Chamber Method for Estimating the Primary Productivity : of Rice Plant Population

Abstract

When the dry matter productivity estimated by the chamber method (ΔW^") was compared with that observed by the biomass method (ΔW), it was experienced that the former sometimes gave lower estimation than the latter. To clarify the reason of their disagreement in terms of the ventilation rate to the chamber, the continuous measurements of photosynthesis and respiration from 6 to 10 days by using big chambers (1.3 m in width, 10 m in length and 1.5 m in height) were conducted. The amount of air supply to the chambers were 19.1 m³/min, 75.6 m³/min, and 122 m³/min, respectively. Results are summarized as follows. 1. By comparing the environmental factors inside the chamber with those under natural conditions, it was found that the difference increased with solar radiation. The difference decreased as air supply increased and when the ventilation rate exceeded 156 times/hr, environmental factors inside the chamber almost coincided with the outside one. 2. When the air volume supplied to the chamber was small, the light-photosynthesis curve showed a light saturation form, but the photosynthetic rate under strong light intensity increased with air volume supplied, and shape of the light curve became more linear. 3. CGR obtained by the biomass method (ΔW) and CGR estimated by the chamber method (ΔW^") agreed well at a certain amount of air supply to the chamber, and it was 75.6 m³/min (156 times/hr) in this experiment. But, when the amount of air supply to the chamber was too small or too large, ΔW^" was often under estimated. 4. Using the mathematical model, we tried to find the causes to decrease the photosynthesis in the chamber under the small amount of air supply. The experiment shows that main cause was high temperature inside the chamber and thc decrease of CO₂ concentration also affectcd it a little. The effect of wind speed, which changes the boundary layer resistance to CO₂ transfer of a leaf, on photosynthesis was found not so much. 5. The photosynthetic rate observed in the afternoon was somewhat smaller than that in the morning. As the daliy total radiation became higher, not only photosynthesis during the daytime (P_net), but also the respiratory loss during the succeeding night (R_N) increased. 6. R_N/P<net> ratios were 16.7 and 14.4% in botting and heading stages. These values were obtained when ΔW^" was almost coincided with ΔW, and they were considerably smaller than the Values ever estimated