Non-limiting Nutrient Conditions Research Articles

Errata and re-visitation of “What is the limit for photoautotrophic plankton growth rates?” (Flynn and Raven, 2017)

Abstract An error in our original work prompts a revisitation of factors constraining photoautotrophic plankton growth rates (μmax). Ribulose-1,5-bisphosphate Carboxylase-Oxygenase does not itself provide that constraint, but we identify other factors that result in our previously suggested value of ~2 doublings per day still likely being representative of the maximum for most photoautotrophs. μmax likely evolves to balance the advantage of possessing a high competitive value while minimizing the stresses incurred when the organism is incapable of routinely achieving a higher μmax due to various limiting factors. Organisms with extreme high μmax are thus expected to grow under conditions that provide the necessary environment (stable pH, non-limiting nutrients and light) for sufficient time that the evolution of higher μmax becomes advantageous. Conditions in nature allowing the evolution of higher μmax include the exploitation of an exceptional opportunity and then entering stasis (e.g. desert microalgae), or a situation where high grazing pressures match high phytoplankton growth, thus maintaining non-limiting nutrient and light conditions. The latter, however, conflicts with the paradox of enrichment, as only under resource limitation would the necessary stability be attained in the predator–prey dynamic. Ultimately, ecology, not biophysics, constrains phototroph μmax.

The role of a microbial consortium on gas exchange and water relations in Allium cepa L. under water and nutritional deficit conditions

Synergistic studies of microorganisms in the last decade have been mostly directed towards their biofertilizing effects on growth and crop yield. Our research examines the role of a microbial consortium (MC) on physiological responses of Allium cepa hybrid F1 2000 under water and nutritional deficit in a semi-arid environment. An onion crop was established with normal irrigation (NIr) (100% ETc) and water deficit (WD) (67% ETc) and different fertilization treatments (MC with 0%, 50% and 100% NPK). Gas exchange (Stomatal conductance (Gs), transpiration (E) and CO2 assimilation rates (A)) and leaf water status were evaluated throughout its growth cycle. The MC + 50% NPK treatment with NIr maintained similar A rates to the production control. A. cepa decreased Gs by approximately 50% in the WD treatment. The highest water use efficiency (WUE) and an increase in the modulus of elasticity in response to water stress were obtained for the 100% NPK treatment under non-inoculated WD. The onion hybrid F1 2000 was tolerant to water stress and under non-limiting nutrient conditions, irrigation may be reduced. The MC facilitated the availability of nutrients under NIr allowing a 50% reduction in the application of high doses of fertilization without affecting yield, resulting in a suitable agroecological strategy for this crop.

Effect of residence time in continuous photobioreactor on mass and energy balance of microalgal protein production

There is increasing interest in new protein sources for the food and feed industry and for the agricultural sector, and microalgae are considered a good alternative, having a high protein content and a well-balanced amino acid profile. However, protein production from microalgae presents several unsolved issues, as the biomass composition changes markedly as a function of cultivation operating conditions. Continuous systems, however, may be properly set to boost the accumulation of protein in the biomass, ensuring stable production. Here, two microalgae and two cyanobacterial species were cultivated in continuous operating photobioreactors (PBR) under nonlimiting nutrient conditions, to study the effects of light intensity and residence time on both biomass and protein productivity at steady state. Although light strongly affected biomass growth inside the PBR, the overall protein pool did not vary in response to irradiance. On the other hand, shorter residence times resulted in protein accumulation of up to 68 % in cyanobacteria, in contrast with green algae, where a minor influence of residence time on biomass composition was observed. Energy balance showed that light conversion to protein decreased with light intensity. Protein content was also related to energy costs for cell maintenance. In conclusion, it is shown that residence time is the key variable to increase protein content and yield of protein production, but its effect depends on the specific species.

