Impact of defoliation intensity and frequency on N uptake and mobilization in Lolium perenne

The aim of the study was to evaluate the impact of defoliation intensity, defoliation frequency, and interactions with N supply on N uptake, N mobilization from and N allocation to roots, adult leaves, and growing leaves. Plants of Lolium perenne were grown under two contrasted N regimes. Defoliation intensity treatments consisted of a range of percentage leaf area removal (0, 25, 50, 75, or 100%). These treatments were applied in parallel to a set of plants previously undefoliated, and to a second set of plants which had been defoliated several times at a constant height. A (15)N tracer technique was used to quantify N uptake, mobilization, and allocation over a 7 d period. A significant reduction in plant N uptake was observed with the removal of more than 75% of lamina area, but only with high N supply. As defoliation intensity increased, the amount of N taken up and subsequently allocated to growing leaves during the labelling period was maintained at the expense of N allocation to roots and adult leaves. Increasing defoliation intensity increased the relative contribution of roots supplying mobilized N to growing leaves and decreased the relative contribution of adult leaves. Defoliation frequency did not substantially alter N uptake, mobilization, and allocation between roots, adult and growing leaves on a plant basis. However, tiller number per plant was largely increased under repeated defoliation, hence indicating that allocation and mobilization of N to growing leaves, on the basis of individual tillers, was decreased by defoliation frequency.     

Reduced atmospheric CO2 inhibits nitrogen mobilization in Festuca rubra

In defoliated grasses, where photosynthesis is reduced due to removal of leaf material, it is well established that remobilization of nitrogen occurs from both older remaining leaves and roots towards the younger growing leaves. In contrast, little is known about the movement of nitrogen within intact grass plants experiencing prolonged inhibition of photosynthesis. We tested the following hypotheses in Festuca rubra L. ssp. rubra cv. Boreal: that both reduction of the atmospheric CO2 concentration and defoliation (1) induce mobilization of nitrogen from roots and older leaves towards growing leaves and (2) elicit similar directional change in the abundance of proteins in roots and older leaves relevant to the process of nitrogen mobilization including, glutamine synthetase (GS), EC 6.3.1.2; papain, EC 3.4.22.2; chymopapain, EC 3.4.22.6; ribulose bisphosphate carboxylase/oxygenase (Rubisco), EC 4.1.1.39; and the light harvesting complex of photosystem II (LHCPII). After growth at ambient atmospheric CO2 concentration, plants of F. rubra were subject to atmospheres containing either ambient (350 micro l l-1) or deplete (< 20 micro l l-1) CO2. Concurrently, plants were either left intact or defoliated on one occasion. Steady state 15N labelling coupled with a series of destructive harvests over a 7-day period enabled changes in the nitrogen dynamics of the plants to be established. Proteins pertinent to the process of nitrogen mobilization were quantified by immunoblotting. Irrespective of defoliation, plants in ambient CO2 mobilized nitrogen from older to growing leaves. This mobilization was inhibited by deplete CO2. Greater concentration of Rubisco and reduced chymopapain abundance in older remaining leaves of intact plants, in deplete compared with ambient CO2, suggested the inhibition of mobilization was due to inhibition of protein degradation, rather than to the export of degradation products. Both deplete CO2 and defoliation induced nitrogen mobilization from roots to growing leaves. In plants at ambient CO2, defoliation did not affect nitrogen uptake or its allocation. Therefore in F. rubra nitrogen mobilization can occur independently of any downregulation of nitrogen uptake. This suggests either different signal compounds may act to downregulate uptake and upregulate mobilization, or if one particular signalling compound is used its concentration threshold differs for induction of mobilization and downregulation of uptake. The abundance of the cysteine proteases papain and chymopapain was low in roots suggesting that they were not involved in protein degradation in this tissue.     

