Shade-induced response and recovery of the seagrass Posidonia sinuosa

The effect of shading on the seagrass Posidonia sinuosa Cambridge et Kuo was investigated to identify mechanisms that prolong its survival during periods of low light and permit its subsequent recovery. We also tested whether the responses were consistent in plants growing at different depths. Shade treatments were low (LS; 70 – 100% of ambient Photosynthetic Photon Flux Density), medium (MS; 12 – 39%) and heavy (HS; 5 – 4%) at the shallow (3 – 4 m) site, whilst the deep (7 – 8 m) site had no HS treatment. HS at the shallow and MS at the deep site were below minimum light requirements (MLR) for the long-term survival of P. sinuosa. Physiological, morphological and growth attributes were repeatedly measured during 198 d of shade treatments and a subsequent 384 d recovery period at ambient PPFD. Shoot density declined by 82% within 105 d under HS treatment, though 6% of shoots remained after 198 d. We estimate that complete shoot loss in HS would have taken 2 years. Rhizome sugar concentrations declined to 32 – 52% of the controls at the end of the most severe shading treatments but after shoot loss, sugar concentrations declined more slowly or increased, suggesting a return to positive carbon balance. In the treatments below MLR, shading induced changes in physiological, morphological and growth characteristics, including reduced leaf length and width, reduced δ¹³C and photosynthetic adaptation to low light (increased α, reduced E_k and ETR_max), though not consistently. After removal of shading, photosynthetic characteristics became more typical of high light adaptation, possibly induced by greater light penetration through the thinned canopy, including reversal of the changes in α, E_k and ETR_max and induction of non-photochemical quenching. Carbohydrate concentrations increased to ambient concentrations within 115 d at ambient PPFD. Recovery of shoot density was slow, remaining significantly reduced in the MS and HS treatments after 384 d recovery. Shoot density at the end of shading is an important determinant of the rate seagrass meadows will recover and we estimated that the moderately and heavily shaded meadows would require 3.5 to 5 years to recover.     

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.     

Effect of shading of Zostera marina (eelgrass) on sulfur cycling in sediments with contrasting organic matter and sulfide pools

Eelgrass Zostera marina was collected in spring and autumn from a light-saturated environment with low-organic sediments and a light-limited environment with organic-rich sediments in Denmark. The eelgrass and sediment responses to reduced light conditions were studied in 2-week shading experiments. Z. marina responded to reduced light conditions by decreasing growth rates and a loss of above-ground biomass. The spring plants were most sensitive to light reductions and the relative leaf elongation rates were reduced with up to 58% and the shoot densities with 33-36%. There was no difference in light response in relation to sediment organic matter contents. The sulfate reduction rates were reduced in the shaded low-organic sediments with up to 67%, whereas there was no effect of shading on rates in the organic-rich sediments. The lack of effect of shading in the organic-rich sediments was attributed to a limited coupling between Z. marina production and sediment bacterial carbon cycling. In contrast to the sulfate reduction rates, the pools of reduced sulfur were increased with up to 89% in the shaded, low-organic sediments, suggesting that the reoxidation of sulfides was reduced. Shading had no effect on the pools of sulfides in the organic-rich sediments due to much larger pools of sulfides. The enhanced sensitivity of spring plants to shading was probably due to a low above- to below-ground ratio compared to the autumn plants, which limited the plant-mediated oxidation of the sediments and thus the reoxidation of sulfides. The shaded plants were possibly more exposed to anoxic and sulfidic conditions affecting their growth and survival.     

