Search

The distribution of seagrass meadows in intertidal zone is determined by the intensity of desiccation stress. The physiological characteristics of plants are the important basis to reflect their environmental stress effects. In order to explore the effects of the duration of photothermal desiccation stress on the physiological characteristics of seagrass during the tidal cycle, taking the tropical dominant seagrass Thalassia hemprichii as the research object, a simulation experiment was carried out in a solar greenhouse with high temperature (41.0-44.5℃) and strong light (633-1 682 μmol·m-2·s-1) environmental conditions. The effects of different desiccation time (0, 10, 20, 40, 60, 90 min·d-1, continuous for 7 d) on the growth rate, photosynthetic pigment content and photosynthetic fluorescence parameters of the leaves, as well as on the activities of antioxidant enzymes and the contents of nonstructural carbohydrates and secondary metabolites of leaves and roots were studied. The results were as follows: (1) The diurnal periodic photothermal desiccation of ≤10 min·d-1 had no significant effect on the growth rate and physiological characteristics of T. hemprichii (P>0.05). (2) Under the photothermal desiccation conditions, the damage of T. hemprichii leaves increased, the color of leaves changed from green to brown, and the growth rate of leaves decreased with the extension of desiccation time. Meanwhile, the photosynthetic pigment content, chlorophyll a/chlorophyll b and the chlorophyll fluorescence parameters of leaves showed a downward trend; the contents of MDA and soluble sugar and the activities of POD, SOD and CAT of leaves and roots showed an upward trend; whereas the contents of starch, total phenol and total flavonoids in leaves increased first and then decreased. (3) The diurnal periodic photothermal desiccation of 90 min·d-1 was the upper limit of incomplete inactivation for T. hemprichii chloroplast PS Ⅱ photosynthetic reaction center. In summary, a long term photothermal desiccation stress during tidal cycle has negative impacts on the growth and physiological characteristics for T. hemprichii. The results provide the reference for understanding the distribution of tropical seagrasses in the intertidal zone and the location selection of seagrass meadows for ecological restoration.

IntroductionThe transplantation of seagrass fragments with shoots and rhizomes is the most common method for the ecological restoration of damaged seagrass meadows.MethodsThe aim of this study was to explore the effects of exogenous addition (10 mg per month for 3 months) of three commonly used plant growth regulators (PGRs), including indoleacetic acid (IAA), gibberellin (GA3), and paclobutrazol (PP333) on the growth and physiology of transplanted Thalassia hemprichii fragments (TTFs), with and without the rhizome apex (RA), using a simulation test. [Results] IAA and GA3 elevated the photosynthetic capacity and growth rate of TTF leaves but did not significantly alter leaf length and width. In contrast, PP333 reduced the leaf photosynthetic capacity and growth rate, while greatly increasing the leaf width and root viability. Additionally, PP333 treatment led to an increase in antioxidant enzyme activities (peroxidase [POD], superoxide dismutase [SOD], and catalase [CAT]), as well as malondialdehyde (MDA) and total phenol contents in TTFs, indicating some stress effects on the seagrass. Furthermore, IAA and GA3 decreased soluble sugar and protein contents and increased starch content in TTF tissues, whereas PP333 treatment elevated both nonstructural carbohydrate and soluble protein contents. The presence of RA positively affected the growth and physiology of T. hemprichii compared with TTFs without RA. There was a significant interaction between the PGRs and RA on leaf growth rate, chlorophyll fluorescence variables, and photosynthetic pigment content of the TTFs (p < 0.05).DiscussionThis study provides guidelines for the application of PGRs during the transplantation of T. hemprichii fragments for the restoration of seagrass meadows.