141
wavelengths (absorption maxima 500–550 nm Novak and Short 2011a, b) and
reflecting across a wide region (600–640 nm Gausman 1982). Such absorbed light
energy is not transferred to the photosynthetic pigments, thus reducing excitation
energy reaching the photosystems. Production of anthocyanin is also stimulated
upon the exposure to UV-B, indicating that UV-B plays a critical role in seagrass
light signal transduction pathways (Novak and Short 2011b). On the other hand, no
clear indication of the photoprotective function of anthocyanin was observed in the
seagrass H. ovalis (Southern Thailand, Kaewsrikhaw and Prathep 2014). In this
work, H. ovalis exhibited higher anthocyanin content in the rainy season when irradiance was less strong when compared to that in the dry season.
7.2.2 Salinity Stress and Its Impact on Photosynthesis
Photosynthetic responses to salinity have been quantified in the seagrass species
such as P. oceanica, C. nodosa, T. testudinum, and Z. japonica (Shafer et al. 2011;
Marín-Guirao et al. 2011; Marín-Guirao et al. 2013b; Sandoval-Gil et al. 2012;
Howarth and Durako 2013; Sandoval-Gil et al. 2014; Piro et al. 2015a). Nevertheless,
the mechanisms which salinity induces photosynthetic damage are not yet fully
understood. Chlorophyll fluorescence investigations have shown a reduction in Φ PSII
and F v /F m (Howarth and Durako 2013) in hypersaline treatment. Interestingly, photoprotection appears to contribute to salinity tolerance as demonstrated in the seagrass P. oceanica (Marín-Guirao et al. 2013a, b). In this work, no difference in
maximum photochemical efficiencies was detected in under hypersaline stress
whereas non-photochemical quenching (NPQ) was significantly enhanced. This
suggests that under such unfavorable condition when carbon assimilation is compromised, excess energy dissipation via NPQ plays a significant role in protecting
the photosynthetic apparatus of this seagrass (Marín-Guirao et al. 2013a, b). It has
been suggested that a decrease in chlorophyll density observed under hypersaline
stress could also be a result of a downregulation of light energy harvesting in order
to minimize oxidative stress (Sandoval-Gil et al. 2012).
While photosynthetic rates, measured as oxygen evolution rates, decrease under
salinity stress, many studies have reported an increase in respiratory rates in seagrasses (Shafer et al. 2011; Marín-Guirao et al. 2011, 2013b; Sandoval-Gil et al.
2012). For example, dark respiration rates in salt-stressed plants were almost double
of control plants (Marín-Guirao et al. 2013b). This shift in carbon metabolisms
affects plant carbon balance and might subsequently reduce their growth rates and
lead to mortality. Such an increase in respiratory demands might be a result of more
energy needed to accommodate an increase in the activity of osmoregulatory processes. Piro et al. (2015a) provide evidence of a shift in carbon metabolisms in C.
nodosa under salinity stress from proteomics data. Here, it was shown that the structural proteins and enzymes associated with photosynthesis (PSII, PSI, Rubisco)
became less abundant, whereas the respiratory enzymes (cytosolic glyceraldehyde3- phosphate dehydrogenase, enolase2, and triose phosphate isomerase) became
7 Photobiology of Seagrasses: A Systems Biology Perspective
wavelengths (absorption maxima 500–550 nm Novak and Short 2011a, b) and
reflecting across a wide region (600–640 nm Gausman 1982). Such absorbed light
energy is not transferred to the photosynthetic pigments, thus reducing excitation
energy reaching the photosystems. Production of anthocyanin is also stimulated
upon the exposure to UV-B, indicating that UV-B plays a critical role in seagrass
light signal transduction pathways (Novak and Short 2011b). On the other hand, no
clear indication of the photoprotective function of anthocyanin was observed in the
seagrass H. ovalis (Southern Thailand, Kaewsrikhaw and Prathep 2014). In this
work, H. ovalis exhibited higher anthocyanin content in the rainy season when irradiance was less strong when compared to that in the dry season.
7.2.2 Salinity Stress and Its Impact on Photosynthesis
Photosynthetic responses to salinity have been quantified in the seagrass species
such as P. oceanica, C. nodosa, T. testudinum, and Z. japonica (Shafer et al. 2011;
Marín-Guirao et al. 2011; Marín-Guirao et al. 2013b; Sandoval-Gil et al. 2012;
Howarth and Durako 2013; Sandoval-Gil et al. 2014; Piro et al. 2015a). Nevertheless,
the mechanisms which salinity induces photosynthetic damage are not yet fully
understood. Chlorophyll fluorescence investigations have shown a reduction in Φ PSII
and F v /F m (Howarth and Durako 2013) in hypersaline treatment. Interestingly, photoprotection appears to contribute to salinity tolerance as demonstrated in the seagrass P. oceanica (Marín-Guirao et al. 2013a, b). In this work, no difference in
maximum photochemical efficiencies was detected in under hypersaline stress
whereas non-photochemical quenching (NPQ) was significantly enhanced. This
suggests that under such unfavorable condition when carbon assimilation is compromised, excess energy dissipation via NPQ plays a significant role in protecting
the photosynthetic apparatus of this seagrass (Marín-Guirao et al. 2013a, b). It has
been suggested that a decrease in chlorophyll density observed under hypersaline
stress could also be a result of a downregulation of light energy harvesting in order
to minimize oxidative stress (Sandoval-Gil et al. 2012).
While photosynthetic rates, measured as oxygen evolution rates, decrease under
salinity stress, many studies have reported an increase in respiratory rates in seagrasses (Shafer et al. 2011; Marín-Guirao et al. 2011, 2013b; Sandoval-Gil et al.
2012). For example, dark respiration rates in salt-stressed plants were almost double
of control plants (Marín-Guirao et al. 2013b). This shift in carbon metabolisms
affects plant carbon balance and might subsequently reduce their growth rates and
lead to mortality. Such an increase in respiratory demands might be a result of more
energy needed to accommodate an increase in the activity of osmoregulatory processes. Piro et al. (2015a) provide evidence of a shift in carbon metabolisms in C.
nodosa under salinity stress from proteomics data. Here, it was shown that the structural proteins and enzymes associated with photosynthesis (PSII, PSI, Rubisco)
became less abundant, whereas the respiratory enzymes (cytosolic glyceraldehyde3- phosphate dehydrogenase, enolase2, and triose phosphate isomerase) became
7 Photobiology of Seagrasses: A Systems Biology Perspective