Effects of salinity and water temperature on the ecological performance of Zostera marina

We tested the effects of salinity and water temperature on the ecological performance of eelgrass (Zostera marina L.) in culture-experiments to identify levels that could potentially limit survival and growth and, thus, the spatial distribution of eelgrass in temperate estuaries. The experiments included eight levels of salinity (2.5, 5, 10, 15, 20, 25, 30 and 35‰) and seven water temperatures (5, 10, 15, 20, 25, 27.5 and 30 °C). Low salinity (i.e. 5 and 2.5‰) increased mortality (3–6-fold) and had a strong negative effect on shoot morphology (number of leaves per shoot reduced by 40% and shoot biomass reduced by 30–40%), photosynthetic capacity (P_max—reduced by 30–80%) and growth (production of new leaves reduced by 50–60%, leaf elongation rate reduced by 60–70% and production of side-shoots reduced by 40–60%), whereas eelgrass performed almost equally well at salinities between 10 and 35‰. The optimum salinity for eelgrass was between 10 and 25‰ depending on the response parameter in question. Extreme water temperatures had an overall negative impact on eelgrass, although via different mechanisms. Low water temperatures (5 °C) slowed down photosynthetic rate (by 75%) and growth (production of new leaves by 30% and leaf elongation rate by 80%), but did not affect mortality, whereas high temperatures (25–30 °C) increased mortality (12-fold) and lowered both photosynthetic rate (by 50%) and growth (production of new leaves by 50% and leaf elongation rate by 75%). The optimum water temperature for eelgrass appeared to lie between 10 and 20 °C. These results show that extreme conditions may affect the fitness of eelgrass and, thus, may potentially limit its distribution in coastal and estuarine waters.     

Carbon and nitrogen translocation in response to shading of the seagrass Posidonia sinuosa

Translocation of carbon (C) and nitrogen (N) was investigated in response to shading of the seagrass Posidonia sinuosa in control (ambient light) and shade (below minimum light requirement) treatments after 10 d shading. A mature leaf was incubated in situ in ¹³C- and ¹⁵N-enriched seawater for 2 h and the appearance of the isotopes in the young leaf and adjacent rhizome monitored over 29 d. C and N isotopes gradually reduced in the mature leaf: of ¹⁵N contained in the entire shoot (mature leaf, young leaf and 4 cm rhizome), 95% (control) and 97% (shade) was found in the mature leaf after 2 h incubation and only 75% and 60% remained in the mature leaf after 29 d; 98% and 94% of ¹³C was found in the mature leaf after 2 h, and it had reduced to 36% and 44% after 29 d. This corresponded to an equal increase in the young leaf + rhizome indicating that the mature leaf is a source of these nutrients to the young leaf and rhizome. C translocation from mature leaves was not significantly affected by the shade treatment. In contrast, there was an increase in ¹⁵N taken up by the mature leaves (1.9× higher in the shade), the percent of ¹⁵N translocated to the young leaf and rhizome (24% in control and 40% in shade) and N concentration in the young leaf (1.24% control and 1.41% shade) and rhizome (0.86% control and 0.99% shade). Resorption of C and N was also estimated from changes in the total C and N content of the mature leaf over 29 d. N resorption from the mature leaf contributed up to 63% of young leaf N requirements in the control treatment but only 41% in the shade treatment. We conclude that uptake and translocation of N by mature leaves is a response to shading in P. sinuosa and would provide additional N to growing leaves, enhancing light harvesting efficiency.     

Effects of light availability on growth, architecture and nutrient content of the seagrass Zostera noltii Hornem

