Nitrogen, phosphorus and light effects on growth and allocation of biomass and nutrients in wild rice

Separating plastic from ontogenetic and growth-limiting responses of plants to changes in resource availability can be challenging because there are a total of eight combinations of these three types of responses. These can, however, be uniquely distinguished on plots of root:shoot ratios against total biomass through time. We used this approach to separate ontogenetic, plastic, and growth-limiting responses of wild rice (Zizania palustris L.) to changes in nitrogen, phosphorus, and light availabilities. Relative growth rate was limited primarily by nitrogen but responded to increased light and phosphorus after nitrogen limitations were alleviated. Nitrogen addition increased relative growth rate because it simultaneously increased unit leaf rate, specific leaf area, and leaf weight ratio. Increased light did not change relative growth rate because decreased specific leaf area and leaf weight ratio compensated the increased unit leaf rate. Phosphorus did not change either relative growth rate or its underlying components. Plants responded ontogenetically to increased nitrogen and light availabilities by accelerating their developmental rate, and plastically by decreasing or increasing their root:shoot ratios, respectively. Plants did not respond either ontogenetically or plastically to increased phosphorus availability. Ontogenetic changes in growth can be separated from plastic and growth-limiting responses by plotting root:shoot ratio against total biomass in the context of the eight possible responses identified above, and also by examining how the underlying components of relative growth rate respond.     

Is resource allocation and grain yield of rice altered by inoculation with arbuscular mycorrhizal fungi?

Aims Our study quantified the combined effects of fertilization and inoculation with arbuscular mycorrhizal fungi (AMF) on grain yield and allocation of biomass and nutrients in field-grown rice (Oryza sativa L.).Methods A two-factor experiment was conducted at a field site in northeast of China (in Shuangcheng, Heilongjiang Province, Songhua River basin): six nitrogen–phosphorus–potassium fertilizer levels were provided (0, 20, 40, 60, 80 and 100% of the local norm of fertilizer supply), with or without inoculation with Glomus mosseae. At maturity, we quantified the percentage of root length colonization by AMF, grain yield, shoot:root ratios, shoot N and P contents and nutrients allocated to panicles, leaves and stems.Important findings As expected, inoculation resulted in greatly increased AMF colonization, which in turn led to higher shoot:root ratios and greater shoot N contents. Shoot:root ratios of inoculated rice increased with increasing fertilization while there was a significant interaction between fertilization and inoculation on shoot:root ratio. Additionally, AMF inoculation increased panicle:shoot ratios, panicle N:shoot N ratios and panicle P:shoot P ratios, especially in plants grown at low fertilizer levels. Importantly, inoculated rice exhibited higher grain yield, with the maximum improvement (near 62%) at the lower fertilizer end. Our results showed that (i) AMF-inoculated plants conform to the functional equilibrium theory, albeit to a reduced extent compared to non-inoculated plants and (ii) AMF inoculation resulted in greater allocation of shoot biomass to panicles and increased grain yield by stimulating N and P redistribution to panicles.     

Carbon allocation to shoots and roots in relation to nitrogen supply is mediated by cytokinins and sucrose: Opinion

