Effects of temperature and nitrate on phosphomonoesterase activities between carbon source and sink tissues in Zostera marina L.

Inorganic phosphorus (Pi) is important in the regulation of many carbon and nitrogen metabolic processes of plants. In this study, we examined alterations of phosphomonoesterase activity (PA; both alkaline and acid) in a submersed marine angiosperm, Zostera marina, grown in Pi non-limiting conditions under elevated temperature and/or nitrate enrichment. Control plants (ambient water-column NO3− < 2.5 μM, with weekly mean water temperatures between 26.5–27.0 °C based on a 20-yr data set in a local embayment) were compared to treated plants that were exposed to increased water-column nitrate (8 μM NO3− above ambient, pulsed daily at 0900 h), and/or increased temperature (ca. 3 °C above weekly means) over eight weeks in late summer–fall. Under both nitrate regimes, increased temperature resulted in periodic increased leaf and root-rhizome tissue carbon content, and increased acid and alkaline PA activities (AcPAs and AlPAs, respectively). There was a positive correlation between AlPA and AcPA activities and sucrose synthase activities in belowground structures, and a negative correlation between AlPA activities and sucrose concentrations. There were also periodic changes in PA partitioning between carbon source and sink tissues. In high-temperature and high-nitrate treatments, AcPAs significantly increased in leaves relative to activities in root-rhizome tissues (up to 12-fold higher in aboveground than belowground tissues in as little as 3 weeks after initiation of treatments). These responses were not observed in control plants, which maintained comparable AcPA activities in above- and belowground tissues. In addition, AlPA activity was significantly higher in leaf than in root-rhizome tissues of plants in high-temperature (weeks 3 and 6) and high temperature combined with high nitrate treatments (week 8), relative to AlPA activities in control plants. The observed changes in PAs were not related to Pi growth limitation, and may allow Z. marina to alter its carbon metabolism during periods of increased carbon demand/mobilization. This response would make it possible for Z. marina to meet short-term P requirements to maximize carbon production/allocation. Such a mechanism could help to explain the variability in PA activities that has been observed for many plant species during periods when environmental Pi exceeds requirements for optimal growth.