The growth vs. irradiance response of the seagrass Zostera noltii from Cadiz Bay Natural Park (southwestern Spain) was characterised. Plants were exposed along 14 days to different light treatments (1%, 7%, 42% and 100% surface irradiance, SI), using shade screens in an outdoor mesocosm. Growth at 100% SI (1.6 mg DW plant⁻¹ day⁻¹) was lower than that at 42% SI (2.4 mg DW plant⁻¹ day⁻¹), suggesting photoinhibition. The minimum light requirement estimated was 0.8 mol photons m⁻² day⁻¹ (2% SI). Light availability affected the pattern of plant development and the overall plant growth. The contribution of the apical shoots to the aboveground production was nearly constant (c.a. 1.13 cm plant⁻¹ day⁻¹) regardless of the light level (except at 1% SI). In contrast, recruitment and growth of lateral shoots arising from the main rhizome axes accounted for the observed differences in aboveground growth. Rhizome branching was only observed at 42% SI. The possibility of a light threshold for rhizome branching could explain the seasonality of shoot recruitment, as well as the observed decrease in shoot density along depth (or light) gradients in seagrass meadows. Carbon demands at low irradiances (1% and 7% SI) were partially met by mobilization of carbohydrate reserves (sucrose in belowground and starch in aboveground parts). Plant nitrogen content decreased with increasing light, especially in belowground parts, reaching critical levels for growth.     

Decomposition of Phragmites australis rhizome in a shallow lake

Decomposition of Phragmites australis (Cav. Trin ex Steudel) rhizome was studied at Lake Fert?/Neusiedler See using the litter bag technique. Samples were analysed for rhizome dry mass, fibre (cellulose, hemicellulose, lignin) and nutrient content (C, N, P and S), litter-associated fungal biomass, potential microbial respiration (electron transport activity: ETS) and cellulolitic bacteria. The mass loss of decomposing rhizome was rapid in the initial period and only 13.6% of the dry mass remained at the end of the experiment during 953 days. Substantial quantities of C, N, S and P were lost during 99 days; only 18% C, 19% N, 14% S and 6.4% of the P remained after 953 days. Hemicellulose degraded more rapidly than the other fibres whilst the lignin had the slowest rate of decomposition. Bacteria were found to be the primary colonizers of plant detritus, which was followed by fungal growth. An antagonistic relationship was observed between bacteria and fungi. Fungal biomass as determined by ergosterol concentrations ranged between 4.1 and 420 μg g⁻¹ and peaked every year in September. The number of cellulolitic bacteria varied from 0 to 22 MPN g⁻¹ with higher values in summer. The ETS-activity ranged between 0.1 and 1.6 mg O₂ g⁻¹ h⁻¹. The changes in ETS-activity varied almost in parallel with the in situ temperature of the lake water.     

Change in the thermal biology of tadpoles of Odontophrynus occidentalis from the Monte desert, Argentina: Responses to photoperiod

We determined if the photoperiod regime affects the thermal biology of the tadpoles of Odontophrynus occidentalis from the Monte desert (Argentina). Variables measured were: selected body temperature (T_sel), critical thermal maximum (CT_max) and thermal critical minimum (CT_min). The tadpoles were acclimated to 15±2 °C for 15 days, and they were divided in three experimental groups: 24 h light, 24 h dark and 12 h/12 h light/dark. Data indicate that the photoperiod had an important effect upon the thermal biology of the Odontophrynus occidentalis tadpoles. The treatment group exposed to 24 h of light showed the highest selected temperature and thermal extremes. We suggest that changes in photoperiod may allow these organisms to anticipate the future changes in their thermal environment, as longer days usually involve higher temperatures.     

Physiological responses to flooding and light in two tree species native to the Amazonian floodplains

