Materials and methods

Two trials were conducted in northern Nevada between May and Oct 2022 (Fallon) and 2023 (Reno) under high-tunnel settings (Supplemental Table 1). Tomato cultivar BHN-589 was used as the nongrafted control and the scion for grafted plants. The commercial rootstock (Commercial) was Beaufort in 2022, which was replaced by Maxifort in 2023 because of seed availability and given their similar characteristics (Bristow et al. 2021; Suchoff et al. 2017). The comparative rootstock (OH-SG18-197) was a backcross between the tomato OH8245 (Berry et al. 1991) and a wild relative (Solanum galapagense LA1141), with abiotic stress tolerance (Fenstemaker et al. 2022b).

Plants were grafted by Plug Connections (Vista, CA, USA) in 2022 and at UNR in 2023 as in Bristow et al. (2021). In both years, transplanting occurred the last week of May in drip irrigated beds and with a 44-cm plant spacing.

The N treatments were surface applied in the first 56 d after planting (DAP) in three applications. Diluted urea (46–0–0) was used in Fallon (previously organic, currently conventional management), while a 4:6 diluted mixture of blood (13–0–0) and feather meal (13–0–0) was used in Reno (organic management). Two treatments were applied: (1) suboptimal N fertilization (N-) of 75 kg·ha⁻¹ N and (2) optimal N fertilization (N+) of 150 kg·ha⁻¹ N. The experiments were conducted in a randomized complete block design with a split-plot structure. The main plot (i.e., N treatment) had three subplots (one per rootstock; four plants per subplot). Six and four replicate subplots were included in Fallon and Reno, respectively. The two central plants in each subplot were used for measurements. A supplementary field trial was conducted in Reno Main Station Field Lab (MSFL) with a 0 kg·ha⁻¹ N as N- and 75 kg·ha⁻¹ N as N+ treatments.

Total shoot fresh biomass was weighed between 119 and 126 DAP and subsamples dried at 60 °C to calculate dry weight (DW). Tomatoes were harvested weekly between 77 and 126 DAP. Leaf [N], SPAD and NDVI were measured between 35 and 96 DAP as in Farnisa et al. (2023). Linear discriminant analysis (LDA) and mixed-effects analysis of variance, followed by unrestricted least significant difference test were performed in R (v. 4.3.1) to analyze the effects of rootstock, nitrogen, and location on each parameter as described in Bonarota et al. (2024).

Results and discussion

Nitrogen had no consistent effect on tomato growth and yield (this study; Buajaila et al. 2021). Only in Reno, the N+ treatment increased shoot biomass for the nongrafted BHN-589 and the OH-SG18-197 rootstock (70% and 52%, respectively) (Fig. 1A). Interestingly, a higher N rate increased yield in the commercial rootstock by 42% (Reno; Fig. 1B). In Fallon, the overall effect of the N treatment was to increase leaf [N] (Fig. 1C), but it did not increase biomass or yield, as in Reno-MSFL where N- received no N fertilization (Supplemental Fig. 1). Although the results were not consistent across locations, the commercial rootstocks maintained higher biomass even under the N- treatment (e.g., Reno). This lack of response to N may be from processes such as the pretransplant soil-N content, soil-N mineralization rates, and loss through leaching and denitrification that are dependent on temperature and moisture (Hartz and Johnstone 2006; Jackson et al. 2012). The LDA evaluated whether grafting affected the shoot nutrient profile within each location. Leaf [N] was not a main driver of differentiation among phenotypes, although it was somehow important in Reno, which had lower soil-N at pre-planting (Supplemental Table 1). In Reno, the separation of phenotypes was driven by increases in leaf [Mn], [N], and [Cu] toward the right (e.g., commercial rootstock), and increases of [Na] and [Ca] toward the left of the LD1 axis (e.g., nongrafted BHN-589) (Supplemental Fig. 2A and Supplemental Table 2). In Fallon and at Reno-MSFL, with relatively more soil-N at pre-planting than Reno, no clear distinction among phenotypes was identified (Supplemental Fig. 2B and C). Thus, the commercial rootstock influenced more the scion nutrient profile in locations with lower pre-planting soil-N. Moreover, the nutrient profile of each phenotype changed based on location (Supplemental Table 3).

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SPAD and NDVI only explained 28% of the variance in leaf [N] (P < 0.0001) (Fig. 2A and B). The NDVI and SPAD indices were not accurate in predicting [N] status when leaf [N] was suboptimal (i.e., <3.0%; Hartz et al. 1999). Even in Reno, where leaf [N] fluctuated more, these indices did not clearly discern between treatments.

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Conclusion

This study shows that crop responses to N fertilization are complex and dependent on genotype-by-environment interactions. High N input was not a driver for higher yields under high-tunnel production, suggesting that fertilization management requires an integrated approach throughout the season. The use of rootstocks (Djidonou et al. 2013) and attention to plant–soil N cycling within location (Bowles et al. 2015) could improve the economic and ecological sustainability of tomato high-tunnel production. The N spectral indices (e.g., NDVI and SPAD) are influenced by environmental conditions and cultivar (Farnisa et al. 2023; Xiong et al. 2015), and their capacity to support N fertilization management is not warranted (Padilla et al. 2015; Rehman et al. 2019).