Gerardo Rubio

Effect of phosphorus availability on basal root shallowness in common bean

Root gravitropism may be an important element of plant response to phosphorus availability because it determines root foraging in fertile topsoil horizons, and thereby phosphorus acquisition. In this study we seek to test this hypothesis in both two dimensional paper growth pouch and three-dimensional solid media of sand and soil cultures. Five common bean (Phaseolus vulgaris L.) genotypes with contrasting adaptation to low phosphorus availability were evaluated in growth pouches over 6 days of growth, and in sand culture and soil culture over 4 weeks of growth. In all three media, phosphorus availability regulated the gravitropic response of basal roots in a genotype-dependent manner. In pouches, sand, and soil, the phosphorus-inefficient genotype DOR 364 had deeper roots with phosphorus stress, whereas the phosphorus-efficient genotype G19833 responded to phosphorus stress by producing shallower roots. Genotypes were most responsive to phosphorus stress in sand culture, where relative root allocation to the 0–3- and 3–6-cm horizons increased 50% with phosphorus stress, and varied 300% (3–6 cm) to 500% (0–3 cm) among genotypes. Our results indicate that (1) phosphorus availability regulates root gravitropic growth in both paper and solid media, (2) responses observed in young seedlings continue throughout vegetative growth, (3) the response of root gravitropism to phosphorus availability varies among genotypes, and (4) genotypic adaptation to low phosphorus availability is correlated with the ability to allocate roots to shallow soil horizons under phosphorus stress.

The importance of root gravitropism for inter-root competition and phosphorus acquisition efficiency: results from a geometric simulation model.

We have observed that low soil phosphorus availability alters the gravitropic response of basal roots in common bean (Phaseolus vulgaris L.), resulting in a shallower root system. In this study we use a geometric model to test the hypotheses that a shallower root system is a positive adaptive response to low soil P availability by (1) concentrating root foraging in surface soil horizons, which generally have the highest P availability, and (2) reducing spatial competition for P among roots of the same plant. The growth of nine root systems contrasting in gravitropic response over 320 h was simulated in SimRoot, a dynamic three-dimensional geometric model of root growth and architecture. Phosphorus acquisition and inter-root competition were estimated with Depzone, a program that dynamically models nutrient diffusion to roots. Shallower root systems had greater P acquisition per unit carbon cost than deeper root systems, especially in older root systems. This was due to greater inter-root competition in deeper root systems, as measured by the volume of overlapping P depletion zones. Inter-root competition for P was a significant fraction of total soil P depletion, and increased with increasing values of the P diffusion coefficient (De), with root age, and with increasing root gravitropism. In heterogenous soil having greater P availability in surface horizons, shallower root systems had greater P acquisition than deeper root systems, because of less inter-root competition as well as increased root foraging in the topsoil. Root P acquisition predicted by SimRoot was validated against values for bean P uptake in the field, with an r2 between observed and predicted values of 0.75. Our results support the hypothesis that altered gravitropic sensitivity in P-stressed roots, resulting in a shallower root system, is a positive adaptive response to low P availability by reducing inter-root competition within the same plant and by concentrating root activity in soil domains with the greatest P availability.

Genetic mapping of basal root gravitropism and phosphorus acquisition efficiency in common bean

Root gravitropism determines the relative distribution of plant roots in different soil layers, and therefore, may influence the acquisition of shallow soil resources such as phosphorus (P). Growth pouch and field studies were conducted to evaluate root gravitropism of common bean (Phaseolus vulgaris L.) in response to P deficiency and to detect quantitative trait loci (QTL) associated with this trait. A deep-rooted genotype, DOR364, was crossed with a shallow-rooted genotype, G19833, to obtain 86 F5.7 recombinant inbred lines (RILs). Root gravitropic traits were measured as basal root growth angle (BRGA), shallow basal root length (SBRL, basal root length in the top 0-3 cm of soil) and relative shallow basal root length (RSBRL, percentage of basal root length in the top 0-3 cm of soil relative to total basal root length). Large genetic variability for these traits was found in the parents and RILs, with BRGA ranging from -18.73 to 56.69º and SBRL ranging from 0.42 to 2.63 m per plant. The parents and six RILs with contrasting root gravitropism were further evaluated in the field, where root shallowness was significantly correlated with plant growth and P uptake. QTL were detected by single point analysis (SPA), interval mapping (IM) and composite interval mapping (CIM) techniques with a genetic map for the DOR364 × G19833 population consisting of 236 molecular markers. The IM / CIM QTL were detected among the 11 linkage groups of common bean, with 16 QTL controlling the above root traits and six QTL controlling P acquisition efficiency (PAE) in the field study. At least three of the root trait QTL were associated with QTL for PAE, suggesting that root gravitropic traits are associated with PAE and that QTL for these traits can be used to facilitate selection and breeding for higher P efficiency in common bean and other crops.

