Responses Of Physiological Characteristics Of Wheat/Maize Seedling To Elevated CO₂ Concentration

Effects of elevated atmospheric CO₂ on plants receive much attention at home and abroad. As the material of photosynthesis of plant, the growth and development of plants would be changed with the CO₂ concentration increased. By comparison, Chinese research start of this topic was much later. The present researches in this field mainly focus on morphological characters, photosynthetic physiology and the efficiency of nutrient elements. And the adaptability of result had its limitations for different crops and periods. Some of results had contradictory findings. So some studies should be carried out for verifying the reported conclusion. At present, the responses of roots secretion to elevated CO₂ concentration still lacked enough investgation. This experiment was carried out to study the responses of physiological characteristics of wheat/maize to elevated CO₂ concentration under different nitrogen nutrition levels.The seedlings of wheat and maize were planted in Open top chamber (OTCs) under two atmospheric CO₂ concentrations (375 and 750μmol·mol^-1) and 4 nitrogen nutrition levels (2.5, 5, 10 and 15 mmol·L^-1). The test aimed to investgate the effects of elevated CO₂ concentration on C3/C4 crops and organic carbon secretion with a view to further enrich the responses of crops to doubled CO₂ concentration. Also, it could provide certain theoretical and practical references for further crops cultivation under high CO₂ concentration. The main results showed as follows:1. Elevated CO₂ concentration had a significant influence on the characters and biomass of wheat and maize seedling. Compared with AT, enhanced CO₂ concentration would increase plant height, leaf area of single plant and biomass of root and shoot at different growth stage. While, the R/S ratio decreased with elevated CO₂ concentration. Increasing nitrogen level could improve plant height, leaf area of single plant, biomass of root and shoot and dry matter accumulation of whole plant in the ET treatment. With the increase of nitrogen level, the effect of elevated CO₂ on leaf area of single plant increased, and then decreased. In conclusion, elevated CO₂ concentration could promote the growth of root and shoot and dry matter accumulation of wheat/maize seedlings. With the improvement of nitrogen nutrition, these indexes could be further enhanced in ET treatment. The right nitrogen level could promote the root growth.2. Elevated CO₂ concentration had a significant influence on root growth and the organic excretion and the degrees of influence were relevant to the nitrogen level and the species of the crops. Enhanced CO₂ concentration increased the root length and area. Increasing nitrogen level could promote the root length and area in the ET treatment. Compared with maize, the degrees of influence on elevated CO₂ concentration of wheat was greater. The right amount of nitrogen level could improve the root growth, while the excessive amounts of nitrogen fertilizer would restrain the root growth. In conclusion, elevated CO₂ concentration could promote the organic carbon secretion of wheat/maize. The effect of wheat was obvious in low nitrogen treatments. In three-leaf and jointing stage, the effects of maize were both positive obviously in low nitrogen treatment. At jointing stage high nitrogen treatment, the effect of maize was negatively.3. Elevated CO₂ concentration had a significantly positive influence on chlorophyll a and chlorophyll b, especially in abundant nitrogen treatment. Enhanced CO₂ concentration could decrease the chlorophyll a/b of wheat seedlings, especially in low nitrogen treatment, while the responses of chlorophyll a/b of maize seedlings to elevated CO₂ concentration were not obvious. In conclusion, increasing nitrogen level could improve chlorophyll a/b in the ET treatment and maintain a balance between chlorophyll a and chlorophyll b.4. Compared with maize, the responses of Pn of wheat to elevated CO₂ concentration were greater. Increasing nitrogen level could improve Pn of wheat and maize in the ET treatment, meanwhile, the right amount of nitrogen level could improve the positive effect. Elevated CO₂ concentration had a significant negatively influence on Gs. Increasing nitrogen level could increase the Gs in high CO₂ concentration treatment.5. Elevated CO₂ concentration had a significantly negative influence on F0 of wheat/maize seedlings. Increasing nitrogen level could improve this negative influence. Enhanced CO₂ concentration had a positive influence on Fm,Fv,Fv/Fm and Fv/F0, especially in wheat treatment. Increasing nitrogen level could improve this positive influence. Elevated CO₂ concentration had a positive influence on NR activity of wheat/maize seedlings, especially, in wheat treatment. Increasing nitrogen level could further improve the NR activity and nitrogen assimilation. Enhancing nitrogen level could increase the content of soluble sugar and promote the response of crops to elevated CO₂ concentration.These results showed that elevated CO₂ concentration had a positive influence on the growth and development of crops and root organic carbon secretion. The effects of nitrogen level on the growth and development of crops and root organic carbon secretion were relevant to nitrogen level, the species of the crops and growth phases. All these results fully proved the basic assumptions that increasing nitrogen level could improve the growth of root and shoot, nitrogen assimilation and R/S ratio in the CO₂ enrichment,and reduce carbon loss of root for organic carbon excretion. They also proved that elevated CO₂ concentration had a significant effect on wheat seedlings, while, the effect on maize was no obvious, even no difference. But the effect of high CO₂ concentration, nitrogen level and interaction between them on crops still needs further study.