In this paper we firstly show some general responses of biomass partitioning upon nitrogen deprivation. Secondly, these responses are explained in terms of allocation of carbon and nitrogen, photosynthesis and respiration, using a simulation model. Thirdly, we present a hypothesis for the regulation of biomass partitioning to shoots and roots.Shortly after nitrogen deprivation, the relative growth rate (RGR) of the roots generally increases and thereafter decreases, whereas that of the shoot decreases immediately. The increased RGR of the root and decreased RGR of the shoot shortly after a reduction in the nitrogen supply, cause the root weight ratio (root weight per unit plant weight) to increase rapidly.We showed previously that allocation of carbon and nitrogen to shoots and roots can satisfactorily be described as a function of the internal organic plant nitrogen concentration. Using these functions in a simulation model, we analyzed why the relative growth rate of the roots increases shortly after a reduction in nitrogen supply. The model predicts that upon nitrogen deprivation, the plant nitrogen concentration and the rate of photosynthesis per unit plant weight rapidly decrease, and the allocation of recently assimilated carbon and nitrogen to roots rapidly increases. Simulations show that the increased relative growth rate of the root upon nitrogen deprivation is explained by decreased use of carbon for root respiration, due to decreased carbon costs for nitrogen uptake. The stimulation of the relative growth rate of the root is further amplified by the increased allocation of carbon and nitrogen to roots. Using the simple relation between the plant nitrogen concentration and allocation, the model describes plant responses quite realistically.Based on information in the literature and on our own experiments we hypothesize that allocation of carbon is mediated by sucrose and cytokinins. We propose that nitrogen deprivation leads to a reduced cytokinin production, a decreased rate of cytokinin export from the roots to the shoot, and decreased cytokinin concentrations. A reduced cytokinin concentration in the shoot represses cell division in leaves, whereas a low cytokinin concentration in roots neutralizes the inhibitory effect of cytokinins on cell division. A reduced rate of cell division in the leaves leads to a reduced unloading of sucrose from the phloem into the expanding cells. Consequently, the sucrose concentration in the phloem nearby the expanding cells increases, leading to an increase in turgor pressure in the phloem nearby the leaf's division zone. In the roots, cell division continues and no accumulation of sugars occurs in dividing cells, leading to only marginal changes in osmotic potential and turgor pressure in the phloem nearby the root's cell division zone. These changes in turgor pressure in the phloem of roots and sink leaves affect the turgor pressure gradients between source leaf-sink leaf and source leaf-root in such a way that relatively more carbohydrates are exported to the roots. As a consequence RWR increases after nitrogen deprivation. This hypothesis also explains the strong relationship between allocation and the plant nitrogen status.     

水稻根系在根袋处理条件下对氮养分的反应

通过大田试验对 1 0个水稻品种根系与产量的关系研究表明 ,抽穗期和成熟期根冠比与产量呈极显著的负相关关系 ,相关系数分别为 - 0 .861 6和 - 0 .8889。随之在大田试验基础上选择根冠比大的品种粳籼89,设计水分和养分能自由通过 ,而根系不能穿过的根袋 ,根袋从小到大不同 ,以便产生不同大小的水稻植株根冠比。通过水培实验研究在根袋处理后对不同养分条件的反应。水培液设 3种氮素养分水平 ,即2 0 mg/kg,40 mg/kg,60 mg/kg。结果表明 ,在不同氮素养分条件下 ,经过根袋处理后在抽穗期根系干重都有下降趋势 ,根冠比显著降低 ,而根系活性吸收面积在抽穗期有不同程度的增加 ,茎鞘贮存性碳水化合物含量明显增加 ,叶绿素含量则无明显影响。在抽穗期较大的根袋处理根系总吸收面积、活跃吸收面积及所占比例与对照相比增加效果较为明显 ,而较小的根袋处理根系吸收的能力降低 ,根系吸收能力大小顺序为 :大袋 >中袋 >对照 >小袋。随养分浓度的增加 ,不同根袋处理在抽穗期的根系总吸收面积和活跃吸收面积有下降的趋势。较大的根袋处理在 2 0 mg/kg和 60 mg/kg氮素养分条件下能适当减少根系直径 ,增强根系的活性吸收比例 ,从而提高根系的活力 ;但在成熟期根袋处理对根系的活性吸收无明显影响     

Carbon Balance of Tall Fescue (Festuca arundinacea Schreb.): Effects of Nitrogen Fertilization and the Growing Season

The C balance of a tall fescue sward grown under different ratesof N fertilization in summer, autumn, and spring was calculatedusing models derived from measurements of shoot growth, canopygross photosynthesis, shoot respiration and of C partitioningto the roots. Under the diverse growing conditions associatedwith the seasons and the N fertilization, C utilization forabove- and below-ground biomass accumulation never exceeded39 and 14% of the canopy gross photosynthesis, respectively.Carbon losses attributed to root respiration and exudation,which were estimated by difference between canopy net photosynthesisand total growth, ranged between 3 and 30% of canopy gross photosynthesis.Seasonal differences in shoot growth could be attributed tothe amount of intercepted radiation, the radiation-use efficiencyand the C partitioning to the roots. The effect of N deficiencyon shoot growth can be attributed to its effects on canopy photosynthesis(principally resulting from changes in intercepted photosyntheticallyactive radiation) and C partitioning. In comparison with theeffect on shoot growth, the effect of the N deficiency on thecanopy gross photosynthesis per unit of light intercepted overthe regrowth cycle was limited. It is concluded that most ofthe effect of N fertilization on shoot growth is due to changesin C partitioning which result in faster leaf area developmentand greater light interception.Copyright 1994, 1999 AcademicPress Tall rescue, Festuca arundinacea Schreb., carbon balance, nitrogen, grass, fertilization     

