Characterization of the Relationship of Manganese Nutrition to Photosynthesis of Pecan in the Mesilla Valley, New Mexico

In the past several decades, there has been a shift in U.S. pecan (Carya illinoinensis) plantings from the southeastern to the southwestern U.S. In acidic soils manganese (Mn) may reach toxic levels in pecan trees. Soils in the southwest pecan growing areas are typically alkaline and calcareous; thus phosphorus and most micronutrients, including Mn, are poorly available for root uptake. Manganese is essential for photosynthesis specifically in the oxidation side of the PSII complex. Mn also acts as a coenzyme for biosynthesis of chlorophyll.  The Extension recommendations for New Mexico (NM) pecans are 100–300 ppm Mn in July sampled leaflet tissue. A published survey of NM pecan orchards showed, on average, only 85 ppm Mn in leaf tissue.  Previous research showed that severe Mn deficiency (11 ppm in leaf tissue) negatively impacts photosynthesis, but the level of Mn at which photosynthesis is optimum is not yet known. Our objective was to characterize Mn impacts on photosynthesis over a broad range of leaf Mn concentrations. In 2011 and 2012, an experiment was conducted on 24 second leaf ‘Pawnee’ pecan trees in Las Cruces, NM.  There were four treatments in which Mn (as an amino acid chelate) was applied foliarly with three applications in the first season and five in the second season at four different concentrations: 3% (High), 1.5% (Medium), 0.75% (Low), and 0% (Control) Mn. Gas exchange was measured using a portable photosynthesis system and correlated to leaf Mn tissue concentrations.  Mean leaf Mn concentration in 2011 was 38, 52, 149, and 302 ppm in the Control, Low, Medium, and High treatments, respectively.  Concentrations in 2012 were 53, 84, 147, and 329 ppm in the Control, Low, Medium, and High, respectively. All other nutrients were within normal ranges. Leaf Mn concentrations were not significantly different across treatments on May 2012 (prior to 2012 Mn applications), indicating no carryover of Mn from 2011. Analyzed across dates the Medium Mn treatment had significantly higher photosynthesis and stomatal conductance (α = 0.05) than the other treatments. Our data confirm a relationship between photosynthesis in pecan and Mn nutrition. Furthermore, our results suggest that photosynthesis in NM pecan orchards is limited by Mn.