Ecological Studies on the Photosynthesis of Winter Cereals : I. Diurnal changes in the photosynthesis of two-rowed barley under field condiditions in winter

Abstract

Diurnal changes in apparent photosynthesis of two-rowed barley (Hordium distichum c.v. Azuma-goruden) were measured with assimilation chambers under field conditions during the period from December 1969 to February 1970. The assimilation chamber used was 25×25 cm in area and 20 cm high. Sampling of the air inside the chamber was made hourly (exp.B) or at intervals of half an hour (exp.A and C) for five minutes using vinyl ball to measure CO₂ concentration of the air. Three types of experiments were designed as indicated below: Experiment A: The assimilation chamber was placed over the plot from the nightfall before the day of each experiment and kept until the end of the experiment. Experiment B: The chamber was placed over the plot ten minutes before each sampling of air and then removed soon after the sampling, for the purpose of putting the crop under national conditions as long as possible. Experiment C: The chamber was placed over the plot ten minutes before the first sampling of the air, but kept throughout the day until the end of the experiment. Four plots were used as replicates to measure the rates of photosynthesis in most of the experiments, but two plots in some cases. Soil temperature was measured only in the experiment B. 1. Results of the experiments A, B and C showed that apparent photosynthetic rates in the morning of clear winter days were mostly lower than those in the afternoon at equivalent solar radiation level. In the experiment B, on the other hand, there were some occasions in which photosynthesis was inhibited throughout the whole day. 2. Two assumptions were made, based on the results of the experiment B, about the relation between the inhibition of apparent photosynthesis and temperature regime. (1) In the morning when air temperature remains below O°C long time, soil layer surrounding the main root zone may be frozen. When the freezing of soil is prolonged in the daytime, the inhibition of photosynthesis may be large and continue until evening. (2) 0n the day when the minimum air temperature is below O°C but the soil freezing does not continue long time, the inhibition of photosynthesis may occur only in the morning. 3. It was found in the present experiments that the air temperature inside the chamber was higher than the ambient air temperature, especially at midday with difference of from 5°C to 8°C among the temperature inside and outside the chamber. This suggests the necessity of improving on the assimilation chamber