Increased Defoliation Frequency Depletes Remobilization of Nitrogen for Leaf Growth in Grasses

Plants ofLolium perenneandFestuca rubrawere grown in sand culturereceiving all nutrients as a complete nutrient solution containing1.5 mMNH₄NO₃, and subjected to one of three defoliation treatments:undefoliated, defoliated on one occasion, or defoliated weekly.¹⁵Nlabelling was used to determine the rate of N uptake, allowingthe amount of N remobilized from storage for the growth of thetwo youngest leaves (subsequently referred to as ‘newleaves’) growing over a 14 d period after defoliationto be calculated. The total plant N uptake by both species wasreduced, compared with undefoliated plants, by both a singleand repeated defoliation, although neither caused complete inhibitionof uptake. Regularly defoliatedL. perennehad a greater reductionin root mass, concomitant with a greater increase in N uptakeper g root than did regularly defoliatedF. rubra. In both species,the amount of N derived from uptake recovered in the new leaveswas unaffected by the frequency of defoliation. BothL. perenneandF.rubramobilized nitrogen to the new leaves after a single defoliation,mobilization being sufficient to supply 50 and 41%, respectively,of the total nitrogen requirement. In regularly defoliated plants,no significant nitrogen was mobilized to the new leaves inL.perenne, and only a small amount was mobilized inF. rubra. Plantsachieved greater leaf regrowth when only defoliated once. Weconclude that increasing the frequency of defoliation of bothL.perenneandF. rubrahad little effect on the uptake of nitrogenby roots which was subsequently supplied to new leaves, butdepleted their capacity for nitrogen remobilization, resultingin a reduction in the rate of growth of new leaves. Lolium perenne; Festuca rubra; defoliation frequency; mobilization; root uptake; nitrogen     

Growth response to step-decrease in nutrient availability in small birch (Betula pendula Roth)

Abstract Changes in the uptake and allocation of carbon and nitrogen, after a step-decrease in nutrient availability, were investigated in small birch (Betula pendula Roth). By demonstrating stable nutrition, before and after the decrease in nutrient supply, it was possible to eliminate the effects of plant size and age. Immediately following the step-decrease in nutrient availability, net nitrogen uptake to leaves and the relative rate of increase in shoot area tended to zero. Although photosynthetic rate per shoot area decreased, carbon uptake remained in excess of that used in structural growth and respiration. More of the excess carbon was accumulated as starch in leaves than in roots. After a lag phase, the relative rates of increase in plant dry matter, starch amount, net nitrogen uptake to leaves and shoot area development equalled that of the reduced rate of nutrient supply. It is concluded that the reduction in plant relative growth rate was much more attributable to the reduced allocation of photosynthate to leaf area growth than to the reduction in photosynthesis per shoot area.     

The phosphorus budget of a field-grown strawberry (Fragaria x ananassa cv. Hapil) crop: evidence for a mycorrhizal contribution

The phosphorus content of a field-grown strawberry crop was monitored over one growing season and the allocation of P to different plant parts determined. In the early part of the season, leaves were the major sink for P and this was partly supplied by redistribution from rhizomes. Once flowers were initiated, these became the major sink and their P requirement was satisfied by new uptake. The calculated rates of P inflow to the roots in this phase of growth were so large that it was necessary to postulate not only that P uptake was assisted by mycorrhizas, but that the whole root system, including the old, brown roots, was active. The implications of this conclusion for concepts of vesicular-arbuscular mycorrhizal function under field conditions are discussed.     

Frost damage and winter nitrogen uptake by the grass Poa pratensis L.: consequences for vegetative versus reproductive growth

Frost damage can decrease nitrogen uptake by grasses over winter, and it can also decrease biomass production over the following growing season. However, it is not clear to what extent reduced nitrogen uptake over winter decreases grass production, or whether is it merely a symptom of root damage. We examined the growth response of the grass Poa pratensis L. (Kentucky bluegrass) to variation in the timing of freezing and nitrogen availability over winter in London, Ontario, Canada. All tillers were transplanted into untreated soil in early spring, and at peak seed maturation, root, shoot, and reproductive biomass were measured. There was an interaction between freezing and increased winter nitrogen availability, whereby nitrogen addition increased tiller biomass under ambient temperatures, but decreased tiller biomass in combination with a late winter freeze. The nitrogen response of ambient temperature tillers occurred primarily via increased seed production, whereas for frozen tillers seed production was generally absent. Our results support the hypothesis that nitrogen uptake over winter can increase growing season productivity in P. pratensis, but also demonstrate that increased nitrogen availability increases tiller vulnerability to frost. These results have important implications for grass responses to the alteration of soil freezing dynamics with climate change.     

