Photosynthetic Acclimation, Biomass Allocation, and Water Use Efficiency of Garlic in Response to Carbon Dioxide Enrichment and Nitrogen Fertilization

Abstract

Garlic ( Allium sativum ) is a commercially and culturally important crop worldwide. Despite the importance of garlic, there have been few studies investigating how garlic growth and development will be affected by the atmospheric enrichment of carbon dioxide (CO 2 ). A split-plot experiment with CO 2 concentrations as main plot and nitrogen (N) fertilization as subplot was carried out to examine the effects of elevated CO 2 at (mean ± sd ) 745 ± 63 µmol·mol −1 across three levels of N: high-N (16.0 m m ), mid-N (4.0 m m ), and low-N (1.0 m m ). Three hypotheses were tested: 1) garlic plants will allocate proportionally more biomass to bulb when grown in elevated CO 2 compared with the plants grown in ambient CO 2 ; 2) plants will sustain improved photosynthesis without downregulation in elevated CO 2 , irrespective of N; and 3) elevated CO 2 will improve plant water use efficiency (WUE) across N fertilization levels. We found that proportional biomass allocation to bulb was not significantly enhanced by CO 2 enrichment in garlic. Overall biomass accumulation represented by leaf, stem, and bulb did not respond significantly to CO 2 enrichment but responded strongly to N treatments ( P < 0.001). Contrary to our hypothesis, photosynthetic downregulation was apparent for garlic plants grown in elevated CO 2 with a decrease in Rubisco capacity ( P < 0.01). Instantaneous leaf WUE improved in response to elevated CO 2 ( P < 0.001) and also with increasing N fertilization ( P < 0.001). Finally, our results indicate that bulbing ratio is likely to remain unchanged with CO 2 or N levels and may continue to serve as a useful nondesctructive metric to estimate harvest timing and bulb size.