In Amazonian floodplains, plant survival is determined by adaptations and growth strategies to effectively capture sunlight and endure extended periods of waterlogging. By measuring gas exchange, quantum efficiency of photosystem 2 (PSII), and growth parameters, we investigated the combined effects of flooding gradients and light on two common evergreen floodplain tree species, the light-tolerant Cecropia latiloba and the shade-tolerant Pouteria glomerata. Individual plants were subjected to different combinations of light and flooding intensity in short-term and long-term experiments. Plants of C. latiloba lost all their leaves under total submersion treatments (plants flooded to apex and with reduced irradiance) and showed highest maximum assimilation rates (A_max) in not flooded, high light treatments (6.1 μmol CO₂ m⁻² s⁻¹). Individuals of P. glomerata showed similar patterns, with A_max increasing from 1.9 μmol CO₂ m⁻² s⁻¹ under total flooding to 7.1 μmol CO₂ m⁻² s⁻¹ in not flooded, high light treatments. During the long-term flooding experiment, quantum efficiency of PSII (F_v/F_m) of C. latiloba was not affected by partial flooding. In contrast, in P. glomerata F_v/F_m decreased to values below 0.73 after 120 days of total flooding. Moreover, total submergence led P. glomerata to reduce significantly light saturation point (LSP), as compared to C. latiloba. For both species morphological adjustments to long-term flooding, such as the production of adventitious roots, resulted in reduced total biomass, relative growth rate (RGR) and leaf mass ratio (LMR). Growth increase in C. latiloba seemed to be more limited by low-light than by flooding. Therefore, the predominant occurrence of this species is in open areas with high light intensities and high levels of inundation. In P. glomerata flooding induced high reductions of growth and photosynthesis, whereas light was not limiting. This species is more abundant in positions where irradiance is reduced and periods of submergence are slightly modest. We could show that the physiological requirements are directly responsible for the flooding (C. latiloba) and shade (P. glomerata) tolerance of the two species, which explains their local distribution in Amazonian floodplain forests.     

Light-dependent phosphate uptake of a submersed macrophyte Myriophyllum spicatum L.

The uptake kinetics of phosphate (Pi) by Myriophyllum spicatum was determined from adsorption and absorption under light and dark conditions. Pi uptake was light dependent and showed saturation following the Michaelis-Menten relation (in light: V = 16.91 × [Pi](1.335 + [Pi]), R² = 0.90, p < 0.001; in the dark: V = 5.13 × [Pi](0.351 + [Pi]), R² = 0.77, p < 0.001). Around 77% of the loss of Pi in the water column was absorbed into the tissue of M. spicatum, and only 23% was adsorbed on the surface of the plant shoots. Our study shows that M. spicatum shoots have a much higher affinity (in light: 3.9 μmol g⁻¹ dw h⁻¹ μM⁻¹; in the dark: 3.7 μmol g⁻¹ dw h⁻¹ μM⁻¹) and V_max (maximum uptake rate, shoot light) for Pi uptake than many other aquatic macrophytes (in light: 0.002-0.23 μmol g⁻¹ dw h⁻¹ μM⁻¹; in the dark: 0.002-0.19 μmol g⁻¹ dw h⁻¹ μM⁻¹), which may provide a competitive advantage over other macrophytes across a wide range of Pi concentrations.     

Biomass and primary production of a 8–11 m depth meadow versus <3 m depth meadows of the seagrass Cymodocea nodosa (Ucria) Ascherson

Current knowledge about the abundance, growth, and primary production of the seagrass Cymodocea nodosa (Ucria) Ascherson is biased towards shallow (depth <3 m) meadows although this species also forms extensive meadows at larger depths along the coastlines. The biomass and primary production of a C. nodosa meadow located at a depth of 8–11 m was estimated at the time of maximum annual vegetative development (summer) using reconstruction techniques, and compared with those available from shallow meadows of this species. A depth-referenced data base of values at the time of maximum annual development was compiled to that end. The vegetative development of C. nodosa at 8–11 m depth was not different from that achieved by shallow (depth <3 m) meadows of this species. Only shoot density, which decreased from 1637 to 605 shoots m⁻², and the annual rate of elongation of the horizontal rhizome, which increased from 23 to 71 cm apex⁻¹ year⁻¹, were different as depth increased from <3 to 8–11 m. Depth was a poor predictor of the vegetative development and primary production of C. nodosa. The biomass of rhizomes and roots decreased with depth (g DW m⁻² = 480 (±53, S.E.) − 32 (±15, S.E.) depth (in m); R² = 0.12, F = 4.65, d.f. = 35, P = 0.0381) which made total biomass of the meadow to show a trend of decrease with depth but the variance of biomass data explained by depth was low. The annual rate of elongation of the horizontal rhizome showed a significant positive relationship with depth (cm apex⁻¹ year⁻¹ = 18 (±5.1, S.E.) + 5.0 (±1.33, S.E.) depth (in m); R² = 0.50, F = 14.07, d.f. = 14, P = 0.0021). As shoot size and growth did not change significantly with depth, the reduction of shoot density should drive any changes of biomass and productivity of C. nodosa as depth increases. The processes by which this reduction of C. nodosa abundance with depth occur remain to be elucidated.     

