Biomass and photosynthetic efficiency of Allium species grown in elevated carbon dioxide levels, with differing plant densities and harvest schemes

Abstract

This research evaluated the effect of carbon dioxide (CO2) level, plant density, Allium species phenology, and harvest scheme on edible biomass of green salad onions. The ontogeny of A. fistulosum, A. cepa, and A. schoenoprasum grown at 10, 15, and 20-mm spacings harvested weekly and bi-monthly was compared to plants harvested once at 70 days after planting (DAP). Plants were grown in ambient (~400 μL•L-1) or elevated (1200 μL•L-1) CO2 levels. Weekly removal of shoots began 28 DAP with leaves removed 5-cm above planting surface; destructive harvest was 70 DAP. Leaves harvested from Allium grown in 1200 μL•L-1 weighed more at 28 DAP than leaves grown in ~400 μL•L-1 CO2; there was no difference in leaf weight of Allium grown in ~400 μL•L-1 and 1200 μL•L-1 CO2 for the remaining harvests. At 20-mm spacings, leaves harvested from A. cepa weighed the most; leaves harvested from chives weighed the least. The species and spacing producing a large amount of biomass at one harvest is A. fistulosum harvested 70 DAP; the species and spacing selected is A. cepa grown at 20-mm for biomass desired over time. It has been observed in our research that plants grown in elevated CO2 have initial increase in shoot biomass, but as plants age, chlorosis and necrosis was evident on leaves. Allium fistulosum was grown in 400, 1200, and 2000 μL•L-1 CO2 in the same manner above. Leaves from four harvests were analyzed for photosynthetic efficiency (fluorescence yield), photosynthetic pigments (chlorophyll a, b, and carotenoids), and C:N. Leaves from plants grown in each treatment were photosynthetically efficient; chlorophyll was higher in leaves grown in 1200 and 2000 rather than 400 μL•L-1 CO2 at 28 DAP. Chlorophyll amounts decreased in plants grown in both elevated CO2 levels over 70 days while it increased in leaves grown in ambient at 35 and 49 DAP, then decreased at 70 DAP. A. fistulosum plants grown in elevated CO2 levels, although photosynthetically efficient, had reduction of N and an increase of carbohydrate accumulation in the leaves. Elevated CO2 levels do not increase biomass of Allium, but do not cause decreases in biomass weight; however, plants grown in elevated CO2 appeared more chlorotic due to reduced N in leaves.