Respiratory energy requirements of roots vary with the potential growth rate of a plant species

The rates of growth, net rate of nitrate uptake and root respiration of 24 wild species were compared under conditions of optimum nutrient supply. The relative growth rate (RGR)of the roots of these species varied between 110 and 370 mg g^-1 day^-1 and the net rate of nitrate uptake between 1 and 7 mmol (g root dry weight)^-1 day^-1. The rate of root respiration was positively correlated with the RGR of the roots. Root respiration was also calculated from the measured rate of growth and nitrate uptake, using previously determined values for the costs of maintenance, growth and ion uptake of two slow-growing species. The calculated rate of respiration was slightly lower than the measured one for slow-growing species, but twice as high as measured rates for rapid-growing species. This discrepancy was not due to a relatively smaller electron flow through the alternative pathway and, consequently, a more efficient ATP production in the fast-growing species. Neither could variation in specific costs for root growth or maintenance explain these differences. Therefore, we conclude that fast-growing species have lower specific respiratory costs for ion uptake than slow-growing ones. Due partly to these lower specific costs of nutrient uptake, the fraction of respiration that rapid-growing species spend on anion uptake is lower than that of slow-growing species, in spite of the much higher rate of ion uptake of the fast-growing ones.     

Plant respiration in relation to growth, maintenance, ion uptake and nitrogen assimilation

Abstract Respiration in plants is generally observed to comprise two components: one proportional to the growth rate and the other to the plant dry mass. These components are usually interpreted as being related to the growth of new plant material and maintenance of existing plant material, respectively. By analysing data in this way, the respiratory costs of both structural synthesis and maintenance are observed to be greater in the root than the shoot. This contradicts current understanding of the biochemistry of the processes involved. The basic model is developed to incorporate three additional processes. The first is the cost of ion uptake for plant growth. The second allows for the fact that the site of nitrogen assimilation into amino acids may differ from the site of utilization for protein synthesis: when ammonium is supplied, this is incorporated immediately into amino acids owing to its toxicity to the plants; when nitrate is supplied it may be reduced either in the shoot or root, or both, and subsequently transported around the plant for utilization. The third process to be included is an energy cost for the uptake of ions to balance efflux from the root system. The theory is consistent with experimental observation and provides a means of understanding and interpreting respiration and nitrogen metabolism in plants.     

Energy requirements for maintenance of ion concentrations in roots

Maintenance of ion gradients across plant membranes is considered to be an important process requiring respiratory energy in plant tissues. In order to test this hypothesis, roots of intact plants of potato ( Solanum tuberosum L. cv. Alcmaria and cv. Pimpernel) were incubated in a closed circulation system. Electrical conductivity of the solution surrounding these roots was continuously monitored to determine total ion efflux into demineralized water. Anion efflux rate from the symplast was 35 neq (g dry weight)⁻¹ s⁻¹. In combination with literature data on the specific costs of ion transport, this efflux rate yields the respiration rate associated with re-uptake balancing efflux (i. e. maintenance of cellular ion concentrations). The results suggest that energy costs associated with re-uptake of ions may account for up to 25–50% of the total respiratory costs involved in ion influx.     

Effects of a localized supply of nitrate on NO3 - uptake rate and growth of roots in Lolium multiflorum Lam.

Roots of higher plants are usually exposed to varying spatial and temporal changes in concentrations of soil mineral nitrogen. A split root system was used to see how Lolium multiflorum Lam. roots adapt to such variations to cope with their N requirements. Plants were grown in hydroponic culture with their root system split in two spatially separated compartments allowing them to be fed with or without KNO₃. Net NO₃ ^- uptake, ¹⁵NO₃ ^- influx and root growth were studied in relation to time. Within less than 24 h following deprivation of KNO₃ to half the roots, the influx in NO₃ ^- fed roots was observed to increase (about 200% of the influx measured in plant uniformly NO₃ ^- supplied control plant) thereby compensating the whole plant for the lack of uptake by the N deprived roots. Due to the large NO₃ ^- concentrations in the roots, the NO₃ ^- efflux was also increased so that the net uptake rate increased only slightly (35% maximum) compared with the values obtained for control plants uniformly supplied with NO₃ ^-. This increase in net NO₃ ^- uptake rate was not sufficient to compensate the deficit in N uptake rate of the NO₃ ^- deprived split root in the short term. Over a longer period (>1 wk), root growth of the part of the root system locally supplied with NO₃ ^- was stimulated. An increase in root growth was mainly responsable for the greater uptake of nitrate in Lolium multiflorum so that it was able to fully compensate the deficit in N uptake rate of the NO₃ ^- deprived split root.     

