139
and Johnson 2000). Nevertheless, fast chlorophyll a fluorescence transient (OJIP)
analysis (see review in Stirbet and Govindjee 2011) and investigations at molecular
levels have increased significantly during the recent years.
Large non-photosynthetic tissue in seagrasses resulted in high respiratory
oxygen demand, hence the high minimum light requirement to maintain the wholeplant positive carbon balance. Furthermore, seagrasses depend largely on the
transfer of photosynthetically derived oxygen to the rhizosphere to prevent sulfide
intrusion (Brodersen et al. 2015a, b, discussed further in the section Phytotoxins).
Consequently, seagrasses are regarded as highly sensitive to a decrease in light
intensity (reviewed in Ralph et al. 2007). For this reason, much attention has been
focused on determining the effect of light limitation on seagrass photosynthesis
(reviewed in Ralph et al. 2007). In general, plant responses to low light condition
include an increase in number of chloroplasts, chlorophyll contents, and other lightharvesting pigments per unit leaf area (Murchie and Horton 1997) as well as an
increase in chlorophyll b/a ratios which have been shown to indicate a larger PSII
light-harvesting antenna (Bailey et al. 2001). Modulation of photosynthetic apparatus under a low light condition such as state transition (Mullineaux and Emlyn- Jones
2005), an increase in PSI reaction center components (Bailey et al. 2001) and an
alteration in the activation state of Rubisco (Salvucci and Ogren 1996) have been
observed. Seagrasses grown under low light condition exhibit a common shade
plant response measured as the photosynthesis-irradiance characteristics such as an
increase in a slope of the light-limiting range of the photosynthesis-irradiance curve
(α) and lower light saturation point (Ek) when compared to those of seagrasses
under high light (Silva et al. 2013; Howarth and Durako 2013; Dattolo et al. 2014;
Park et al. 2016). Enhancement of light-harvesting capacity is achieved by means of
increasing chlorophyll a, chlorophyll b, and carotenoid contents and antenna size
(reviewed in Ralph et al. 2007; Silva et al. 2013; Howarth and Durako 2013; Dattolo
et al. 2014). Chlorophyll b/a ratio (a proxy of PSII antenna size) was found to be
higher in seagrasses growing under lower light intensity (Dennison and Alberte
1982; Longstaff and Dennison 1999; Lamote, and Dunton 2006; Dattolo et al.
2014). Accumulating evidence suggests a regulatory role of PSI in shade adaptation
in seagrasses. Thalassia testudinum grown in deeper site exhibited larger PSI
antenna, estimated as total chlorophylls associated with isolated PSI, when compared to that of the shallow site (Major and Dunton 2002). It was proposed that an
increase in PSI antenna size might help to reduce the light requirement to reach
maximum photosynthesis, thus optimizing the photosynthetic efficiency under low
light intensity. In the extreme case of H. stipulacea with exceptionally wide range
of depth distribution (10–50 m), the protein levels of PSI iron-sulfur center (PsaC)
relative to PSII protein D1 (PsbA) as well as the functionality of PSI (estimated
from the peak heights of fluorescence emission at 717 nm at low-temperature using
excitation wavelength at 435 nm (Sharon et al. 2011) increase in the deep water
population. A similar result was observed in Arabidopsis thaliana (Bailey et al.
2001), and it was proposed to be a result of an increase in ATP demand in relation
to NADPH which can be supplied by cyclic electron transports around PSI (Bailey
et al. 2001; Sharon et al. 2011). In contrast, opposing results have been reported in
recent studies in the seagrass P. oceanica and Z. muelleri (Dattolo et al. 2014;
7 Photobiology of Seagrasses: A Systems Biology Perspective
and Johnson 2000). Nevertheless, fast chlorophyll a fluorescence transient (OJIP)
analysis (see review in Stirbet and Govindjee 2011) and investigations at molecular
levels have increased significantly during the recent years.
Large non-photosynthetic tissue in seagrasses resulted in high respiratory
oxygen demand, hence the high minimum light requirement to maintain the wholeplant positive carbon balance. Furthermore, seagrasses depend largely on the
transfer of photosynthetically derived oxygen to the rhizosphere to prevent sulfide
intrusion (Brodersen et al. 2015a, b, discussed further in the section Phytotoxins).
Consequently, seagrasses are regarded as highly sensitive to a decrease in light
intensity (reviewed in Ralph et al. 2007). For this reason, much attention has been
focused on determining the effect of light limitation on seagrass photosynthesis
(reviewed in Ralph et al. 2007). In general, plant responses to low light condition
include an increase in number of chloroplasts, chlorophyll contents, and other lightharvesting pigments per unit leaf area (Murchie and Horton 1997) as well as an
increase in chlorophyll b/a ratios which have been shown to indicate a larger PSII
light-harvesting antenna (Bailey et al. 2001). Modulation of photosynthetic apparatus under a low light condition such as state transition (Mullineaux and Emlyn- Jones
2005), an increase in PSI reaction center components (Bailey et al. 2001) and an
alteration in the activation state of Rubisco (Salvucci and Ogren 1996) have been
observed. Seagrasses grown under low light condition exhibit a common shade
plant response measured as the photosynthesis-irradiance characteristics such as an
increase in a slope of the light-limiting range of the photosynthesis-irradiance curve
(α) and lower light saturation point (Ek) when compared to those of seagrasses
under high light (Silva et al. 2013; Howarth and Durako 2013; Dattolo et al. 2014;
Park et al. 2016). Enhancement of light-harvesting capacity is achieved by means of
increasing chlorophyll a, chlorophyll b, and carotenoid contents and antenna size
(reviewed in Ralph et al. 2007; Silva et al. 2013; Howarth and Durako 2013; Dattolo
et al. 2014). Chlorophyll b/a ratio (a proxy of PSII antenna size) was found to be
higher in seagrasses growing under lower light intensity (Dennison and Alberte
1982; Longstaff and Dennison 1999; Lamote, and Dunton 2006; Dattolo et al.
2014). Accumulating evidence suggests a regulatory role of PSI in shade adaptation
in seagrasses. Thalassia testudinum grown in deeper site exhibited larger PSI
antenna, estimated as total chlorophylls associated with isolated PSI, when compared to that of the shallow site (Major and Dunton 2002). It was proposed that an
increase in PSI antenna size might help to reduce the light requirement to reach
maximum photosynthesis, thus optimizing the photosynthetic efficiency under low
light intensity. In the extreme case of H. stipulacea with exceptionally wide range
of depth distribution (10–50 m), the protein levels of PSI iron-sulfur center (PsaC)
relative to PSII protein D1 (PsbA) as well as the functionality of PSI (estimated
from the peak heights of fluorescence emission at 717 nm at low-temperature using
excitation wavelength at 435 nm (Sharon et al. 2011) increase in the deep water
population. A similar result was observed in Arabidopsis thaliana (Bailey et al.
2001), and it was proposed to be a result of an increase in ATP demand in relation
to NADPH which can be supplied by cyclic electron transports around PSI (Bailey
et al. 2001; Sharon et al. 2011). In contrast, opposing results have been reported in
recent studies in the seagrass P. oceanica and Z. muelleri (Dattolo et al. 2014;
7 Photobiology of Seagrasses: A Systems Biology Perspective