Maximum axial growth pressures of the lateral roots of pea and eucalypt

Although lateral roots may contribute significantly towards growth and nourishment of plants, the mechanics of their elongation behaviour in strong soils is not well known. The aim of this study is to report maximum axial growth pressures (p) and maximum elongation rates (E) of the lateral roots of an annual herbaceous plant (pea) and a woody perennial (eucalypt). As such measurements have not been reported previously, measurements of P and E for lateral roots were compared with the primary roots of pea for which reports are widespread. Values of P were estimated from the measured maximum values of axial force and root diameter on single, intact roots of seedlings in the laboratory. Additional measurements of both P and E were made for the lateral roots of pea when the growth of the remaining root axes was stopped (with removal of tips) to determine the overall effects of root-growth-inhibition on P and E of single roots.Values of P and E for lateral roots of pea were significantly greater than those for the lateral roots of eucalypt. Although root diameter for the primary roots of pea were similar to those for the lateral roots of eucalypt, the former exerted nearly twice as much pressure as the latter. The lateral roots of pea elongated significantly slower than the primary roots; however, P of lateral roots was significantly lower than the primary roots when elongation of all other roots was inhibited during the measurements. Production and/or development of lateral roots increased when elongation of the remaining roots (both primary and lateral roots) of pea seedlings was restricted due to the removal of root tips and exposure of one of the lateral roots to high strength. In general, maximum axial force exerted by primary and lateral roots was similar for roots of <1 mm diameter. However, primary roots exerted greater maximum axial force than the lateral roots when root diameter was >1 mm. As axial pressure of lateral roots was independent of root diameter, thickening of root tips is less likely to assist penetration of lateral roots in strong soils.     

水氮处理下不同品种水稻根系生长分布特征

为明确不同栽培条件下水稻(Oryza sativa)根系生长分布特征, 通过不同水氮处理和不同品种的水稻桶栽试验, 采用内置根架法, 于拔节期和抽穗期取样, 获取根系总干重(TRW)、不定根数(ARN)以及各类根(不定根、细分枝根和粗分枝根)的形态指标(长度、表面积和体积), 并分析植株根系生长状况和根系分布特征。结果显示: (1)各试验条件下抽穗期各项根系指标较拔节期均呈增长趋势。同一时期, 各项根系指标在3个施氮水平间均差异显著, 且随施氮量的增加而增加。不同水分处理下, 两个时期的ARN在湿润灌溉(W2)与保持水层(W1)之间差异均不显著, 而其他指标上W2处理均显著最高; 干旱处理 (W3)下, 仅拔节期的TRW和粗分枝形态指标与W1处理接近, 而在其他指标上均显著最低。不同品种间, ‘扬稻6号’ (V3)的各项根系指标均最高, 而‘日本晴’ (V1)和‘武香粳14’ (V2)间差异不显著。(2)各试验条件下, 抽穗期较拔节期根系下扎生长比例增加, 多分布于表层(0-5 cm)土中; 减少氮素和水分供应可提高根系在5 cm以下土层中的分布比例, 且分枝根反应最为明显; 品种V1和V2的深扎根性较V3明显。结果表明, 合理施氮与控水可优化水稻不同类型根的生长与分布特征, 但需考虑不同品种之间的差异。     

Competitive inhibition of the auxin-induced elongation by α-D-oligogalacturonides in pea stem segments

Branca, C, De Lorenzo, G. and Cervone, F. 1988. Competitive inhibition of the auxin-induced elongation by α-D-oligogalacturonides in pea stem segments. - Physiol. Plant. 72: 499–504.
α-D-galacturonide oligomers (OG) were prepared by partial hydrolysis of sodium polypectate with an homogeneous Aspergillus niger endopolygalacturonase (EC 3.2.1.15). OG, obtained after digestion for 10, 20, 30, 60, 120 min and 24 h, were assayed for their ability to interfere with the IAA-induced elongation of pea ( Pisum sativum L. cv. Alaska) stems. Maximum inhibiting activity was exhibited by oligomers with an approximate degree of polymerization higher than 8. Inhibition by longer OG was much lower, and the products of the 24 h digestion and the unhydrolysed polypectate were ineffective. The addition of OG to pea stems caused a parallel shift to the right of the IAA dose-effect curve. The shift depended on the amount of OG used, showing that oligogalacturonides behave as competitive antagonists of IAA. The presence of OG caused the disappearance of the second maximum of the elongation rate and reduced the first maximum. OG were also tested for their ability to inhibit IAA-induced ethylene evolution of pea stem segments. Maximal inhibition was obtained with OG of the same size as those that interfered with IAA-induced elongation. Inhibition of the auxin action seemed to be specific as OG did not interfere with the activity of gibberellic acid (GA₃) or kinetin. It was concluded that oligogalacturonides strongly interfere with the activity of IAA, although they are by themselves incapable to influence the elongation of pea stem segments directly.     

