Nitrogen form mediates sink strength and resource allocation of a C3 root crop under elevated CO2

Abstract

Plant physiological processes alter with elevated carbon dioxide concentrations in the atmosphere (eCO₂), which affects the growth and yield potential of crops. Among plants with C₃ carbon fixation, root crops show highest yield response under eCO₂ which is suggested to be linked to their large carbon (C) sink strength. The high C gain under eCO₂ can be limited by processes that constrain sink capacity, such as nitrogen (N) supply. Different N sources may interact with eCO₂ and thus have variable impacts on carboxylation activity, N uptake efficiency and plant development. This study aims at contributing to a better understanding of sink-driven assimilation, re-allocation and finally growth processes under eCO₂ as a function of N-form. Radish (Raphanus sativus L. var. sativus), a C₃ tuber plant with strong sink strength, was used. The plants were grown in pots in climate chambers at 400 ppm (aCO₂) and 1000 ppm CO2 (eCO₂) with either pure nitrate or ammonium-dominated nutrition. A split plot design was applied. Plants were harvested after four weeks and physiological, morphological and chemical parameters in leaves and tubers were assessed. The N-form had no effect on N acquisition, but affected C and N partitioning into plant organs differently under eCO₂. N assimilation processes of nitrate-fed plants were focussed on leaves being both source and sink, and those of ammonium-fed plants on tubers, the strongly pronounced sink. Acclimation of CO₂ fixation due to eCO₂ did not occur for both N forms, probably due to altered sink strength and low N content in leaves. The N-form influences the sink-driven C and N balance and thus enables unrestricted carbon gain as well as the variation of organ development under future eCO₂ conditions. These processes can be utilized in cultivation and breeding.