Effects of increasing temperature on the photosynthetic activity and oxygen balance of sheath-covered seagrass Zostera marina seeds

Abstract

The seed sheaths of eelgrass Zostera marina L. have recently been shown to possess photosynthetic capacity that can alleviate intra-seed hypoxic conditions and thereby enhance biosynthetic activity. However, nothing is known about how increasing seawater temperatures affect physiological responses in developing Z. marina seeds. We used an optical multi-analyte sensor system in combination with O2-sensitive sensor spots to measure rates of photosynthesis and dark respiration within custom-made gas exchange chambers. Exponential saturation models were then applied to determine key photosynthetic parameters, such as maximum photosynthesis rate, light use efficiency, saturating photon irradiance (EK), compensation photon irradiance, and net diel O2 budget. Our results showed that both photosynthetic activity and dark respiration rates in sheath-covered seeds increased with increasing seawater temperature (from 10 to 25°C) but with a 2-fold stronger response in dark respiration than in gross photosynthesis over the measured temperature range. These temperature responses resulted in increasing light requirements (from 47 to 183 µmol photons m-2 s-1 in EK) and decreasing net diel O2 budgets (from -5.4 to -126 nmol O2 mg wet weight [WW]-1 h-1) of the eelgrass seeds with increasing temperature. Eelgrass seed exposure to high temperature led thus to highly reduced net diel O2 balances, which is expected to have detrimental effects on seed development and germination success owing to negative effects on synthesis rates of storage products in the endosperm.