Landscape-level seagrass–sediment relations in a coastal lagoon

We investigated the relationships between sediment (subaqueous soil) properties and eelgrass (Zostera marina L.) distribution to develop landscape-level soil-based strategies for choosing eelgrass restoration locations. Subaqueous soils were sampled and eelgrass cover determined for 14 soil-landscape units within a 116 ha area of Ninigret Pond, a coastal lagoon in Rhode Island, USA. Of the 14 soil-landscape units sampled for eelgrass cover, 52% had virtually no eelgrass cover (<10%), while 18% had high eelgrass cover (>90%). The Lagoon Bottom, Shallow Lagoon Bottom, Flood-tidal Delta Slope, and Barrier Cove subaqueous soil-landscape units had the highest eelgrass cover (66–100%). A weak relationship between eelgrass cover and water depths (r² = 0.10) was observed suggesting that properties other than water depth may also control eelgrass distribution. Subaqueous soils on landscapes with >60% eelgrass cover had relatively high levels of acid-volatile sulfides (>90 μg/g), high soil salinity levels (34–44 ppt), fine textures (silt loam), and relatively high total nitrogen levels (>0.15%). Four principal components accounted for 81% of the variability in eelgrass cover. The first component reflected particle-size distribution (i.e. sand, silt, and clay contents) effects and accounted for 43% of the variability. The other components suggested that eelgrass cover is correlated to carbonaceous remains, non-calcareous rock fragments and soil salinity. These data suggest that the current distribution of eelgrass within the study area is strongly influenced by physical and chemical subaqueous soil characteristics. Soil survey techniques proved useful for the delineation of sediment characteristics (e.g. texture, salinity) that influence eelgrass distribution patterns at landscape-level scales.     

Diurnal variation in relative photosynthetic performance in giant kelp Macrocystis pyrifera (Phaeophyceae, Laminariales) at different depths as estimated using PAM fluorometry

Pulse-amplitude modulated (PAM) fluorometry allows instantaneous estimates of photosynthetic rates, but may well produce variable measurements of photosynthetic activity depending on time of day, recent light history, internal fluctuations, and environmental variability. To investigate this, we compare estimates of diurnal variability in relative photosynthetic performance for the giant kelp, Macrocystis pyrifera (L.) C. Agardh, obtained from PAM fluorometry at three depths during 3 days characterized by different light conditions, and for two different blade ages. Sampling in the mid morning, late morning, early afternoon and late afternoon, we examined diurnal changes in relative photosynthetic performance in meristematic tissue and older blades occurring near the bottom, in the mid water, and at the water surface. Measures of maximum relative electron transport rates (rETR_max), minimum saturating irradiance (E_k), photosynthetic efficiency (α) and maximum quantum yield (F_v/F_m) show that giant kelp blades in the mid water and near the bottom exhibit little to no photosynthetic changes during the day. Near the surface, however, blades exhibit photosynthetic characteristics similar to light-adapted species in that they begin the day acclimated to low light, acclimate to increasing irradiance during the day, and end the day acclimated to low light. Consequently, while estimates of rETR_max were highest during the midday for all sample depths and days, they were also always highest near the surface for both old blades (112.16 ± 8.7, 98.6 ± 14.7, 70.16 ± 5.7) and meristematic tissue (109.0 ± 9.0, 86.9 ± 1.9, 59.2 ± 11.6, surface, mid water and bottom, respectively). Similar patterns were observed for E_k for both old blades (169.2 ± 5.4, 88.0 ± 11.2, 83.8 ± 5.2) and meristematic tissue (138.4 ± 11.5, 96.6 ± 4.69, 68.4 ± 10.6). In contrast, estimates of F_v/F_m were lowest near the surface during the midday for both old blades (0.6 ± 0.02, 0.73 ± 0.69, 0.75 ± 0.01) and meristematic tissue (0.58 ± 0.02, 0.69 ± 0.05, 0.74 ± 0.01, surface, mid water and bottom, respectively). These patterns coincided with similar patterns in ambient light, which was most variable and reached its greatest values near the surface during the midday.     

