Plants utilizing C-4 photosynthetic pathways have high photosynthetic rates, and high water-, potassium- and nitrogen-use efficiencies. Decarboxylating enzymes in two of the three main types of C-4 plants are activated by manganese (Mn), with NAD-malic enzyme (NAD-ME) showing an absolute requirement for Mn. In hydroponic nutrient solutions, Mn concentrations of 1-2 mM are typically used. However, since all carbon fixed in photosynthesis by NAD-ME C-4 plants must go through Mn-activated enzyme decarboxylation, Mn nutrition may be especially important.
Two NAD-ME C-4 species: Pearl millet (Pennisetum glaucum), and purple amaranthus (Amaranthus hypochondricus) were grown in hydroponic solutions designed to determine the effect of varying solution Mn concentration on photosynthesis and growth. Plants were grown in growth chambers at light intensity of 550 uE, 16hr/28o C day, 8hr/20o C night, in complete nutrient solution containing 0, 10, 25, 50, 75 or 100 uM Mn. Field grown plants received 0, 4, 8 or 16 lbs/acre of elemental Mn. Photosynthetic measurements were determined with a LiCOR-6400 photosynthesis meter.
In growth chamber experiments, leaf stomatal conductance, CO2 assimilation rates and plant biomass increased with increasing Mn treatment levels. Plants receiving high Mn treatments had greater photosynthetic rates in response to increasing light levels. Pearl millet and purple amaranthus plants grown with 50 uM Mn had leaf quantum efficiencies that were 100% and 50% greater than the controls. Pearl millet and purple amaranthus grown on a sandy soil showed increased photosynthetic rates and leaf stomatal conductance with increased Mn fertilization treatments. Photosynthetic rates, growth rates and yields of pearl millet and purple amaranthus responded to Mn treatments both in hydroponic and field studies. In hydroponic culture, these C-4 plants response to Mn levels that where 25x higher than those found in most nutrient solutions.