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 lightenvironment is an important and widely investigated adaptive response of plant carbon and nitrogeneconomy. However a general picture of how nitrogen distribution in photosynthetic tissues, not onlyin leaf laminae, is structured at plant scale and is affected by the topology and light environment ofplant modules (i.e. lamina, sheath, internode) is missing. We investigated the spatial patterns of nitrogendistribution in relation with plant botanical structure for wheat (Triticum aestivum L.) culms. Nitrogendistribution between and within laminae, sheaths and internodes was quantified at anthesis and duringgrain filling, for two cultivars with contrasted leaf posture grown in the field under low and high Nfertilization. We found that independently of leaf posture, specific nitrogen mass (i.e. nitrogen mass perunit surface area) was homogeneous within individual laminae and sheaths, although they spanned asignificant depth in the canopy. Sharp changes in nitrogen specific mass at module boundaries wereobserved. At the canopy level, vertical nitrogen gradients resulted from a decrease of mean specificnitrogen mass of individual plant modules with their position along the culm, and laminae and sheathsspecific nitrogen mass decreased linearly with the distance from the top of the canopy. There was nosignificant 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. Thisstudy gives important information to better understand the phenotypic plasticity of nitrogen distributionin wheat and to build a process-based model of N distribution within wheat culms during the postanthesisperiod. 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