Clonal integration improves compensatory growth in heavily grazed ramet populations of two inland-dune grasses

Herbaceous species possess several mechanisms to compensate for tissue loss. For clonal herbaceous species, clonal integration may be an additional mechanism. This may especially hold true when tissue loss is very high, because other compensatory mechanisms may be insufficient. On inland dunes in northern China, we subjected Bromus ircutensis and Psammochloa villosa ramets within 0.5 m×0.5 m plots to three clipping treatments, i.e., no clipping, moderate (50% shoot removal) and heavy clipping (90% shoot removal), and kept rhizomes at the plot edges connected or disconnected. Moderate clipping did not reduce ramet, leaf or biomass density of either species. Under moderate clipping, rhizome connection significantly improved the performance of Psammochloa, but not that of Bromus. Heavy clipping reduced ramet, leaf and biomass density in the disconnected plots of both species, but such negative effects were negated or greatly ameliorated when the rhizomes were connected. Therefore, clonal integration contributed greatly to the compensatory growth of both species. The results suggest that clonal integration is an additional compensatory mechanism for clonal plants and may be important for their long-term persistence in the heavily grazed regions in northern China.     

Stomatal development and associated photosynthetic performance of capsicum in response to differential light availabilities

The mechanisms of capsicum growth in response to differential light availabilities are still not well elucidated. Hereby, we analyzed differential light availabilities on the relationship between stomatal characters and leaf growth, as well as photosynthetic performance. We used either 450–500 μmol m⁻² s⁻¹ as high light (HL) or 80–100 μmol m⁻² s⁻¹ as low light (LL) as treatments for two different cultivars. Our results showed that the stomatal density (SD) and stomatal index (SI) increased along with the leaf area expansion until the peak of the correlation curve, and then decreased. SD and SI were lower under the LL condition after three days of leaf expansion. For both cultivars, downregulation of photosynthesis and electron transport components was observed in LL-grown plants as indicated by lower light- and CO₂-saturated photosynthetic rate (P _max and RuBP_max), quantum efficiency of photosystem II (PSII) photochemistry (Φ_PSII), electron transport rate (ETR) and photochemical quenching of fluorescence (q_p). The observed inhibition of the photosynthesis could be explained by the decrease of SD, SI, Rubisco content and by the changes of the chloroplast. The low light resulted in lower total biomass, root/shoot ratio, and the thickness of the leaf decreased. However, the specific leaf area (SLA) and the content of leaf pigments were higher in LL-treatment. Variations in the photosynthetic characteristics of capsicum grown under different light conditions reflected the physiological adaptations to the changing light environments.     

