In 2016, a fruit-bearing (6^th leaf) orchard (6.5 hectares) was selected in San Simon, AZ in which 12 ‘Western’ and 12 ’Wichita’ cultivar pecan trees were chosen (see 'Introduction' section photo).

Using completely randomized design (CRD) two treatments of 1) Ni Sulfate (HumaGro), was selected consisting of Ni applied (10%) and 2) Control (0%, H₂O only) with three replications per treatment and all foliar applied.

When possible, data measurements were recorded one to two weeks after applications (2 total in 2016, 3 total in 2017) using the LI-6400XT portable photosynthesis unit (LI-COR Biosciences, Lincoln, NE; Fig. 3). A third year (2018) was added, with enlarged treatment areas in order to prevent drift, after results from 2016-17 were inconsistent. Photosynthesis (Pn), stomatal conductance, and other variables were recorded on three full sun exposed leaflets per tree.

Soil Plant Analytical Development (SPAD) was recorded on the same leaflets measured for Pn using a portable SPAD 502 chlorophyll meter. Mid-day stem water potential (MDSWP) was recorded on the same dates using a portable plant moisture stress pressure chamber.

Leaf tissue analysis was performed by Motzz Laboratory (Phoenix, AZ) with one collection in 2016, 2017, and 2018. Statistical analysis was calculated using JMP PRO 13.0, CRD ANOVA at alpha 0.05 for Mn leaf tissue concentrations on each leaflet collection date.

Ni treatments applied during 2016 supplied increased leaf tissue Ni concentration compared to the non-treated control, meeting the current recommendation of >2-3 ppm in leaf tissue for pecan trees (Fig. 4). In 2017, there was a triple increase in Ni concentration in both treated and non-treated control, suggesting either spray drift or air-blast sprayer applicator error (Fig. 4). In 2018, after expanding the experimental plot to reduce drift, the Ni treatments again supplied increased leaf tissue in only the Ni treated pecan trees (Fig. 5). All other elemental concentrations in the leaf were of adequate or normal range that is recommended in peer reviewed literature (data not shown).

Photosynthetic rates in 2016 had a slightly higher photosynthetic capacity of the Ni treated pecans compared to the non-treated when analyzed across dates (Fig. 6). This is a 8.9% increase in photosynthesis compared with the control. However, in 2017 these photosynthesis differences were not observed (Fig. 6). In 2018, photosynthetic rates had a marginal higher photosynthesis rate than the non-treated control pecan trees (Fig. ).

MDSWP and SPAD chlorophyll readings (data not shown) were not significantly different among treatments indicating no relationship of the Pn measurements due to plant moisture stress or greenness of the leaves.

The differences observed in years 2016 & 2018 suggest that there is relationship between nickel and photosynthetic capacity that may be in turn related to the inability to synthesize urease which would limit the nitrogen cycle and form available for normal leaf function. More investigation is necessary to determine the most adequate Ni concentration where Pn is optimal.

Figure 4. Leaf Ni concentration of 2016 and 2017 showing treatment application effect. Two treatments: 1)Control (Ni-) and 2)10% Nickel Sulfate (Ni+) replicated three times. 24 total trees were randomly selected with 12 ‘Western’ and 12 ‘Wichita’ cultivars.

Figure 5. Leaf Ni concentration of 2018 showing treatment application effect. Two treatments: 1)Control (Ni-) and 2)10% Nickel Sulfate (Ni+) replicated three times. 24 total trees were randomly selected with 12 ‘Western’ and 12 ‘Wichita’ cultivars.

Figure 6. Mean leaflet photosynthesis on ‘Western’ and ‘Wichita’ pecans combined with 2 dates in 2016 and 3 dates in 2017. These data comprised of 3 subsamples from each tree within the treatment rows.

Figure 7. Mean leaflet photosynthesis on ‘Western’ and ‘Wichita’ pecans combined with one date of measurements in 2018. Due to larger experimental plot these data comprised of 2 subsamples from each tree within the treatment rows.