Carbon-11 Radiotracing Reveals Physiological and Metabolic Responses of Maize Grown under Different Regimes of Boron Treatment

Abstract

In agriculture, boron is known to play a critical role in healthy plant growth. To dissect the role of boron in maize metabolism, radioactive carbon-11 (t½ 20.4 min) was used to examine the physiological and metabolic responses of 3-week-old B73 maize plants to different levels of boron spanning 0 mM, 0.05 mM, and 0.5 mM boric acid (BA) treatments. Growth behavior, of both shoots and roots, was recorded and correlated to plant physiological responses. 11CO2 fixation, leaf export of [11C]-photosynthates, and their rate of transport increased systematically with increasing BA concentrations, while the fraction of [11C]-photosynthates delivered to the roots under 0 mM and 0.5 mM BA treatments was lower than under 0.05 mM BA treatment, likely due to changes in root growth. Additionally, solid-phase extraction coupled with gamma counting, radio-fluorescence thin layer chromatography, and radio-fluorescence high-performance liquid chromatography techniques applied to tissue extracts provided insight into the effects of BA treatment on ‘new’ carbon (as 11C) metabolism. Most notable was the strong influence reducing boron levels had on raising 11C partitioning into glutamine, aspartic acid, and asparagine. Altogether, the growth of maize under different regimes of boron affected 11CO2 fixation, its metabolism and allocation belowground, and altered root growth. Finally, inductively coupled plasma mass spectrometry provided insight into the effects of BA treatment on plant uptake of other essential nutrients. Here, levels of boron and zinc systematically increased in foliar tissues with increasing BA concentration. However, levels of magnesium, potassium, calcium, manganese, and iron remained unaffected by treatment. The rise in foliar zinc levels with increased BA concentration may contribute to improved 11CO2 fixation under these conditions.