CO2 response in nitrogen concentration of seeds, embryo and albumen in rice cultivars grown at free-air CO2 enrichment (FACE)

Seed quality is a key characteristic for plant life for their fitness and also important for human and animal as a food source. Previous studies have shown that seed nitrogen concentration ([N]) decreases at elevated [CO₂] in grass species such as rice and wheat whereas legumes and other dicot species exhibit less or no decrease in seed [N]. We hypothesized that the difference in CO₂ response of seed [N] came from the difference in seed structure; seed of grass species consists of embryo and albumen whereas seeds of most of the dicot species consist only of the embryo, and therefore, CO₂ response may be different between [N] of albumen and embryo. Here, we examined [N] in seed, albumen and embryo in eight cultivars of rice grown at free-air CO₂ enrichment (FACE). There was a significant variation in CO₂ response of seed [N] among rice cultivars. The reduction of seed [N] by elevated [CO₂] was significantly correlated to the decrease in the albumen [N], but not with the embryo [N]. As expected, embryo [N] was less responsive to elevated [CO₂]. It is concluded that presence of albumen and CO₂ response in albumen [N] may be a key for understanding variation in seed [N] response to elevated [CO₂] among species and among cultivars. 

Key Words: elevated [CO₂], seed, nitrogen, albumen, embryo, rice.

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Nitrogen (N) allocation in Stands of High-Yielding Rice Cultivars Takanari at Free-Air CO2 Enrichment (FACE)

Productivity and its responses to elevated [CO₂] concentration often differ even among genotypes of a species. Recent study has been identified that an indica rice variety (Oryza sativa L. cv. Takanari) has a higher productivity than other cultivar, especially at elevated [CO₂], because of greater photosynthetic capacity in canopy leaves, which is achieved by a higher leaf N per area. In the present study, we addressed a question, why does Takanari consist of higher leaf N per area in canopy leaves? Higher leaf N per area in canopy leaves is potentially achieved by (1) a higher N in plant, (2) a greater proportion nitrogen leaves relative to nitrogen plant, (3) a greater leaf N per area in the upper leaf and (4) a greater canopy leaf area / leaf area index. Takanari and Koshihikari (a normal cultivar) were grown in Free Air CO₂ enrichment (FACE) facilities with 200 ppm above the ambient CO₂ and it's compared. The experiment was arranged in completed randomized design with four replicates each growth condition. Sampling of the whole plant and stratified clipping methods were applied and measured in mass, leaf area and N content. The superiority of Takanari in N plant appeared under both ambient and FACE growth, contributing to high N and photosynthetic capacity in canopy leaves. However, Takanari allocated less N to leaves at an early stage. N Top leaf per area also showed smaller in Takanari during early stage, but larger at later stages under ambient and FACE. The effect of upper leaves on canopy photosynthesis in a high-yielding rice cultivar Takanari was shown in the vertical gradient of N profiles between cultivars. Clearly, The N coefficient of canopy leaves under both ambient and FACE growth was higher in Takanari through out the growth periods. The gradient of N profiles relative to the light gradient in both cultivars, with Takanari was steeper than Koshihikari in ambient and FACE growth. The other side of Takanari had a greater canopy leaf / leaf area index under ambient and FACE growth condition. Cultivars response to FACE of N plant and N leaf per ground area was higher in Takanari at latest stage, regardless leaf nitrogen fraction and top leaves nitrogen in response to FACE was lower in Takanari during early growth. There was interaction growth CO₂ and cultivars in the top leaves N. We conclude that Takanari achieved higher canopy photosynthetic capacity with large investment of nitrogen in the upper leaves, along with light attenuation. These results suggest optimal nitrogen distribution more influenced by light extinction than is nitrogen plant or leaf area expansion. Further improvement in understanding how optimal nitrogen distribution achieve with the model of canopy photosynthesis in order to take full advantage to maximize grain output under current and future high [CO₂] environment.

Keywords: nitrogen uptake, canopy leaves, Takanari, free-air CO₂ enrichment (FACE), nitrogen distribution