Rapid decline in nitrate uptake and respiration with age in fine lateral roots of grape: implications for root efficiency and competitive effectiveness

Changes in function as an individual root ages has important implications for understanding resource acquisition, competitive ability and optimal lifespan. Both nitrate uptake and respiration rates of differently aged fine roots of grape (Vitis rupestris x V. riparia cv. 3309 C) were measured. The resulting data were then used to simulate nitrate uptake efficiency and nutrient depletion as a function of root age. Both nitrate uptake and root respiration declined remarkably quickly with increasing root age. The decline in both N uptake and root respiration corresponded with a strong decline in root N concentration, suggesting translocation of nitrogen out of the roots. For simulations where no nutrient depletion occurs at the root surface, daily uptake efficiency was maximal at root birth and lifetime nitrate uptake efficiency slowly increased as the roots aged. Simulations of growth of roots into unoccupied soil using a solute transport model indicated the advantage of high uptake capacity in new roots under competitive conditions where nitrate availability is very transitory.     

Changes in growth and nutrient uptake in Brassica oleracea exposed to atmospheric ammonia

BACKGROUND AND AIMS: Plant shoots form a sink for NH3, and are able to utilize it as a source of N. NH3 was used as a tool to investigate the interaction between foliar N uptake and root N uptake. To what extent NH3 can contribute to the N budget of the plant or can be regarded as a toxin, was investigated in relation to its concentration and the N supply in the root environment. METHODS: Brassica oleracea was exposed to 0, 4 and 8 microL L(-1) NH3, with and without nitrate in the nutrient solution. Growth, N compounds, nitrate uptake rate, soluble sugars and cations were measured. KEY RESULTS: In nitrate-sufficient plants, biomass production was not affected at 4 microL L(-1) NH3, but was reduced at 8 microL L(-1) NH3. In nitrate-deprived plants, shoot biomass was increased at both concentrations, but root biomass decreased at 8 microL L(-1) NH3. The measured nitrate uptake rates agreed well with the plant's N requirement for growth. In nitrate-sufficient plants nitrate uptake at 4 and 8 microL L(-1) NH3 was reduced by 50 and 66 %, respectively. CONCLUSIONS: The present data do not support the hypothesis that NH3 toxicity is caused by a shortage of sugars or a lack of capacity to detoxify NH3. It is unlikely that amino acids, translocated from the shoot to root, are the signal metabolites involved in the down-regulation of nitrate uptake, since no relationship was found between changes in nitrate uptake and root soluble N content of NH3-exposed plants.     

Nitrite in the root zone and its effects on ion uptake and growth of wheat seedlings

The immediate and posteffects of various concentrations of NaNO₂ on ion uptake of wheat ( Triticum aestivum L. cv. GK Öthalom) seedlings were studied at different pH values. Without pretreatment, the higher the concentration of NaNO₂ the greater was the decrease in uptake of K⁺ into the roots, both at pH 4 and pH 6. At pH 6 but not at pH 4 the reverse was true when the seedlings were pretreated with NaNO₂. Due to the high Na⁺ content of the roots, an effect of Na⁺ in this process cannot be excluded. Nitrite was taken up by the roots more rapidly than nitrate. Nitrite at 0.1 m M in the medium induced the development of an uptake system for both NO⁻₂ and NO⁻₃ in wheat roots. At higher concentrations pretreatment with NO⁻₂ decreased NO⁻₃ uptake by the roots, but NO₃ did not inhibit the uptake of NO₂. The toxic effect of NO⁻₂ was strongly pH dependent. Lower pH of the external solution led to an increased inhibition by NO⁻₂ of both ion uptake and growth of seedlings. The inhibitory effect of NO⁻₂ differed considerably for roots and shoots. The roots and especially the root hairs were particularly sensitive to NO⁻₂ treatment.     

Cultivar differences in the rate of nitrate uptake by intact wheat plants as related to growth rate

Six Argentinian wheat ( Triticum aestivum L.) cultivars grown in nutrient solutions in controlled environment were compared for their nitrate uptake rates on a root dry weight basis. Up to 3-fold differences were observed among the cultivars at 16, 20 and 24 days from germination, either when measured by depletion from the nutrient solution in short-term experiments, or by total N accumulation in the tissue during 8 days.
No differences in total N concentration in root or shoots were found among cultivars. Although the different cultivars showed significant differences in shoot/root ratio and nitrate reductase activity (EC 1.6.6.1) in the roots, none of these parameters was correlated with the nitrate uptake rate. However, nitrate uptake was found to be positively correlated (r = 0.99) with the shoot relative growth rate of the cultivars. The three cultivars with the highest nitrate uptake rates and relative growth rates showed a positive correlation between root nitrate concentration and uptake. However, this correlation was not found in the cultivars with the lowest growth and uptake rates.
Our results indicate that the difference in nitrate uptake rate among these cultivars may only be a consequence of their differences in growth rate, and it is suggested that at least two mechanisms regulate nitrate uptake, one working when plant demand is low and another when plant demand is high.     

