Red light mitigates Cd toxicity in Egeria densa by restricting Cd accumulation and modulating antioxidant defense system.

Controlling light qualities have been acknowledged as an effective method to enhance the efficiency of phytoremediation, as light has a significant impact on plant growth. This study examined the effects of light qualities on cadmium (Cd) tolerance in aquatic plant Egeria densa using a combination of biochemical and transcriptomic approaches. The study revealed that E. densa exhibits higher resistance to Cd toxicity under red light (R) compared to blue light (B), as evidenced by a significant decrease in photosynthetic inhibition and damage to organelle ultrastructure. After Cd exposure, there was a significantly reduced Cd accumulation and enhanced levels of both glutathione reductase (GR) activity and glutathione (GSH), along with an increase in jasmonic acid (JA) in R-grown E. densa compared to B. Transcriptional analysis revealed that R caused an up-regulation of Cd transporter genes such as ABCG (G-type ATP-binding cassette transporter), ABCC (C-type ATP-binding cassette transporter), and CAX2 (Cation/H+ exchanger 2), while down-regulated the expression of HIPP26 (Heavy metal-associated isoprenylated plant protein 26), resulting in reduced Cd uptake and enhanced Cd exportation and sequestration into vacuoles. Moreover, the expression of genes involved in phytochromes and JA synthesis was up-regulated in Cd treated E. densa under R. In summary, the results suggest that R could limit Cd accumulation and improve antioxidant defense to mitigate Cd toxicity in E. densa, which might be attributed to the enhanced JA and phytochromes. This study provides a foundation for using light control methods with aquatic macrophytes to remediate heavy metal contamination in aquatic systems.

Subcellular accumulation and source of O2- and H2O2 in submerged plant Hydrilla verticillata (L.f.) Royle under NH4+-N stress condition.

In this study, the effects of excess NH4+-N on the subcellular accumulation of O2- and H2O2 in submerged plant Hydrilla verticillata (L.f.) Royle were investigated using both histochemical and cytochemical methods. Treatments with ≥ 2.00 and ≥ 5.00 mg L-1 NH4+-N for 5 d significantly increased production of O2- and H2O2, respectively. The activities of plasma membrane-bound NADPH (nicotinamide adenine dinucleotide phosphate) oxidases and antioxidant enzymes (superoxide dismutase, peroxidase, ascorbate peroxidase, catalase, dehydroascorbate reductase and glutathione reductase) were also increased correspondingly. This study also provides the first cytochemical evidence of subcellular accumulation of O2- and H2O2 in the submerged plants. In the leaves of H. verticillata treated with 20.0 mg L-1 NH4+-N, O2- dependent DAB precipitates were found primarily on the inner side of the plasma membrane, extracellular space and chloroplasts. H2O2-CeCl3 precipitates were mainly localized on the inner side of the plasma membrane and extracellular space of the mesophyll cells. Treatments with the inhibitors of NADPH oxidase (diphenylene iodonium and imidazole) indicate that NH4+-N-induced production of O2- and H2O2 in H. verticillata leaves may involve plasma membrane-bound NADPH oxidase. Moreover, low-light treatment decreased NH4+-induced O2- production, suggesting that alterations in the photosynthetic electron transfer chain due to NH4+ toxicity could lead to O2- production.

Determining light stress responses for a tropical multi-species seagrass assemblage.

Existing mitigations to address deterioration in water clarity associated with human activities are based on responses from single seagrass species but may not be appropriate for diverse seagrass assemblages common to tropical waters. We present findings from a light experiment designed to determine the effects of magnitude and duration of low light on a mixed tropical seagrass assemblage. Mixed assemblages of three commonly co-occurring Indo-West Pacific seagrasses, Cymodocea serrulata, Halodule uninervis and Halophila ovalis were grown in climate-controlled tanks, where replicate pots were subjected to a gradient in light availability (0.9-21.6 mols PAR m-2 day-1) for 12 weeks. Increased shading resulted in declines in growth and changes in cellular and photosynthesis responses for all species, although time-scale and magnitude of response were species-specific. Applying management criteria (e.g. thresholds) relevant to one species may under- or over-estimate potential for impact on other species and the meadow as a whole.

Elodea nuttallii exposure to mercury exposure under enhanced ultraviolet radiation: Effects on bioaccumulation, transcriptome, pigment content and oxidative stress.