The effect of neighbors on the root system of the desert annualEremobium aegyptiacum

Intra-specific competition with a primary focus on root competition between plants living in an area with low resource levels, was studied using the natural monotypic population of a desert annual plantEremobium aegyptiacum (Cruciferae). We tested the effect of neighboring distance on shoot and root biomass, and such root parameters as root length, diameter of root neck, number of first order lateral roots and number of lateral roots per unit of main root length. Our results indicate a strong negative relationship between neighboring plant density and such plant parameters as shoot and root biomass, and root neck diameter. The number of first order lateral roots and the number of lateral roots per unit of main root length were negatively related to the distance between neighboring plants. Contrary to predictions, there was no influence of competition on node distribution: neither root overlap nor root avoidance was observed in pairs of adjacent plants.     

Soil nutrient patchiness and plant genotypes interact on the production potential and decomposition of root and shoot litter: evidence from short-term laboratory experiments with Triticum aestivum

Aims

The purpose of this study was to test the hypotheses that soil nutrient patchiness can differentially benefit the decomposition of root and shoot litters and that this facilitation depends on plant genotypes.

Methods

We grew 15 cultivars (i.e. genotypes) of winter wheat (Triticum aestivum L.) under uniform and patchy soil nutrients, and contrasted their biomass and the subsequent mass, carbon (C) and nitrogen (N) dynamics of their root and shoot litters.

Results

Under equal amounts of nutrients, patchy distribution increased root biomass and had no effects on shoot biomass and C:N ratios of roots and shoots. Roots and shoots decomposed more rapidly in patchy nutrients than in uniform nutrients, and reductions in root and shoot C:N ratios with decomposition were greater in patchy nutrients than uniform nutrients. Soil nutrient patchiness facilitated shoot decomposition more than root decomposition. The changes in C:N ratios with decomposition were correlated with initial C:N ratios of litter, regardless of roots or shoots. Litter potential yield, quality and decomposition were also affected by T. aestivum cultivars and their interactions with nutrient patchiness.

Conclusions

Soil nutrient patchiness can enhance C and N cycling and this effect depends strongly on genotypes of T. aestivum. Soil nutrient heterogeneity in plant communities also can enhance diversity in litter decomposition and associated biochemical and biological dynamics in the soil.     

A Balanced Quantitative Model for Root: Shoot Ratios in Vegetative Plants

The vegetative growth of a two-component plant consisting ofroot and shoot only is considered in terms of the transportand utilization of two required substrates, one providing carbonand the other providing nitrogen. The model provides a quantitativescheme for examining how root: shoot ratios depend upon thespecific activities of root and shoot and hence environment.It has been shown that the total shoot activity is proportionalto the total root activity in a plant undergoing steady-stategrowth.     

Effect of Root and Shoot Meristem Temperature on Shoot to Root Dry Matter Partitioning and the Internal Concentrations of Nitrogen and Carbohydrates in Maize and Wheat

