or inflorescence development stage (IDS). Days from leaf emergence to first open flower for the inflorescence initiated in that leaf axil decreased from 86 to 55 as ADT increased from 18 to 26C.

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D.G. Mortley, P.A. Loretan, W.A. Hill, C.K. Bonsi, and C.E. Morris

Two sweetpotato [Ipomoea batatas (L.) Lam] genotypes (`Georgia Jet' and the breeding clone TI-155) were grown at 12-, 15-, 18-, and 21-h light/12-, 9-, 6-, 3-h dark cycles, respectively, to evaluate their growth and elemental concentration responses to duration and amount of daily lighting. Vine cuttings (15 cm long) of both genotypes were grown in rectangular nutrient film technique channels for 120 days. Conditions were as follows: photosynthetic photon flux (PPF) mean 427 μmol·m–2·s–1, 28C day/22C night air cycle, and 70% ± 5% relative humidity. The nutrient solution used was a modified half-strength Hoagland's solution. Storage root count per plant and per unit area, yield (in grams per square meters per day), and harvest index increased, while production efficiency (in grams per mole) decreased with increased daily PPF. Stomatal conductance for both genotypes declined with increased daily PPF. Leaves were smallest for both genotypes at the 21-h light period, while storage root yield declined as leaf area index increased. Except for a linear decrease in leaf N and K with increased light period, elemental concentration was not significantly influenced.

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David L. Bubenheim, Raman Sargis, and David Wilson

Electronic dimming of high-intensity discharge lamps offers control of photosynthetic photon flux (PPF) but is often characterized as causing significant spectral changes. Growth chambers with 400-W metal halide (MH) and high-pressure sodium (HPS) lamps were equipped with a dimmer system using silicon-controlled rectifiers (SCR) as high-speed switches. Phase control operation turned the line power off for some period of the alternating current cycle. At full power, the electrical input to HPS and MH lamps was 480 W (root mean squared) and could be decreased to 267 W and 428 W, respectively, before the arc was extinguished. Concomitant with this decrease in input power, PPF decreased by 60% in HPS and 50% in MH. The HPS lamp has characteristic spectral peaks at 589 and 595 nm. As power to the HPS lamps was decreased, the 589-nm peak remained constant while the 595-nm peak decreased, equaling the 589-nm peak at 345-W input, and the 589-nm peak was almost absent at 270-W input. The MH lamp has a broader spectral output but also has a peak at 589 nm and another smaller peak at 545 nm. As input power to the MH lamps decreased, the peak at 589 diminished to equal the 545-nm peak. As input power approached 428 W, the 589-nm peak shifted to 570 nm. While the spectrum changed as input power was decreased in the MH and HPS lamps, the phytochrome equilibrium ratio (Pfr: Ptot) remains unchanged for both lamp types.

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Jocelyn L. Catley and Ian R. Brooking

Flowering responses of Heliconia psittacorum L.f. × H. spathocircinata Aristeguieta `Golden Torch' to temperature and photosynthetic photon flux (PPF) were determined in controlled-environment conditions using a 2 × 2 factorial combination of temperature (32C day/20C night and 24C day/20C night) and PPF (475 and 710 μmol·m–2·s–1). Temperature had no significant effect on new shoot production, with an average of 9.3 shoots per plant being produced over the 248 days of treatment. More shoots, however, were produced at the higher PPF level (10.1 compared with 8.3 shoots). The proportion of shoots that initiated flowers (85%) was similar in all treatments. The duration from shoot until inflorescence emergence was significantly less at 32C day/20C night than at 24C day/20C night (140 and 146 days, respectively) and was unaffected by PPF. This duration also was significantly affected by the interacting effects of order of shoot appearance and the number of leaves subtending the inflorescence. The second shoots to emerge had the shortest duration from shoot emergence to inflorescence emergence. The number of leaves subtending the inflorescence increased at the higher temperature and decreased as shoot order increased but was unaffected by PPF. Temperature and PPF levels influenced total leaf area at flowering, with highest areas being achieved in the high temperature–low PPF combination. Acceptable flower quality with at least two, opened, well-formed, well-colored bracts was obtained in all treatments, although flower stems were taller and thicker at 32C day/20C night and these dimensions increased further with increasing order of shoot appearance. Stem diameters tended to be thinner at the lower PPF level. Overall, temperature was more dominant than light in influencing production and quality of flowers, but developmental factors associated with the order of shoot appearance also played a significant role. Flower production of `Golden Torch' should be feasible in temperature-controlled glasshouses in temperate regions where mean air temperatures can be maintained at ≈20C.

