Effects of elevated CO₂ and nitrogen application on photosynthetic area and gain-leaf ratio of winter wheat

Either CO₂ enrichment or nitrogen (N) application influences growth and development of wheat (Triticum aestivum L.). The effects of elevated CO₂ concentration and nitrogen addition on the photosynthetic area of source organs (green leaves, stem & sheath, spike and awns), grain number and weight per spike, and grain-leaf ratio of winter wheat at heading stage were investigated in this study. The pot experiment was carried out in open top chambers (OTCs) with two wheat varieties, "Xiaoyan 6" and "Xiaoyan 22", as materials in 2008-2009. Wheat plants were grown under different treatments of two CO₂ concentrations (375 μL·L^-1 and 750 μL·L^-1) and three nitrogen application levels 0, 0.15 g(N)·kg^-1(soil) and 0.30 g(N)·kg^-1(soil). The results showed that nitrogen application significantly increased leaf length and width, stem node length and diameter, spike length and width, awn length. The photosynthetic area, grain number and weight per spike, and grain-leaf ratio of wheat were also enhanced under nitrogen application. Compared with ambient CO₂ concentration (375 μL·L ^-1), elevated CO₂ concentration (750 μL·L^-1) positively influenced leaf and stem node length, photosynthetic area of leaf, stem node and awn (P < 0.05); but it insignificantly affected leaf width, stem node diameter and spike area (P> 0.05). CO₂ concentration elevation increased the photosynthetic area per stem of "Xiaoyan 6" and "Xiaoyan 22" by 8.1%~15.1% and 2.8%~13.2%, respectively, with the biggest variable amplitude under the nitrogen application level of 0.30 g(N)·kg^-1(soil). Under three nitrogen levels, CO₂ enrichment increased spike grain number and the ratio of grain number-leaf area in varying degrees. The grain number-leaf area ratio of two varieties increased most obviously under 0.30 g(N)·kg ^-1(soil) and 0.15 g(N)·kg^-1(soil), with 44.2% and 41.4% increments compared with the ambient CO₂ concentration. Moreover, high CO₂ concentration reduced spike grain weight and the ratio of grain weight-leaf area under no nitrogen application, but raised them by 43.6% and 20.7% averagely under nitrogen application. The awn area was only 3% of the total photosynthetic area, greatly smaller than those of other resources organs. It suggested that CO₂ enrichment enlarge the photosynthetic area of wheat mainly through elongating leaves and stem. It improved the relationship between sources and sink to increase sink number and mater translocation captivity per unit leaf area simultaneously. Nitrogen addition would promote the response of wheat source and sink growth to CO₂ enrichment.