Maize (Zea mays L.) and spring wheat (Triticum aestivum L.)were grown in nutrient solution at uniformly high air temperature(20 °C), but different root zone temperatures (RZT 20, 16,12 °C). To manipulate the ratio of shoot activity to rootactivity, the plants were grown with their shoot base includingthe apical meristem either above (i.e. at 20 °C) or withinthe nutrient solution (i.e. at 20, 16 or 12 °C). In wheat, the ratio of shoot:root dry matter partitioning decreasedat low RZT, whereas the opposite was true for maize. In bothspecies, dry matter partitioning to the shoot was one-sidedlyincreased when the shoot base temperature, and thus shoot activity,were increased at low RZT. The concentrations of non-structuralcarbohydrates (NSC) in the shoots and roots were higher at lowin comparison to high RZT in both species, irrespective of theshoot base temperature. The concentrations of nitrogen (N) inthe shoot and root fresh matter also increased at low RZT withthe exception of maize grown at 12 °C RZT and 20 °Cshoot base temperature. The ratio of NSC:N was increased inboth species at low RZT. However this ratio was negatively correlatedwith the ratio of shoot:root dry matter partitioning in wheat,but positively correlated in maize. It is suggested that dry matter partitioning between shoot androots at low RZT is not causally related to the internal nitrogenor carbohydrate status of the plants. Furthermore, balancedactivity between shoot and roots is maintained by adaptationsin specific shoot and root activity, rather than by an alteredratio of biomass allocation between shoot and roots.Copyright1994, 1999 Academic Press Wheat, Triticum aestivum, maize, Zea mays, root temperature, shoot meristem temperature, biomass allocation, shoot:root ratio, carbohydrate status, nitrogen status, functional equilibrium     

Nutrient Relations of Groundsel (Senecio vulgaris) Infected by Rust (Puccinia lagenophorae) at a Range of Nutrient Concentrations II. Uptake of N, P and K and Shoot-Root Interactions

Groundsel (Senecio vulgaris), healthy or infected with rust,Puccinia lagenophorae, was grown at a range of nutrient concentrationsin sand culture. Specific absorption rates calculated on thebasis of root dry weight (SAR_W) were greater in rusted thancontrol groundsel for nitrogen, potassium and phosphorus. Whilethe magnitudes of these stimulations varied, they occurred acrossthe whole range of nutrient concentrations. By contrast, specificabsorption rate on the basis of root length (SAR_L) were littlechanged by rust at any external nutrient concentration; SAR_Lfor phosphate and potassium were slightly reduced when nutrientswere freely available. Water flux per unit dry root weight and length was stimulatedby rust because transpiration per unit leaf area was more rapidin infected plants after fungal sporulation. However, water-fluxand the rate of uptake of nutrients were correlated only whenexpressed on the basis of root weight and increased transpirationdid not appear to be the mechanism underlying increased rootactivity. Rather, increased SAR_W for N, P and K could very largelybe attributed to increased shoot demand per unit root, whichresulted from the higher shoot: root (S: R) ratios of infectedindividuals. Changes in S: R accounted for 92, 81 and 57% oftotal variation in SAR_W for K, P and N respectively. Greatervalues for SAR_W were possible because specific root length (SRL)increased, producing more functional root per unit root weight.The lack of stimulation in SAR_L in response to rust could beexplained since the higher SRL of infected plants resulted instable values of shoot weight per unit root length, i.e. shootdemand was not increased by infection on this basis. Senecio vulgaris, Puccinia lagenophorae, rust infection, nutrient uptake, water uptake, shoot: root interactions     

Using an ecophysiological analysis to dissect genetic variability and to propose an ideotype for nitrogen nutrition in pea

BACKGROUNDS AND AIMS: Nitrogen nutrition of legumes, which relies both on atmospheric N2 and soil mineral N, remains a major limiting factor of growth. A decade ago, breeders tried to increase N uptake through hypernodulation. Despite their high nodule biomass, hypernodulating mutants were never shown to accumulate more nitrogen than wild types; they even generally displayed depressed shoot growth. The aim of this study was to dissect genetic variability associated with N nutrition in relation to C nutrition, using an ecophysiological framework and to propose an ideotype for N nutrition in pea. METHODS: Five pea genotypes (Pisum sativum) characterized by contrasting root and nodule biomasses were grown in the field. Variability among genotypes in dry matter and N accumulation was analysed, considering both the structures involved in N acquisition in terms of root and nodule biomass and their efficiency, in terms of N accumulated through mineral N absorption or symbiotic N2 fixation per amount of root or nodule biomass, respectively. KEY RESULTS: Nodule efficiency of hypernodulating mutants was negatively correlated to nodule biomass, presumably due to the high carbon costs induced by their excessive nodule formation. Root efficiency was only negatively correlated to root biomass before the beginning of the seed-filling stage, suggesting competition for carbon between root formation and functioning during the early stages of growth. This was no longer the case after the beginning of the seed-filling stage and nitrate absorption was then positively correlated to root biomass. CONCLUSIONS: Due to the high C costs induced by nodule formation and its detrimental effect on shoot and root growth, selecting traits for the improvement of N acquisition by legumes must be engineered (a) considering inter-relationships between C and N metabolisms and (b) in terms of temporal complementarities between N2 fixation and nitrate absorption rather than through direct increase of nodule and/or root biomass.     

