No abstract

High CO2 was applied to field‐grown winter wheat (Triticum aestivum L. cv. Arthur) during different growth periods in order to identify when photosynthesis was most limiting grain yield. Wheat was grown in open top Mylar® polyester chambers which were provided with circulated air (340 μL/L CO2 = control treatment) or air enriched with sufficient CO2 to expose the foliage to 1200 μL/L (high CO2 treatment). The CO2 enrichment treatments were applied from jointing to anthesis, from jointing to maturity, or from anthesis to maturity. Seed yield and total dry matter were influenced most by CO2 during the period from jointing to anthesis. The 17% increase in seed yield resulted from more seed per meter2, which was associated with an increase in heads per meter2. Total dry matter at maturity was increased by 11%. Neither harvest index nor nitrogen content of plant parts at harvest were altered by CO2 enrichment. In order to characterize the effect of CO2 on seed yield, several physiological parameters of leaf function were monitored using the flag leaves of plants grown in the control chambers and the high CO2 chambers treated from jointing to maturity. High CO2 had no significant effect on flag leaf chlorophyll and protein content or the onset of their decline during senescence. In addition, there was no effect on the level of ribulose bisphosphate carboxylase or its decline during senescence or on total proteolytic activity. High CO2 did, however, cause a 50% increase in apparent photosynthesis of the flag leaf. This resulted in a doubling of the sucrose and starch content of the flag leaf prior to seed growth. With the onset of seed growth this difference in sucrose and starch levels rapidly disappeared, suggesting a rapid mobilization to the head and developing seeds.

Soybeans [Glycine max (L.) Merr. ‘Kent’ and ‘Ware’], grown under field conditions, were exposed to a CO2‐enriched atmosphere (1232 μL L−1) from 22 days after planting to maturity. Seed yield increased 81% in Kent and 56% in Ware in response to CO2 enrichment. Plants grown at high CO2 concentrations maintained higher rates of apparent photosynthesis and lower leaf conductances throughout ontogeny than those grown at low CO2 concentrations. Carbon dioxide‐enriched plants accumulated larger pools of soluble sugars and starch in the leaves compared with control plants. Rapid depletion of the stored carbohydrates was evident at the time flower buds were developing and flowering commenced. Carbon dioxide‐enriched plants also maintained large pools of starch and sucrose in the leaves during the grain‐filling periods, suggesting that these were not utilized for seed development. In contrast, control plants were virtually devoid of sucrose and starch at plant maturity. Examination of leaf chlorophyll, protein and proteolytic enzyme activity revealed that CO2 enrichment did not delay senescence in soybeans.

No abstract

The N2‐fixing legume nodule has an O2 requirement for ATP production to support nitrogenase (ECI.7.99.2) activity, however, the internal partial pressure of O2 (pO2) in the nodule must be maintained at low level because of the O2 sensitivity of nitrogenase. Effects on N2 fixation of short‐term exposures of the rhizosphere to altered external pO2's using both excised nodulated soybean [Glycine max (L.) Merr.] roots and intact plants were investigated utilizing acetylene (C2‐H2) reduction. Exposures of 30 min to supra‐ambient pO2's of 0.26 to 0.41 atm stimulated N2[C2H2] fixation by excised roots, but not intact plants while short exposures to a sub‐ambient pO2 of 0.05 atm reduced N2[C2H2] fixation by excised roots more than by intact plants. Short exposures of 30 rain to a supra‐optimum pO2 of 0.65 to 0.90 atm reduced N2[C2H2] fixation in both excised roots and intact plants, and in contrast to exposure to other O2 concentrations, there was only partial recovery of N2[C2H2] fixation by the intact plants and a further decline by the excised roots on reincubation in ambient O2. With excised roots, altered pO2 effects were age‐dependent with a decline in the optimum pO2 from 0.41 to 0.26 atm and intensification of the detrimental effects of high pO2. It is concluded that; 1) the excised root is more sensitive to altered pO2 than intact plants, 2) measurements of the effects of stress, such as altered pO2, on N2 fixation should be made on intact plants, and 3) ambient pO2 is not limiting for N2 fixation but short‐term exposures to sub‐ambient pO2 will limit N2 fixation.