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In order to meet increasing food demands in the future, we will need to improve the current crop productivity. Abiotic stresses like drought and salinity are major factors resulting in crop yield losses and soil degradation worldwide. Recent studies suggest that seaweed-based biostimulants could be a solution for this problem. Here we summarise the current findings of using these biostimulants and highlight current knowledge gaps. Seaweed extracts were shown to enhance nutrient uptake and improve growth performance in crops under stressed and normal conditions. Seaweed extracts contain phytohormones, polysaccharides, polyphenols, lipids, amino acids and proteins. Although it has been shown that some of these compounds are active and have growth-promoting properties on plants, their underlying molecular mechanism of action and optimal applications especially in crops exposed to abiotic stress remains understudied. Seaweed extracts were shown to also improve protein content of crops and contribute to a healthy soil by facilitating water retention, soil aeration and nutrient availability, thereby promoting plant growth. In this paper we review the role of these extracts and their bioactive compounds as plant biostimulants. The targeted application to improve crop performance and the impact of seaweed extracts for enhancing the protein content of crops are discussed.

Accurate species identification often relies on public repositories to compare the barcode sequences of the investigated individual(s) with taxonomically assigned sequences. However, the accuracy of identifications in public repositories is often questionable, and the names originally given are rarely updated. For instance, species of the Sea Lettuce (Ulva spp.; Ulvophyceae, Ulvales, Ulvaceae) are frequently misidentified in public repositories, including herbaria and gene banks, making species identification based on traditional barcoding unreliable. We DNA barcoded 295 individual distromatic foliose strains of Ulva from the North-East Atlantic for three loci (rbcL, tufA, ITS1). Seven distinct species were found, and we compared our results with all worldwide Ulva spp. sequences present in the NCBI database for the three barcodes rbcL, tufA and the ITS1. Our results demonstrate a large degree of species misidentification, where we estimate that 24%–32% of the entries pertaining to foliose species are misannotated and provide an exhaustive list of NCBI sequences reannotations. An analysis of the global distribution of registered samples from foliose species also indicates possible geographical isolation for some species, and the absence of U.lactuca from Northern Europe. We extended our analytical framework to three other genera, Fucus, Porphyra and Pyropia and also identified erroneously labelled accessions and possibly new synonymies, albeit less than for Ulva spp. Altogether, exhaustive taxonomic clarification by aggregation of a library of barcode sequences highlights misannotations and delivers an improved representation of species diversity and distribution.

Sea Lettuce (Ulva spp.; Ulvophyceae, Ulvales, Ulvaceae) is an important ecological and economical entity, with a worldwide distribution and is a well-known source of near-shore blooms blighting many coastlines. Species of Ulva are frequently misidentified in public repositories, including herbaria and gene banks, making species identification based on traditional barcoding hazardous. We investigated the species distribution of 295 individual distromatic foliose strains from the North East Atlantic by traditional barcoding or next generation sequencing. We found seven distinct species, and compared our results with all worldwide Ulva spp sequences present in the NCBI database for the three barcodes rbcL, tufA and the ITS1. Our results demonstrate a large degree of species misidentification in the NCBI database. We estimate that 21% of the entries pertaining to foliose species are misannotated. In the extreme case of U. lactuca, 65% of the entries are erroneously labelled specimens of another Ulva species, typically U. fenestrata. In addition, 30% of U. rigida entries are misannotated, U. rigida being relatively rare and often misannotated U. laetevirens. Furthermore, U. armoricana and U. scandinavica present as being synonymous to U. laetevirens. An analysis of the global distribution of registered samples from foliose species also indicates possible geographical isolation for some species, and the absence of U. lactuca from Northern Europe. Altogether, exhaustive taxonomic clarification by aggregation of a library of barcode sequences highlights misannotations, and delivers an improved representation of Ulva species diversity and distribution. This approach could be easily adapted to other taxa.

