Studies on Matter Production in Wheat Plant

III. Changes with growth in photosynthetic capacity and respiratory capacity of wheat stand

Abstract

Using the data on diurnal changes of CO₂ exchange and environmental factors obtained at each stage of growth in two experiments of 1969-70 and 1971-72, the authors investigated the developmental changes in photosynthetic capacity and respiratory capacity of wheat stand and their related factors. The results obtained are summarized as follows:
1. In both experiments, photosynthetic capacity of wheat stand (rate of net CO₂ uptake per unit land area at the radiation level of 0.6 ly min^-1 and at comparatively favourable temperatures) gradually increased in winter and then rapidly attained the maximum value (ca. 5 g CO₂ m^-2 hr^-1) at flowering, and thereafter decreased rapidly (Fig.2).
2. The time trend in photosynthetic capacity was closely related with that of LAI throughout the whole growth period in the two experiments (Fig.2).
3. From the investigation of LAI-photosynthetic capacity curve, which was drawn from the data obtained at various times of growth, it was shown that photo synthetic capacity increased with increasing LAI in its range up to 4-5. Above this range of LAI, photosynthetic capacity did not seem to increase with increasing LAI (Fig.3).
4. According to two comparisons, namely, one between photosynthetic capacity of stand before and after heading, and the other between apparent photosynthetic rate of stand before and after ear removal, wheat ears contributed to the photosynthetic capacity of stand by 10-20% (Fig.3, 4).
5. The respiratory capacity of stand (rate of CO₂ release per unit land area at 10°Ctemperature) increased with growth and attained the maximum value (ca. 0.4 g CO₂ m^-2 hr^-1, 1969-70; ca. 0.3 g CO₂ m^-2 hr^-1, 1971-72) at flowering or a little earlier, and thereafter decreased rapidly (Fig.2).
6. The respiratory activity of plant (rate of CO₂ release per unit plant dry weight under 10°C temperature) remained relatively low and constant during winter. Then, the activity began to increase at the middle of February and attained the maximum value at the beginning or middle of March when the stand just started to grow vigorously, decreasing thereafter with the progress of growth (Fig.5).
7. In the former half of the growth period, the change with growth in respiratory capacity of stand was mainly attributed to that in total plant dry weight per unit land area, and in the latter half, to that in respiratory activity of plant (Fig.2, 5).