GreenLab Course
Production-Expansion
Light interception (reminder).
Light interception.
-
PAR
-
Leaves intercept the light.
Chlorophyll, the most abundant plant pigment, is most efficient in capturing red and blue light. Photosynthetically active radiation, often abbreviated PAR, designates the spectral range of solar radiation (from 400 to 700 nanometres) that photosynthetic organisms are able to use in the photosynthesis process.
This spectral region corresponds more or less to the range of light visible to the human eye.
LAI
-
The
LAI or Leaf Area Index is the ratio of total upper leaf surface of vegetation divided by the
surface area of the land on which the vegetation grows. LAI is a dimensionless value, typically ranging
from 0 for bare ground to 9 for a dense forest.
Beer-Lambert Law
-
Detailed numerical simulation of radiation absorption has shown that the following approximation
is excellent, and never deviates more than 1 or 2% from a detailed simulation with sunlit and shaded leaves:
IabsLAI = I0 (1-e-k*LAI)
In this formula, called the Beer-Lambert Law, k stands for the extinction coefficient, IabsLAI and I0 for the radiation level at canopy depth LAI (expressed in overlying LAI) and at depth 0 above the canopy, respectively.
Typical values for k are in the range of 0.5 to 0.9
For a LAI above a value of 3, almost 90% of PAR is absorbed by the canopy. White cover on the soil surface may reflect 50 to 80% of PAR and increase crop photosynthesis by at least 7% for a LAI of 3 (as long as CO2 is not a limiting factor) (Gijzen, 1995a).
Influence of the Leaf Area Index on the fraction of light intercepted by a tomato crop (Graph E. Heuveulink, WAGENINGEN UNIVERSITY)
- In this example, the extinction coefficient k is experimentally found from three sets
of data respectively, represented by the green and pink symbols.
Definition
PAR (Photosynthetically Active Radiation)
Physiology (abrev.). PAR defines the wave band of solar radiation that plants are able to use in the process of photosynthesis. In agronomy, PAR is usually expressed in energy units as an irradiance per area. Unit: W.m-2.Definition
LAI (Leaf Area Index)
Physiology. (abrev.). LAI is a dimensionless quantity that characterizes plant canopies, defined as the one-sided green leaf area per unit ground surface.Bibliography
Gijzen, H. 1995. CO2 uptake by the crop. Crop growth and development. In: J.C. Bakker, G. P. A. Bot, H. Challa, and Van de Braak, N.J. (eds), Greenhouse climate control-an integrated approach, Wageningen Pers, The Netherlands. pp. 16-35