Weed competition in organic and no-till conventional soils under nonlimiting nutrient conditions

AbstractSome well-managed organic soils are known to have higher crop yield potential than conventionally managed soils due to the greater soil quality and the ability to tolerate weed competition. However, low available soil mineral N and P in some organic systems may mask such soil quality–related benefits. We hypothesize that when plant-available N and P are not limiting, tillage-based highly diverse organic crop rotations have less yield loss (better crop tolerance) due to weed competition and higher crop yields than no-till conventional systems with low-diversity rotations. A greenhouse study was carried out in Saskatoon, Canada, using long-term (18-yr) organically managed soils (ORG) and no-till conventional soils (CONV) with three crop rotation diversities (LOW, MEDIUM, and HIGH) to compare the crop tolerance to weed competition under standard soil nutrient management conditions and under excess supply of mineral N and P. Under fertilized conditions, crop biomass increased by 50% and 69% in ORG and CONV systems, respectively. Weed biomass was similar between ORG and CONV systems under nonfertilized conditions but was 14% greater in CONV when excessive N and P were supplied. Crop biomass loss (crop tolerance) was not different among cropping systems under excess fertilizer or under standard fertilizer levels. Even with greater weed biomass under fertilized conditions, the CONV system showed crop tolerance similar to that of the ORG system. Under nonfertilized conditions, the crop biomass yield was 43% lower in ORG compared with CONV, and even after mineral N and P were applied, ORG systems showed less (17%) crop biomass than CONV. Further, differences in crop tolerance were not identified among crop rotations under both fertilizer levels. Overall, this study revealed that there were no yield benefits or better crop tolerance to weed competition in organically managed soils compared with no-till conventional soils, even under nonlimiting soil macronutrient conditions.

Metabolic Rearrangements Causing Elevated Proline and Polyhydroxybutyrate Accumulation During the Osmotic Adaptation Response of Bacillus megaterium.

For many years now, Bacillus megaterium serves as a microbial workhorse for the high-level production of recombinant proteins in the g/L-scale. However, efficient and stable production processes require the knowledge of the molecular adaptation strategies of the host organism to establish optimal environmental conditions. Here, we interrogated the osmotic stress response of B. megaterium using transcriptome, proteome, metabolome, and fluxome analyses. An initial transient adaptation consisted of potassium import and glutamate counterion synthesis. The massive synthesis of the compatible solute proline constituted the second longterm adaptation process. Several stress response enzymes involved in iron scavenging and reactive oxygen species (ROS) fighting proteins showed higher levels under prolonged osmotic stress induced by 1.8 M NaCl. At the same time, the downregulation of the expression of genes of the upper part of glycolysis resulted in the activation of the pentose phosphate pathway (PPP), generating an oversupply of NADPH. The increased production of lactate accompanied by the reduction of acetate secretion partially compensate for the unbalanced (NADH/NAD+) ratio. Besides, the tricarboxylic acid cycle (TCA) mainly supplies the produced NADH, as indicated by the higher mRNA and protein levels of involved enzymes, and further confirmed by 13C flux analyses. As a consequence of the metabolic flux toward acetyl-CoA and the generation of an excess of NADPH, B. megaterium redirected the produced acetyl-CoA toward the polyhydroxybutyrate (PHB) biosynthetic pathway accumulating around 30% of the cell dry weight (CDW) as PHB. This direct relation between osmotic stress and intracellular PHB content has been evidenced for the first time, thus opening new avenues for synthesizing this valuable biopolymer using varying salt concentrations under non-limiting nutrient conditions.

Marine diatoms change their gene expression profile when exposed to microscale turbulence under nutrient replete conditions

Diatoms are a fundamental microalgal phylum that thrives in turbulent environments. Despite several experimental and numerical studies, if and how diatoms may profit from turbulence is still an open question. One of the leading arguments is that turbulence favours nutrient uptake. Morphological features, such as the absence of flagella, the presence of a rigid exoskeleton and the micrometre size would support the possible passive but beneficial role of turbulence on diatoms. We demonstrate that in fact diatoms actively respond to turbulence in non-limiting nutrient conditions. TURBOGEN, a prototypic instrument to generate natural levels of microscale turbulence, was used to expose diatoms to the mechanical stimulus. Differential expression analyses, coupled with microscopy inspections, enabled us to study the morphological and transcriptional response of Chaetoceros decipiens to turbulence. Our target species responds to turbulence by activating energy storage pathways like fatty acid biosynthesis and by modifying its cell chain spectrum. Two other ecologically important species were examined and the occurrence of a morphological response was confirmed. These results challenge the view of phytoplankton as unsophisticated passive organisms.