Efficiency of nitrate uptake in spinach: impact of external nitrate concentration and relative growth rate on nitrate influx and efflux

Regulation of nitrate influx and efflux in spinach (Spinacia oleracea L., cv. Subito), was studied in short-term label experiments with 13N- and 15N-nitrate. Nitrate fluxes were examined in relation to the N demand for growth, defined as relative growth rate (RGR) times plant N concentration. Plants were grown at different nitrate concentrations (0.8 and 4 mM), with mineral composition of growth and uptake solutions identical. Nitrate influx, efflux and net nitrate uptake rate (NNUR) were independent of the external nitrate concentration, despite differences in internal nitrate concentration. At both N regimes, NNUR was adequate to meet the N demand for growth. RGR-related signals predominantly determined the nitrate fluxes. At high RGR (0.25 g g-1 day-1), nitrate influx was 20 to 40% lower and nitrate efflux was 50 to 70% lower than at lower RGR (0.17 g g-1 day-1); efflux:influx ratio (E:I) declined from 0.5 at low RGR to 0.2 at higher RGR. Thus, the efficiency of NNUR substantially increased with increasing RGR. Differences in nitrate translocation between morning and afternoon coincided with differences in nitrate efflux, which is in accordance with the suggested regulation of nitrate efflux by the root cytoplasmic nitrate concentration. This revised version was published online in July 2006 with corrections to the Cover Date.     

A model relating the maximum nitrate inflow of lettuce (Lactuca satvia L.) to the growth of roots and shoots

The net inflow of nitrate can be calculated from the nitrate concentration at the root surface by means of the Michaelis-Menten equation. Because of maximum inflow (Imax) is not constant but varies with plant age and growing conditions, a model for calculating Imax during plant growth was derived. Lettuce was grown in nutrient solution. Variations in temperature, radiation and plant age were used to vary growth rates and N-demand of plants. There was a linear relationship between relative growth rates (RGR) and maximum nitrate inflow (Imax), that could be described by the following regression function: Imax = 0.24 + 6.57 RGR. A residual analysis showed a further influence on Imax from the root:shoot-ratio (RSR), the effects of which could be accounted for by including an e-function in the relationship: Imax = (0.27 + 10.63 RGR) e^{(–0.0017 RSR)}. This model for calculating Imax was validated in two further experiments.     

Relation between relative growth rate, endogenous gibberellins, and the response to applied gibberellic acid for Plantago major

Relationships between relative growth rate (RGR), endogenous gibberellin (GA) concentration and the response to application of gibberellic acid (GA₃) were studied for two inbred lines of Plantago major L., which differed in RGR. A4, the fast-growing inbred line, had a higher free GA concentration than the slow-growing W9, as analyzed by enzyme immunoassay. GA₃ application increased total plant weight and RGR₃ particularly for the slow-growing line. Chlorophyll a content and photosynthetic activity per unit leaf area were decreased, while transpiration rate was unaffected by GA₃ application. The increase in RGR by GA₃ application was associated with an increased leaf weight ratio; specific leaf area and percentage of dry matter in the leaves were only temporarily affected. Root respiration rate per unit dry weight was unaffected.
The correlation between low RGR, low GA concentration and high responsiveness to applied GA₃ supports the contention that gibberellins are involved in the regulation of RGR. However, the transient influence of GA₃ application on some growth components suggests the involvement of other regulatory factors in addition to GA.     

