2.2. Light interception
Photosynthetically active radiation interception was measured
with a 0.8-m long ceptometer (Decagon Devices Inc., Pullman,
Washington) between 11:30 and 14:00 h on clear days. Six
measurements were taken in each replicate. The first and last measurements
were taken above the canopy to determine incident PAR
(Io). The other four measurements were taken at soil surface level
placing the sensor below the canopy and moving it parallel to rows
at regular intervals in order to determine transmitted PAR (It) as
indicated by Charles-Edwards and Lawn (1984). The fraction of PAR
intercepted at midday (F) was calculated as:
where Io and It are themeans of themeasurements above and below
the canopy.
Daily fraction of PAR intercepted (Fd) was calculated from midday
values, as proposed by Charles-Edwards and Lawn (1984) as: