Dissolved and particulate organic matter in contrasting Zostera marina (eelgrass) sediments

The influence of seagrass Zostera marina on sediment characteristics was examined in two contrasting sediments, one organic-rich and one organic-poor. The presence of plants leads to reduced sediment redox potential in both sediment types compared to bare sediment with the largest effects in the organic-poor sediment. Z. marina stimulated the sulfate reduction rates in organic-poor sediment with ∼50% and higher pools of dissolved organic carbon (DOC) were found. In contrast, sulfate reduction rates were lower in vegetated compared to bare sites in the organic-rich sediment. Despite a low contribution of dissolved carbohydrate (DCHO) to the DOC pool (<5%), the seagrass vegetation was responsible for an increase of ∼50% in DCHO pools with a peak in the root zone suggesting that Z. marina supplied DCHO to the pore waters. The Z. marina meadows also enhanced the contribution of particulate carbohydrate (PCHO) to sedimentary particulate organic carbon (POC) pools by 6-14% compared to bare sediment. Although the PCHO pools were higher in organic-rich than organic-poor sediments, the analyses of carbohydrate composition revealed that three groups of neutral sugars including glucose, galactose and mannose+xylose were the major compounds of PCHO and contributed with >60% to sedimentary carbohydrate pools at both sites. Only glucose showed depletion with depth in the vegetated sediments, whereas the percentage of ribose and rhamnose increased indicating a selective degradation of labile carbohydrates in the meadows. Galactose and mannose+xylose appeared to represent a refractory part of carbohydrate that remained after degradation of the more labile components. The sugar content was rather constant with depth at the bare organic-rich sediment indicating that only recalcitrant carbohydrate pools were buried. There was less difference in the PCHO composition profiles between vegetated and bare organic-poor sediments.     

Effects of filamentous algal mats on sulfide invasion in eelgrass (Zostera marina)

The effect of filamentous algae invasion into Zostera marina meadows on water quality, sediment sulfur pools and sulfide invasion into plant tissues was studied experimentally. Sulfide invasion was assessed through analysis of sulfur isotopic composition (δ³⁴S) and total sulfur (TS) concentrations in plant tissues. The algal mats (5 and 10 cm thickness) depleted oxygen in the mats and increased the pools of sulfides in the sediments. Plants exposed to algal mats had δ³⁴S signals closer to the δ³⁴S of sediment sulfide, whereas plants with no mats present had δ³⁴S signals closer to the δ³⁴S of seawater sulfate, indicating a higher sulfide invasion in plants exposed to algal mats. The δ³⁴S varied between the plant tissues with the leaves having more positive δ³⁴S signals than roots and rhizomes, indicating that sulfide was invading into the roots and moved to the other tissues through the lacunae. TS concentrations were higher in plants exposed to algal mats suggesting that sulfur derived from sediment sulfide accumulated in the plants. F_sulfide showed that up to 50% of the sulfides in the plants were derived from sedimentary sulfides. The combined effect of water column anoxia in the lower parts of the meadow and high sulfide invasion into the plants lead to significantly reduced growth rates after 3 weeks and the below-ground tissues showed signs of degradation suggesting that algal mats invasion in to Zostera marina meadows can result in seagrass decline.     

Sulfur accumulation in eelgrass (Zostera marina) and effect of sulfur on eelgrass growth

Eelgrass (Zostera marina) was grown under exposure to high levels of sediment sulfides to examine their ability to reoxidize sulfides intruding into the plants. The plants were kept under full light (control and high sulfide level) and at 10% of light saturation (high sulfide level) for 3 weeks and growth and accumulation of elemental sulfur (S⁰) in the plants were examined. The growth rate was reduced with ∼75% in the low light treatment, whereas there was no significant difference between the rates at full light saturation. S⁰ was accumulating in the below-ground structures of the plants exposed to high sulfide concentrations with highest concentration in the youngest roots and oldest internodes. There was no accumulation of S⁰ in the leaves, suggesting that the intruding sulfides were reoxidized in the below-ground structures before reaching the leaves. The accumulation of S⁰ was higher in the roots of the low light treatment (up to two times) suggesting a larger intrusion of sulfides. These plants also appeared highly affected by the treatment with rotting meristems and increased mortality after the 3-week growth period. These results are the first to show an accumulation of sulfur compounds internally in seagrasses as a result of reoxidation of sulfides. The reoxidation is facilitated by the internal transport of oxygen and is an example of the advantage of the internal lacunae system in seagrasses.     

Sulfur cycling and seagrass (Posidonia oceanica) status in carbonate sediments

Sulfur cycling was investigated in carbonate-rich and iron-poor sediments vegetated with Posidonia oceanica in oligotrophic Mediterranean around Mallorca Island, Spain, to quantify sulfate reduction and pools of sulfide in seagrass sediments. The oxygen penetration depth was low (< 4.5 mm) and sulfate reduction rates were relatively high (0.7–12 mmol m^–2d^–1). The total pools of reduced sulfides were remarkably low (< 5 mol S m^–2) indicating a fast turnover of reduced sulfides in these iron-poor sediments. The sulfate reduction rates were generally higher in vegetated compared to bare sediments possible due to enhanced sedimentation of sestonic material inside the seagrass meadows. The sulfate reduction rates were positively correlated with the seasonal variation in water temperature and negatively correlated with the shoot density indicating that the microbial activity was controlled by temperature and release of oxygen from the roots. The pools of reduced sulfides were low in these iron-poor sediments leading to high oxygen consumption for reoxidation. The sediments were highly anoxic as shown by relatively low oxygen penetration depths (< 4.5 mm) in these low organic sediments. The net shoot recruitment rate was negative in sediments enriched with organic matter, suggesting that organic matter enrichment may be an important factor for seagrass status in these iron-depleted carbonate sediments.     

Experimental test of biodeposition and ammonium excretion from blue mussels (Mytilus edulis) on eelgrass (Zostera marina) performance

Blue mussels and eelgrass have been found to coexist in many locations. However, knowledge of the interactions between these species is limited. Two experiments were conducted in the laboratory, a “Deposit” and an “Epiphyte” experiment. The Deposit experiment examined possible effects of increasing load of blue mussel (Mytilus edulis) biodeposits on sediment biogeochemistry and eelgrass (Zostera marina) performance. Z. marina mesocosms received normal or high loads of mussel biodeposits (Normal and High), while no biodeposits were added to the Control. High dosage had overall negative effects on Z. marina, which was reflected as lower leaf numbers and biomass and accumulation of elemental sulphur in rhizomes. The sediment biogeochemical conditions were altered, as the mussel biodeposits enhanced sulphate reduction rates and increased sulphide concentrations in the porewater, which may result in sulphide invasion and reduced growth of Z. marina.In the Epiphyte experiment effects of mussel excretion, with particular emphasis on ammonium, on the growth of Z. marina and their epiphytes were examined. A thick cover of epiphytes developed on Z. marina growing together with M. edulis, and the relative growth rate was reduced with 20% compared to plants from control without mussels. Overall the experiments showed negative effects on Z. marina growing together with M. edulis, thereby supporting a preceding field study by Vinther et al. [Vinther, H.F., Laursen, J.S., Holmer, M. 2008. Negative effects of blue mussel (Mytilus edulis) presence in eelgrass (Zostera marina) beds in Flensborg fjord, Denmark. Est. Coast Shelf. Sci. 77, 91-103.].     

Physiological Responses of Zostera marina and Cymodocea nodosa to Light-Limitation Stress

The effects of light-limitation stress were investigated in natural stands of the seagrasses Zostera marina and Cymodocea nodosa in Ria Formosa coastal lagoon, southern Portugal. Three levels of light attenuation were imposed for 3 weeks in two adjacent meadows (2–3 m depth), each dominated by one species. The response of photosynthesis to light was determined with oxygen electrodes. Chlorophylls and carotenoids were determined by high-pressure liquid chromatography (HPLC). Soluble protein, carbohydrates, malondialdehyde and phenol contents were also analysed. Both species showed evident signs of photoacclimation. Their maximum photosynthetic rates were significantly reduced with shading. Ratios between specific light harvesting carotenoids and the epoxidation state of xanthophyll cycle carotenoids revealed significantly higher light harvesting efficiency of C. nodosa, a competitive advantage in a low light environment. The contents of both soluble sugars and starch were considerably lower in Z. marina plants, particularly in the rhizomes, decreasing even further with shading. The different carbohydrate energy storage strategies found between the two species clearly favour C. nodosa''s resilience to light deprivation, a condition enhanced by its intrinsic arrangement of the pigment pool. On the other hand, Z. marina revealed a lower tolerance to light reduction, mostly due to a less plastic arrangement of the pigment pool and lower carbohydrate storage. Our findings indicate that Z. marina is close to a light-mediated ecophysiological threshold in Ria Formosa.     

Sulfate reduction in lake sediments inhabited by the isoetid macrophytes Littorella uniflora and Isoetes lacustris

Sulfur cycling was examined in sediments inhabited with the isoetids Littorella uniflora and Isoetes lacustris in the oligotrophic soft-water Lake Kalgaard, Denmark. Based on short-term tracer incubations sulfate reduction was measured along a transect from the shore (0.6 m) to profundal sediments (4.6 m). The sulfate reduction rates were low (0.008–0.8 mmol m⁻² d⁻¹) in the sandy shallow sediments with low organic content (<1.3 mmol C g⁻¹ sed DW) and high redox potentials (>100 mV), whereas sulfate reduction was higher at the deeper sites (2.7–4.6 mmol m⁻² d⁻¹) with high organic content (max. 11.5 mmol C g⁻¹ sed DW) and lower redox potentials (<100 mV). High concentrations of dissolved organic carbon (DOC) were found in the low particulate organic sediments (up to 18.4 mM), and most of the DOC pool consisted of acetate (40–77%). Reoxidation of sulfides due to root oxygen release was probably important at all sites and a positive efflux of sulfate across the sediment–water interface was measured, attaining rates (up to 4.8 mmol m⁻² d⁻¹) similar to the sulfate reduction rates. Reoxidation of sulfides was also manifested by high fraction (>80%) of reduced sulfides being accumulated as elemental sulfur or pyrite (chromium reducible sulfur, CRS). The largest pools of CRS were found in high organic sediment with vertical distributions resembling those of the sulfate reduction rates. The overall effect of isoetid growth on sulfur cycling in the rhizosphere is a suppression of sulfate reduction in low organic sediments and the governing of sulfide reoxidation in sediments with higher organic content.     

Local weak light induces the improvement of photosynthesis in adjacent illuminated leaves in maize seedlings

To copy with highly heterogeneous light environment, plants can regulate photosynthesis locally and systemically, thus, maximizing the photosynthesis of individual plants. Therefore, we speculated that local weak light may induce the improvement of photosynthesis in adjacent illuminated leaves in plants. In order to test this hypothesis, maize seedlings were partially shaded, and gas exchange, chlorophyll a fluorescence and biochemical analysis were carefully assessed. It was shown that local shading exacerbated the declines in the photosynthetic rates, chlorophyll contents, electron transport and carbon assimilation‐related enzyme activities in shaded leaves as plants growth progressed. While, the decreases of these parameters in adjacent illuminated leaves of shaded plants were considerably alleviated compared to the corresponding leaves of control plants. Obviously, the photosynthesis in adjacent illuminated leaves in shaded plants was improved by local shading, and the improvement in adjacent lower leaves was larger than that in adjacent upper ones. As growth progressed, local shading induced higher abscisic acid contents in shaded leaves, but it alleviated the increase in the abscisic acid contents in adjacent leaves in shaded plants. Moreover, the difference in sugar content between shaded leaves and adjacent illuminated ones was gradually increased. Consequently, local weak light suppressed the photosynthesis in shaded leaves, while it markedly improved the photosynthesis of adjacent illuminated ones. Sugar gradient between shaded leaves and adjacent illuminated ones might play a key role in photosynthetic regulation of adjacent illuminated leaves.     

Seasonal growth dynamics of Ruppia maritima L.s.l. and Halodule wrightii aschers. In Southern Texas and evaluation of sediment fertility status

Annual growth dynamics of Ruppia maritima L.s.l. and Halodule wrightii Aschers. at two southern Texas, U.S.A., coastal sites were compared using plant biomass to monitor production. While sparse Ruppia normally coexists with dense Halodule in southern Texas bays, these two sites contained extensive stands of dense Ruppia mixed with or adjacent to Halodule. Corresponding measurements of water-column salinity and temperature revealed that vigorous Ruppia growth correlated positively with cool spring temperatures and not with low water salinities. In contrast, Halodule growth increased only after warm summer temperatures were reached. Partitioning of biomass into above-ground and below-ground tissues is suggested as an important autecological factor in resource competition between the two species. Ruppia, with a maximum 31% below-ground biomass in mid-spring, could be out-competed by Halodule, with a minimum of 66% below-ground biomass, when sediment or other conditions become favorable for Halodule growth.Analyses of sediment interstitial water from Ruppia or Halodule grass beds showed differences in rhizosphere nutrient pools characteristic of each species. Ruppia-dominated beds contained up to 15 μM nitrate during spring, whereas Halodule beds showed only a trace (< 1 μM). Halodule-dominated beds showed consistently higher levels of sediment H₂S compared to Ruppia areas (130 μM vs. 50 μM, respectively, average autumn concentration in top 10 cm). Based on these measurable edaphic differences, growth responses to sediment fertilization with two distinct fertilizer formulations were tested: (1) Osmocote^TM, an ammonium nitrate-based inorganic mixture, and (2) Hyponex^TM an organic mixture derived from fish meal. Contrasting responses to these two sediment fertilizers were observed. Halodule showed growth stimulation in early autumn by the organic fish meal only, while Ruppia responded to both types of fertilizer in both spring and early autumn. It is postulated that Ruppia normally grows on low-nutrient sediments, whereas Halodule prefers organic-rich sediments with substantial sulfate reduction activity.     

Photosynthetic and morphological responses of eelgrass (Zostera marina L.) to a gradient of light conditions

Zostera marina L. (eelgrass) from Great Bay Estuary, New Hampshire and Maine (USA), was transplanted in outdoor mesocosms and subjected to four light treatments (100, 58, 34 and 11% surface irradiance, SI) between May and September 2003 to investigate the relationship between light availability and the growth and survival of eelgrass. Evaluating eelgrass seedlings and adult mature plants demonstrated no differences in photosynthetic response after 22 days of acclimation. During at least the first 19 days of shading, maximum electron transport rate (ETR_max) rate of eelgrass did not differ significantly between light treatments. After 40 days, a significant reduction in ETR_max and minimum saturating light was observed in plants growing at 34% SI and below. Morphological responses exhibited a linear increasing trend with greater light. 34% SI exhibited drastic reductions (to less than 25% of control) in rhizome growth, shoot density, shoot production, number of nodes per plant and plant weight at the end of the study (81 days). Shoot to root ratio at 34% SI increased by > 50%. Plants shaded to 58% SI showed no significant difference from the control in plant parameters except an increased rate of rhizome elongation. Our results link the lower shoot densities with shading to the slow growth rate of horizontal rhizomes and a total lack of lateral expansion at 11% SI. ETR_max declined over time in plants at 11% SI resulting in 81% mortality, no lateral branching and no morphological development, indicating that the minimum light required for long-term eelgrass growth and survival is greater than the previously suggested 11% SI. We demonstrate that eelgrass plants at these latitudes can persist at light levels of 58% SI and above, and are light-limited at 34% SI and below.     

Impact of fiddler crabs (Uca spp.) on rates and pathways of benthic mineralization in deposited mangrove shrimp pond waste

The impact of fiddler crabs (Uca spp.) on benthic mineralization rates and pathways in deposits of shrimp pond waste (SPW) with planted mangrove trees (Rhizophora apiculata) were determined in the Ranong mangrove forest, Thailand. Sediment metabolism, measured as CO₂ flux, increased by 2- to 3-fold when either fiddler crabs or mangrove trees were present compared to control plots. Sulfate reduction rates (SRR) were always high and partitioning of various electron acceptors to total carbon oxidation revealed that sulfate reduction contributed by >90%, with iron reduction being important only near the sediment-water interface. However, significant iron reduction appeared down to 7 cm when bioturbation and plant roots were combined in the easily oxidizable mangrove sediment (MS), indicating that infaunal activity and plant roots were able to alter the substratum. Microprofiles around individual Uca burrows showed 46% lower SRRs in a 15-mm-thick oxidized layer around the burrows compared to the surrounding sediment. The burrow wall environment appears to be a zone of intense reoxidation of reduced compounds as indicated by low pools of reduced sulfide compounds and a high Fe(III) content. Despite the decreased SRRs near the burrow, and the introduction of Fe(III) to deeper sediment layers, fiddler crabs and mangrove trees have only limited impact on rates and partitioning of anaerobic carbon mineralization in the SPW. This lack of response was attributed to the relative small volume of sediment affected by crab activities.     

Improved growth and water use efficiency of cherry saplings under reduced light intensity

Cherry (Prunus avium L.) saplings were grown under natural sunlight (controls) or moderate shading (up to 30%, depending on the incident light intensity and the hour of the day). Reduced light intensity increased the dry mass of each of the plant components studied. Consequently, the total dry mass of shaded plants was significantly greater than that of controls at the end of the growing season. However, the diurnal trend in the level of photosynthesis (per unit of leaf area) of shaded plants was similar to the controls in August, but lower in September. As the growing season proceeded, reduced photosynthetic rates, thinner mesophyll and larger specific leaf area in the shaded plants indicated that leaf development had adapted to shaded conditions throughout the growing season. It is suggested that increased growth of shaded plants was caused by a higher initial relative growth rate and a greater whole-plant photosynthesis. Shading consistently reduced transpiration over the season, therefore improving water use efficiency of shaded leaves. Our results suggest that a moderate reduction in light intensity can be a useful method for improving growth and saving water in hot and dry environments.     

Response of soybean photosynthesis and chloroplast membrane function to canopy development and mutual shading

The effect of natural shading on photosynthetic capacity and chloroplast thylakoid membrane function was examined in soybean (Glycine max. cv Young) under field conditions using a randomized complete block design. Seedlings were thinned to 15 plants per square meter at 20 days after planting. Leaves destined to function in the shaded regions of the canopy were tagged during early expansion at 40 days after planting. To investigate the response of shaded leaves to an increase in available light, plants were removed from certain plots at 29 or 37 days after tagging to reduce the population from 15 to three plants per square meter and alter the irradiance and spectral quality of light. During the transition from a sun to a shade environment, maximum photosynthesis and chloroplast electron transport of control leaves decreased by two- to threefold over a period of 40 days followed by rapid senescence and abscission. Senescence and abscission of tagged leaves were delayed by more than 4 weeks in plots where plant populations were reduced to three plants per square meter. Maximum photosynthesis and chloroplast electron transport activity were stabilized or elevated in response to increased light when plant populations were reduced from 15 to three plants per square meter. Several chloroplast thylakoid membrane components were affected by light environment. Cytochrome f and coupling factor protein decreased by 40% and 80%, respectively, as control leaves became shaded and then increased when shaded leaves acclimated to high light. The concentrations of photosystem I (PSI) and photosystem II (PSII) reaction centers were not affected by light environment or leaf age in field grown plants, resulting in a constant PSII/PSI ratio of 1.6 ± 0.3. Analysis of the chlorophyll-protein composition revealed a shift in chlorophyll from PSI to PSII as leaves became shaded and a reversal of this process when shaded leaves were provided with increased light. These results were in contrast to those of soybeans grown in a growth chamber where the PSII/PSI ratio as well as cytochrome f and coupling factor protein levels were dependent on growth irradiance. To summarize, light environment regulated both the photosynthetic characteristics and the timing of senescence in soybean leaves grown under field conditions.     