Role of Arbuscular Mycorrhizal Fungi in Uptake of Phosphorus and Nitrogen From Soil

Abstract

Colonization of plant roots by arbuscular mycorrhizal fungi can greatly increase the plant uptake of phosphorus and nitrogen. The most prominent contribution of arbuscular mycorrhizal fungi to plant growth is due to uptake of nutrients by extraradical mycorrhizal hyphae. Quantification of hyphal nutrient uptake has become possible by the use of soil boxes with separated growing zones for roots and hyphae. Many (but not all) tested fungal isolates increased phosphorus and nitrogen uptake of the plant by absorbing phosphate, ammonium, and nitrate from soil. However, compared with the nutrient demand of the plant for growth, the contribution of arbuscular mycorrhizal fungi to plant phosphorus uptake is usually much larger than the contribution to plant nitrogen uptake. The utilization of soil nutrients may depend more on efficient uptake of phosphate, nitrate, and ammonium from the soil solution even at low supply concentrations than on mobilization processes in the hyphosphere. In contrast to ectomycorrhizal fungi, nonsoluble nutrient sources in soil are used only to a limited extent by hyphae of arbuscular mycorrhizal fungi. Side effects of mycorrhizal colonization on, for example, plant health or root activity may also influence plant nutrient uptake.     

Responses of wetland graminoids to the relative supply of nitrogen and phosphorus

The biomass production of wetland vegetation can be limited by nitrogen or phosphorus. Some species are most abundant in N-limited vegetation, and others in P-limited vegetation, possibly because growth-related traits of these species respond differently to N versus P supply. Two growth experiments were carried out to examine how various morphological and physiological traits respond to the relative supply of N and P, and whether species from sites with contrasting nutrient availability respond differently. In experiment 1, four Carex species were grown in nutrient solutions at five N:P supply ratios (1.7, 5, 15, 45, 135) combined with two levels of supply (geometric means of N and P supply). In experiment 2, two Carex and two grass species were grown in sand at the same .ve N:P supply ratios combined with three levels of supply and two light intensities (45% or 5% daylight). After 12-13 weeks of growth, plant biomass, allocation, leaf area, tissue nutrient concentrations and rates and nutrient uptake depended signi.cantly on the N:P supply ratio, but the type and strength of the responses differed among these traits. The P concentration and the N:P ratio of shoots and roots as well as the rates of N and P uptake were mainly determined by the N:P supply ratio; they showed little or no dependence on the supply level and relatively small interspeci.c variation. By contrast, the N concentration, root mass ratio, leaf dry matter content and speci.c leaf area were only weakly related to the N:P supply ratio; they mainly depended on plant species and light, and partly on overall nutrient supply. Plant biomass was determined by all factors together. Within a level of light and nutrient supply, biomass was generally maximal (i.e. co-limited by N and P) at a N:P supply ratio of 15 or 45. All species responded in a similar way to the N:P supply ratio. In particular, the grass species Phalaris arundinacea and Molinia caerulea showed no differences in response that could clearly explain why P. arundinacea tends to invade P-rich (N-limited) sites, and M. caerulea P-limited sites. This may be due to the short duration of the experiments, which investigated growth and nutrient acquisition but not nutrient con­servation.     

Allocation strategies of carbon,nitrogen and phosphorus following a gradient of wildfire severities

植物碳、氮、磷在不同火烧强度下的分配策略 森林野火是影响北方针叶林演替过程中养分分配规律的重要因素。然而,植物叶片和细根之间 的碳(C)、氮(N)、磷(P)分配策略在不同强度森林野火后的研究尚不充分。本研究旨在探讨不同野火强度下叶片和细根间C、N、P的分配策略。运用化学计量学理论和异速生长方程,选取中国东北大兴安岭地区的4个不同火烧强度(未火烧、低、中、高)恢复10年后的火烧迹地为研究样地,比较不同火烧强 度下各物种叶片和细根的C、N、P含量。研究结果表明,与未受到火烧的样地相比,轻度火烧迹地的植物叶片和细根C浓度增加,重度火烧迹地植物叶片N浓度最高,但是细根N浓度最低。N:P比值的平均值大于16的结果表示植物养分利用策略在高火烧强度下趋于P限制。更重要的是,随着火烧严重程度的增加,细根与叶片间的C、N、P分配规律出现由异速生长向等速生长的转变,即随着火烧强度的增加,元素分配表现为对叶片的分配多于细根。这些结果表明,植物叶片和细根之间的元素分配策略在受到不同强度的野火干扰以后发生了失衡。本研究加深了我们对火后森林生态系统演替过程中植物与土壤养分动态的认识。     