Topsoil foraging and its role in plant competitiveness for phosphorus in common bean

or leaf acid phosphatase activity (Lynch and Beebe, 1995; Yan et al., 2001). We evaluated the effect of root shallowness on interplant competiIn most natural soils, phosphorus bioavailability is tion for phosphorus in common bean (Phaseolus vulgaris L.). Recombinant inbred lines (RILs) segregating for basal root gravitropism greater in surface or near-surface horizons than in the were evaluated in monogenetic and polygenetic stands with varying subsoil, because of deposition of phosphorus onto the phosphorus availability in the field in South China and in solution soil surface in decayed leaves and other plant residues, culture and solid media in controlled environments. In the field, shalas well as biological, chemical, and physical factors in low-rooted RILs were more productive than deep-rooted RILs. Shoot the topsoil that favor phosphorus bioavailability. In agbiomass of these RILs almost doubled the deep-rooted ones when in ricultural soils, fertilization and cultivation increase competition. In the greenhouse, three treatments representing differphosphorus bioavailability in the topsoil, with only very ent soil phosphorus distributions were compared. Root shallowness slow movement of phosphorus into the subsoil in most did not confer any competitive advantage when phosphorus availabilcases. As a result, phosphorus availability usually deity was uniformly low or uniformly high, but did confer a competitive advantage when phosphorus availability was concentrated in the topclines substantially with soil depth (Chu and Chang, soil. Shallow and deep-rooted RILs did not differ in response to 1966; Keter and Ahn, 1986; Pothuluri et al., 1986). Root phosphorus availability in solution culture where phosphorus is mixed architectural traits that enhance the exploration and and uniformly available. Our results demonstrate that basal root graviexploitation of surface horizons may therefore enhance tropism, which is a specific root architectural trait under genetic conphosphorus acquisition (Lynch and Brown, 2001). trol, is important for belowground competition in low phosphorus Root gravitropism, one of the principal components soils. of root architecture, may be an important mechanism responsible for phosphorus efficiency in bean. Root gravitropism is the tendency of a root to grow at a C bean is the most important food legume on specific orientation with respect to gravity, or Graviearth, providing essential nutrients for hundreds of tropic Setpoint Angle (“GSA,” Firn and Digby, 1997). millions of people in developing countries (CIAT, 1987; The GSA of the various classes of roots in a root system Wortmann et al., 1998). Over half of global bean producis an important determinant of root foraging at various tion occurs on severely phosphorus-deficient soils (Thung, depths in the soil profile. Root gravitropism may there1990; Lynch and Beebe, 1995; Wortmann et al., 1998). fore be an important factor in topsoil foraging and thereApplication of phosphate fertilizers is not an adequate fore phosphorus acquisition in infertile soils (Lynch and solution to this problem because of rural poverty, poor Brown, 2001). The root system of common bean is comaccess to appropriate fertilizers, and limited fertilizer posed of four root types: adventitious roots, basal roots, efficacy in highly weathered soils. An alternative or taproot, and lateral roots arising from the first three complementary approach is the development of cultitypes (Fig. 1). The tap root has strong positive gravitropvars with superior growth and yield in soils with low ism and usually goes straight downwards. Adventitious phosphorus availability, or “phosphorus efficiency.” roots arise from the hypocotyl and explore soil volumes Phosphorus efficient genotypes would yield better withclose to the soil surface. Basal roots arise from the basal out fertilizers and would respond better to fertility inpart of the root system. In conjunction with the lateral puts (Lynch, 1998). Significant genetic variation in phosroots that emerge from them, basal roots usually comphorus efficiency exists in bean (Lynch and Beebe, 1995; prise the majority of total root length. Basal root graviBeebe et al., 1997). Phosphorus efficiency in bean aptropism is a key determinant of the overall shallowness pears to be associated primarily with enhanced phosof the root system, since basal roots form the scaffold phorus acquisition from the soil through superior root on which most of the bean root system develops (Fig. 1) growth and architecture rather than through microbial (Zobel, 1975). The growth of the basal roots with respect associations, chemical modification of the rhizosphere, to gravity over time determines whether this part of the root system descends rapidly into the subsoil or remains Jonathan P. Lynch, Department of Horticulture, The Pennsylvania in the topsoil (Lynch and Van Beem, 1993). Basal root State University, University Park, PA 16802, USA; Gerardo Rubio, gravitropism can be measured by the growth angle of Faculty of Agronomy, University of Buenos Aires, 1417 Buenos Aires, the root axis or by the proportion of basal roots in Argentina; Hong Liao and Xiaolong Yan, Laboratory of Plant Nutritional Genetics and Root Biology Center, South China Agricultural the topsoil relative to the total amount of basal roots University, Guangzhou 510642, China. Received 19 Sept. 2001. *Cor(Bonser et al., 1996, Liao et al., 2001). responding author (jpl4@psu.edu). Since basal root gravitropism is important for topsoil exploration, it is interesting that in bean and other lePublished in Crop Sci. 43:598–607 (2003).