Short- and long-term effects of a change in the spatial distribution of nitrate in the root zone on N uptake, growth and root development of young lettuce plants

Abstract. The effects of a change in the distribution of nitrate within the root zone on N uptake and growth were studied using young lettuce plants after reducing the proportion of their root systems supplied with nitrate from 100 to ca 10% in split-root experiments in the glasshouse. The main effects of the localized nitrate supply were concentrated in a 2-week period immediately after the treatment was imposed, when a temporary reduction in nitrate uptake caused the gradual development of N deficiency and a decline in plant growth rate. The plants adapted to the change in nitrate distribution, initially by increasing unit absorption rates (uptake rates per unit weight of root) and more gradually by increasing production of new roots in the high-nitrate zone. As a result, relative N uptake rates and relative growth rates were restored to the same levels as for control plants (given a spatially uniform N supply throughout) after ca 12d, even though only ca 12–15% of their roots were exposed to nitrate at this time. Thereafter, the plants continued to adapt by concentrating new root growth in the nitrate-containing zone, ultimately allowing unit absorption rates to return to normal. There was no evidence of any significant N deficiency in the plants after the initial adaptive response was complete, even though the total-N concentrations of the plants given the localized supply were consistently less than those given the uniform N treatment, and nitrate concentrations in the petiole sap were generally lower in leaves on one side of the plant (because of limited lateral movement of nitrate between xylem vessels during its transport to the shoot). The delay in the initiation of an adaptive response caused a significant check in growth, and the resulting relative weight differences were maintained throughout the subsequent life of the plant. Plants in all treatments matured on the same date, so yields for those grown with the localized supply were less than those of the control, and could not be recovered by delaying final harvest without unacceptable loss of quality. The pattern of the changes in N uptake and plant growth, and the effect on final yield, were similar to those exhibited by young lettuce plants subjected to a temporary interruption in nitrate supply, suggesting that the reduction in final yield for plants grown with the localized supply was largely the effect of the check in growth which occurred whilst the Plants were adapting to the change in nitrate distribution during the early part of the experiment. This implies that the rate of dry matter production of young lettuce plants can be altered by N treatment without affecting their rate of physiological development.     

毛果苔草湿地枯落物及地下生物量动态

采用网袋法和土柱法分别对三江平原湿地毛果苔草（Carex lasiocarpa）种群枯落物及地下生物量的季节动态变化规律进行分析。结果表明,毛果苔草的立枯物总的变化趋势是其拟合曲线符合指数方程。以其凋落物的失重率表示分解速率,而日失重率是随着时间增长而不断减少,且日失重率的变化在0.7058％-0.2372％之间。毛果苔草全生长季（1999年5月2日-10月10日）枯落物总量为210.8876g·m^-2。毛果苔草地下生物量具有明显的垂直结构,呈倒金字塔形,数学模拟近于抛物线型。     

An in situ approach for measuring root-associated respiration and nitrate uptake of forest trees

The ecophysiological characteristics of fine roots of mature forest plants are poorly understood because of difficulties of measurement. We explored a root in-growth approach to measure respiration and nitrate uptake of woody plant roots in situ. Roots of seven species were grown into sand-filled chambers. Root-associated respiration was measured as CO ₂ emission on four dates and nitrate uptake was quantified using ¹⁵N. All the roots were younger than 3 months at the time of measurement. Fine root respiration measured over the temperature range of 14.5–15.5 °C averaged 18.9–36.5 nmol gDM ^–1 s ^–1 across species. Nitrate uptake rates by these fine roots (1.3–6.8 nmol gDM ^–1 s ^–1) were comparable to other studies of forest trees. The root respiration rates were several times higher than measurements on detached roots of mature trees, concurring with literature observations that young roots respire much more rapidly than older roots. The root in-growth approach appears promising for providing information on the metabolic activity of fine roots of mature forest trees growing in soil.     

Changes in root-shoot ratio and ion uptake of maize (Zea mays L.) from soil as influenced by a plant growth regulator

A pot experiment with maize cv. Limac was conducted to investigate the influence of BAS 110.. W, a plant growth regulator (PGR), on root and shoot development and nutrient uptake. The PGR was applied via the soil with 0, 5, 10, 20, and 40 mg a.i. per pot. Shoot dry matter production was reduced to a higher degree than root length, resulting in a higher root-shoot ratio (RSR) of the treated plants. Shoots of treated plants contained higher concentrations of N, P, Ca, Mg, and unchanged K concentrations. The alterations in concentration could be explained by the changes in RSR induced by the plant growth retardant. The effect was strongest with P (+40%) which was limited by soil supply. N, Ca, and Mgconcentrations were positively influenced (+20%), there was no increase for Kvs RSR.     