GROWTH AND NUTRITION OF TIMOTHY (PHLEUM PRATENSE L.): IV. THE EFFECT OF NITROGEN, PHOSPHORUS AND POTASSIUM SUPPLY ON GROWTH DURING THE FIRST YEAR

In a sand-culture experiment lasting 21 weeks plants of timothy grown from seed were maintained at all combinations of three levels of nitrogen, phosphorus and potassium. The number and weight of tillers, leaves and ears, and the weight of roots, were determined at the end of the experiment, and for some of the treatments at intervals of 4 weeks. All three nutrients had significant effects, especially N which at its lowest level of supply tended to mask the effect of the other two elements. Potassium influenced tiller numbers least, especially those of primary tillers, but in the presence of high concentrations of N and P it had a large effect on leaf area and dry weight. Relative growth and net assimilation rates responded to varying nutrient supply only in the early stages of growth, so that in general variations in dry weight were associated with nutrient effects on leaf area.     

Morphological and physiological responses of rice (Oryza sativa) to limited phosphorus supply in aerated and stagnant solution culture

BACKGROUND AND AIMS: Rain-fed lowland rice commonly encounters stresses from fluctuating water regimes and nutrient deficiency. Roots have to acquire both oxygen and nutrients under adverse conditions while also acclimating to changes in soil-water regime. This study assessed responses of rice roots to low phosphorus supply in aerated and stagnant nutrient solution. METHODS: Rice (Oryza sativa 'Amaroo') was grown in aerated solution with high P (200 micro m) for 14 d, then transferred to high or low (1.6 micro m) P supply in aerated or stagnant solution for up to 8 d. KEY RESULTS: After only 1 d in stagnant conditions, root radial oxygen loss (ROL) had decreased by 90 % in subapical zones, whereas near the tip ROL was maintained. After 4 d in stagnant conditions, maximum root length was 11 % less, and after 8 d, shoot growth was 25 % less, compared with plants in aerated solution. The plants in stagnant solution had up to 19 % more adventitious roots, 24 % greater root porosity and 26 % higher root/shoot ratio. Rice in low P supply had fewer tillers in both stagnant and aerated conditions. After 1-2 d in stagnant solution, relative P uptake declined, especially at low P supply. Aerated roots at low P supply maintained relative P uptake for 4 d, after which uptake decreased to the same levels as in stagnant solution. CONCLUSIONS: Roots responded rapidly to oxygen deficiency with decreased ROL in subapical zones within 1-2 d, indicating induction of a barrier to ROL, and these changes in ROL occurred at least 2 d before any changes in root morphology, porosity or anatomy were evident. Relative P uptake also decreased under oxygen deficiency, showing that a sudden decline in root-zone oxygen adversely affects P nutrition of rice.     

Nitrate supply and plant development influence nitrogen uptake and allocation under elevated CO<Subscript>2</Subscript> in durum wheat grown hydroponically

Growth in elevated CO₂ often leads to decreased plant nitrogen contents and down-regulation of photosynthetic capacity. Here, we investigated whether elevated CO₂ limits nitrogen uptake when nutrient movement to roots is unrestricted, and the dependence of this limitation on nitrogen supply and plant development in durum wheat (Triticum durum Desf.). Plants were grown hydroponically at two N supplies and ambient and elevated CO₂ concentrations. Elevated CO₂ decreased nitrate uptake per unit root mass with low N supply at early grain filling, but not at anthesis. This decrease was not associated with higher nitrate or amino acid, or lower non-structural carbohydrate contents in roots. At anthesis, elevated CO₂ decreased the nitrogen content of roots with both levels of N and that of aboveground organs with high N. With low N, elevated CO₂ increased N allocation to aboveground plant organs and nitrogen concentration per unit flag leaf area at anthesis, and per unit aboveground dry mass at both growth stages. The results from the hydroponic experiment suggest that elevated CO₂ restricts nitrate uptake late in development, high N supply overriding this restriction. Increased nitrogen allocation to young leaves at low N supply could alleviate photosynthetic acclimation to elevated CO₂.     