Experimental manipulation of sediment organic content and water column aeration reduces Zostera marina (eelgrass) growth and survival

The effect of synergy between sediment organic enrichment and lack of night oxygen renewal in the water column on growth and survival of Zostera marina, and how it is reflected in the sulfur parameters in the plants (δ³⁴S, TS and S⁰) was studied experimentally. An experiment consisting of Z. marina mesocosms with different levels of organic enrichment and water column aeration was established, and the effects on sediment conditions, sulfide invasion and growth and survival of Z. marina were examined over a 4 week period. Shoots growing in Ambient Organic matter-sediments showed signs of sulfide invasion, as TS increased in all plant compartments and δ³⁴S of the plant tissues decreased during the experiment, but neither growth rate nor survival were significantly affected. The lack of night oxygen renewal had no evident effects in non-enriched sediments as porewater sulfide concentrations, AVS- and CRS-pools were not different from the corresponding 24 h aeration treatment. Plant growth rate and survival were neither different from the corresponding 24 h aeration treatment. On the contrary, shoots growing on High Organic matter-sediments suffered a massive sulfide invasion and it was directly correlated to the observed decrease in growth rates. Even though the lack of night oxygen renewal had no evident effects on sediment variables there were, however, strong indications that the different aeration levels affected plant performance, suggesting a lower sulfide oxidation capacity and confirming that low water column oxygen concentrations reduces the defense capacity of the shoots against sulfide invasion.Although δ³⁴S, TS and S⁰ concentrations together provided a powerful set of indicators to detect the invasion of sulfide in Z. marina shoots, this study enlightens the need for a deeper investigation of sulfide intrusion in seagrasses and the relation between plant sulfur parameters and sediment conditions.     

Stimulation of sulfate reduction rates in Mediterranean fish farm sediments inhabited by the seagrass <Emphasis Type="Italic">Posidonia oceanica</Emphasis>

Sulfate reduction rates and biogeochemical parameters of fish farm sediments across the Mediterranean were investigated in the order to evaluate the potential effects of organic matter inputs on habitat quality for the common seagrass Posidonia oceanica. Four study sites were selected in Spain, Italy, Greece and Cyprus to represent the Mediterranean basin. P. oceanica was found in immediate vicinity of all the farms, which were located at physically exposed sites about 1 km from the shore lines. Organic matter accumulation, sulfate reduction rates and sulfur pools were measured in depth profiles along transects from the farms in both bare and vegetated sediments. Results show that although the organic matter accumulation was minor at the sites (POC < 2.8% DW), the sulfate reduction rates were high, in particular at the largest farm in Italy (up to 212 mmol m⁻² d⁻¹), similar to rates found at shallower, temperate fish farm sites, where higher sedimentation rates can be expected. Sulfate reducing bacteria in these low-organic, carbonate-rich Mediterranean sediments respond strongly to organic matter loadings and cause habitat degradation. Sulfate reduction rates measured in the P. oceanica sediments were among the highest recorded (7.8–42.0 mmol m⁻² d⁻¹) similar to rates found in degrading meadows impacted by organic matter loadings. As sulfate reduction rates were correlated with the sedimentation rates along the transects rather than organic matter pools this suggests mineralization processes were controlled by organic matter loading in fish farm sediments. The vegetated sediments near the net cages were more reduced due to accumulation of sulfides compared to control sites, which is a possible contributing factor to the observed seagrass decline in the farm surroundings. It is recommended that Mediterranean fish farms are placed in areas with rapid dispersal of particulate waste products to minimize organic matter loading of the sediments and thereby preserve habitat quality for benthic fauna and flora.     