Medium chain length polyhydroxyalkanoates biosynthesis in Pseudomonas putida mt-2 is enhanced by co-metabolism of glycerol/octanoate or fatty acids mixtures

The synthesis of medium chain length polyhydroxyalkanoates (mcl-PHAs) by Pseudomonas putida mt-2 was investigated under nitrogen-rich then deficient conditions with glycerol/octanoate or long-chain fatty acids (LCFAs) as carbon sources. When mixed, glycerol and octanoate were co-assimilated regardless of nitrogen availability but provided that glycerol uptake has been already triggered under non-limiting nutrient conditions. This concomitant consumption allowed to enhance mcl-PHAs accumulation (up to 57% of cell dry weight (CDW)) under both non-limiting and nitrogen deficient conditions. Octanoate then mostly drove anabolism of the polyester with 3-hydroxyoctanoate (3HO) synthesized as the main monomer (83%). If the preferred PHA precursor octanoate was supplied, glycerol was mainly involved in cell growth and/or maintenance but very little in PHA production even under nitrogen starvation. P. putida cells accumulated higher amounts of mcl-PHAs when grown on mixtures of LCFAs compared to LCFAs supplied as single substrate (25% and 9% of CDW, respectively). However, only a weak enrichment of the polyester was observed after transfer of cells in a fresh nitrogen-free medium containing the same combination of LCFAs. Some typical units within the polyester were related to the LCFAs ratio supplied in the medium indicating that tailor-made monomers could be synthesized.

Temperature affects the morphology and calcification of <i>Emiliania huxleyi</i> strains

Abstract. The global warming debate has sparked an unprecedented interest in temperature effects on coccolithophores. The calcification response to temperature changes reported in the literature, however, is ambiguous. The two main sources of this ambiguity are putatively differences in experimental setup and strain specificity. In this study we therefore compare three strains isolated in the North Pacific under identical experimental conditions. Three strains of Emiliania huxleyi type A were grown under non-limiting nutrient and light conditions, at 10, 15, 20 and 25 °C. All three strains displayed similar growth rate versus temperature relationships, with an optimum at 20–25 °C. Elemental production (particulate inorganic carbon (PIC), particulate organic carbon (POC), total particulate nitrogen (TPN)), coccolith mass, coccolith size, and width of the tube element cycle were positively correlated with temperature over the sub-optimum to optimum temperature range. The correlation between PIC production and coccolith mass/size supports the notion that coccolith mass can be used as a proxy for PIC production in sediment samples. Increasing PIC production was significantly positively correlated with the percentage of incomplete coccoliths in one strain only. Generally, coccoliths were heavier when PIC production was higher. This shows that incompleteness of coccoliths is not due to time shortage at high PIC production. Sub-optimal growth temperatures lead to an increase in the percentage of malformed coccoliths in a strain-specific fashion. Since in total only six strains have been tested thus far, it is presently difficult to say whether sub-optimal temperature is an important factor causing malformations in the field. The most important parameter in biogeochemical terms, the PIC : POC ratio, shows a minimum at optimum growth temperature in all investigated strains. This clarifies the ambiguous picture featuring in the literature, i.e. discrepancies between PIC : POC–temperature relationships reported in different studies using different strains and different experimental setups. In summary, global warming might cause a decline in coccolithophore's PIC contribution to the rain ratio, as well as improved fitness in some genotypes due to fewer coccolith malformations.

Technical Note: A generic law-of-the-minimum flux limiter for simulating substrate limitation in biogeochemical models

Abstract. We present a generic flux limiter to account for mass limitations from an arbitrary number of substrates in a biogeochemical reaction network. The flux limiter is based on the observation that substrate (e.g., nitrogen, phosphorus) limitation in biogeochemical models can be represented as to ensure mass conservative and non-negative numerical solutions to the governing ordinary differential equations. Application of the flux limiter includes two steps: (1) formulation of the biogeochemical processes with a matrix of stoichiometric coefficients and (2) application of Liebig's law of the minimum using the dynamic stoichiometric relationship of the reactants. This approach contrasts with the ad hoc down-regulation approaches that are implemented in many existing models (such as CLM4.5 and the ACME (Accelerated Climate Modeling for Energy) Land Model (ALM)) of carbon and nutrient interactions, which are error prone when adding new processes, even for experienced modelers. Through an example implementation with a CENTURY-like decomposition model that includes carbon, nitrogen, and phosphorus, we show that our approach (1) produced almost identical results to that from the ad hoc down-regulation approaches under non-limiting nutrient conditions, (2) properly resolved the negative solutions under substrate-limited conditions where the simple clipping approach failed, (3) successfully avoided the potential conceptual ambiguities that are implied by those ad hoc down-regulation approaches. We expect our approach will make future biogeochemical models easier to improve and more robust.