Genetic variation in pea (Pisum sativum L.) demonstrates the importance of root but not shoot C/N ratios in the control of plant morphology and reveals a unique relationship between shoot length and nodulation intensity

Nodule numbers are regulated through systemic auto‐regulatory signals produced by shoots and roots. The relative effects of shoot and root genotype on nodule numbers together with relationships to organ biomass, carbon (C) and nitrogen (N) status, and related parameters were measured in pea (Pisum sativum) exploiting natural genetic variation in maturity and apparent nodulation intensity. Reciprocal grafting experiments between the early (Athos), intermediate (Phönix) and late (S00182) maturity phenotypes were performed and Pearson's correlation coefficients for the parameters were calculated. No significant correlations were found between shoot C/N ratios and plant morphology parameters, but the root C/N ratio showed a strong correlation with root fresh and dry weights as well as with shoot fresh weight with less significant interactions with leaf number. Hence, the root C/N ratio rather than shoot C/N had a predominant influence on plant morphology when pea plants are grown under conditions of symbiotic nitrogen supply. The only phenotypic characteristic that showed a statistically significant correlation with nodulation intensity was shoot length, which accounted for 68.5% of the variation. A strong linear relationship was demonstrated between shoot length and nodule numbers. Hence, pea nodule numbers are controlled by factors related to shoot extension, but not by shoot or root biomass accumulation, total C or total N. The relationship between shoot length and nodule numbers persisted under field conditions. These results suggest that stem height could be used as a breeding marker for the selection of pea cultivars with high nodule numbers and high seed N contents.     

A Model Allocating Growth Among Leaf Proteins, Shoot Structure, and Root Biomass to Produce Balanced Activity

A model is developed that considers the allocation of carbonand nitrogen substrates to a protein compartment in the shoots,shoot structural components, and root biomass. Inclusion ofa shoot-protein compartment allows variation in shoot-specificactivity to be modelled as a function of leaf nitrogen concentration.Allocation to the biomass compartments is controlled by twopartitioning variables that are defined by explicitly usingthe balanced activity hypothesis. The model produces balancedactivity where the shoot-specific activity, as well as rootand shoot biomass, vary in response to the above-ground (lightand CO₂) and below-ground (nitrogen) environments. The predictedpatterns of both root: shoot ratio and leaf nitrogen concentrationin response to environmental resource availability are qualitativelyconsistent with general trends observed in plants. Biomass allocation, plant growth, modelling, leaf nitrogen, root: shoot ratio, balanced activity     

Biosensor reporting of root exudation from Hordeum vulgare in relation to shoot nitrate concentration

The aim of this study was to determine the relationship between shoot nitrate concentration, mediated by nitrate supply to roots, and root exudation from Hordeum vulgare. Plants were grown for 14 d in C-free sand microcosms, supplied with nutrient solution containing 2 mM nitrate. After this period, three treatments were applied for a further 14 d: (A) continued supply with 2 mM nitrate (zero boost), (B) supply with 10 mM nitrate (low boost), and (C) supply with 20 mM nitrate (high boost). At the end of the treatment period, a bacterial biosensor (Pseudomonas fluorescens 10586 pUCD607, marked with the lux CDABE genes for bioluminescence) was applied to the microcosms to report on C-substrate availability, as a consequence of root exudation. The nitrate boost treatments significantly affected shoot nitrate concentrations, in the order C>B>A. In treatments receiving a nitrate boost (B, C), increased shoot nitrate concentration was correlated with increased plant biomass, reduced root length, reduced number of root tips, and increased mean root diameter, relative to the no boost treatment (A). Imaging of biosensor bioluminescence (proportional to metabolic activity in response to availability of root exudates) indicated that root exudation increased with decreasing shoot nitrate concentration. Biosensor reporting of root C-flow indicated that exudation was greater from root tip regions than from the whole root, but that specific exudation rates for all sites were unaffected by treatments. Total root exudation across treatments was found to be closely correlated with total root length, indicating that increased root exudation, per unit root biomass, with decreasing nitrate supply was associated with altered root morphology, as a consequence of systemic plant responses to internal N-status.     

