A field experiment was conducted in 2005 at the Environmental Science Field Laboratory (24g 27" N, 90Q 50" E, elevation 18 m), Bangladesh Agricultural University, Mymensingh. The soil belongs to Sonatola series of non-calcareous silty loam texture. The rice cultivar "BR11" was transplanted on 10 July, 2005 with three seedlings maintaining 15cm x 15cm spacing in E-W orientation in 400 m2 field.Field experiment was conducted in 2005 at the Environmental Science Field Laboratory (24g 27" N, 90Q 50" E, elevation 18 m), Bangladesh Agricultural University, Mymensingh. The soil belongs to Sonatola series of non-calcareous silty loam texture. The rice cultivar "BR11" was transplanted on 10 July, 2005 with three seedlings maintaining 15cm x 15cm spacing in E-W orientation in 400 m2 field.
Plant samples were taken randomly from one square meter area at 10 day intervals beginning from five weeks after transplanting. Then samples were separated into leaves, shoot and root for growth analysis. The leaf area was measured using a automatic leaf area meter (LI-COR Squarecentimeters, LI 3100 area meter, USA) then leaf area index was calculated. The leaf area index (LAI) was determined as leaf area/ unit land area. Dry weight of each plant part was also measured by oven drier at temperature of 80 0C for 48 hrs. From five weeks after transplanting, two photosynthetically active radiation (PAR) sensors (LI-190023553) and (S91023.117) was placed on the tower at a height of 170 cm above the soil surface to measure the incident PAR flux density (c1), measured above the canopy) and reflected PAR (c1), measured above the canopy). Transmitted PAR (TpAR) was measured by 1m long line quantum sensor (L0A1692, LICOR, USA), which was positioned below the canopy at the soil surface. All the sensors were carefully leveled and calibrated at the beginning and end of the growing season.
In addition, intercepted PAR (PAR,) was calculated at weekly intervals using following equation as suggested by Kemanian et al (2004):
IPAR = PAR [1 — exp (-k x LAI)] (1)
Where, !PAR= Intercepted Photosynthetically Active Radiation,
PAR = Incident Photosynthetically Active Radiation, k = Extinction Coefficient and LAI= Leaf Area Index
Fractional intercepted and reflected photosynthetically active radiations were determined by using the following equation:
flPAR = IPAR/PAR, (2)
fPARR = PARR/PAR° (3)
The Extinction Coefficient of intercepted radiation at different height of canopy level was measured by Bouguer- Lambert Law as proposed by Monsi and Saeki (1953)-
-In (PARO/PAR) I = k LAI (4)
Where, PAR°= Incident PAR PAR!= Intercepted PAR
Intercepted light was measured by using the following equation:
PAR, = PAR0_ PART - PARR (5)
Finally, crop Radiation Use Efficiency (RUE) was obtained as the slope of the linear regression between cumulative total biomass and cumulative intercepted radiation in gMJ-1