Light–nitrogen relationships within reproductive wheat canopy are modulated by plant modular organization

Within dense canopies, the spatial distribution of nitrogen between leaves in relation to the local light environment is an important and widely investigated adaptive response of plant carbon and nitrogen economy. However a general picture of how nitrogen distribution in photosynthetic tissues, not only in leaf laminae, is structured at plant scale and is affected by the topology and light environment of plant modules (i.e. lamina, sheath, internode) is missing. We investigated the spatial patterns of nitrogen distribution in relation with plant botanical structure for wheat (Triticum aestivum L.) culms. Nitrogen distribution between and within laminae, sheaths and internodes was quantified at anthesis and during grain filling, for two cultivars with contrasted leaf posture grown in the field under low and high N fertilization. We found that independently of leaf posture, specific nitrogen mass (i.e. nitrogen mass per unit surface area) was homogeneous within individual laminae and sheaths, although they spanned a significant depth in the canopy. Sharp changes in nitrogen specific mass at module boundaries were observed. At the canopy level, vertical nitrogen gradients resulted from a decrease of mean specific nitrogen mass of individual plant modules with their position along the culm, and laminae and sheaths specific nitrogen mass decreased linearly with the distance from the top of the canopy. There was no significant gradient of N concentration on a dry mass basis within and between the enclosed internodes, only the distal part of the ear peduncle, which was exposed to the light, showed a strong N gradient. This study gives important information to better understand the phenotypic plasticity of nitrogen distribution in wheat and to build a process-based model of N distribution within wheat culms during the post-anthesis period. It strongly supports the idea that processes should be formalized at the module scale and that a similar formalization can be used for individual laminae and sheaths.

► N distribution between and within wheat laminae, sheaths and internodes was quantified at anthesis and mid grain-filling. ► Specific N mass of individual laminae and sheaths decreased with lower position along the stem ► Specific N mass was homogeneous within individual laminae and sheaths, although they spanned a significant depth in the canopy. ► There was no significant gradient of N concentration on a dry mass basis within and between the enclosed internodes. ► This study gives important information to build a process-based model of N distribution within reproductive wheat canopy.