The upscaling of offshore wind farms in the North Sea creates opportunities for seaweed aquaculture that has the potential to meet part of future resource needs, provided that it is done sustainably. Here a follow-up study of the MIP project in 2018 “Development of Offshore Seaweed Cultivation: food safety, cultivation, ecology and economy” with a focus on ecology and cultivation is presented. In order to ensure a sustainable development of seaweed farming in Dutch offshore and coastal regions in the future, it is essential to collect empirical data on the interaction of seaweed cultivation with marine ecosystems for realistic impact assessments. In subproject 1 “Ecology – Fauna associated with seaweed aquaculture in the North Sea Farm” ecosystem services and impacts of seaweed farming in the North Sea were investigated on the basis of biodiversity, a key parameter for the functioning of ecosystems. Therefore in 2019 the associated fauna on growing seaweed biomass (Saccharina latissima) and cultivation ropes was assessed at the North Sea Farm. A high number of individuals was detected on the seaweeds and cultivation ropes in general (up to 7679 individuals per rope), but species richness was low. Abundance in fauna increased from May to June and all detected species are also known from other hard substrates in the North Sea. Compared to previous assessments of biodiversity with eDNA metabarcoding at the same site, the biodiversity detected in 2019 was very low. However, biodiversity levels may differ from year to year. Moreover, the samples were not taken at the same time points and are therefore not directly comparable and the methodology only included organisms that could be collected by hand (visible to the eye) with a focus on fauna attached to the rope and kelp. It is advised to combine classical morphological biodiversity assessment and eDNA metabarcoding in future assessments to compare results in order to determine the best-suited methodology for biodiversity assessments. Biodiversity in the seaweed farm should be assessed repeatedly every 5 years to check for temporal alterations in fauna composition, especially when cultivation structures, such as anchors, are deployed throughout several years. In subproject 2 “Cultivation” seasonal variation in biomass productivity and chemical composition of kelp was evaluated in order to determine the optimal time point for harvesting in relation to the desired end product. Biomass production at the test-farm was very low in 2019, compared to previous years and a seaweed farm test location near Helgoland in the North Sea. Below 4m environmental conditions for growth were unfavourable (mainly light limitation) for Saccharina latissima. Both in 2018 and 2019 large differences in standing crops over time and depth were observed. Contrary, true protein levels varied only slightly over time. If protein is the target product, final biomass yield of S. latissima will determine the profitability of the mariculture. A combination of economic analyses and growth experiments may assist in determining the optimal cultivation technique. The 2018 experiments performed at the North Sea Farm showed large seasonal variability in the chemical composition of seaweed tissue, and high amounts of nitrogen-containing compounds besides proteins variations. Therefore in 2019, nitrogen, starch and nitrate content in the seaweed tissue were analysed. Nitrate content in S. latissima varied throughout the season and could not fully explain the difference between N measured by Dumas and true protein content in the 2019 samples. Therefore, other seaweed components containing nitrogen must explain this variation, e.g. its accumulation in cellular nitrate pools. As a final note, in order to improve the understanding of environmental conditions in the farm it is recommended that nitrate and phosphate concentrations, two essential macronutrients for growth in seaweeds, should be assessed in the water column at different depths and time points.

Background: Since available arable land is limited and nitrogen fertilizers pollute the environment, cropping systems ought to be developed that do not rely on them. Here we investigate the rapidly growing, N2 -fixing Azolla/Nostoc symbiosis for its potential productivity and chemical composition to determine its potential as protein feed. Results: In a small production system, cultures of Azolla pinnata and Azolla filiculoides were continuously harvested for over 100 days, yielding an average productivity of 90.0-97.2 kg dry weight (DW) ha-1 d-1 . Under ambient CO2 levels, N2 fixation by the fern's cyanobacterial symbionts accounted for all nitrogen in the biomass. Proteins made up 176-208 g kg-1 DW (4.9 × total nitrogen), depending on species and CO2 treatment, and contained more essential amino acids than protein from soybean. Elevated atmospheric CO2 concentrations (800 ppm) significantly boosted biomass production by 36-47%, without decreasing protein content. Choice of species and CO2 concentrations further affected the biomass content of lipids (79-100 g kg-1 DW) and (poly)phenols (21-69 g kg-1 DW). Conclusions: By continuous harvesting, high protein yields can be obtained from Azolla cultures, without the need for nitrogen fertilization. High levels of (poly)phenols likely contribute to limitations in the inclusion rate of Azolla in animal diets and need further investigation. © 2018 The Authors. Journal of the Science of Food and Agriculture published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