The hypothesis that increased UV radiation result in co-tolerance to Hg toxicity in aquatic plants was studied at the physiological and transcriptomic level in Elodea nuttallii. At the transcriptomic level, combined exposure to UV+Hg enhanced the stress response in comparison with single treatments, affecting the expression level of transcripts involved in energy metabolism, lipid metabolism, nutrition, and redox homeostasis. Single and combined UV and Hg treatments dysregulated different genes but with similar functions, suggesting a fine regulation of the plant to stresses triggered by Hg, UV and their combination but lack of co-tolerance. At the physiological level, UV+Hg treatment reduced chlorophyll content and depleted antioxidative compounds such as anthocyanin and GSH/GSSG in E. nuttallii. Nonetheless, combined exposure to UV+Hg resulted in about 30% reduction of Hg accumulation into shoots vs exposure to Hg alone, which was congruent with the level of expression of several transporter genes, as well as the UV effect on Hg bioavailability in water. The findings of the present work underlined the importance of performing experimentation under environmentally realistic conditions and to consider the interplay between contaminants and environmental variables such as light that might have confounding effects to better understand and anticipate the effects of multiple stressors in aquatic environment.

Quantitative imaging of radial oxygen loss from Valisneria spiralis roots with a fluorescent planar optode.

Oxygen (O2) availability within the sediment-root interface is critical to the survival of macrophytes in O2-deficient sediment; however, our knowledge of the fine-scale impact of macrophyte roots upon the spatiotemporal dynamics of O2 is relatively limited. In this study, a non-invasive imaging technology was utilized to map O2 micro-distribution around Vallisneria spiralis. Long-term imaging results gathered during a 36day-period revealed an abundance of O2 spatiotemporal patterns ranging from 0 to 250µmolL(-1). The root-induced O2 leakage and consequent oxygenated area were stronger in the vicinity of the basal root compared to that found in the root tip. The O2 images revealed V. spiralis exhibited radial O2 loss (ROL) along the entire root, and the O2 distribution along the root length showed a high degree of small-scale spatial heterogeneity decreasing from 80% at the basal root surface to 10% at the root tip. The oxygenated zone area around the roots increased as O2 levels increased with root growth and irradiance intensities ranging from 0 to 216µmol photons m(-2)s(-1). A weak ROL measuring <20% air saturation around the basal root surface was maintained in darkness, which was presumably attributed to the O2 supply from overlying water via plant aerenchyma. The estimated total O2 release to the rhizosphere of V. spiralis was determined to range from 8.80±7.32 to 30.34±17.71nmolm(-2)s(-1), which is much higher than many other macrophyte species. This O2 release may be an important contribution to the high-capacity of V. spiralis for quickly colonizing anaerobic sediment.

Heat stress of two tropical seagrass species during low tides - impact on underwater net photosynthesis, dark respiration and diel in situ internal aeration.

Seagrasses grow submerged in aerated seawater but often in low O2 sediments. Elevated temperatures and low O2 are stress factors. Internal aeration was measured in two tropical seagrasses, Thalassia hemprichii and Enhalus acoroides, growing with extreme tides and diel temperature amplitudes. Temperature effects on net photosynthesis (PN ) and dark respiration (RD ) of leaves were evaluated. Daytime low tide was characterized by high pO2 (54 kPa), pH (8.8) and temperature (38°C) in shallow pools. As PN was maximum at 33°C (9.1 and 7.2 µmol O2  m(-2) s(-1) in T. hemprichii and E. acoroides, respectively), the high temperatures and reduced CO2 would have diminished PN , whereas RD increased (Q10 of 2.0-2.7) above that at 33°C (0.45 and 0.33 µmol O2  m(-2)  s(-1) , respectively). During night-time low tides, O2 declined resulting in shoot base anoxia in both species, but incoming water containing c. 20 kPa O2 relieved the anoxia. Shoots exposed to 40°C for 4 h showed recovery of PN and RD , whereas 45°C resulted in leaf damage. These seagrasses are 'living near the edge', tolerant of current diel O2 and temperature extremes, but if temperatures rise both species may be threatened in this habitat.

Integrated role of ROS and Ca+2 in blue light-induced chloroplast avoidance movement in leaves of Hydrilla verticillata (L.f.) Royle.