ACCLIMATION OF PHOTOSYNTHESIS AND PIGMENTS DURING AND AFTER SIX MONTHS OF DARKNESS IN PALMARIA DECIPIENS (RHODOPHYTA): A STUDY TO SIMULATE ANTARCTIC WINTER SEA ICE COVER1

The influence of seasonally fluctuating photoperiods on the photosynthetic apparatus of Palmaria decipiens (Reinsch) Ricker was studied in a year‐round culture experiment. The optimal quantum yield (F_v/F_m) and the maximal relative electron transport rate (ETR_max), measured by in vivo chl fluorescence and pigment content, were determined monthly. During darkness, an initial increase in pigment content was observed. After 3 months in darkness, ETR_max and F_v/F_m started to decrease considerably. After 4 months in darkness, degradation of the light‐harvesting antennae, the phycobilisomes, began, and 1 month later the light harvesting complex I and/or the reaction centers of PSII and/or PSI degraded. Pigment content and photosynthetic performance were at their minimum at the end of the 6‐month dark period. Within 24 h after re‐illumination, P. decipiens started to accumulate chl a and to photosynthesize. The phycobiliprotein accumulation began after a time lag of about 7 days. Palmaria decipiens reached ETR_max values comparable with the values before darkness 7 days after re‐illumination and maximal values after 30 days of re‐illumination. Over the summer, P. decipiens reduced its photosynthetic performance and pigment content, probably to avoid photodamage caused by excess light energy. The data show that P. decipiens is able to adapt to the short period of favorable light conditions and to the darkness experienced in the field.     

Salt sensitivity of the vegetative and reproductive stages in chickpea (Cicer arietinum L.): Podding is a particularly sensitive stage

Soil salinity is an increasing problem, including in regions of the world where chickpea is cultivated. Salt sensitivity of chickpea was evaluated at both the vegetative and reproductive phase. Root-zone salinity treatments of 0, 20, 40 and 60 mM NaCl in aerated nutrient solution were applied to seedlings or to older plants at the time of flower bud initiation. Even the reputedly tolerant cultivar JG11 was sensitive to salinity. Plants exposed to 60 mM NaCl since seedlings, died by 52 d without producing any pods; at 40 mM NaCl plants died by 75 d with few pods formed; and at 20 mM NaCl plants had 78-82% dry mass of controls, with slightly higher flower numbers but 33% less pods. Shoot Cl exceeded shoot Na by 2-5 times in both the vegetative and reproductive phase, and these ions also entered the flowers. Conversion of flowers into pods was sensitive to NaCl. Pollen from salinized plants was viable, but addition of 40 mM NaCl to an in vitro medium severely reduced pollen germination and tube growth. Plants recovered when NaCl was removed at flower bud initiation, adding new vegetative growth and forming flowers, pods and seeds. Our results demonstrate that chickpea is sensitive to salinity at both the vegetative and reproductive phase, with pod formation being particularly sensitive. Thus, future evaluations of salt tolerance in chickpea need to be conducted at both the vegetative and reproductive stages.     

Environmental factors regulating the radial oxygen loss from roots of Myriophyllum spicatum and Potamogeton crispus

Myriophyllum spicatum and Potamogeton crispus are common species of shallow eutrophic lakes in north-eastern Germany, where a slow recovery of the submersed aquatic vegetation was observed. Thus, the characterisation of the root oxygen release (ROL) as well as its implication for geochemical processes in the sediment are of particular interest. A combination of microelectrode measurements, methylene blue agar and a titanium(III) redox buffer was used to investigate the influence of the oxygen content in the water column on ROL, diel ROL dynamics as well as the impact of sediment milieu. Oxygen gradients around the roots revealed a maximum oxygen diffusion zone of up to 250 μm. During a sequence with a light/dark cycle as well as alternating aeration of the water column, maximum ROL with up to 35% oxygen saturation at the root surface occurred under light/O₂-saturated conditions. A decrease to about 30% was observed under dark/O₂-saturated conditions, no ROL was detected at dark/O₂-depleted conditions and only a weak ROL with 5–10% oxygen saturation at the root surface was measured under light but O₂-depleted water column. These results indicate, that during darkness, ROL is supplied by oxygen from the water column and even during illumination and active photosynthesis production, ROL is modified by the oxygen content in the water column. Visualisation of ROL patterns revealed an enhanced ROL for plants which were grown in sulfidic littoral sediment in comparison to plants grown in pure quartz sand. For both plant species grown in sulfidic littoral sediment, a ROL rate of 3–4 μmol O₂ h⁻¹ plant⁻¹ was determined with the Ti(III) redox buffer. For plants grown in pure quartz sand, the ROL rate decreased to 1–2 μmol O₂ h⁻¹ plant⁻¹. Hence, aside from the oxygen content in the water column, the redox conditions and microbial oxygen demand in the sediment has to be considered as a further major determinant of ROL.     

