Studies on Matter Production in Wheat Plant : II. Carbon dioxide balance and efficiency of solar energy utilization in wheat stand

Abstract

Using the data on diurnal changes of environmental factors and CO2 exchange, which were obtained in the experiment reported in the first part of this series14), the authors investigated the balance of CO2 and the efficiency of solar energy utilization in wheat stand. The results obtained are summarized as follows: 1) Gross production per day increased markedly with the increase of solar radiation per day. Respiration per day also increased with solar radiation, but the amount of increase was far less than that of gross production. Consequently, net production increased much with the increase of solar radiation. progress of growth. The increase of gross production was very slow in winter and became rapid from the beginning of March. Gross production attained to the maximum value at the beginning of May (flowering stage) and maintained the high level for about two weeks and thereafter decreased. Respiration per day varied in a similar way to that of gross production though the decrease after the peak was slower than that of gross production. Net production varied in parallel with gross production though the former attained to the peak somewhat earlier than the latter. The amounts of gross production and net production on fine day during flowering stage were ca. 53 gCO2·m-1·day-1 (36 g dry matter·m-1·day-1) and ca. 38 gCO2·m-1·day-1 (26 g dry matter·m-1·day-1), respectively. 3) The ratio of respiration to gross production was very low (ca. 12%) on fine day in winter while it increased gradually with the progress of the season. From the beginning of ripening stage the ratio increased rapidly and reached to 40% at the latter stage of ripening. 4) The ratio of respiration to gross production was in average about 27% at the period of maximum LAI consisting of various weathers. Therefore, it seemed that the ratio was similar to or a little less than those of many other crops at the corresponding stage. 5) Efficiency of solar energy utilization for gross production (Eg; energy fixed by gross photosynthesis/solar energy incident on unit area of land) decreased remarkably with the increase of solar radiation. Efficiency of solar energy utilization for net production (En; energy contained in net production/solar energy incident on unit area of land) also decreased with the increase of solar radiation, although considerably slower than in the case of Eg. 6) Efficiencies of solar energy utilization both for gross and net productions were held at low and almost constant level from the end of January to the beginning of March. After that, those efficiencies began to increse and attained to the peak at the beginning of April. After maintaining the high level for a month or more, these efficiencies began to decrease. 7) In the above mentioned period when Eg and En were maximum, their values were about 3.9% and 2.9% in average under various weathers, respectively. Therefore, the authers concluded that wheat had medium efficiency in solar energy utilization among the various field crops.