Effects of decreasing source/sink ratio in soybeans on photosynthesis, photorespiration, transpiration and yield

Abstract. Shading of all side leaflets of a determinate soybean cultivar during pod filling significantly increased rates of photosynthesis in the unshaded centre leaflets, compared to centre leaflets of controls. Higher rates were associated with both higher stomatal and mesophyll conductances, and were reversible within 2 days when shades were removed. These higher rates of photosynthesis were not associated with decreased percentage enhancement by low oxygen, indicating that treatment effects were probably not associated with changes in photorespiration relative to photosynthesis. Percentage enhancement did, however increase as the plants approached physiological maturity, chiefly because of a decrease in photosynthesis.
In spite of these increases in rates of photosynthesis seed weight per plant was decreased by 37% in plants with side leaflets shaded for the entire pod-filling period and by 28% in plants shaded for only the second half of the period. In plants where shades were removed during the second half of pod filling seed yield was reduced by only 19% because shade removal delayed leaf senescence. The four treatments reduced yield by different mechanisms. Plants shaded continuously during pod filling produced fewer seeds than controls, but the weight per seed was similar. When shading was applied during the second half of pod fillings seed number was unchanged but weight per seed was significantly reduced. In contrast when shades were removed for the second half of pod filling, seed number remained similar to that of continuously shaded plants, but seed weight increased.
Although all shading treatments reduced yield, the reduction was not proportional to the 63% reduction in leaf area available for photosynthesis. This was because (1) photosynthetic rates in the centre leaflet of shaded plants were higher than rates in controls, (2) stem and lower surface photosynthesis in shaded leaf-lets contributed to whole leaflet photosynthesis.     

不同颜色遮阳网遮光对丘陵茶园夏秋茶和春茶产量及主要生化成分的影响

为研究不同颜色遮阳网遮光对夏秋茶与春茶产量和品质的影响,在中国科学院桃源农业生态试验站以“碧香早”茶树为试验材料,选用不遮光和中度遮光（遮光率(50?4)%）的黑色、绿色、银灰色遮阳网开展遮光试验。结果表明：不同季节使用黑色遮阳网能显著增加氨基酸和咖啡碱含量,降低茶多酚含量和酚氨比,与对照茶园比较,夏季、秋季、春季茶叶氨基酸含量分别增加了14.22%、17.07%、6.23%,茶多酚含量分别降低了4.40%、6.43%、27.66%,咖啡碱含量分别增加了9.48%、8.10%、7.28%。夏秋季银灰色遮阳网效果最差,绿色遮阳网效果介于黑色和对照之间,春季三种颜色遮阳网效果均优于对照;茶树遮光后品质明显优于对照,夏秋茶经遮光可达到制高档茶的要求,春茶遮光可达到制名优茶的要求。     

Photosynthetic carbon metabolism in potato leaves under changes in light intensity

Photosynthetic assimilation of ¹⁴CO₂ was examined in leaves of potato (Solanum tuberosum L.) plants that were grown under direct sunlight and then transferred to 50% irradiance for various periods. The rate of ¹⁴CO₂ assimilation correlated with light intensity: the photosynthetic rate reduced by 52% after 5-day shading and by 70% after 30-min shading. In all shaded and shade-adapted plants, the sucrose/hexose ratio decreased by a factor of 3.5–4.1; furthermore, the radioactivity of glycolate cycle metabolites and the serine/glycine ratio were lowered. In plants shaded for 5 days or 30 min, the radioactivity of aspartate and malate was higher than at continuous high irradiance, especially in plants shaded for 30 min, whereas a sudden illumination of the shaded plants reduced the radioactivity of these substances. We suppose that low irradiance averted the reentry of glycolate path carbon into the Calvin cycle and redirected this carbon source for the production of four-carbon acids that acidified the apoplast. This acidification activated the apoplastic invertase, which enhanced sucrose hydrolysis and hindered the sucrose export from the leaf. Hydrolysis of sucrose promoted the increase in osmolarity of the apoplastic solution, this increase being stronger at close distances to the stomatal pores where water is intensely evaporated. The increase in osmolarity of extracellular medium led to closing of stomata and the suppression of photosynthesis.     