Directional chloroplast photorelocation is a major physio-biochemical mechanism that allows these organelles to realign themselves intracellularly in response to the intensity of the incident light as an adaptive response. Signaling processes involved in blue light (BL)-dependent chloroplast movements were investigated in Hydrilla verticillata (L.f.) Royle leaves. Treatments with antagonists of actin filaments [2,3,5-triiodobenzoic acid (TIBA)] and microtubules (oryzalin) revealed that actin filaments, but not microtubules, play a pivotal role in chloroplast movement. Involvement of reactive oxygen species (ROS) in controlling chloroplast avoidance movement has been demonstrated, as exogenous H2O2 not only accelerated chloroplast avoidance but also could induce chloroplast avoidance even in weak blue light (WBL). Further support came from experiments with different ROS scavengers, i.e., dimethylthiourea (DMTU), KI, and CuCl2, which inhibited chloroplast avoidance, and from ROS localization using specific stains. Such avoidance was also partially inhibited by ZnCl2, an inhibitor of NADPH oxidase (NOX) as well as 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), a photosynthetic electron transport chain (ETC) inhibitor at PS II. However, methyl viologen (MV), a PS I ETC inhibitor, rather accelerated avoidance response. Exogenous calcium (Ca+2) induced avoidance even in WBL while inhibited chloroplast accumulation partially. On the other hand, chloroplast movements (both accumulation and avoidance) were blocked by Ca+2 antagonists, La3+ (inhibitor of plasma membrane Ca+2 channel) and ethylene glycol-bis(2-aminoethylether)-N,N,N',N'-tetraacetic acid (EGTA, Ca+2 chelator) while LiCl that affects Ca+2 release from endosomal compartments did not show any effect. A model on integrated role of ROS and Ca+2 (influx from apolastic space) in actin-mediated chloroplast avoidance has been proposed.

Interaction of Tritium with Elodea canadensis and Lemna minor.

Tritium is the least toxic radionuclide. The main contribution into the total tritium content in ecosystems is made by technogenic tritium, which is due to the operation of nuclear fuel cycle enterprises. The tritium content in the ecosystem of the River Yenisei is connected with its background values as well as with tritium entering the water ecosystem as a result of the operation of the Mining and Chemical Combine, MCC Rosatom. Presented here are the investigations of the possible transformation of tritium interacting with certain species of aqueous plants - submerged rnacrophyte Elodea canadensis and an aqueous plant floating on the surface of water reservoirs Lemna minor. Elodea sampling was made in a real water reservoir - the River Yenisei, while lemna was grown in the laboratory conditions. The experiments show that with the chronic exposure of young elodea shoots to tritium, the latter transforms from HTO to OBT. Optimal conditions were also obtained for the maximum transformation of tritium ≈35% from the total content: at 25°C and the light period 6/18 (day/night). In the experiments with duckweed, observed a significant increase in area of fronds in introducing tritium into the system.

Pollutant tracking for 3 Western North Atlantic sea grasses by remote sensing: Preliminary diminishing white light responses of Thalassia testudinum, Halodule wrightii, and Zostera marina.

Sea grasses are foundation species for estuarine ecosystems. The available light for sea grasses diminishes rapidly during pollutant spills, effluent releases, disturbances such as intense riverine input, and tidal changes. We studied how sea grasses' remote-sensing signatures and light-capturing ability respond to short term light alterations. In vivo responses were measured over the entire visible-light spectra to diminishing white-light on whole-living-plants' spectral reflectance, including 6h of full oceanic-light fluences from 10% to 100%. We analyzed differences by various reflectance indices. We compared the sea grasses species responses of tropical vs. temperate and intertidals (Halodule wrightii, and Zostera marina) vs. subtidal (Thalassia testudinum). Reflectance diminished with decreasing light intensity that coincided with greater accessory pigment stimulation (anthocyanin, carotenoids, xanthins). Chlorophyll a and Chlorophyll b differed significantly among species (Thalassia vs. Halodule). Photosynthetic efficiency diminished at high light intensities. The NDVI index was inadequate to perceive these differences. Our results demonstrate the leaf-level utility of data to remote sensing for mapping sea grass and sea grass stress.

Effects of copper-oxide nanoparticles, dissolved copper and ultraviolet radiation on copper bioaccumulation, photosynthesis and oxidative stress in the aquatic macrophyte Elodea nuttallii.