Inoculation of Rhizobium (VR-1 and VA-1) induces an increasing growth and metal accumulation potential in Vigna radiata and Vigna angularis L. growing under fly-ash

Fly-ash-tolerant Rhizobium strains were isolated from plants grown in fly-ash-contaminated soil, axenically under laboratory conditions. Saplings of both plants were raised in N₂-free Jenson medium and inoculated with 2.6 × 10⁸ cell ml⁻¹ and 5.2 × 10⁸ cell ml⁻¹ of culture after 10 d of growth. Plants were transferred into 100% fly-ash under natural condition. Rhizobium-inoculated plants grown on 100% fly-ash showed marked increase in relation to root-shoot length, biomass yield, photosynthetic pigment, protein content and nodulation frequency compared to uninoculated plant grown in control (100% fly-ash). Inoculation of fly-ash-tolerant Rhizobium increased the accumulation of Fe, Zn, Cu Cd and Cr in different tissues vis-à-vis enhanced translocation of metals to the aboveground part of plant. Although inoculation of fly-ash-tolerant Rhizobium strains (VR-1 and VA-1) enhanced the translocation of more Fe to shoot parts, nevertheless, the amount of Rhizobium inoculants supplied to the plant was found to be very important since it has a positive role in increasing plant growth through increased N₂ supply via nitrogenase activity. Results suggest that an integrated approach employing biotechnological means and inoculation of plants with host-specific fly-ash-tolerant Rhizobium strain may prove a stimulus to a fly-ash management programme.     

Light response characteristics of a morphologically diverse group of southern hemisphere conifers as measured by chlorophyll fluorescence

Unlike northern hemisphere conifer families, the southern family, Podocarpaceae, produces a great variety of foliage forms ranging from functionally broad-, to needle-leaved. The production of broad photosynthetic surfaces in podocarps has been linked qualitatively to low-light-environments, and we undertook to assess the validity of this assumption by measuring the light response of a morphologically diverse group of podocarps. The light response, as apparent photochemical electron transport rate (ETR), was measured by modulated fluorescence in ten species of this family and six associated species (including five Cupressaceae and one functionally needle-leaved angiosperm) all grown under identical glasshouse conditions. In all species, ETR was found to increase as light intensity increased, reaching a peak value (ETR_max) at saturating quantum flux (PPFD_sat), and decreasing thereafter. ETR_max ranged from 217 μmol electrons · m⁻² · s⁻¹ at a PPFD_sat of 1725 μmol photons · m⁻² · s⁻¹ in Actinostrobus acuminatus to an ETR of 60 μmol electrons · m⁻² · s⁻¹ at a PPFD_sat of 745 μmol electrons · m⁻² · s⁻¹ in Podocarpus dispermis. Good correlations were observed between ETR_max and both PPFD_sat and maximum assimilation rate measured by gas-exchange analysis. The effective quantum yield at light saturation remained constant in all species with an average value of 0.278 ± 0.0035 determined for all 16 species. Differences in the shapes of light response curves were related to differences in the response of non-photochemical quenching (q _n), with q _n saturating faster in species with low PPFD_sat. Amongst the species of Podocarpaceae, the log of average shoot width was well correlated with PPFD_sat, wider leaves saturating at lower light intensities. This suggests that broadly flattened shoots in the Podocarpaceae are an adaptation to low light intensity. Received: 15 April 1996 / Accepted: 30 September 1996