Establishment and two-year growth of a bio-energy plantation with fast-growing Populus trees in Flanders (Belgium): Effects of genotype and former land use

Abstract In April 2010, a large-scale Short Rotation Coppice (SRC) plantation was established with mainly poplar ( Populus spp.) on a former agricultural site (cropland and pasture) in Flanders. The 12 selected genotypes planted were assessed on establishment and production characteristics during the first two years of growth and were found highly productive, with a volume index ranging between 1.00 (±0.68) and 1.93 (±0.97) dm³ in growing season 1 (GS1) and between 2.75 (±1.70) and 11.91 (±6.33) dm³ in growing season 2 (GS2). Despite high survival rates of the cuttings after planting, competitive weeds and management operations increased tree mortality during the growing season from 3.4 % up to 18.2 % averaged over the entire plantation. Weed control therefore turned out to be the key factor in the establishment success. Only a minor influence of former land use was observed during GS1, which is explained by the non-limiting nutrient conditions on both former cropland and pasture, and which disappeared during GS2. These productive soils also explained the high growth rates, with an average tree height of 247 cm and 445 cm and stem diameter (at 22 cm height) of 25.21 mm and 40.73 mm after GS1 and GS2, respectively. Genotypic and parentage variations were found to be less pronounced during GS1, and increased during GS2 as expected. The maximum leaf area index and total leaf area duration were shown to be good indicators of production and growth performance. The results of this paper confirm the high potential of SRC with poplar on agricultural land for bio-energy purposes.

Filamentous green algae inhibit phytoplankton with enhanced effects when lakes get warmer

1. Filamentous green algae (FGA) may represent an alternative state in high-nutrient shallow temperate lakes. Furthermore, a clear water state is sometimes associated with the dominance of FGA; however, the mechanisms involved remain uncertain. 2. We hypothesised that FGA may promote a clear water state by directly suppressing phytoplankton growth, mostly via the release of allelochemicals, and that this interaction may be affected by temperature. 3. We examined the relationships between FGA, phytoplanktonic chlorophyll a concentrations and zooplankton in a series of mesocosms (2.8 m3) mimicking enriched shallow ponds now and in a future warmer climate (0 and c. 5 °C above ambient temperatures). We then tested the potential allelopathic effects of FGA (Cladophora sp. and Spirogyra sp.) on phytoplankton using several short-term microcosms and laboratory experiments. 4. Mesocosms with FGA evidenced lower phytoplanktonic chlorophyll a concentrations than those without. Zooplankton and zooplankton : phytoplankton biomass ratios did not differ between mesocosms with and without FGA, suggesting that grazing was not responsible for the negative effects on phytoplanktonic biomass (chlorophyll a). 5. Our field microcosm experiments demonstrated that FGA strongly suppressed the growth of natural phytoplankton at non-limiting nutrient conditions and regardless of phytoplankton initial concentrations or micronutrients addition. Furthermore, we found that the negative effect of FGA on phytoplankton growth increased up to 49% under high incubation temperatures. The experiment performed using FGA filtrates confirmed that the inhibitory effect of FGA on phytoplankton may be attributed to allelochemicals. 6. Our results suggest that FGA control of phytoplankton growth may be an important mechanism for stabilising clear water in shallow temperate lakes dominated by FGA and that FGA may play a larger role when lakes get warmer.