In this study, the uptake and sub-toxic effects of CuO nanoparticles (CuO-NPs), dissolved Cu(II) alone or in combination with UV radiation on the aquatic macrophyte Elodea nuttallii were studied. Emphasis was on Cu accumulation, growth, photosynthesis and the oxidative stress related enzymes peroxidase (POD) and superoxide dismutase (SOD). The results showed stronger Cu accumulation in plants exposed to 10 mg L(-1) CuO-NPs, corresponding to 1.4-2 mg L(-1) dissolved Cu(II), than to 256 µg L(-1) Cu(II). However, the ratio between the accumulated Cu and dissolved Cu in CuO treatments was lower than in Cu(II) treatments. Additional UV exposure increased accumulation in both treatments, with the effect being stronger for Cu accumulation from CuO-NPs than for dissolved Cu(II). Photosynthetic capacity was strongly reduced by UV treatment, whereas remained unaffected by Cu(II) or CuO-NP treatments. Similarly, the increase of SOD activity was more pronounced in the UV treatments. On the other hand, POD activity enhancement was strongest in the plants exposed to CuO-NPs for 24 h. Expression of the copper transporter COPT1 as revealed by RT-qPCR was inhibited by Cu(II) and CuO-NP treatment, limiting the uptake of excess Cu into the cells. Overall, the combined exposure of E. nuttallii to UV radiation with CuO-NPs or Cu(II) has a higher impact than exposure to CuO-NPs or Cu(II) alone. The results imply that heavy pollution of natural water with CuO-NPs or dissolved Cu might have stronger effects in combination with natural UV irradiation on organisms in situ.

Blue-light-induced rapid chloroplast de-anchoring in Vallisneria epidermal cells.

In the outer periclinal cytoplasm of leaf epidermal cells of an aquatic angiosperm Vallisneria, blue light induces "chloroplast de-anchoring", a rapid decline in the resistance of chloroplasts against centrifugal force. Chloroplast de-anchoring is known induced within 1 min of irradiation with high-fluence-rate blue light specifically, preceding the commencement of chloroplasts migration toward the anticlinal cytoplasm. However, its regulatory mechanism has remained elusive, although pharmacological analysis suggested that a calcium release from intracellular calcium stores is necessary for the response. In search of the responsible photoreceptors, immunoblotting analysis using antibodies against phototropins demonstrated that cross-reactive polypeptides of 120-kDa exist in the plasma-membrane fraction prepared from the leaves. In vitro phosphorylation analysis revealed that 120-kDa polypeptides were phosphorylated by exposure to blue light in a fluence-dependent manner. The blue-light-induced phosphorylation activity was sensitive to a Ser/Thr kinase inhibitor, staurosporine, and unusually was retained at a high level for a long time in darkness. Furthermore, phototropin gene homologs (Vallisneria PHOTOTROPIN1 and PHOTOTROPIN2) expressed in leaves were isolated. We propose that calcium-regulated chloroplast de-anchoring, possibly mediated by phototropins, is an initial process of the blue-light-induced avoidance response of chloroplasts in Vallisneria.

Chronic light reduction reduces overall resilience to additional shading stress in the seagrass Halophila ovalis.

Seagrasses have substantial capacity to survive long periods of light reduction, but how acclimation to chronic low light environments may influence their ability to cope with additional stress is poorly understood. This study examines the effect of temporal light reduction by adding two levels of shading to Halophila ovalis plants in two meadows with different light histories, one characterized by a low light (turbid) environment and the other by a relatively high light (clear) environment. Additional shading resulted in complete mortality for both shading treatments at the turbid site while the clear site showed a pattern of decreased shoot density and increased photochemical efficiency (Fv/Fm) with increased shading. These contrasting results for the same species in two different locations indicate that acclimation to chronic low light regimes can affect seagrass resilience and highlights the importance of light history in determining the outcome of exposure to further (short-term) stress.

[Cytogenetic studies on submerged plants from the Yenisei river area in the zone of radioactive contamination].

Cytogenetic studies on three species of submerged plants from different parts of the Yenisei river area subjected to radioactive impact of the Krasnoyarsk Mining-and-Chemical Plant and the Electrochemical Factory have been conducted. A high level of irregularities in anatelophase and metaphase of mitoses has been revealed in test samples compared to the control: agglutination and fragmentation of chromosomes, lagging chromosomes, bridges, fragments, misdivisions, and others. The natuie of the disorders indicates that they are related in part to the direct damage to the chromosome structure and in part to damage to the spindle.

Novel seed adaptations of a monocotyledon seagrass in the wavy sea.

