Studies on the Effects of Oxygen Concentration on the Photosynthesis and the Growth of Crop Plants : II. Relationship between Photo respiration and Expansion of Leaf Area

Abstract

In this study, three experiments were conducted to make clear whether or not the expansion of leaf area is closely related to photorespiration in C3-plant, and the effect of oxygen concentration on the growth of C4-plant. Materials used were two row barley (Hordeum vulgare) in C3-plant, and vasey gass (Paspalum urvillei) and sorghum (Sorghum vulgare) in C4-plant. Exp. I. Seedlings of vasey grass and sorghum were grown for 5 days in specially devised growth chamber which is equipped with water culture facilities, under 2.5% O2-0.03% CO2 and 210% O2-0.03% CO2, and light intensity of 45 Klx at 25°C leaf temperature under a regime of 10 hr light and 14 hr darkness. Exp. II. Seedlings of two row barley were cultured under 2.5, 5, 10 and 21% O2(control) at 20°C leaf temperature. And other experimental procedures were followed to Exp. I. Exp. III. Seedlings of two row barley were grown under 21% O2 in light and 2.5% O2 in darkness (treatment A), and the reverse condition (treatment B). The experimental results obtained are summarized as follows: 1. No major differences in dry rnatter production (ΔW and RGR) and NAR were found between the plants grown under 2.5% O2 and 21% O2 in both vasey grass and sorghum. These results corresponded roughly to the phenomenon that C4-plant lacks an apparent enhancement of CO2 uptake by low oxygen concentration below normal air. Expansion of leaf area (ΔLeaf area, RLGR), however, decreased at low oxygen concentration in both species, especially in sorghum, as well as C3-plants. 2. In Exp. II, the greatest NAR of two row barley was found at 2.5% O2, but NAR at 10% O2 was still greater than one at 21% O2. It is suggested that the photorespiration is tolerably suppressed even at 10% O2. On the other hand, RLGR rand ΔLeaf area decreased at 2.5% O2 and 5% O2, but at 10% O2 they did not decrease as compared with control. It call be deduced that at O2 concentration below 10%, expansion of leaf area is influenced and above 10%, it is independent to oxygen concentration. 3. In Exp. III, ΔW and RGR increased extemely in treatment B as compared with treatment A. On the other hand, no major differences in ΔLeaf area and RLGR were found between both treatments. 4. As for the question of whether or not expansion of leaf area is closely related to photorespiration in C3-plant, the answer may be given as follows. In wheat, rice plant (preceeding report5)) and two row barley (Exp. II.), suppression of photorespiration and expansion of leaf area occurred together under low concentration (2.5% O2). Whereas, even in C4-plants possessing no apparent photorespiration, expansion of leaf area was reduced under low oxygen concentration (2.5% O2). Furthermore, in two row barley in Exp. II, photorespiration was considerably suppressed even at 10% O2, but expansion of leaf area did not decrease at this concentration. In addition to these results, as seen in Exp. III, there were no major differences in the effect of low oxygen concentration on expansion of leaf area under between light and darkness. From the results mentioned above, it can be concluded that there exists no direct causal-relationship between photorespiration and expansion of leaf area in C3-plant.