In many countries wetlands are constructed or restored for removing nutrients from surface water. At the same time vegetated wetlands can act as carbon sinks when CO 2 is sequestered in biomass. However, it is well known that wetlands also produce substantial amounts of greenhouse gasses CH 4 and N 2 O. Especially N 2 O, resulting from nitrification and denitrification, is a very potent GHG. To assess the environmental sustainability of constructed wetlands the benefit of carbon sink and the downside of GHG emissions have to be evaluated. Since nutrient and carbon cycles in wetlands are complex and variable among wetlands and in time such a balance always contains uncertainties. Several studies have addressed this issue and indicated that CW can be either a sink or a source of CO 2 equivalents depending on the time scale of research and the environmental and management conditions involved. Here we balance carbon sequestration with CH 4 and N 2 O emissions in a multi-functional constructed wetland, dominated by emergent Phragmites vegetation. Detailed measurements were combined with a nitrogen budget, and all fluxes were expressed as a range indicating the uncertainties in measurements and extrapolation techniques. Measured methane emissions were variable and showed clear relationship with temperature and density of the emergent vegetation. Average CH 4 emissions in the vegetation were 7.8 at 15 °C and 24.5 mg m −2 h −1 at 24 °C. Estimated N 2 O emissions ranged from 0.5 to 1.9 g m −2 y −1 . After converting the fluxes to CO 2 equivalents we concluded that the Lankheet constructed wetland is most likely a sink of CO 2 in the present conditions. Annual net sequestration of CO 2 amounts 0.27–2.4 kg m −2 y −1 which represents 12–67% of the CO 2 fixation in the biomass. N 2 O emissions represent a substantial part of the total effect of GHG emissions (12–29%) and should not be disregarded in budget studies. We acknowledge the limitations and uncertainties of our estimates, however, we are confident that our findings contribute to assessing the environmental sustainability of constructed wetlands.

Sustainable agriculture demands reduced input of man-made nitrogen (N) fertilizer, yet N2 fixation limits the productivity of crops with heterotrophic diazotrophic bacterial symbionts. We investigated floating ferns from the genus Azolla that host phototrophic diazotrophic Nostoc azollae in leaf pockets and belong to the fastest growing plants. Experimental production reported here demonstrated N-fertilizer independent production of nitrogen-rich biomass with an annual yield potential per ha of 1200 kg−1 N fixed and 35 t dry biomass. 15N2 fixation peaked at noon, reaching 0.4 mg N g−1 dry weight h−1. Azolla ferns therefore merit consideration as protein crops in spite of the fact that little is known about the fern’s physiology to enable domestication. To gain an understanding of their nitrogen physiology, analyses of fern diel transcript profiles under differing nitrogen fertilizer regimes were combined with microscopic observations. Results established that the ferns adapted to the phototrophic N2-fixing symbionts N. azollae by (1) adjusting metabolically to nightly absence of N supply using responses ancestral to ferns and seed plants; (2) developing a specialized xylem-rich vasculature surrounding the leaf-pocket organ; (3) responding to N-supply by controlling transcripts of genes mediating nutrient transport, allocation and vasculature development. Unlike other non-seed plants, the Azolla fern clock is shown to contain both the morning and evening loops; the evening loop is known to control rhythmic gene expression in the vasculature of seed plants and therefore may have evolved along with the vasculature in the ancestor of ferns and seed plants.

In Western-Europe, agricultural practices have contributed to environmental problems such as eutrophication of surface and ground water, flooding, drought and desiccation of surrounding natural habitats. Solutions that reduce the impact of these problems are urgently needed. Common reed (Phragmites australis) is capable of sanitizing surface water and may function as green energy source because of its high productivity. Here, the results of an experiment in a constructed wetland in the Netherlands are presented where two different sanitation treatments were compared. Depending on the residence time and volume per unit area, reed is capable to reduce the total amount of nitrogen in the water with average efficiencies from 32 to 47% and the total amount of phosphorous with 27–45%. Although biomass production still varies largely between different parts of the constructed wetland, a rapid increase in biomass was observed since planting. Constructed wetlands with reed provide opportunities to improve water quality and reed produces enough biomass to serve as green energy source. Moreover, these wetlands also function as a flood water reservoir and are possibly advantageous for biodiversity. The optimal moment of reed harvesting depends on the goal of the owner. This moment should be chosen wisely, as it may have consequences for reed filter regeneration, biomass production, biodiversity, methane emission and water sanitation efficiency.

