New Insights into calcium availability in apple trees
Nanjing Agricultural University The Academy of Science
image: Effects of N and Ca supply on phenotype and antioxidant metabolism in apple leaves. L−: low N without Ca (0.5 mM NO3− + 0 mM Ca), L+: low N with Ca (0.5 mM NO3− + 5 mM Ca), H−: high N without Ca (10 mM NO3− + 0 mM Ca), and H+: high N with Ca (10 mM NO3− + 5 mM Ca).
Credit: Horticulture Research
A new study has revealed how excessive nitrogen disrupts calcium availability in apple leaves, shedding light on a key factor behind calcium deficiency disorders in apple trees. Researchers found that high nitrogen levels drive the biosynthesis of oxalate, a compound that locks calcium in an unusable form. This discovery is a major step forward in addressing calcium-related fruit disorders, which can significantly impact apple quality and yield. By deciphering the molecular mechanisms governing calcium sequestration, this study paves the way for smarter nutrient management strategies and the development of calcium-efficient apple rootstocks, potentially revolutionizing apple cultivation worldwide.
Apple cultivation is a cornerstone of global agriculture, with China leading the world in production. However, apple trees frequently suffer from calcium deficiency, a major challenge exacerbated by excessive nitrogen fertilization. Calcium is essential for cell wall integrity and fruit development, but its mobility within plants is limited. When nitrogen levels rise, calcium becomes increasingly locked in forms that trees cannot use, leading to physiological disorders such as bitter pit and cork spot. These issues not only reduce fruit quality but also cause substantial economic losses. Given the widespread use of nitrogen fertilizers, understanding their impact on calcium metabolism is critical for the future of apple farming.
On July 30, 2024, researchers from Shandong Agricultural University published a pioneering study (DOI: 10.1093/hr/uhae208) in Horticulture Research, unveiling the intricate relationship between nitrogen and calcium in apple leaves. The study employed advanced techniques such as electron probe microanalysis and transcriptome sequencing to explore how nitrogen-driven oxalate synthesis influences calcium dynamics. Their findings provide a new perspective on how nutrient imbalances contribute to calcium deficiency disorders in apple trees.
The researchers discovered that under high nitrogen conditions, oxalate levels in apple leaves surged by up to 40.79 times compared to low-nitrogen environments. This spike was driven by the upregulation of genes responsible for oxalate biosynthesis—MdICL, MdOXAC, and MdMDH—while the gene controlling oxalate breakdown, MdAAE3, was downregulated. Using Fourier transform infrared spectroscopy (FTIR) and electron microscopy, they confirmed that calcium predominantly accumulates as calcium oxalate (CaOx) crystals in the phloem, with CaOx accounting for as much as 93.54% of total calcium in high-nitrogen conditions. Further analysis pinpointed oxaloacetate as a crucial precursor in the "photosynthesis/glycolysis – oxaloacetate – oxalate – CaOx" pathway, illustrating how nitrogen indirectly depletes bioavailable calcium.
Dr. Shun-Feng Ge, one of the study’s lead authors, emphasized the significance of these findings: "Unraveling the nitrogen-calcium interaction is essential for enhancing apple quality. Our research provides a blueprint for developing smarter fertilization strategies that can mitigate calcium deficiency and improve fruit quality."
The implications of this study extend far beyond basic plant biology. By fine-tuning nitrogen and calcium application, farmers can prevent calcium deficiency disorders, leading to healthier, more resilient apple crops. This research also opens new doors for breeding apple rootstocks with enhanced calcium efficiency, potentially yielding hardier, higher-quality fruit varieties. Furthermore, these insights could contribute to more sustainable farming practices by reducing nitrogen overuse, curbing its environmental impact, and promoting a more balanced nutrient management approach. As global agriculture faces increasing challenges, such innovations could play a crucial role in ensuring the future of high-quality apple production.
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References
DOI
Original Source URL
https://doi.org/10.1093/hr/uhae208
Funding information
This work was supported by the National Key R&D Program of China (2023YFD2301000), the Special Fund for the Natural Science Foundation of Shandong Province (ZR2021MC093), the earmarked fund for CARS (CARS-27), the Taishan Scholar Assistance Program from Shandong Provincial Government (TSPD20181206), and Xin lianxin Innovation Center for Efficient Use of Nitrogen Fertilizer (2020-apple).
About Horticulture Research
Horticulture Research is an open access journal of Nanjing Agricultural University and ranked number one in the Horticulture category of the Journal Citation Reports ™ from Clarivate, 2022. The journal is committed to publishing original research articles, reviews, perspectives, comments, correspondence articles and letters to the editor related to all major horticultural plants and disciplines, including biotechnology, breeding, cellular and molecular biology, evolution, genetics, inter-species interactions, physiology, and the origination and domestication of crops.
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