Adaptations and biomass production of two grasses in response to waterlogging and soil nutrient enrichment

We analysed the response of two grass species, Danthonia montevidensis and Paspalum dilatatum to waterlogging, soil-nutrient enrichment and the combination of both factors. Waterlogging did not affect total biomass of D. montevidensis, but it slightly promoted growth of P. dilatatum. Most analysed variables showed no significant interaction between fertilization and waterlogging. Therefore, waterlogging does not produce a detrimental effect either in the growth of these species or in their response capacity to stimulating growth factors, such as fertilization.

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

Abstract 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.

Heavy metals in soils of Argentina: Comparison between urban and agricultural soils

Abstract Trace metals, including heavy metals, can be harmful to the biota and human beings. This leads to study the accumulation of those elements in soils. In the Pampean region (Argentina) this knowledge is scarce. Our objectives were to (i) determine the trace metal concentration in soils of Buenos Aires City and agricultural areas, (ii) start to establish the soil trace metals baseline concentration, and (iii) find relationships between soil properties and those elements Topsoil samples were taken in Buenos Aires City and on farms along an arc 50 to 250 km away from the metropolis. All studied soils were Mollisols. Soil samples were analyzed for their cadmium (Cd), copper (Cu), zinc (Zn), chromium (Cr), cobalt (Co), lead (Pb), and nickel (Ni) contents by acid extraction. Soil properties were determined using standard methods. The soils of Buenos Aires City show the highest average concentrations of Cd, Cu, Pb, and Zn. The further the sampling sites were from Buenos Aires, the lower the metal concentr...

Effect of retention of run-off water and grazing on soil and on vegetation of a temperate humid grassland

Abstract A 4-year field trial was carried out on a Typic Natraqualf to modify the surface run-off, to change the soil water regime and improve forage productivity. Water was retained by earth banks which were built along contour lines. The area was grazed by cattle at a density of six animal units per hectare during five or six occupation periods per year. To study the effect of cattle trampling, 1 ha within the water retention area was excluded from grazing. It was found that surface accumulation of water led to higher soil water contents and prevented salt ascension by capillarity from the water table (Electrical Conductivity of A 1 horizon, 1.4 dS·m −1 against 3.4 dS·m −1 in the control area). Soil salinization in the control area was associated with soil water evaporative losses and the water table depth, when it was less than 1.5 m deep. Soil alkalinity (pH and SAR) showed variations closely related to salinity. The already impaired soil physical properties were not significantly affected by livestock trampling in the water retention area. A dramatic change in plant community composition was observed. Most halophitic species disappeared and the area was covered by hydrophilous grasses. This determined a 4-fold increase in higher quality forage. Run-off water retention proved to be a promising way to change temporarily the status of the soil and to cause a large change in grassland characteristics and productivity.