Returning to the sea, just like invasion of land, has occurred in many groups of animals and plants. For flowering plants, traits adapted to the terrestrial environments have to change or adopt a new function to allow the plants to survive and prosper in the sea where water motion tends to rotate and move seeds. In this investigation, how seeds of the seagrass Thalassia hemprichii (Hydrocharitaceae), a common monocotyledon in the Indo-Pacific, adapt to the wavy environment was studied. Mature seeds were collected from Dongsha Atoll in South China Sea. The effects of light qualities on seed germination, the seed morphology, the unipolar distribution of starch granules in the endosperms and growth of root hair-like filamentous cells from basal surface of the seeds were all found to differ from those of terrestrial monocotyledons. Physiologically, germination of the seeds was stimulated by blue light rather than red light. Morphologically, the bell-shaped seeds coupled with the unipolar distribution of starch granules in the enlarged bases helped maintain their upright posture on the tidal seafloor. Growth of root hair-like filamentous cells from the basal surface of the seeds prior to primary root growth served to attach onto sediments, providing leverage and attachment required by the primary roots to insert into sediments. These filamentous cells grasped coral sand but not silicate sand, demonstrating a habitat preference of this species.

Regulatory effect of stems on sucrose-induced chlorophyll degradation and anthocyanin synthesis in Egeria densa leaves.

Detached green leaves of the aquatic plant Egeria densa showed chlorophyll degradation and turned red due to induced anthocyanin synthesis incubated in 0.1 M sucrose under continuous light for 7-10 days. If the leaves were placed in water, only chlorophyll degradation occurred and the detached leaves turned yellow. The levels of endogenous total carbohydrates increased in detached leaves cultured in the sucrose solution but only increased marginally in water. If the leaves were still attached to a piece of stem, with a node on either side of the single leaf whorl, then they did not accumulate anthocyanin in culture with 0.1 M sucrose. These leaves showed a similar increase in total carbohydrates and degradation of chlorophyll as detached leaves. Attached leaves, in which the midrib had been cut in situ, showed localized accumulation of anthocyanin in the leaf tissue distal to the cut in the midrib when cultured in 0.1 M sucrose. These results suggest that the stem plays a regulatory role in anthocyanin synthesis in attached leaves cultured in a sucrose solution but does not influence chlorophyll degradation or carbohydrate accumulation.

[Luminescent control of the functional condition of Vallisneria spiralis L. treated by microwave irradiation].

The influence of microwave emission on the Vallisneria spiralis L. photoluminescence (PL) is studied. It is found that the irradiation leads to degradation of all the luminescence spectrum bands. It is shown that the influence on the chlorophyll PL at 715-735 and 800 nm is not limited by accompanied temperature effect. The degradation of PL has a temperature threshold at the thermal treatment. The 725 nm luminescence band of chlorophyll is shifted to short waves at both the irradiation and thermal treatment. The shift dependence from the irradiation dose and temperature is non-monotonous.

Flavones and flavone glycosides from Halophila johnsonii.

Halophila johnsonii Eiseman is a shallow-water marine angiosperm which contains UV-absorbing metabolites. Studies on methanol extracts of H. johnsonii by means of HPLC-UV, NMR, HPLC-MS resulted in isolation and identification of seven previously unknown flavone glycosides: 5,6,7,3',4',5'-hexahydroxyflavone-7-O-beta-glucopyranoside (1), 5,6,7,3',4',5'-hexahydroxyflavone-7-O-(6''-O-acetyl)-beta-glucopyranoside (2), 6-hydroxyluteolin-7-O-(6''-O-acetyl)-beta-glucopyranoside (3), 6-hydroxyapigenin-7-O-(6''-O-acetyl)-beta-glucopyranoside (4), 6-hydroxyapigenin-7-O-(6''-O-[E]-coumaroyl)-beta-glucopyranoside (5), 6-hydroxyapigenin-7-O-(6''-O-[E]-caffeoyl)-beta-glucopyranoside (6) and 6-hydroxyluteolin-7-O-(6''-O-[E]-coumaroyl)-beta-glucopyranoside (7). Also isolated were three known flavone glycosides, 6-hydroxyluteolin 7-O-beta-glucopyranoside (8), scutellarein-7-O-beta-glucopyranoside (9), and spicoside (10), and five known flavones, pedalitin (11), ladanetin (12), luteolin (13), apegenin (14) and myricetin (15). Qualitative comparison of the flavonoid distribution in the leaf and rhizome-root portions of the plant was also investigated, with the aim of establishing the UV-protecting roles that flavonoids played in the sea grass.