Mechanisms for the increase in phosphorus uptake of waterlogged plants: soil phosphorus availability, root morphology and uptake kinetics

Waterlogging frequently reduces plant biomass allocation to roots. This response may result in a variety of alterations in mineral nutrition, which range from a proportional lowering of whole-plant nutrient concentration as a result of unchanged uptake per unit of root biomass, to a maintenance of nutrient concentration by means of an increase in uptake per unit of root biomass. The first objective of this paper was to test these two alternative hypothetical responses. In a pot experiment, we evaluated how plant P concentration of Paspalum dilatatum, (a waterlogging-tolerant grass from the Flooding Pampa, Argentina) was affected by waterlogging and P supply and how this related to changes in root-shoot ratio. Under both soil P levels waterlogging reduced root-shoot ratios, but did not reduce P concentration. Thus, uptake of P per unit of root biomass increased under waterlogging. Our second objective was to test three non-exclusive hypotheses about potential mechanisms for this increase in P uptake. We hypothesized that the greater P uptake per unit of root biomass was a consequence of: (1) an increase in soil P availability induced by waterlogging; (2) a change in root morphology, and/or (3) an increase in the intrinsic uptake capacity of each unit of root biomass. To test these hypotheses we evaluated (1) changes in P availability induced by waterlogging; (2) specific root length of waterlogged and control plants, and (3) P uptake kinetics in excised roots from waterlogged and control plants. The results supported the three hypotheses. Soil P avail-ability was higher during waterlogging periods, roots of waterlogged plants showed a morphology more favorable to nutrient uptake (finer roots) and these roots showed a higher physiological capacity to absorb P. The results suggest that both soil and plant mechanisms contributed to compensate, in terms of P nutrition, for the reduction in allocation to root growth. The rapid transformation of the P uptake system is likely an advantage for plants inhabiting frequently flooded environments with low P fertility, like the Flooding Pampa. This advantage would be one of the reasons for the increased relative abundance of P. dilatatum in the community after waterlogging periods. Received: 15 February 1997 / Accepted: 20 May 1997     

Carbon cost of plant nitrogen acquisition: global carbon cycle impact from an improved plant nitrogen cycle in the Community Land Model

Plants typically expend a significant portion of their available carbon (C) on nutrient acquisition – C that could otherwise support growth. However, given that most global terrestrial biosphere models (TBMs) do not include the C cost of nutrient acquisition, these models fail to represent current and future constraints to the land C sink. Here, we integrated a plant productivity‐optimized nutrient acquisition model – the Fixation and Uptake of Nitrogen Model – into one of the most widely used TBMs, the Community Land Model. Global plant nitrogen (N) uptake is dynamically simulated in the coupled model based on the C costs of N acquisition from mycorrhizal roots, nonmycorrhizal roots, N‐fixing microbes, and retranslocation (from senescing leaves). We find that at the global scale, plants spend 2.4 Pg C yr^?1 to acquire 1.0 Pg N yr^?1, and that the C cost of N acquisition leads to a downregulation of global net primary production (NPP) by 13%. Mycorrhizal uptake represented the dominant pathway by which N is acquired, accounting for ~66% of the N uptake by plants. Notably, roots associating with arbuscular mycorrhizal (AM) fungi – generally considered for their role in phosphorus (P) acquisition – are estimated to be the primary source of global plant N uptake owing to the dominance of AM‐associated plants in mid‐ and low‐latitude biomes. Overall, our coupled model improves the representations of NPP downregulation globally and generates spatially explicit patterns of belowground C allocation, soil N uptake, and N retranslocation at the global scale. Such model improvements are critical for predicting how plant responses to altered N availability (owing to N deposition, rising atmospheric CO₂, and warming temperatures) may impact the land C sink.     

The reallocation of carbon in P deficient lupins affects biological nitrogen fixation

It is not known how phosphate (P) deficiency affects the allocation of carbon (C) to biological nitrogen fixation (BNF) in legumes. The alteration of the respiratory and photosynthetic C costs of BNF was investigated under P deficiency. Although BNF can impose considerable sink stimulation on host respiratory and photosynthetic C, it is not known how the change in the C and energy allocation during P deficiency may affect BNF. Nodulated Lupinus luteus plants were grown in sand culture, using a modified Long Ashton nutrient solution containing no nitrogen (N) for ca. four weeks, after which one set was exposed to a P-deficient nutrient medium, while the other set continued growing on a P-sufficient nutrient medium. Phosphorus stress was measured at 20 days after onset of P-starvation. During P stress the decline in nodular P levels was associated with lower BNF and nodule growth. There was also a shift in the balance of photosynthetic and respiratory C toward a loss of C during P stress. Below-ground respiration declined under limiting P conditions. However, during this decline there was also a shift in the proportion of respiratory energy from maintenance toward growth respiration. Under P stress, there was an increased allocation of C toward root growth, thereby decreasing the amount of C available for maintenance respiration. It is therefore possible that the decline in BNF under P deficiency may be due to this change in resource allocation away from respiration associated with direct nutrient uptake, but rather toward a long term nutrient acquisition strategy of increased root growth.     