Compared phosphorus efficiency in soybean, sunflower and maize.

ABSTRACT A more comprehensive understanding of the mechanisms of phosphorus (P) efficiency is agronomically significant to advance in the design of crop management schemes that increase P efficiency and reduce the need of fertilizers. Phosphorus efficiency is defined as the ability of a plant to acquire P from the soil and/or to utilize it in the production of biomass or the harvestable organ. Because most parameters related to P efficiency vary according to the growth conditions and isolation of the individual effect of P efficiency is not straightforward; plants must be grown in uniform experimental conditions to obtain a fair comparison of their nutrient acquisition and utilization. In this work, we compare the ability of soybean, sunflower, and maize to utilize and acquire soil P. Field and greenhouse experiments including different P levels were conducted. The general observation was that the three species ranked differently according to the specific parameter of P efficiency considered. Maize clearly showed higher P utilization efficiency than soybean and sunflower, either expressed as biomass or as grain produced per unit of absorbed P. In turn, soybean and sunflower exhibited higher acquisition efficiency than maize. Soybean showed the shallowest root system: 69% of the total root length was concentrated in the top 20 cm of the soil. Phosphorus uptake per unit root length was rather similar among the three species, but soybean and sunflower had higher P uptake per unit of root weight. This can be explained by the higher specific root length (SRL) and specific root area (SRA) of both dicots. For example, SRL averaged 59, 94, and 34 m g−1 in field grown soybean, sunflower, and maize, respectively. The more favorable root morphology determined that soybean and sunflower can explore more soil with the same belowground biomass and absorb more P per unit of carbon invested belowground. Since the three species exhibited similar values of P uptake per unit root length, we hypothesize that the capacity of each segment of root to deplete soil P fractions is similar.

Soil volumetric changes in natric soils caused by air entrapment following seasonal ponding and water table rises

Soil volumetric changes have been seldom studied in seasonally ponded soils subjected to periodic water table rises. In the Flooding Pampa of Argentina the topsoils develop significant swelling and shrinkage, despite their low percentages of total and expansible clay. We tested the hypothesis that: (a) the swelling of a Natraquoll and a Natraqualf of this region is caused by the wide change in water contents during ponding–drying cycles; and (b) soil swelling is accentuated by the effect of air entrapment ahead of the advance of soil wetting fronts. The relationship between the reciprocal of bulk density (i.e. soil specific volume), ν, and water content, θ, was determined in the laboratory (clod shrinkage curves) and in the field (repeated core sampling). Soil clods behaved in accordance to their inherent soil properties, with zero and residual shrinkage (slope n=δν/δθ<1) in both top horizons, and normal shrinkage (slope n=δν/δθ≈1) throughout the water content range of Bt horizons. Unlike the clods, in the field the slope, n, was as high as 1.47–1.48 in top horizons, and 1.93–1.98 in both Bt horizons, showing the occurrence of abnormal soil swelling processes. Taking into account the narrow volumetric water content range found in the field (i.e. 0.25 v/v in both Bt horizons), this rejects our first proposed hypothesis. Soil air became trapped ahead of the advance of two field wetting fronts: (a) water table rises from depth and (b) surface ponded water. As a result, pore air volume increased during soil wetting, and was as high as 0.24–0.34 v/v, and 0.35 v/v at the maximum swelling limit of top and Bt horizons, respectively. Results show that air entrapment caused the swelling or “inflation” of soils, which agrees with our second hypothesis. However, the influence of air entrapment was more pronounced than a simple accentuation of swelling in Bt horizons. Air entrapment caused the whole soil to a depth of about 0.4 m to expand.