Carbon Gain, Biomass Allocation, and Water Use by Garlic in Response to Elevated CO2 and Nitrogen Availability

The carbon-nitrogen balance is central to crop yield, and can be altered by nutrient deficient soils and atmospheric enrichment of carbon dioxide.  Although aboveground responses to carbon dioxide and nitrogen (N) have been reported for a number of crops, little is known about the response of belowground components that function as storage and vegetative propagules such as tubers, rhizomes, and bulbs.  We investigated growth, biomass allocation, leaf gas-exchange, and water use efficiency (WUE) in a hardneck garlic and tested whether elevated CO₂ and nitrogen availability alters carbon gain and allocation to bulbs, and plant WUE. The garlic plants were grown at three different N levels (Low-N, Mid-N, and Full-N) in sunlit CO₂ enrichment chambers. We quantified whole-plant growth and allocation by destructive harvests, determined leaf nitrogen content and stable carbon isotope (¹³C) fractionations, and performed leaf gas-exchange analyses.  The results show that nitrogen deficiency primarily affected the aboveground plant parts with significant decreases in aboveground growth (P < 0.05) in the Low-N compared with the Full-N. Whereas CO₂ enrichment increased stem biomass (P < 0.05) and WUE as corroborated by both leaf gas exchange (P < 0.05) and stable carbon isotope analyses (P < 0.001).  Significant interaction in WUE between CO₂ and N were only detected in stable carbon isotopes (P < 0.01).  Biomass partitioning to bulb was similar across the CO₂ and N treatments. The consistency in biomass allocation patterns across all N and CO₂ treatments suggest that the use of bulbing ratio for making cropping decisions such harvest scheduling may remain a robust method across N fertilizations as well as atmospheric enrichment of CO₂.