Recovery after deficiency: Systemic copper prioritization and partitioning in the leaves and stems of hybrid poplar

Authors: Hunter, Cameron; WARE, MAXWELL - Colorado State University; Gleason, Sean; PILON-SMITS, ELIZABETH - Colorado State University; PILON, MARINUS - Colorado State University

Submitted to: Tree Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/5/2022
Publication Date: 4/8/2022
Citation: Hunter, C., Ware, M.A., Gleason, S.M., Pilon-Smits, E., Pilon, M. 2022. Recovery after deficiency: Systemic copper prioritization and partitioning in the leaves and stems of hybrid poplar. Tree Physiology. Article etpac038. https://doi.org/10.1093/treephys/tpac038.
DOI: https://doi.org/10.1093/treephys/tpac038

Interpretive Summary: Copper (Cu) is important for many aspects of plant function. Cu is especially important for photosynthesis in young leaves. Young leaves are prioritized for Cu delivery when Cu is deficient in hybrid poplar. Stable Cu isotopes were fed to Cu-deficient plants and tracked in their leaves and stems. Most of the Cu was traced to younger leaves rather than mature leaves 72 hours after the isotope was fed to the plants. We then measured the effect of Cu delivery on photosyntheses using chlorophyll fluorescence imaging. Photosynthesis recovered faster in areas of leaves that were closer to large veins. Finally, we measured water use (~stomatal conductance) on leaves for 6 weeks during Cu deficiency and also when Cu was fed back to plants during a 3-day period. During deficiency, mature leaves maintained higher water use than younger leaves, but three days after Cu resupply the younger leaves that had recovered exhibited the highest water use. These results show that Cu moves quickly to young leaves in deficient poplar and that this is effective in improving water use and photosynthesis. Water use and photosynthesis are important physiological processes underpinning plant growth, and as such, these results may prove useful towards improving the growth of poplar and other agriculturally important angiosperm species.

Technical Abstract: Copper (Cu) is important for many aspects of plant function. In photosynthesis, Cu is a cofactor in the protein plastocyanin required for electron flow to Photosystem I. It has been demonstrated that photosynthesis, especially in young leaves are prioritized for Cu delivery after deficiency in hybrid poplar (Shahbaz et al., 2015). Cu has two stable isotopes, 63Cu and 65Cu that naturally occur at 69.2% and 30.8%, respectively. To determine relative Cu delivery prioritization, we enriched hydroponic plant growth media of Cu deficient poplar with 98% 65Cu and tracked Cu delivery after deficiency to young leaves, mature leaves, and stems. Most of the 65Cu was traced to the younger leaves rather than mature leaves 72 hours after isotope enrichment; furthermore, stems had higher 65Cu compared to young and mature leaves. Next, isotope enrichment was coupled with chlorophyll fluorescence imaging in young and mature leaves as a proxy to visualize photosynthetic recovery as a consequence of Cu delivery from the leaf vasculature to photosynthetic electron carriers. The lamina between the primary and secondary veins exhibited more rapid and complete recovery of FPSII upon pulse with 65Cu. Additionally, stomatal conductance (gs) was measured on leaves for 6 weeks and during a 3-day 65Cu pulse period. During deficiency, mature leaves maintained a higher gs than younger leaves but three days after Cu resupply the younger leaves that had recovered showed the highest gs. In conclusion, these results provide a quantitative understanding of how Cu is systemically transported and distributed to photosynthetic and stem tissues.