Inter- and intraspecific relationships between shade tolerance and shade avoidance in temperate trees

Patterns of physiological and architectural adaptation and acclimation to decreasing light availability were investigated along a light gradient for saplings of 12 common species of temperate deciduous trees in southeastern Ontario, Canada. Physiological adaptation and acclimation (shade tolerance physiology) were quantified at the leaf level by measuring leaf mass per unit area (LMA), dark respiration per unit leaf nitrogen, chlorophyll per unit leaf nitrogen and the chlorophyll a:b ratio for the newest fully expanded leaf on the leader. Architectural adaptation and acclimation (shade avoidance) were quantified by measuring branching intensity and side shoot:main shoot length ratios for the most recent three years of growth on the leader and selected side branches. Within species, increases in LMA, chlorophyll a:b ratio and respiration per unit nitrogen and decreases in chlorophyll per unit nitrogen indicated that shade tolerance physiology generally increased with decreasing canopy openness. Increases in the branching intensity and side shoot:main shoot ratios of the leader and side branches indicated that shade avoidance also increased with decreasing canopy openness for the majority of species; however, in some species, stem bending under deep shade resulted in lateral growth. Interspecific variation in shade tolerance physiology was minimal when species were compared under equal amounts of canopy openness. In contrast, interspecific variation in shade avoidance variables was relatively high under equal canopy openness, with saplings of shade tolerant canopy species exhibiting higher shade avoidance than saplings of shade tolerant understory species.     

Changes in plant chemical defenses and nutritional quality as a function of ontogeny in <Emphasis Type="Italic">Plantago lanceolata</Emphasis> (Plantaginaceae)

Numerous empirical studies have examined ontogenetic trajectories in plant defenses but only a few have explored the potential mechanisms underlying those patterns. Furthermore, most documented ontogenetic trajectories in plant defenses have generally concentrated on aboveground tissues; thus, our knowledge regarding whole plant trends in plant defenses throughout development or potential allocation constraints between growth and defenses is limited. Here, we document changes in plant biomass, nutritional quality and chemical defenses for below- and aboveground tissues across seven age classes of Plantago lanceolata (Plantaginaceae) to evaluate: (1) partial and whole plant ontogenetic trajectories in constitutive chemical defenses and nutritional quality, and (2) the role of resource allocation constraints, namely root:shoot (R:S) ratios, in explaining whole plant investment in chemical defenses over time. Overall investment in iridoid glycosides (IGs) significantly increased, while water and nitrogen concentrations in shoot tissues decreased with plant age. Significant variation in IG content between shoot and root tissues across development was observed: allocation of IGs into root tissues linearly increased from younger to older plants, while non-linear shifts in allocation of IGs during ontogeny were observed for shoot tissues. Finally, R:S ratios only weakly explained overall allocation of resources into defenses, with young stages showing a positive relationship, while older stages showed a negative relationship between R:S ratios and IG concentrations. Ontogenetic trajectories in plant quality and defenses within and among plant tissues can strongly influence insect herbivores’ performance and/or predation risk; thus, they are likely to play a significant role in mediating species interactions.     