Growth and maintenance respiratory costs of cucumber fruits as affected by temperature, and ontogeny and size of the fruits

The rates of dry weight increase and respiration of fruits were measured throughout fruit ontogeny at 20, 25 and 30°C in cucumber ( Cucumis sativus L. cv. Corona). By maintaining one or five fruits per plant, which strongly affected fruit dry weight but not ontogeny, the effects of fruit size and ontogeny on respiration could be studied separately. The respiration rate per fruit followed a sigmoid curve during fruit ontogeny, while the specific respiration rate (respiration rate per unit dry weight) declined with time after anthesis. The specific respiration rate was almost linearly related to the relative growth rate. The specific respiratory costs for both growth and maintenance were highest in young fruits, but were not affected by fruit size. The average specific respiratory costs for growth and maintenance at 25°C were 3.3–3.9 mmol CO₂ g⁻¹ and 4.0 nmol CO₂ g⁻¹ s⁻¹, respectively. An increase in temperature had no effect on the specific respiratory costs for growth, while the costs for maintenance increased with a Q₁₀ of about 2. The costs for growth agreed reasonably well with theoretical estimates based on the chemical composition of the fruits but not with estimates based on only the carbon and ash content. The respiratory losses as a fraction of the total carbon requirement of a fruit changed during fruit ontogeny, but were independent of temperature and were similar for slow- and fast-growing fruits. The cumulative respiratory losses accounted for 13–15% of the total carbon requirement.     

Reduction, assimilation and transport of N in normal and gibberellin-deficient tomato plants

A fast-growing normal and a slow-growing gibberellin-deficient mutant of Lycopersicon esculentum (L.) Mill. cv. Moneymaker were used to test the hypothesis that slow-growing plants reduce NO₃^? in the root to a greater extent than do fast-growing plants. Plants that reduce NO₃^? in the root may grow more slowly due to the higher energetic and carbon costs associated with root-based NO₃^? reduction compared to photosynthetically driven shoot NO₃^? reduction. The plants were grown hydroponically with a complete nutrient solution containing 10 mM NO₃^? and the biomass production, gas exchange characteristics, root respiratory O₂ consumption, nitrate reductase activity and translocation of N in the xylem were measured. The gibberellin-deficient mutants accumulated more total N unit^?1 dry weight than did the faster-growing normal plants. There were no significant differences between the genotypes in the rates of photosynthesis expressed on a leaf dry weight basis. The plants differed in the proportion of photosynthetic carbon available to growth due to a greater proportion of daily photo-synthate production being consumed by respiration in the slow-growing genotype. This difference in allocation of carbon was associated with differences in the specific leaf area and specific root length. In addition, a greater leaf weight ratio in the fast-growing than in the slow-growing plants indicates a greater investment of carbon into biomass supporting photosynthetic production in the former. We did not find differences in the activity or distribution of nitrate reductase or in the N composition of the xylem sap between the genotypes. We thus conclude that the growth rate was determined by the efficiency of carbon partitioning and that the site of NO₃^? reduction and assimilation was not related to the growth rate of these plants.     

Nutrient uptake and allocation at steady-state nutrition

Ingestad, T. and Ågren, G. I. 1988. Nutrient uptake and allocation at steady-state nutrition. - Physiol. Plant. 72: 450–459. Net nutrient uptake and translocation rates are discussed for conditions of steady-state nutrition and growth. Under these conditions, the relative uptake rate is equal to the relative growth rate, for whole plants as well as for plant parts, since the root/shoot ratio and internal concentrations remain stable. The nutrient productivity and the minimum internal concentration are parameters characteristic for the plant and the nutrient. A conceptual, mathematical model, based on these two fundamental parameters is used for calculation and prediction of the net nutrient uptake rate, which is required to maintain steady-state nutrition at a specified internal nutrient concentration or relative growth rate. When uptake rate is expressed on the basis of the root growth rate, there is, up to optimum, a strong linear relationship between uptake rate and the internal concentration of the limiting nutrient. More complicated and less consistent relationships are obtained when uptake rate is related to root biomass. The limiting factor for suboptimum uptake is the amount of nutrients becoming available at the root surface. When replenishment is efficient, e.g. with vigorous stirring, the concentration requirement at the root surface appears to be extremely low, even at optimum. In the suboptimum range of nutrition, the effect of nutrient status on root growth rate is a critical factor with a strong feed-back on nutrition, growth and allocation. At supraoptimum conditions, the uptake mechanism is interpreted as a protection against too high uptake rates and internal concentrations at high external concentration. In birch (Betula pendula Roth.), the allocation of nitrogen to the shoots is high compared to that of potassium and also to that of phosphorus at low nitrogen or phosphorus status. With decreasing stress, phosphorus allocation becomes more and more similar to nitrogen allocation. The formulation of a mathematical model for calculation of allocation of biomass and nutrients requires more exact information on the quantitative dependence of the growth-regulating processes on nutrition.