Light photosynthetically active radiation

High light intensity may have decreased the concentration of secondary metabolites if the plants were stressed by the high irradiance (see the environmental stress theory, Section IV.D). The plants grown at 100% of surface irradiance probably received high doses of ultraviolet radiation as well as photosynthetically active radiation. Ultraviolet radiation can stress Dictyota ciliolata, leading to decreased concentrations of secondary metabolites.183... 

Zangerl, A.R. and Berenbaum, M.R., Furanocoumarins in wild parsnip effects of photosynthetically active radiation, ultraviolet light, and nutrients, Ecology, 68, 516, 1987. 

Since 0.47 MJ of solar energy is trapped as chemical energy in this process, the maximum efficiency for total white light absorption is 28.1%. Further adjustments are usually made to account for the percentages of photosyntheti-cally active radiation in white light that can actually be absorbed, and respiration. The fraction of photosynthetically active radiation in solar radiation that reaches the earth is estimated to be about 43%. The fraction of the incident light absorbed is a function of many factors such as leaf size, canopy shape, and reflectance of the plant it is estimated to have an upper limit of 80%. This effectively corresponds to the utilization of 8 photons out of every 10 in the active incident radiation. The third factor results from biomass respiration. A portion of the stored energy is used by the plant, the amount of which... 

Light quality Light intensity Warm white light 80 fiE m 2 photosynthetically active radiation s 1 (850 to 1000 fc)... 

Figure 2. The ratio of UV-A (380 nm) to photosynthetically active radiation (PAR, 400-700 nm) in terms of W m. Data were collected during the summer of 2001 with a UV radiometer (model Biospherical GUV-521) located at the Lacawac Sanctuary in the Pocono Mts., PA, USA (41.23 N, 75.21 W). Sunrise (5.27 h) and sunset (20.37 h) for 17 June 2001 are denoted by the vertical dashed lines. Sunrise and sunset on 8 July 2001 occurred at 5.36 h and 20.37 h, respectively. During crepuscular periods, the UVrPAR ratio is higher because the light field is mostly composed of skylight (see Section 14.1). As the sun s elevation increases, the amount of PAR increases and the light field in dominated by solar radiation. Note that a similar increase in UV-A-to-PAR occurs when patches of clouds pass over the sun. This is shown between 17.0-19.0 h on 8 July 2(X)1.

A xenon arc (ORC 10OOW lamp) was used from which infrared and UV-C radiation were filtered out. To remove UV radiation (0.035 mW cm 2) a polycarbonate screen was placed in the light path leaving only photosynthetically active radiation (PAR)(160 pE m 2 s ). 

The quantum yield is expressed as the number of moles of O2 evolved (or CO2 fixed) per mole absorbed quanta of photosynthetically active radiation (400-700 nm). Obviously the quantum yield is constrained by biophysical/biochemical processes. Healthy leaves of C3 plants show almost identical quantum yields for O2 evolution at a saturating partial pressure of C02f irrespective of taxa and habitats of the materials, which approach the theoretical maximum of around 0.11 mol O2 mol absorbed quanta (2,3). Compared with this, the quantum yield is considerably lower at the CO2 partial pressure of around 350 bar due to photorespiration. Nevertheless, variation in the quantum yield among various C3 plants remains small also at this CO2 partial pressure. Thus, initial slopes of the light response curves are not markedly different among "healthy C3 plants. 

The data presented in this report are from the mature leaves of a single stem measured hourly from sunrise to sunset. Correlation analyses between A and environmental variables showed the highest correlation coefficient between A and PPFD (R = 0.88). In the field, light saturation of assimilation was not apparent (Fig. 1). By integrating the areas under the diurnal time course plots of A and PPFD, the total C assimilated and total photosynthetically active radiation (PAR) intercepted by each leaf was calculated. Total C assimilated by a leaf over the course of a day was linearly related to the total PAR intercepted (Fig. 2). 

Bertram, L. and Lercari, B., Kinetics of stem elongation in light grown tomato plants. Responses to different photosynthetically active radiation levels by wild type and aurea mutant tomato,... 

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