On the efficiency of legume supernodulating mutants

The supernodulating mutants of legumes lack the internal regulation of the number of symbiotic root nodules that harbour N₂-fixing nodule bacteria. On one hand, these mutants represent an efficient tool for dramatic increase in the degree of rhizobial symbiosis development. The trait of released nodulation is often associated with the desirable resistance of nodule initiation and functioning to the inhibition by ambient nitrate. On the other hand, the more intense and stable atmospheric nitrogen fixation of supernodulated plants is devalued by plant growth depression that results from the disproportion between the photosynthetic capacity of the shoot and the catabolic demands of symbiotic nodules. The deleterious effects of excessive nodulation can be neutralised or alleviated by a breeding strategy aimed at creating an ideotype of N₂-fixing legume. The growth depression can be diminished by the reduction in the nodule number typical for supernodulators, that is, 6–10-fold of the wild type, to the level found permissive for the particular crop. This shift should be accompanied with breeding aimed at the increased photosynthetic capacity of the shoot. Forage varieties of legumes represent a reserve of high photosynthetic and shoot growth capacity, thanks to a long-term breeding history for green biomass accumulation. Moreover, the deleterious effects of supernodulation are less perceived after introgression into the background of forage varieties in view of different criteria in their evaluation, such as nitrogen accumulation and biomass production per crop area unit. The growth of supernodulators can be further corrected by breeding for auxiliary traits such as long-vine shoot architecture, a longer vegetation period and late flowering. The same strategy is applicable to the compensation for inherent pleiotropic changes in plant development, which are often associated with primarily symbiotic mutations. Supporting evidence for the efficiency of the described approach has already been reported.     

Susceptibility and tolerance in hybrid and pure‐line rice varieties to herbivore attack: biomass partitioning and resource‐based compensation in response to damage

Hybrid rice has been noted for its susceptibility to insects and diseases compared to pure‐line (conventional) rice varieties. We investigated herbivory by Nilaparvata lugens, Sogatella furcifera and Scirpophaga incertulas on replicated three‐line hybrid sets (parental and hybrid lines) in field and greenhouse experiments. In a field experiment, caterpillar densities and stemborer damage was similar among hybrid and parental lines. In field and greenhouse experiments, the cytoplasmic male sterile (CMS)‐lines and maintainer lines had higher densities of planthoppers (including N. lugens and S. furcifera) than restorer or hybrid lines likely because of their wild abortive CMS‐lineage. High nitrogen levels increased plant mortality due to N. lugens, but often reduced mortality from S. furcifera and S. incertulas: this was similar between hybrid and pure‐line varieties. The hybrids were generally more tolerant of herbivory (lower biomass reductions per unit weight of insect) than the inbred parental lines. The addition of nitrogen to both the hybrid and pure‐line varieties had contrasting effects on tolerance depending on the nature of the attacking insect: fertiliser increased tolerance to S. furcifera (lower losses of yield and shoot biomass per mg insect) and S. incertulas (lower yield, shoot and root biomass loss) but fertiliser reduced tolerance to N. lugens (higher loss of root biomass and no effects on yield and shoot biomass loss). Our results indicate that hybrid rice is not physiologically more susceptible to herbivores than are pure‐line varieties even under high nitrogen conditions, but does have higher tolerance to insect damage.     

Theory for growth of plants derived from the nitrogen productivity concept

A theory is developed on the assumption that growth of plants is determined by the current amount of nitrogen in the plants. The nitrogen-growth relation is formalized in the nitrogen productivity concept (amount of biomass produced per amount of nitrogen in the biomass and per unit of time), which is essentially a constant for a given species under fixed environmental conditions. A number of results follow for increases in whole plant biomass: (A) The relative growth rate is a linear function of the internal nitrogen concentration. (B) The maximal relative growth rate uniquely determines the scaling of the time axis. (C) Exponential growth is consistent only with stable internal nitrogen concentration. Dose-response curves expressed in reduced variables (the ratio between a variable and the same variable for a plant growing under optimal conditions) are universal, so that all species and all environmental conditions yield the same curve. This is confirmed by experimental data. The shape (linear, exponential, etc.) of the nitrogen uptake curve is the only parameter differentiating these universal curves. The Mitscherlich curve or variations of it can be fitted very closely to the derived dose-response curves, except under exponential growth. A conclusion drawn from the analysis is that the results of nutrition experiments cannot be properly interpreted unless the variation with time of the amount of nitrogen in the plant is known. The theory can be extended to more complex situations, for example, time-varying environmental conditions.