Summary Results are presented of a grassland restoration experiment on a humic sandy soil and on a nutrient rich humic clay. The effect of cutting time and frequency, sod stripping and a rise in ground water level is studied on the decrease in dry matter and nutrient yield and changes in botanical composition. When the annual production is lower than 6 tonnes a second cut hardly contributes to a faster soil impoverishment. But a second cut creates a favourable vegetation structure for seedling establishment. Sod stripping to 5 cm caused only for a short period a lower production, but it stimulates the establishment of species from the soil seedbank. The yield of N and P is lower in the longer run, that of K is very low with and without sod stripping. The changes in yield and concentration of K in the biomass show that the availability of K is very low and possibly limits the further decrease in the dry matter production. Establishment and generative spread of plant species in the undisturbed sward are low...

Are there intrinsic differences in the rates of photosynthesis, shoot- and root-respiration between inherently fast- and slow-growing monocotyledons at high and low nitrogen supply? To analyze this question we grew 5 monocotyledons, widely differing in their inherent relative growth rate at high and low nitrogen supply in a growth room. Nitrate was exponentially added to the plants, enabling us to compare inherent differences in plant characteristicr, without any effect of species differences in the ability to take up nutrients. At high nitrogen supply, the fast-growing species from productive habitats had a higher photosynthetic nitrogen use efficiency and rate of root respiration than the slow-growing ones from unproductive habitats

Leprince, C, van der Werf, A., Deltour, R. and Lambers, H. 1992. Respiratory pathways in germinating maize radicles correlated with desiccation tolerance and soluble sugars. - Physiol. Plant. 85: 581-588. We have assessed the activities of tbe cytochrome and alternative pathways in total respiration and their role in each stage of germination of Zea mays L. radicles. Throughout imbibition, the salicylhydroxamic acid (SHAM) concentration needed to inhibit the cyanide-resistant pathway, without any side effects, decreased from 15 mM in quiescent embryos to 5 mM at 72 h after imbibition. Electrons predominantly flowed through the cytochrome pathway although the alternative pathway was already present at early imbibition. The capacity of the alternative path was about 70% of the control rate of respiration, Its engagement progressively increased from 18% after 10 min of imbibition to 70% at the radicle emergence and then decreased to 50% at 96 h after imbibition, concomitant with the onset of radicle growth. The alternative pathway was. however, not essential for germination. The observed activity of the alternative path correlated with the monosaceharide (glucose -\- fructose) content, suggesting that the alternative pathway could be acting according to the 'energy overflow model'. On the other hand, up to 24 b after imbibition at !6°C. maize radicles tolerate a severe desiccation, becoming intolerant at 72 h. On reimbibition of tolerant radicles, respiration increased immediately and the alternative pathway was rapidly engaged. At 72 h, no respiration was measured, indicating a total loss of the respiratory systems. The possible correlation between carbohydrate content, loss of desiccation tolerance and activity of the two respiratory pathways is discussed.

Contains fulltext : 35312.pdf (Publisher’s version ) (Closed access) The results of a single publication stating that terrestrial plants emit methane has sparked a discussion in several scientific journals, but an independent test has not yet been performed. Here it is shown, with the use of the stable isotope C-13 and a laser-based measuring technique, that there is no evidence for substantial aerobic methane emission by terrestrial plants, maximally 0.3% (0.4 ng g(-1) h(-1)) of the previously published values. Data presented here indicate that the contribution of terrestrial plants to global methane emission is very small at best. Therefore, a revision of carbon sequestration accounting practices based on the earlier reported contribution of methane from terrestrial vegetation is redundant.

Canopy photosynthesis models have predicted an optimal leaf area index (LAI; leaf area per unit surface area) and leaf nitrogen distribution at which whole-plant carbon gain per unit N is maximized. In this study we experimentally tested these models, using transgenic PSAG12-IPT tobacco (SAG; Nicotiana tabacum L.) plants with delayed leaf senescence and therefore a greater LAI and more uniform N distribution than the wild type (WT). In a competition experiment, the increased density of surrounding WT plants caused a greater reduction in dry mass of mature SAG target plants than in that of WT target plants, indicating negative effects of delayed leaf senescence on performance at high canopy density. Vegetative SAG plants achieved a lower calculated daily carbon gain than competing WT plants because the former retained leaves with a negative carbon gain in the shaded, lower part of the canopy. Sensitivity analyses showed that the carbon gain of SAG plants would increase if these lower leaves were shed and the N reallocated from these leaves were used to form additional leaf area at the canopy top. This strategy, which is adopted by the WT, is most advantageous because it results in the shading of competing neighbors. Keywords: carbon gain, senescence, transgenic plants, competition, canopy light gradient.

