Elina J. Negwer (Christian-Albrechts-Universität zu Kiel)
The findings suggest that caffeoylquinic acids and lignin are no considerable sink for surplus, photosynthetically assimilated carbon in nitrogen deficient Helianthus annuus, which challenges the Carbon-Nutrient Balance Hypothesis.
Elina J. Negwer's Master thesis was awarded with the Prize for the Best Plant Science Master Thesis, which was carried out at Christian-Albrechts-Universität zu Kiel in the year 2022
Title: Induction of UV-screening compounds by N-deficiency in Helianthus annuus
Major secondary compounds are no sink for carbon excess in nitrogen starved sunflowers: A reassessment of the Carbon-Nutrient Balance Hypothesis
In 1983, Bryant et al. published the controversial Carbon-Nutrient Balance Hypothesis (CNB) suggesting, inter alia, that the amount of carbon-based secondary compounds (CBSCs) in plants increases upon nitrogen deficiency in response to surplus photosynthetic carbon caused by reduced growth.
To add clarity to the debate whether the CNB is correct, carbon balances were used in this new approach to calculate the amount of carbon that was channeled into the biosynthesis of caffeoylquinic acids (CQAs) and lignin, each in comparison to the respective carbon gain of cell wall carbohydrates, or individual plant parts, to assess if CBSCs are a considerable carbon sink. The carbon assimilation of hydroponically grown, 13 days old sunflowers was measured via gas exchange and C/N-analysis, whereas the concentration of individual compounds was determined by chlorophyll fluorescence and HPLC (CQAs), as well as with the acetyl bromide assay (lignin and cell wall carbohydrates).
Although the concentration of the CQAs increased significantly in the leaves already within 24 h since transfer of the sunflowers into nitrogen deficient nutrient solution, the lignin concentration did not increase even at three days of nitrogen deficiency for any part of the plants (primary leaves, first secondary leaf pair, cotyledons, stem, or root). However, the induction period could not be extended, because at day three the photosynthesis rate started to decrease significantly due to nitrogen deficiency, which is not compatible with the assumptions of the CNB.
The net amount of carbon that was channeled into CQAs (<2%) or lignin (<7%) was much lower as compared to the portion gained by the cell wall carbohydrates (>60%). Interestingly, these amounts were not significantly influenced by the nitrogen status of the plants. However, the carbon allocation to different plant parts suggests that the carbon excess caused by reduced leaf expansion upon nitrogen deficiency was compensated by increased root growth.
Ultimately, the findings indicate that CQAs and lignin cannot function as a considerable sink for excess carbon caused by nitrogen deficiency, which is in conflict with the CNB. Bottom line, a single cause for the increased CQA concentration in nitrogen starved plants might be unlikely. Instead, previous findings allow for the conclusion that CQAs might serve multiple purposes in the context of nitrogen deficiency, such as antioxidative properties, modulation of growth and differentiation, herbivore defense and allelopathic effects.
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Elina J. Negwer conducted this work at the Botanical Institute in the Department of Ecophysiology in the working goup of Professor Wolfgang Bilger.