Comparative performance of invasive alien Eichhornia crassipes and native Ludwigia stolonifera under non-limiting nutrient conditions in Lake Naivasha, Kenya

The ability of Ludwigia stolonifera to thrive in the presence of Eichhornia crassipes was investigated in Lake Naivasha, Kenya. L. stolonifera (indigenous) and E. crassipes (invasive alien) were grown in outdoor experimental boxes in monocultures and mixtures under non-limiting nutrient conditions. An additive series design with eight combinations of planting densities and four replicates was used. Competitive interactions between the two species were determined by assessing the final total biomass and above–below-ground biomass allocation after 98 days of growth. Biomass accumulation and allocation were significantly affected by competition in relation to species, with L. stolonifera accumulating more biomass than E. crassipes. ANOVA analysis indicated that there was no significant difference in Relative Growth Rate (RGR) and root/shoot ratio between monocultures and mixtures with E. crassipes. However, significant differences in RGRs were observed between monocultures and mixtures of L. stolonifera. Multiple regressions on species RGRs revealed that increasing initial biomass of a con-specific neighbour resulted to a greater reduction in species RGR in relation to increasing initial biomass of a hetero-specific neighbour. Thus, a stronger intra- than inter-specific competition coupled with the significantly higher RGR of L. stolonifera relative to that of E. crassipes enabled L. stolonifera to outperform E. crassipes.

Direct effects of Daphnia-grazing, not infochemicals, mediate a shift towards large inedible colonies of the gelatinous green alga Sphaerocystis schroeteri

The influence of Daphnia galeata× hyalina grazing and of infochemicals released by the daphnids on the colony size and growth rate of the colonial gelatinous green alga Sphaerocystis schroeteri (Chlorococcales) was investigated in laboratory batch experiments run for 96 h. High zooplankton grazing pressure was exerted by a final concentration of 100 daphnids L −1 in the Daphnia treatments. Infochemicals were obtained by filtration (0.2 μm) of water from D. galeata× hyalina cultures (200 ind. L −1 exposed for 24 h). This filtrate was added to the S. schroeteri cultures in two concentrations corresponding to 7 and 50 daphnids L −1, respectively. The growth rate of S. schroeteri was neither affected significantly by direct Daphnia grazing nor by the presence of Daphnia infochemicals, in comparison to the control. However, the portion of inedible S. schroeteri colonies (diameter>50 μm) increased under direct grazing pressure, whereas the Daphnia infochemicals did not influence the colony size significantly. We conclude that the shift in colony size by direct zooplankton grazing denotes an effective defence mechanism against size selective feeding for colonial gelatinous green algae. This effective defence in combination with unchanged growth rates of the larger colonies (under non-limiting nutrient and light conditions) falsifies the assumption of a trade-off between minimising grazing losses and maximising growth by optimising the colony size.

Calorimetric investigation of anaerobic digestion: Biomass adaptation and temperature effect

A high-resolution bench-scale calorimeter (Bio-RC1), specially suited for microbiological studies, has been used to investigate the response of an anaerobic granular sludge to repetitive glucose additions at various temperatures. Through the analysis of heat-evolution profiles resulting from substrate degradation, significant biological events have been identified: the feeble net heat uptake related to aceticlastic methanogenesis has been evaluated, and the detrimental effect of a nutrient deficiency, which led up to 75% decrease in heat production rate, has been detected. After several glucose pulses under non-limiting nutrient conditions, a reliable and reproducible reference thermal response, which is an important condition for the application of calorimetric measurements to detect inhibitory effects, was finally obtained. Afterward, glucose additions were repeated at various temperatures, and 57% biomass activity reduction, with respect to the reference value at 35 degrees C, was observed when temperature was fixed at 20 degrees C. Additionally, a preliminary toxicity test was also performed by exposing the biomass to increasing formaldehyde concentrations.

Nutrient availability influences UV-B sensitivity of Plantago lanceolata

Seeds of Plantago lanceolata were collected in a dune grassland ecosystem in the Netherlands. Plants were grown in a greenhouse for 61 days under either low or high nutrient conditions and were exposed to four different levels of biologically effective UV-B radiation. The highest UV-B exposure level simulated 30% reduction of the stratospheric ozone layer during summertime in the Netherlands. Total biomass production of plants at low nutrient supply was 50% lower compared to plants grown at high nutrient supply, while net photosynthesis was decreased by only 12%. Increased levels of UV-B reduced biomass production under non-limiting nutrient conditions only. Biomass production of plants grown at limited nutrient supply was not affected by UV-B. This response was correlated to increased accumulation of carbohydrates under nutrient limitation, which agrees well with the carbon/nutrient balance hypothesis. It is concluded that the increased accumulation of carbon in nutrient-stressed plants, may lead to a reduction of UV-B induced damage because of increased foliar UV-B absorbance by enhanced accumulation of phenolic compounds and leaf thickening.