A reappraisal of nitrate inhibition of nitrogenase in A317, a nitrate reductase-deficient mutant of pea (Pisum sativum)

Symbiotic plants of Pisum sativum L. cv. Juneau and its nitrate reductase-(EC 1. 6. 6. 1)-deficient mutant, A317, were exposed to nitrate for up to 8 days and assessed for nitrate assimilation, nitrogenase activity and nodule carbohydrate status. The mutant, A317, was not impaired in its ability to absorb nitrate over up to 8 days, but was leakier with respect to nitrate reduction ability than previously realized, as 63% of the nitrate absorbed by the plant over 8 days was assimilated (in contrast to 93% in the wild type). After 2 days exposure to 5 m M nitrate, nitrogenase (EC 1.18.2.1) activity was less affected in A317 (84% of initial) than in Juneau (46% of initial): nodule starch reserves were less depleted in A317 (70% of initial) than in Juneau (26% of initial). It was concluded that nitrate reduction is a major cause of nitrate inhibition of nodule activity, and that its effect may be mediated through a decrease in the availability of carbohydrate to the nodules. Longer term (> 4 day) exposure of A317 plants to nitrate resulted in accumulation of nitrate in plant tissues, an associated necrosis of shoot tissue, a marked decrease in nodule starch content and a severe inhibition of nodule activity. This consideration of the effect of the duration of exposure to nitrate is used to resolve a discrepancy between previous reports on the sensitivity to nitrate of nitrogenase activity in nitrate reductase-deficient mutants of pea.     

Mechanisms Coordinating Wheat Seedling Growth Response as Affected by Shoot/Root Ratio

Excision of four out of five roots in 7-day-old wheat seedlings (Triticum durum Desf.) rapidly suppressed shoot growth promoted biomass accumulation by the remained root largely due to its expanded branching. Next, the rate of shoot growth increased although was not completely recovered. After the reduction of the root system, the rate of photosynthesis in the leaves of seedlings did not decrease. As compared to the intact plants, auxins and cytokinins accumulated in the remained root, whereas in the growing part of the shoot, the level of auxins rapidly declined. Shoot growth rate was assumed to decrease after the excision of a part of the root system due to lower extensibility of growing tissue, and the promotion of lateral root formation on the remained root apparently resulted from active redistribution of phytohormones and assimilates between plant organs. The prime role of hormonal signals in the coordination of shoot and root growth is discussed.     

Maize lateral root developmental plasticity induced by mild water stress. I: Genotypic variation across a high‐resolution series of water potentials

Lateral root developmental plasticity induced by mild water stress was examined across a high‐resolution series of growth media water potentials (Ψ_w) in two genotypes of maize. The suitability of several media for imposing near‐stable Ψ_w treatments on transpiring plants over prolonged growth periods was assessed. Genotypic differences specific to responses of lateral root growth from the primary root system occurred between cultivars FR697 and B73 over a narrow series of water stress treatments ranging in Ψ_w from ?0.25 to ?0.40 MPa. In FR697, both the average length and number of first‐order lateral roots were substantially enhanced at a Ψ_w of ?0.25 MPa compared with well‐watered controls. These effects were separated spatially, occurring primarily in the upper and lower regions of the axial root, respectively. Furthermore, first‐order lateral roots progressively increased in diameter with increasing water stress, resulting in a maximum 2.3‐fold increase in root volume at a Ψ_w of ?0.40 MPa. In B73, in contrast, the length, diameter, nor number of lateral roots was increased in any of the water stress treatments. The genotype‐specific responses observed over this narrow range of Ψ_w demonstrate the necessity of high‐resolution studies at mild stress levels for characterization of lateral root developmental plasticity.     