Leaf photoacclimatory responses of the tropical seagrass Thalassia testudinum under mesocosm conditions: a mechanistic scaling-up study.

Here, the leaf photoacclimatory plasticity and efficiency of the tropical seagrass Thalassia testudinum were examined. Mesocosms were used to compare the variability induced by three light conditions, two leaf sections and the variability observed at the collection site. The study revealed an efficient photosynthetic light use at low irradiances, but limited photoacclimatory plasticity to increase maximum photosynthetic rates (P(max)) and saturation (E(k)) and compensation (E(c)) irradiances under high light irradiance. A strong, positive and linear association between the percentage of daylight hours above saturation and the relative maximum photochemical efficiency (F(V)/F(M)) reduction observed between basal and apical leaf sections was also found. The results indicate that T. testudinum leaves have a shade-adapted physiology. However, the large amount of heterotrophic biomass that this seagrass maintains may considerably increase plant respiratory demands and their minimum quantum requirements for growth (MQR). Although the MQR still needs to be quantified, it is hypothesized that the ecological success of this climax species in the oligotrophic and highly illuminated waters of the Caribbean may rely on the ability of the canopy to regulate the optimal leaf light environment and the morphological plasticity of the whole plant to enhance total leaf area and to reduce carbon respiratory losses.

[Radioecological monitoring of the Yenisei River and citological characterization of a submerged aquatic plant Elodea canadensis].

The study was devoted to investigation of the contents of radionuclides and of heavy metals and to evaluate the frequency of chromosomal aberrations in samples of Elodea canadensis, a submerged plant, collected in different parts of the Yenisei River. The samples were collected in the area subjected to radioactive impact of the Mining-and-Chemical Combine (MCC) at Zheleznogorsk and in the control area, upstream of the MCC. The investigations shown that elodea biomass in the area affected by MCC operation contained a long inventory of artificial radionuclides typical for the MCC discharges. The upstream of the MCC, in the control sampling area, the sediments and the elodea biomass contained only one artificial radionuclide--137Cs. Thus, the exposure doses to elodea shoots and roots upstream of the MCC are small (not more than 8 microGy/d) and the main contribution info the dose is made by natural radionuclides. At the MCC discharge site (the village of Atamanovo) and at the downstream of it, the total dose rate increases almost an order of magnitude, reaching its maximal values--72 microGy/d for elodea shoots and 58 microGy/d for its roots. Cytogenetic investigations of elodea roots shown that at the MCC discharge site (the village of Atamanovo) and at downstream of it the occurrence of chromosomal aberrations in ana-telophase and in metaphase cells of elodea was considerably higher than in the control area. It is highly probable that this simultaneous dramatic increase in the total exposure rate and the occurrence of chromosomal aberrations in elodea is associated with the radiation factor. It is suggested that elodea is affected not only by the radiation factor but also by the chemical factor--toxicity of heavy metals.

Form-function analysis of the effect of canopy morphology on leaf self-shading in the seagrass Thalassia testudinum.

The variation in seagrass morphology and the magnitude of leaf self-shading within the canopy of Thalassia testudinum, were compared among nine sites in a fringing reef lagoon. We found a significant variation in the growth-form of T. testudinum reflected in a 5.4-fold variation in the attenuation coefficient (K (d)) within the canopy. The largest morphological variation was observed in shoot density. Leaf biomass, leaf area index (LAI), and shoot density were positively associated with canopy-K (d) and with the percentage of surface irradiance received by the top of the seagrass canopy (% Es). These results provide an explanation for the consistent pattern of depth reduction in seagrass leaf biomass and shoot density reported in the literature. Shoot density and shoot size are two descriptors of the growth-form of T. testudinum related to its clonal life-form. Shoot size was not significantly correlated with canopy-K (d), nevertheless, it showed a significant effect on the slope of the relationship between shoot density and canopy-K (d). According to this model, shoot size also contributes to light attenuation within the seagrass canopy by increasing the effect of shoot density. This form-function analysis suggests that light may have a relevant role in the regulation of the optimal plant balance between horizontal (variation in shoot density) and vertical (variation in shoot size) growth of seagrasses. Other environmental factors and interactions also need to be examined to fully understand the mechanistic bases of the morphological responses of seagrasses to the environment.