Biomass Allocation in Cyperus papyrus in a Tropical Wetland,Lake Naivasha,Kenya1

The nutrient status of Lake Naivasha, a freshwater lake in southeastern Kenya, has been rising since at least 1982. A potential effect of increases in nutrient supply to the lake's floating papyrus is increasing of the plants’ investment in above–water material and reduction of the amount of energy invested in uptake and storage. Biomass and its allocation between culms, panicles, roots, and rhizomes was measured in 17 sites around the 150–km² lake. Although above–water biomass was greatest in sites closest to the lake's major nutrient inflow, the River Malewa, there was little evidence of corresponding decreases in the biomass of uptake and storage tissues. In August 1995, the average ± SD biomass of papyrus in the lake was 11,540 ± 3020 g/m², with the papyrus containing about 4500 ± 1900 g total carbon/m² and about 100 ± 70 g total nitrogen/m². Plant nitrogen contents did not vary with distance from the main external nutrient supply. Together with low nitrogen concentrations in the plants (0.60 ± 0.26 in culms and 0.99 ± 0.50% in rhizomes), very high carbon to nitrogen ratios (49 ± 20:1) and nitrogen fixation in the rhizosphere explaining only about half of the plants’ nitrogen, papyrus is a likely net sink for nitrogen supplied from the lake's increasingly cultivated watershed. Despite this role, clearance of papyrus in favor of agriculture partly explains the reductions in the area of papyrus within the lake basin from 48 km² in the late 1960s to 14 km² in 1995.     

Genotypic variation in partitioning of dry matter and manganese between source and sink organs of rice under manganese stress

Key message

Genetic variability in dry matter and manganese partitioning between source and sink organs was the key mechanism for Mn efficient rice genotypes to cope with Mn stress.

Abstract

Considerable differences exist among cereal genotypes to cope manganese (Mn) deficiency, but the underlying mechanisms are poorly understood. Minimal information regarding partitioning and/or remobilization of dry matter and Mn between source and sink organs exists in rice genotypes differing in Mn efficiency. The present study was aimed to assess the growth dynamics in terms of dry matter and Mn remobilization in the whole plant (leaves and tillers as source and panicles and grains as sink) during the grain development in diverse rice genotypes. The efficient genotypes accumulated higher dry matter than inefficient genotypes under low Mn level. The translocation index i.e., uptake in grain/total uptake was 0.11 in efficient genotype (PR 116) and 0.04 in inefficient genotypes (PR 111). The efficient genotype had higher grain Mn utilization efficiency of 0.71 in comparison to 0.48 of inefficient genotype indicating that in efficient genotype, Mn in grain produces more dry matter than inefficient genotypes. The efficient genotypes also had higher flag leaf area and nitrate reductase activity. The source of efficient genotypes contributed to a greater extent to developing sink but further mobilization to grain was hindered by panicle. The panicle of inefficient genotypes had higher per cent of Mn uptake than efficient genotypes indicating that Mn was least mobilized from panicle to grain in inefficient genotypes. The lower per cent uptake of Mn in efficient genotypes indicated that Mn was mobilized from panicle to developing grain and this led to higher Mn translocation index in grain of efficient genotypes. The uptake partitioning revealed that source of all genotypes mobilized the Mn towards the sink to almost same extent but it was the panicle where highest per cent uptake per plant was in inefficient genotypes and lowest in efficient genotypes. The lowest per cent uptake in panicle of efficient genotypes revealed that it supported developing grain to have highest translocation index.     