Biomass allocation in old‐field annual species grown in elevated CO2environments: no evidence for optimal partitioning

SummaryIncreased atmospheric carbon dioxide supply is predicted to alter plant growth and biomass allocation patterns. It is not clear whether changes in biomass allocation reflect optimal partitioning or whether they are a direct effect of increased growth rates. Plasticity in growth and biomass allocation patterns was investigated at two concentrations of CO2([CO2]) and at limiting and nonlimiting nutrient levels for four fast‐ growing old‐field annual species.Abutilon theophrasti,Amaranthus retroflexus,Chenopodium album, andPolygonum pensylvanicumwere grown from seed in controlled growth chamber conditions at current (350 μmol mol−1, ambient) and future‐ predicted (700 μmol mol−1, elevated) CO2levels. Frequent harvests were used to determine growth and biomass allocation responses of these plants throughout vegetative development. Under nonlimiting nutrient conditions, whole plant growth was increased greatly under elevated [CO2] for three C3 species and moderately increased for a C4 species (Amaranthus). No significant increases in whole plant growth were observed under limiting nutrient conditions. Plants grown in elevated [CO2] had lower or unchanged root:shoot ratios, contrary to what would be expected by optimal partitioning theory. These differences disappeared when allometric plots of the same data were analysed, indicating that CO2‐induced differences in root:shoot allocation were a consequence of accelerated growth and development rates. Allocation to leaf area was unaffected by atmospheric [CO2] for these species. The general lack of biomass allocation responses to [CO2] availability is in stark contrast with known responses of these species to light and nutrient gradients. We conclude that biomass allocation responses to elevated atmospheric [CO2] are not consistent with optimal partitioning predictions.

Short and long-term responses of whole-plant gas exchange to elevated CO 2 in four herbaceous species

Four Mediterranean herbaceous species, the two grasses Bromus madritensis (annual) and B. erectus (perennial), and the two legumes Medicago minima (annual) and M. glomerata (perennial) were grown in glasshouses at two levels of atmospheric CO 2 (350 and 700 μmol mol −1), under non-limiting nutrient conditions. After 6 months of growth, short and long-term responses of whole plant photosynthesis and stomatal conductance to elevated CO 2 were measured, together with changes in leaf total non-structural carbohydrate concentration, leaf nitrogen concentration and specific leaf area. Short-term exposure to elevated CO 2 increased whole plant photosynthesis by 30% on average. However, this stimulation did not persist in the long term, indicating a down-regulation of photosynthesis in plants grown at elevated CO 2. By contrast, stomatal conductance was similarly or more decreased after long-term than after short-term exposure to elevated CO 2. As a result, the short-term effect of CO 2 on instantaneous water use efficiency was conserved in the long-term and the c i/ c a ratio remained nearly constant after both short and long-term exposure to elevated CO 2. Analysis of the main leaf components revealed that when grown at elevated CO 2, leaves of the two grass species showed a large accumulation of total non-structural carbohydrates and a decrease in their nitrogen concentration, while leaf total non-structural carbohydrate and nitrogen concentrations of the two legume species were unaffected by elevated CO 2. Species-specific differences in down-regulation of photosynthesis were positively correlated with the long-term response of stomatal conductance and negatively correlated with changes in total non-structural carbohydrate concentration. This suggests that source–sink relationship may play a role in the control of photosynthetic response to high CO 2 concentration.