Carex acutiformis and Brachypodium pinnatum were grown with a uniform distribution of photosynthetic photon flux density (PFD) with height, and in a vertical PFD gradient similar to the PFD gradient in a leaf canopy. Distribution of organic leaf N and light-saturated rates of photosynthesis were determined. These parameters were also determined on plants growing in a natural vegetation stand. The effect of a PFD gradient was compared with the effect of a leaf canopy. In Brachypodium, plants growing in a vegetation stand had increasing leaf N with plant height. However, distribution of leaf N was not influenced by the PFD gradient treatment. The gradient of leaf N in plants growing in a leaf canopy was not due to differences within the long, mostly erect, leaves but to differences between leaves. In Carex, however, the PFD gradient caused a clear increase of leaf N with height in individual leaves and thus also in plants. The leaf N gradient was similar to that of plants growing in a leaf canopy. Leaf N distribution was not affected by nutrient availability in Carex. In most cases, photosynthesis was positively related to leaf N. Hence, lightsaturated rates of photosynthesis increased towards the top of the plants growing in leaf canopies in both species and, in Carex, also in the PFD gradient, thus contributing to increased N use efficiency for photosynthesis of the whole plant. It is concluded that in Carex the PFD gradient is the main environmental signal for leaf N allocation in response to shading in a leaf canopy, but one or more other signals must be involved in Brachypodium.

Why do inherently fast-growing species from productive habitats generally have a higher rate of biomass production in short-term low-nitrogen experiments than slow-growing species from unproductive habitats, whereas the opposite is found in long-term experiments? Is this mainly due to inherent differences in biomass allocation, leaf characteristics or the plants' physiology? To analyse these questions we grew five monocotyledonous species from productive and unproductive habitats in a climate chamber at both high and low nitrogen supply. Nitrate was supplied exponentially, enabling us to compare inherent differences in morphological and physiological traits between the species, without any interference due to differences in the species' ability to take up nutrients. At high nitrogen supply, we found major inherent differences in specific leaf area and nitrogen productivity, i.e. daily biomass increment per unit plant nitrogen, where-as there were only small differences in net assimilation rate, i.e. daily biomass increment per unit leaf area, and biomass partitioning. We propose that the higher specific leaf area and nitrogen productivity of inherently fast-growing species are the key factors explaining their high abundance in productive habitats compared with inherently slow-growing ones. At low nitrogen supply, the net assimilation rate was decreased to a similar extent for all species, compared with that at high nitrogen supply. The nitrogen productivity of the inherentlyfast-growing species decreased with decreasing nitrogen supply, whereas that of the inherently slow-growing species remained constant. There were no inherent differences in nitrogen productivity in this treatment. At this low nitrogen supply, the inherently fast-growing species invested relatively more biomass in their roots that the slow-growing ones did. The inherently fast-growing species still had a higher specific leaf area at low nitrogen supply, but the difference between species was less than that at high nitrogen supply. Based on the present results and our optimization model for carbon and nitrogen allocation (Van der Werf et al. 1993a), we propose that the relatively large investment in root biomass of fast-growing species is the key factor explaining their higher biomass production in short-term experiments. We also propose that in the long run the competitive ability of the slow-growing species will increase due to a lower turnover rate of biomass. It is concluded that the plant's physiology (net assimilation rate and nitrogen productivity), only plays a minor role in the species' competitive ability in low-nitrogen environments.