Extraction of apoplastic sap from plant roots by centrifugation

A centrifugal method for extracting apoplastic sap from roots of lupin ( Lupinus angustifolius ) and pea ( Pisum sativum ) plants, and a method to analyse malic dehydrogenase in the sap using capillary electrophoresis, are described. Osmolality of apoplastic sap was relatively constant at relative centrifugal forces (RCFs) between 600 and 3000 g for lupin, and between 600 and 4000 g for pea. Electropherograms of a marker enzyme (malic dehydrogenase) and other components in apoplastic and symplastic saps revealed that contamination occurred at 7000 g . It is suggested that apoplastic sap expelled from plant roots at RCF between 600 and 3000 g is free from symplastic contamination, and is regarded as being of apoplastic origin. The proposed method was used to measure apoplastic pH changes in the plant roots in response to external pH, ammonium, nitrate and vanadate.     

Measurement of viscoelastic properties of root cell walls affected by low pH in lateral roots of Pisum sativum L.

Mechanical extensibility of the cell wall limits the elongation growth of roots. Low pH, ranging from pH 3–4.5, induces rapid elongation of excised roots, a phenomenon known as acid growth. The creep-extension analysis was carried out to measure and elucidate the viscoelastic properties of root cell walls in the acidic environment in vitro. The viscoelastic properties were determined at the elongation zone of the lateral roots of pea (Pisum sativumL. cv. Alaska) and described by the physical parameters of three elastic (E₀, E₁, E₂) and three viscosity (₀, ₁, ₂) parameters using a Kelvin–Voigt–Burgers' model. The present method could measure the viscoelasticity of 1-mm long root zones from 2 to 9 mm behind the tip. Among the parameters, E₀ and ₀ were the most significant parameters to represent the whole extensibility of the roots. The parameter ₀ markedly declined in response to the environmental low pH (acid growth), whereas other parameters were not much affected by low pH. Relationship between the change in these physical parameters and the change in cell wall extensibility under low pH was discussed in order to elucidate the rheological processes taking place in the elongating cell walls.     

Role of oxygen limitation and nitrate metabolism in the nitrate inhibition of nitrogen fixation by pea

The impact of nitrate (5–15 m M , 2 to 7 days) on nitrogenase activity and nodule-oxygen limitation was investigated in nodulated, 21-day-old plants of a near-isogenic nitrate reductase-deficient pea mutant (A3171) and its wild-type parent ( Pisum sativum L. cv. Juneau). Within 2 days, 10 or 15 m M nitrate, but not 5 m M nitrate, inhibited the apparent nitrogenase activity (measured as in situ hydrogen evolution from nodules of intact plants) of wild-type plants; none of these nitrate levels inhibited the apparent nitrogenase activity of A3171 plants. Nodule-oxygen limitation, measured as the ratio of total nitrogenase activity to potential nitrogenase activity, was increased in both wild-type and A3171 plants by all nitrate treatments. By 3 to 4 days the apparent nitrogenase activity of A3171 and wild-type plants supplied with 5 m M nitrate declined to 53 to 69% of control plants not receiving nitrate. By 6 to 7 days the apparent nitrogenase activity of A3171 plants was similar to the control value whereas that of the wild-type plants continued to decline. From 3 to 7 days, no significant differences in nodule-oxygen limitation were observed between the nitrate (5 m M ) and control treatments. The results are interpreted as evidence for separate mechanisms in the initial (O₂ limitation) and longer-term (nitrate metabolism) effects of nitrate on nitrogen fixation by effectively nodulated pea.     

Intraspecific variation in home range exclusivity by female green razorfish, Xyrichtys splendens (family Labridae), in different habitats

Female green razorfish, Xyrichtys splendens, were studied in the field in three shallow tropical marine habitats: sandbed, Thalassia grassbed, and coral rubble bottom. Individuals showed varying degrees of home range overlap (one measure of territoriality) in each habitat, due to differences in predator pressure, the availability of refuges, and water turbidity. In a large scale field manipulation, a 68 m² bed of artificial seagrass (300 'grass' blades per m²) was planted at one site with low turbidity, thereby increasing the availability of refuges. The females' home ranges overlapped more, whereas control fish did not change their home range use, indicating that home range exclusivity in this species is a function of refuge distribution. Female X. splendens apparently have the phenotypic plasticity to adapt their social behavior to local conditions; in this case, the availability of economically inexpensive refuges, and the risk of predation as a function of the turbidity of the water.