Relative Nitrogen Limitation at Steady-state Nutrition as a Determinant of Plasticity in Five Grassland Plant Species

Partitioning of biomass between roots and different shoot parts has often been used to explain the response of plants to variations in resource availability. There are still many uncertainties in the importance of this trait for plant performance, and clear guidelines on how partitioning should be quantified in relation to growth rate and resource supply are of fundamental importance for such an understanding. This paper reports an attempt to show how plant nitrogen status relates to root:shoot partitioning and other plastic responses, in a manner that can be used for quantitative predictions. The reactions to nitrogen limitation of five grassland plant species, with different ecological demands, were compared. The species used were the forbs Polygala vulgaris and Crepis praemorsa, and the grasses Danthonia decumbens, Agrostis capillaris and Dactylis glomerata. The experiment was conducted in a climate chamber where the plants were grown hydroponically (1) under non-limiting nutrient conditions and (2) at a steady-state nitrogen limitation, which enabled the plants to express half of their growth potential. The relative growth rate (RGR) of the species was strongly related to plant nitrogen concentration (PNC) and leaf area ratio (LAR), whereas the effects on net assimilation rate (NAR) were very small. Despite large differences in maximum relative growth rate, the species showed remarkable similarities in dry matter partitioning between root and shoot. It is concluded that root:shoot partitioning can be treated as a direct function of the relative resource limitation of the plant. The difficulty of attaining well-defined levels of resource limitation in soil, other solid substrates and many hydroponic systems may be the most important reason for the divergent results in earlier studies. Better knowledge of soil-root interactions, and plant responses to the whole span of resource-supply levels, is required for a thorough understanding of how nutrients limit growth.

Growth characteristics of apple cultivar Gravenstein plants grafted onto the transformed rootstock M26 with rolA and rolB genes under non-limiting nutrient conditions

The growth characteristics of the apple cultivar ( Malus domestica) Gravenstein (GR) grafted onto the transformed rootstock M26 were investigated. The transformed rootstocks used in the experiments included two clones with the rolA gene ( rolA1 and rolA2) and one clone with the rolB gene ( rolB). The grafted plants consisted of GR/ rolA1, GR/ rolA2 and GR/ rolB and the control plants GR/M26 (the untransformed rootstock). Ingestad’s experimental system was employed for growing the plants to maintain non-limiting nutrient conditions and to achieve accurate and comparable results. The results show that the grafted GR/ rolA1, GR/ rolA2, and GR/ rolB plants had a similar relative growth rate to the GR/M26 plants, indicating no effect of the transformed rootstock on relative growth rate under non-limiting nutrient conditions. Moreover, the transformed rootstocks influenced neither the leaf area ratio, specific leaf area values nor dry matter allocation. However, the GR/ rolA1 plants did show a reduced stem length and a decreased internode length compared to the control and the other two transgenic clones. Also, the GR/ rolA1 and GR/ rolA2 showed a significant lower specific root length value compared to the control. The present results suggest that the dwarfing effect does not relate to the relative growth rate of the rootstock. The transformed clone rolA1 and rolA2, especially rolA1, might be potential dwarfing rootstocks for apple production.

Growth rates and biomass production of micropropagated apple plants of M26 and Gravenstein on their own roots and in different micrografted combinations under non-limiting and limiting nutrient conditions

Micropropagated apple plants of semi-dwarf rootstock M26 (M26) and vigorous cultivar Gravenstein (GR) on their own roots and different micrograft combinations were used in the experiments. The combinations included GR scion on GR root (GR/GR), M26 on M26 (M26/M26), GR on M26 (GR/M26), and M26 on GR (M26/GR). The plants were grown under non-limiting and limiting relative addition rates of nutrients. Under non-limiting nutrient conditions, M26 and GR showed a similar relative growth rate (RGR). Grafting reduced RGR significantly in the combination of M26/M26 compared to M26. The relative growth rate for the combination of GR/M26 was similar to the GR/GR plants, but it increased greatly compared to the M26/M26 plants. For the reverse combination of M26/GR, the RGR value decreased significantly compared to either the M26/M26 or the GR/GR plants. The RGR value and specific root length were lower for M26/GR than for GR/M26. No clear relationship between carbohydrate allocation and growth parameters of different plants was found under non-limiting nutrient conditions. Nutrient limitation resulted in increased dry weights and soluble sugars in the roots for all plants except for M26/GR. A reduced leaf area ratio and a lower RGR than the set relative addition rate of nitrogen were found for M26 and GR/M26 under limiting conditions. These results suggest that the dwarfing effect is not directly related to RGR of rootstocks or scions, but rather associated with root morphology of grafted plants and the ability of roots to absorb nutrients.