This study describes the O2 uptake characteristics of intact roots of Brachypodium pinnatum. In the presence of 25 mM salicylhydroxamic acid (SHAM), concentrations of KCN below 3.5 νM had no effect on the rate of root respiration, whereas in the absence of 25 mM SHAM a significant inhibition of approx. 18% was observed. This indicates that an O2-consuming reaction, not associated with the cytochrome pathway, the alternative pathway or the “residual component”, operates in the absence of any inhibitors in roots of B. pinnatum. We demonstrate here that this fourth O2-consuming reaction is mediated by a peroxidase. A peroxidase which catalyzed O2 reduction in the presence of NADH was readily washed from the roots of B. pinnatum. This peroxidase was stimulated by 5 mM SHAM, whereas ascorbic acid, catalase, catechol, gentisic acid, low concentrations potassium cyanide (3.5 μM), sodium azide, sodium sulfide, superoxide dismutase and high concentrations SHAM (25 mM) inhibited this reaction. Except for high concentrations of SHAM and concentrations of KCN higher than approx. 3.5 μM, these effectors could not be used to inhibit the peroxidase-mediated O2 uptake in intact roots of B. pinnatum. Concentrations of SHAM below 10 mM stimulated O2 uptake up to 15% of the control rate, depending on concentration, whereas 25 mM SHAM inhibited O2 uptake by 35%. The stimulation at low concentrations resulted from a SHAM-stimulated peroxidase activity, whereas 25 mM SHAM completely inhibited both the peroxidase-mediated O2 uptake and the activity of the alternative pathway. A method is presented for determining the relative contributions of each of the four O2-consuming reactions, i.e. the cytochrome pathway, the alternative pathway, the “residual component” and the peroxidase-mediated O2 uptake. The peroxidase-mediated O2 uptake contributed 21% to the total rate of oxygen uptake in roots of B. pinnatum, the cytochrome pathway contributed 41%, the alternative pathway 14% and the “residual component” 24%.

van der Werf, A., Kooijman, A., Welschen, R. and Lambers, H. 1988. Respiratory energy costs for the maintenance of biomass, for growth and for ion uptake in roots of Carex diandra and Carex acutiformis. - Physiol. Plant. 72: 483–491. The respiratory characteristics of the roots of Carex diandra Schrank and Carex acutiformis Ehrh. were investigated. The aims were, firstly to determine the respiratory energy costs for the maintenance of root biomass, for root growth and for ion uptake, and secondly to explain the higher rate of root respiration and ATP production in C. diandra. The three respiratory energy components were derived from a multiple regression analysis, using the relative growth rate and the net rate of nitrate uptake as independent variables and the rate of ATP production as a dependent variable. Although the rate of root respiration and ATP production was significantly higher in C. diandra than in C. acutiformis, the two species showed no significant difference in their rate of ATP production for the maintenance of biomass, in the respiratory energy coefficient for growth (the amount of ATP production per unit of biomass produced) and the respiratory energy coefficient for ion uptake (amount of ATP production per unit of ions absorbed). It is concluded that the higher rate of root respiration of C. diandra is caused by a higher rate of nitrate uptake. At relatively high rates of growth and nitrate uptake, the contribution of the rate of ATP production for ion uptake to the total rate of ATP production amounted to 38 and 25% for C. diandra and C. acutiformis, respectively. At this growth rate, the respiratory energy production for growth contributed 37 and 50%, respectively, to the total rate of ATP production. The relative contribution of the rate of ATP production for the maintenance of biomass increased from 25 to 70% with increasing plant age for both species. The results suggest that ion uptake is one of the major sinks for respiratory energy in roots. These experimentally derived values for the rate of ATP production for the maintenance of biomass, the respiratory energy coefficient for growth and the respiratory energy coefficient for ion uptake are discussed in relation to other experimentally and theoretically derived values.

Effects of vesicular-arbuscular mycorrhizal (VAM) infection and P on root respiration and dry matter allocation were studied in Plantago major L. ssp. pleiosperma (Pilger). By applying P, the relative growth rate of non-VAM controls and plants colonized by Glomus fasciculatum (Thaxt. sensu Gerdemann) Gerdemann and Trappe was increased to a similar extent (55-67%). However, leaf area ratio was increased more and net assimilation rate per unit leaf area was increased less by VAM infection than by P addition. The lower net assimilation rate could be related to a 20 to 30% higher root respiration rate per unit leaf area of VAM plants. Root respiration per unit dry matter and specific net uptake rates of N and P were increased more by VAM infection than by P addition. Neither the contribution of the alternative respiratory path nor the relative growth rate could account for the differences in root respiration rate between VAM and non-VAM plants. It was estimated that increased fungal respiration (87%) and ion uptake rate (13%) contributed to the higher respiratory activity of VAM roots of P. major.

A significant portion (12–29%) of the carbohydrate produced daily in photosynthesis is respired in the roots. This value varies with age of the plants, growth conditions and the species under investigation (Lambers, 1987). Not only the proportion of photosynthates used in root respiration, but also the rate of root respiration, measured as oxygen uptake, varies two- to threefold between species, even when plants are grown under similar conditions (Lambers et al., 1983). This variation does not correlate with that of the alternative path in root respiration, though there are major variations of this parameter as well (Lambers et al., 1983). What then could explain these differences in the rate of respiration?