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Marc W. van Iersel and David Gianino

greenhouses. There appears to be little past research on optimizing the economic return of supplemental lighting methods for greenhouse production. Heuvelink and Challa (1989) used a crop photosynthesis model to predict assimilate production, assimilate

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Asmita Paudel and Youping Sun

solute balance, nutrient uptake, water relations, and photosynthesis ( Grattan and Grieve, 1999 ). Salinity can inhibit plant growth in two phases. First, water or osmotic stress leads to a rapid growth reduction because of salts present in the soil. The

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Daniel Warnock, William Randle, and Mark Rieger

Photosynthesis is the very essence of agriculture. Previous photosynthetic and transpirational studies of onion (Allium cepa) have been limited to specific developmental stages. Our study measured photosynthesis and transpiration in sixteen plants of a single short-day cultivar over an eleven week period containing both non- and bulb inductive photoperiods. Differences in weekly means for photosynthesis, leaf conductance, water use efficiency, and intercellular CO, were highly significant. Weekly photosynthetic means increased under a non-inductive photoperiod and peaked one week after initiating a bulb inducing photoperiod. A decrease and leveling period occurred as bulbs developed followed by a decrease as foliage lodged. Weekly photosynthetic and leaf conductance means were correlated and highly significant. Water use efficiency and intercellular CO, means remained fairly constant throughout the study suggesting that photosynthesis in unstressed onions was controlled by internal mechanisms instead of stomata.

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Dariusz Swietlik, R. F. Korcak, and Faust Miklos

Abstract

The effect of polyethylene glycol (PEG)-induced water stress on stomatal and nonstomatal inhibition of photosynthesis of apple seedlings (Malus domestica Borkh.) grown in solution culture was investigated. Water stress was applied gradually by modifying the nutrient solution water potential daily to a minimum of -8.0 bar. Nutrient solution of less than or equal to water potentials -6 bar decreased net and gross photosynthesis rates. Stomatal and nonstomatal factors were responsible for photosynthetic inhibition. Nonstomatal inhibition of photosynthesis appears to be due to decreased capacity for CO2 fixation and not increased photorespiration. The ratio of gross to net photosynthesis was not affected. A higher level of water stress was required to affect mesophyll resistance than stomatal resistance and/or there was a lag time for mesophyll resistance to respond to stress.

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D. D. Mathur and A. S. Bhagsari

Abstract

The influence of photosynthetically active radiation (PAR) and temperature on net photosynthesis and dark respiration was determined on leatherleaf fern [Rumohra adiantiformis (G. Forst) Ching] grown under 73% polypropylene shade. The maximum photosynthesis rate of 7.5 mg CO2 dm-2 hr-1 occurred at 500 μE m-2s-1 PAR. The light compensation point was 44 μE m-2s-1 PAR. A temperature of 20°C and 1000 μE m-2s-1 PAR was most favorable for net photosynthesis of leatherleaf fern. Use of the antitranspirant, a paraffin wax emulsion (Mobilcer A), at a 2% concentration reduced dark respiration by more than 50% and reduced net photosynthesis considerably compared to the control at 40°.

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David P. Miller, G. Stanley Howell, and James A. Flore

85 ORAL SESSION 24 (Abstr. 191–198) Photosynthesis/Vegetables & Fruit

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T.R. Willard, C.M. Peacock, and D.G. Shilling

The effects of sethoxydim, cloproxydim, and fluazifop on photosynthesis and growth of St. Augustinegrass [Stenotaphrum secundatum (Walt.) Kuntze `Floralawn'], bahiagrass (Paspalum notatum var. saurae Parodi `Pensacola'), and centipedegrass [Eremochloa ophiuroides (Munro.) Hack.] were evaluated to determine if photosynthesis could be used as a rapid, nondestructive measure of relative susceptibility. Field and greenhouse studies were conducted using infrared CO2 analysis to estimate photosynthesis. Under field conditions, St. Augustinegrass was susceptible to sethoxydim and fluazifop applications, as indicated by a 40% and 38% reduction in apparent photosynthesis, respectively. Bahiagrass incurred a respective 62% and 51% reduction in apparent photosynthesis from sethoxydim and fluazifop application. Growth of these species, as measured by foliage dry weight, was also inhibited by both herbicides. Centipedegrass growth was unaffected by sethoxydim, but was reduced 48% by fluazifop. Under greenhouse conditions, centipedegrass apparent photosynthesis was reduced by sethoxydim and cloproxydim (41% and 51%, respectively), while fluazifop caused a 71% reduction. Growth of centipedegrass was significantly reduced only by fluazifop (83%). These studies indicated that in vivo photosynthetic measurements may provide a sensitive, rapid, and nondestructive method for determining the susceptibility of turfgrasses to postemergence grass herbicides. chemical names used: 2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio) propyl]-3-hydroxy-2-cyclohexen-l-one (sethoxydim); (E,E) -2-[1-[[(3-chloro-2-propenyl) oxy]imino]butyl] -5-[2-(ethylthio) propyl]-3-hydroxy-2-cyclohexen-l-one (cloproxydim); and butyl ester of 2-[4-[[5-(trifluoromethyl)-2-pyridinyl]oxy]phenoxy]-propanoate(fluazifop).