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Krishna S. Nemali and Marc W. van Iersel

Environmental conditions and incorporation of nutrients into the growing medium can affect the fertilizer needs of bedding plants. To evaluate the effects of photosynthetic photon flux (PPF) and starter fertilizer on the fertilizer requirements of subirrigated plants, we grew wax begonias (Begonia semperflorens-cultorum Hort.) under three PPF levels (averaging 4.4, 6.2, and 9.9 mol·m-2·d-1) and four fertilizer concentrations [electrical conductivity (EC) of 0.15, 0.33, 0.86, and 1.4 dS·m-1] in a normal (with starter fertilizer, EC = 2.1 dS·m-1) and heavily leached (with little starter fertilizer, EC = 0.9 dS·m-1) growing medium. Except for shoot dry mass, we did not find any significant interactions between PPF and fertilizer concentration on any of the growth parameters. There was an interactive effect of fertilizer concentration and starter fertilizer on all growth parameters (shoot dry mass, leaf area, plant height, and number of flowers). When the growing medium contained a starter fertilizer, fertilizer concentration had little effect on growth. When the growing medium was leached before transplanting, growth was best with a fertilizer EC of 0.86 or 1.4 dS·m-1. Water-use efficiency (WUE) was calculated from 24-hour carbon exchange and evapotranspiration measurements, and used to estimate the required [N] in the fertilizer solution to achieve a target tissue N concentration of 45 mg·g-1. Increasing PPF increased WUE and the required [N] (from 157 to 203 mg·L-1 at PPF levels of 4.4 and 9.9 mol·m-2·d-1, respectively). The PPF effect on the required [N] appeared to be too small to be of practical significance, since dry mass data did not confirm that plants grown at high light needed higher fertilizer concentrations. Thus, fertilizer concentrations need not be adjusted based on PPF.

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M.J. McMahon, J.W. Kelly, D.R. Decoteau, R.E. Young, and R.K. Pollock

Abbreviations: B, blue; FR, far red; P, phytochrome; PPF, photosynthetic photon flux; R, red; UV, ultraviolet. South Carolina Experiment Station paper no. 3170. Support for this project was provided by South Carolina Agricultural Experiment Station

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A.M. Armitage, N.G. Seager, I.J. Warrington, D.H. Greer, and J. Reyngoud

Incremental increases in temperature from 14 to 22 to 30C resulted in linear increases in stem length and node number and decreases in stem diameter and stem strength of Oxypetalum caeruleum (D. Don.) Decne. Higher temperatures also resulted in additional flower abortion, reduced time to flowering, and fewer flowering stems per inflorescence. Reduction in the photosynthetic photon flux (PPF) from 695 to 315 μmol·s-1·m-2 had similar effects as increasing the temperature on vegetative characteristics, but had little effect on reproductive ones. The rate of stem elongation was greatest at low PPF for all temperatures and at high temperature for all PPF treatments. Net photosynthesis rose between 14 and 22C and declined at 30C for all PPF treatments. Long photoperiods (12 or 14 hours) resulted in longer internodes, longer stems, and more flowers per cyme than short photoperiods (8 or 10 hours), but photoperiod had little effect on flowering time. Treatments to reduce latex coagulant and silver thiosulfate treatments had no significant effect on vase life.

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Anthony W. Whiley, Christopher Searle, Bruce Schaffer, and B. Nigel Wolstenholme

Leaf gas exchange of avocado (Persea americana Mill.) and mango (Mangifera indica L.) trees in containers and in an orchard (field-grown trees) was measured over a range of photosynthetic photon fluxes (PPF) and ambient CO2 concentrations (Ca). Net CO2 assimilation (A) and intercellular partial pressure of CO2 (Ci) were determined for all trees in early autumn (noncold-stressed leaves) when minimum daily temperatures were ≥14 °C, and for field-grown trees in winter (cold-stressed leaves) when minimum daily temperatures were ≤10 °C. Cold-stressed trees of both species had lower maximum CO2 assimilation rates (Amax), light saturation points (QA), CO2 saturation points (CaSAT) and quantum yields than leaves of noncold-stressed, field-grown trees. The ratio of variable to maximum fluorescence (Fv/Fm) was ≈50% lower for leaves of cold-stressed, field-grown trees than for leaves of nonstressed, field-grown trees, indicating chill-induced photoinhibition of leaves had occurred in winter. The data indicate that chill-induced photoinhibition of A and/or sink limitations caused by root restriction in container-grown trees can limit carbon assimilation in avocado and mango trees.