Seasonal acclimation in metabolism reduces light requirements of eelgrass (Zostera marina)

We investigated the ability of eelgrass (Zostera marina) to adjust light requirements to seasonal changes in temperature, light and nutrient conditions through changes in metabolism, pigment and nutrient content. In agreement with expectations we found that rates of respiration and light saturated photosynthesis of summer acclimated plants peaked at higher temperatures (5 °C and 2 °C higher, respectively), and were lower than of winter acclimated plants, both at sub- and supra-optimal temperatures. Moreover respiration rates were generally more sensitive to increasing temperatures than photosynthetic rates, especially so for cold acclimated plants in February (36% higher Q₁₀-values). These changes were accompanied by a reduction in chlorophyll a and nitrogen concentrations in leaves by 35% and 60% respectively from February to August. The critical light requirement (E_C) of Z. marina to maintain a positive carbon balance increased exponentially with increasing temperature but less so for summer-acclimated than for winter-acclimated plants. However, combining E_C vs temperature models for whole-plants with data on daily light availability showed that seasonal acclimation in metabolism increased the annual period, when light requirements were meet at the 2-5 m depth interval, by 32-66 days. Hence, acclimation is an important mechanism allowing eelgrass to grow faster and penetrate to deeper waters. Critical depth limits estimated for different combinations of summer temperatures and water clarity in a future climate scenario, suggested that expected increases in temperature and nutrient run-off have synergistic negative effects, especially in clear waters, stressing the importance of continued efforts to improve water clarity of coastal waters.     

Black-band disease dynamics: Prevalence, incidence, and acclimatization to light

Black-band disease (BBD) is a temperature- and light-dependent syndrome typically found on shallow-dwelling corals. The bacterial consortium that creates BBD contains a photosynthetic cyanobacterium, which is particularly sensitive to light. High light causes the BBD cyanobacteria to self-shade, which encourages the formation of the anoxic microenvironment essential for the BBD consortium. From May to September 2009, a shallow-reef area was monitored to quantify the epizootiology of BBD on the brain coral Diploria strigosa. The prevalence and incidence of BBD were recorded approximately every two weeks. To test the hypothesis that reduced light would increase the rate of disease progression by reducing photoinhibition in the BBD cyanobacteria, shade structures, made out of plastic sheeting on a polyvinyl chloride (PVC) frame, were installed above D. strigosa colonies with BBD. The shade material reduced photosynthetically available radiation (PAR) by > 80%. The progression of BBD on shaded colonies was compared with cage control and control colonies. A pulse-amplitude-modulation fluorometer was used to measure the amount of PAR and maximum excitation pressure (Q_m) on each colony after two weeks of treatment. Linear progression rates and the band width of BBD were measured weekly and treatments were compared through time. During the summer months, the prevalence of BBD showed a positive, exponential relationship with water temperature, whereas the incidence of BBD displayed a positive linear relationship with the rate of change of water temperature. PAR on, and the Q_m of, shaded corals was significantly lower than the two control treatments; however, Q_m values did not indicate excessive photochemical pressure on the coral host. The progression rates of BBD and the black-band width on shaded colonies significantly increased after one week of shading, whereas the control treatments did not change. During the second week of manipulation, however, black-band width and progression rates were similar to values prior to manipulation. These results suggest that, even under low-light conditions, a compact and dense ‘black’ band is essential for the persistence of the BBD consortium.