Two poplar hybrid clones differ in phenolic antioxidant levels and polyphenol oxidase activity in response to high salt and boron irrigation

Authors: NGUYEN, KHANH - California State University; CUELLAR, CARLOS - California State University; MAVI, PRABHJOT - California State University; LEDUC, DANIKA - California State University; Banuelos, Gary; SOMMERHALTER, MONIKA - California State University

Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/20/2018
Publication Date: 6/20/2018
Citation: Nguyen, K., Cuellar, C., Mavi, P., Leduc, D., Banuelos, G.S., Sommerhalter, M. 2018. Two poplar hybrid clones differ in phenolic antioxidant levels and polyphenol oxidase activity in response to high salt and boron irrigation. Journal of Agricultural and Food Chemistry. 66(28):7256-7264. https://doi.10.1021/acs.jafc.8b01106.
DOI: https://doi.org/10.1021/acs.jafc.8b01106

Interpretive Summary: In some regions of the westside of the San Joaquin Valley of Central California, irrigation with saline and boron (B)-laden water has led to the need to identify alternative crops that can tolerate irrigation with such poor-quality waters. In the last decade, as part of a general phytoremediation strategy, a multi-year screening program was initiated at the USDA-ARS San Joaquin Valley Agricultural Sciences Center (Parlier, CA, USA) to test hybrid poplars for their ability to tolerate high salt/B irrigation water. Hybrid poplars are fast growing trees with economic relevance for the veneer, lumber, and paper industry. Ideally, a poplar plantation could be used to recycle salt- and B- laden waters, e.g., drainage water, while the harvested tree products could provide an economical resource. In the multi-year study, two poplar clones were identified that can grow well under high salt and boron growing conditions. It was unclear as to which mechanisms and plant responses were involved in their boron and salt tolerance. With these two clones, we determined that tolerance antioxidant levels responded respectively differently to cope with stressful growing conditions like high salinity and boron as part of their coping mechanism. This type of evidence suggests there are likely other physiological responses besides increased antioxidant activities that are also participating in the salt and boron tolerance exhibited by these two poplar clones. To acquire a better understanding for identifying plant species for their salt and boron tolerance, more research is needed to evaluate antioxidant and other activities in other plant species that potentially exhibit salt and boron tolerance.

Technical Abstract: Two poplar clones, 345-1 and 347-14, of parentage Populus trichocarpa x nigra x deltoids, were previously both identified as salt- and boron(B)- tolerant. In a multi-year screening study, however, they exhibited significant differences with respect to their antioxidant responses to irrigation with high salt/B water. Leaf extracts of clone 345-1 showed an increase in antioxidant capacity, total soluble phenolic content, as well as caffeic, coumaric, and ferulic acids resulting from high salt/B water treatment compared to control samples from low salt/B water treatment. These increases were accompanied by a significant decrease in polyphenol oxidase (PPO) and phenol peroxidase (POD) activity, whereas L-phenylalanine ammonia lyase (PAL) activity remained unchanged. Clone 347-14 showed different responses after irrigation with waters high in salts and B. Compared to clone 345-1, PPO activity increased, but total soluble phenolic content, levels of specific hydroxycinnamic acids, PAL and POD activity, and total antioxidant capacity either remained unchanged or decreased. Notably, SDS-PAGE, PAGE, and IEF gels stained for PPO activity displayed different PPO isoform patterns for the two clones. We therefore propose that PPO isoform distribution influences phenolic levels in these two salt- and B-tolerant poplar hybrids irrigated with high salt/B waters. Apparently, there are other mechanisms and/or responses that also play a significant role in contributing to the high salt- and B-tolerance exhibited by the respective poplar clones. Future studies should evaluate the PPO isoform distribution and phenolic levels in other salt- and B-tolerant plant species, which may be useful for identifying new salt- and B-tolerant plant species.