Soil-applied zinc-EDTA: Photosynthesis in 'Wichita' pecan grown on an alkaline and calcareous soil

Pecan has higher zinc requirements than other fruit tree species. Zinc deficiency is common in pecan outside its native range, especially in alkaline soils. Zinc-deficient pecan leaves have interveinal chlorosis or necrosis, decreased leaf thickness, and reduced photosynthetic capacity. Low photosynthetic rates contribute to restricted vegetative growth, flowering, and fruiting of zinc-deficient pecan trees. Our objective was to characterize effects of soil-applied zinc-EDTA on photosynthesis of non-bearing 'Wichita' pecan. The study orchard was planted in spring 2011. Soils were alkaline and calcareous. Zinc was applied throughout each growing season as zinc-EDTA through microsprinklers at rates of 0 (untreated control), 2, or 4 kg ha^-1 zinc. Plots were arranged in a randomized complete block design with four blocks. Lightsaturated leaf photosynthesis was measured on 4 dates in 2013: June 25, August 7, September 24, and October 24. On all measurement dates, photosynthesis was significantly increased by soil-applied zinc-EDTA compared to the control, but was not different between the two soil-applied zinc treatments. The differences were most extreme in June when, compared to the untreated control, there was a 72.8 and 65.5% increase in photosynthesis rate for the 4 and 2 kg ha^-1 application treatments, respectively. Intercellular CO₂ concentration was significantly higher for the untreated control than for at least one of the zinc treatments on all measurement dates even on dates when stomatal conductance was significantly lower for the control. Thus, it appears that soil application of zinc-EDTA lessened non-stomatal limitations of photosynthesis caused by zinc deficiency in 'Wichita' pecan.