A comparative study on plant growth and root plasticity responses of two Brachiaria forage grasses grown in nutrient solution at low and high phosphorus supply

Brachiaria forage grasses are widely used for livestock production in the tropics. Signalgrass (Brachiaria decumbens cv. Basilisk, CIAT 606) is better adapted to low phosphorus (P) soils than ruzigrass (B. ruziziensis cv. Kennedy, CIAT 654), but the physiological basis of differences in low-P adaptation is unknown. We characterized morphological and physiological responses of signalgrass and ruzigrass to low P supply by growing both grasses for 30 days in nutrient solution with two levels of P supply using the hydroxyapatite pouch system. Ruzigrass produced more biomass at both levels of P supply whilst signalgrass appears to be a slower-growing grass. Both grasses increased biomass allocation to roots and had higher root acid phosphatase and phytase activities at low P supply. At low P supply, ruzigrass showed greater morphological plasticity as its leaf mass density and lateral root fraction increased. For signalgrass, morphological traits that are not responsive to variation in P supply might confer long-term ecological advantages contributing to its superior field persistence: greater shoot tissue mass density (dry matter content) might lower nutrient requirements while maintenance of lateral root growth might be important for nutrient acquisition in patchy soils. Physiological plasticity in nutrient partitioning between root classes was also evident for signalgrass as main roots had higher nutrient concentrations at high P supply. Our results highlight the importance of analyzing morphological and physiological trait profiles and determining the role of phenotypic plasticity to characterize differences in low-P adaptation between Brachiaria genotypes.     

Carbon and Nutrient Allocation in Elymus farctus Seedlings after Burial with Sand

We report physiological aspects of the response of seedlingsof the strandline grass Elymus farctus to short-term burialwith sand. Seedlings were buried at the two-leaf stage for oneweek and compared with non-buried controls - before, duringand after burial. Photosynthetic CO₂ uptake was measured byinfrared gas analysis and carbon translocation from the youngestexpanded leaf was monitored after exposure to ¹⁴CO₂. The concentrationsof water-soluble carbohydrate, and total nitrogen, phosphorusand potassium in component organs were determined. Net photosynthetic capacity was almost completely inhibitedby 5 d of burial. However, plants uncovered after 7 d of burialrecovered full photosynthetic competence within 24 h. Therewas a sharp, sustained depression in the water-soluble carbohydrateconcentration of the roots and stems after burial, whereas leaves1–3 had much higher concentrations than non-buried plantsfor up to 20 d after uncovering. Burial virtually suppressedthe translocation of ¹⁴C to stem, roots and expanding leaves,and this effect persisted even after full recovery of net photosynthesis.The proportional allocation of total N, P and K to leaves wasincreased after burial, mainly at the expense of the roots.Changes both in nutrient concentration and in the relative massesof organs contributed to this effect. The apparent reversal of the normal source-sink relationshipsfor carbohydrate between photosynthetic and non-photosyntheticorgans, and the differential allocation of inorganic nutrients,may contribute to the maintenance of photosynthetic capacityduring burial. The rapid recovery of net photosynthesis afterre-exposure suggests that these responses may be advantageousfor survival of E. farctus seedlings in the early stages ofgrowth in a physically unstable and unpredictable environment. Sand burial, photosynthesis, translocation, nutrient allocation, Elymus farctus, sand couch grass     

Is root growth under phosphorus deficiency affected by source or sink limitations?

Reduced net photosynthesis (Pn) and decreasing shoot and root biomass are typical effects of phosphorus deficiency in plants. Lower biomass accumulation could be the result of reduced Pn (source limitation), but may also be due to direct negative effects of low P availability on growth (sink limitation). Because of the principal importance of root growth for P uptake, this study specifically examined the question whether source or sink limitations were responsible for reduced root growth rates under P deficiency. Rice plants were grown in nutrient solutions with four levels of P supply and at two light treatments and the effect of Pxlight treatments on growth and carbohydrate distribution was observed. Plants had up to 70% higher Pn when grown with natural (high) light compared with low light. Higher Pn, however, did not lead to additional growth under P deficiency, suggesting that assimilate supply from source leaves to roots was not a limiting factor under P deficiency. This was supported by observations that root starch concentrations increased in P-deficient roots. The comparison of two genotypes with different tolerance to P deficiency showed that the more tolerant one preferentially distributed P to roots where the additional P stimulated root growth and, ultimately, P uptake. The results therefore suggest that source limitation is of little importance under P deficiency. Even at highly sub-optimal tissue P concentrations of below 0.7 mg P g(-1) dry weight, plants were able to produce enough assimilates to sustain growth rates that were directly limited by low P availability.