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Jirong Jiao and Bernard Grodzinski

Photosynthesis and concurrent export rates of expanded leaves on the flowering shoot of Rosa hybrida L. `Samantha' were measured at three stages of shoot and flower bud development. At 35 and 90 Pa CO2 photosynthesis and concurrent export rates of the upper expanded leaves were greater at Stage 3 (i.e., when petal color of the flower bud was visible) than at the two earlier stages of shoot and flower development. The optimum for leaf photosynthesis and concurrent export at ambient CO2 and saturating irradiance were ≈25 °C. Export was more sensitive to increased temperature than was carbon fixation. For example, at 40 °C photosynthesis was 40% lower while the export rate during photosynthesis was reduced by 80%. Increasing the photon fluence flux rate from 200 to 1000 μmol·m-2·s-1 PAR increased the photosynthetic rate and the concurrent export rate at 35 and 90 Pa CO2, but the increase in export was proportionally greater than that of photosynthesis. At 35 Pa CO2, the rate of C export during photosynthesis increased from 31 to 59% of the concurrent C fixation rate. At 90 Pa CO2, export during photosynthesis increased from 38 to 62% of the photosynthesis rate. The importance of irradiance on translocation processes was further demonstrated by comparing the disappearance of label during the feed period and during an extended night period. Plants grown at each CO2 level exported about three times as much of the 14C fixed during a 2-hour feed period in the light as during a subsequent 15-hour dark chase period. The nighttime export and respiration rates of leaves which had been exposed to elevated CO2 levels during the feed were higher than those rates observed at ambient CO2. However, at the end of the chase period, the leaves of plants which had been exposed to CO2 enrichment during the feed also retained more 14C than did the leaves of the plants which were at ambient CO2. Thus, although more 14C was fixed and exported under high CO2, the same proportion of labelled assimilates were exported, respired, and retained in the dark as at ambient CO2.

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Melyssa K. Davis and Jeff S. Kuehny

Herbaceous perennials are one of the fastest growing ornamental sectors in the United States. Current production recommendations do not address the effect of environmental factors, such as high temperature, on growth of herbaceous perennials. The focus of this research was to determine how supra-optimal temperatures effect growth and photosynthesis. Plants were exposed to a high temperature of 35 °C and photosynthesis measurements were recorded over a 6-week period at 1100, 1300, and 1500 hr. Results indicate that the time of day the measurements were taken made little difference on rate of photosynthesis and that there was a similar trend in photosynthetic rate over the 6-week period. Photosynthesis decreased as the plants began to flower and then increased until the onset of flower senescence. Plants grown at supraoptimal and optimal conditions had a similar trend and rate of photosynthesis throughout the 6-week period. Plant growth significantly decreased as the duration of high temperature increased for both species; however, Gaillardia was more heat tolerant then Coreopsis.

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Krishna S. Nemali and M.W. van Iersel

The effect of increasing daily light integral (DLI; 5.3, 9.5, 14.4, and 19.4 mol·m-2·d-1) on photosynthesis and respiration of wax begonia (Begonia semperflorens-cultorum Hort.) was examined by measuring CO2 exchange rates (CER) for a period of 25 d in a whole-plant gas exchange system. Although plant growth rate (GR, increase in dry weight per day) increased linearly with increasing DLI, plants grown at low DLI (5.3 or 9.5 mol·m-2·d-1) respired more carbohydrates than were fixed in photosynthesis during the early growth period (13 and 4 d, respectively), resulting in a negative daily carbon gain (DCG) and GR. Carbon use efficiency [CUE, the ratio of carbon incorporated into the plant to C fixed in gross photosynthesis (Pg)] of plants grown at low DLI was low, since these plants used most of the C fixed in Pg for maintenance respiration (Rm), leaving few, if any, C for growth and growth respiration (Rg). Maintenance respiration accounted for a smaller fraction of the total respiration with increasing DLI. In addition, the importance of Rm in the carbon balance of the plants decreased over time, resulting in an increase in CUE. At harvest, crop dry weight (DWCROP) increased linearly with increasing DLI, due to the increased photosynthesis and CUE at high PPF.