Understanding phosphorus dynamics on wheat plant under split-root system in alkaline soil
© The Author(s) 2016
Received: 19 April 2016
Accepted: 12 July 2016
Published: 20 July 2016
Plants are not uniform in their nutritional requirements, most of them survive under adverse conditions of humidity, temperature and nutrients, because they are genetically adapted to their habitat and even some varieties of the same species present differences in absorption, translocation, accumulation and nutrient use. This study is aimed at examining the phosphorus (P) status and P distribution in the different parts of wheat (Triticum aestivum L.) plant, root and shoot growth response in a split-root soil culture in alkaline soil. The soil having pH 7.9 was collected from the alkaline region of Bangladesh. KH2PO4 was used as the source of phosphorus for the different level of P applications. Two recently BARI developed wheat varieties namely BARI GOM 25 and BARI GOM 26 were used as testing plants with three replications. Result showed the growth parameter increased with the increase of P application. Likewise, P uptake by wheat seedlings also increases with the elevated P application. However, no significant differences were observed between wheat varieties irrespective of growth and P uptake by wheat seedlings. Moreover, elevated P concentrations in the shoot of wheat plants probably provided more P for shoot unloading of P and for P assimilation in the controlled roots, resulting in increased P concentrations in the roots of wheat plants, that means, translocation of P in the roots. These findings indicate that the added soluble P increases the absorption of nutrients from the soil solution. However, application of elevated P is efficient both for increasing shoot development and root growth and plays significant role in the phosphorus dynamics within the wheat plants in split root system in alkaline soil.
Phosphorus (P) availability in most calcareous soils is very low, limiting crop production, because of the formation of sparingly soluble phosphate compounds with Ca in alkaline soils (Marschner 1995). It is estimated that more than 30 % of soils cultivated globally suffer from P deficiency stress, and that the world reserves of P might be depleted by 2050 (Batjes 1997; Vance et al. 2003). Phosphorus deficiency is also a critical nutritional problem in plant growth as it plays a key role in plant growth and is the major plant growth-limiting nutrient despite its abundance in soils in both inorganic and organic forms (Gyaneshwar et al. 1999). It is absorbed by the plants, in orthophosphate (H2PO4 − and HPO4 2−) forms (Hinsinger 2001). Phosphorus is important in several physiological processes of plants, especially in photosynthesis, carbon metabolism, and membrane formation (Wu et al. 2005). The concentrations of inorganic P in soil solution are, however, typically very low, due to inorganic P’s propensity to bind strongly to soil surfaces or form insoluble complexes with cations. This means that inorganic P is often a limiting factor in plant growth and development. This has resulted in a large number of developmental traits amongst plant species that can enhance inorganic P uptake. Physiologically these include the modulation of root elongation (Sánchez-Calderón et al. 2005), branching (Linkohr et al. 2002; López-Bucio et al. 2002), and root hair density (Ma et al. 2001). The root system may also act to enhance inorganic P uptake by exuding protons (Hinsinger 2001), organic acid anions (Ryan et al. 2001), and phosphatases (Tadano and Sakai 1991) into the rhizosphere, or by the formation of symbioses with arbuscular mycorrhizas or ectomycorrhizas (Péret et al. 2011; Smith et al. 2011). Phosphorus is readily translocated within the plants, moving from older to younger tissues as the plant forms cells and develops roots, stems and leaves. Moreover, in inorganic P-deficient plants the restricted supply of P to the shoots from the roots via the xylem is supplemented by increased mobilization of stored P in the older leaves and retranslocation to both the younger leaves and growing roots. To understand the mechanisms controlling these traits is, therefore, of great importance in the pursuit of improved crop inorganic P uptake.
Keeping in view of the above facts, the study aims at following objectives: to understand mechanisms involved in the utilization of inorganic phosphorus by wheat plant under various split-root systems in alkaline soil and to quantify how translocated phosphorus effects on wheat plant within split-root system under different P efficient condition.
Soil and plant
Properties of soils used in different experiments
Total N (%)
Available P (ppm)
Exchangeable K (Cmol/kg)
Available S (ppm)
Available Zn (ppm)
Organic matter (%)
Split-root system with different treatments
Compartment 1 (mg P/kg)
Compartment 2 (mg P/kg)
The soil was incubated at 30 °C for 7 days, then KH2PO4 as per P doses were applied directly to the soil in each cup and mixed thoroughly before sowing. The total experiment was conducted in the Research laboratories, Department of Agronomy and Agricultural Extension Rajshahi University, Rajshahi.
Construction of split-root system
Pots having two compartments or chambers with a fixed partition-wall at the middle of the pot were used for the treatment. Each compartment was filled with 500 g of experimental soil. The soil was compacted. The whole split root system with soil and plant was continued for 28 days.
Seed germination and seedling preparation
Seeds of uniform size were selected for germination. The seeds of BARI GOM-25 and BARI GOM-26 were germinated in moist sand in two separate trays in dark at 25 °C for 70 h. To produce young seedlings, the germinated seeds were allowed to grow for 5 days in those separate trays.
Cultivation of plant
To support the transplanted seedlings, five slots were made on each side of the partition-wall of the pot. Five days old healthy seedlings, were transplanted. Each seeding bearing four seminal roots, (6–7 cm long) after cutting one-uneven root, was taken. A single-seedling was put into each slot keeping two seminal roots in each compartment. Then the roots were covered with the same treated soil and watered immediately after planting. 20 ml water was added to each compartment every day and watering was stopped 3 days before harvesting.
The experimental plants were harvested 27 days after transplanting. The shoots were cut 0.5 cm above the base part of the stem uniformly. Then the roots were cut 0.5 cm below the base part and separated carefully into two halves as previously marked. Soils from two root halves where removed carefully so that the roots could not be tonned or left in the soil. Then the collected bulk soil was air dried and stored in a controlled room temperature (25 °C) until analysis. Then the roots were washed with DI water to remove the adhered soil from roots. The washed roots were oven dried at 70 °C for 3 days. Shoots were also oven dried at the same temperature for the same time. After drying, the root and shoot samples were weighed and stored for analytical experiments.
Measurements of soil physical and chemical properties
Soil textural analyses were conducted by using an abbreviated version of the International Pipette method. Clay content was determined by a pipette method after pretreatment with H2O2 to remove organic matter (Gee and Bauder 1986). The pH of the soil was determined before incubation in deionised water using a soil-to-solution ratio of 1:2.5. Organic carbon of the soil samples was determined by wet oxidation method (Walkley and Black 1934). Soil organic matter content was determined by multiplying the percent value of organic carbon with the conventional Van-Bemmelen’s factor of 1.724 (Piper 1950). The nitrogen content of the soil sample was determined by distilling soil with alkaline potassium permanganate solution (Subbiah and Asija 1956). The distillate was collected in 20 ml of 2 % boric acid solution with methylred and bromocresol green indicator and titrated with 0.02 N sulphuric acid (H2SO4) (Podder et al. 2012). Soil available S (ppm) was determined by calcium phosphate extraction method with a spectrophotometer at 535 nm (Petersen 1996). The soil available K was extracted with 1 N NH4OAC and determined by an atomic absorption spectrometer (Biswas et al. 2012). The available P of the soil was determined by spectrophotometer at a wavelength of 890 nm. The soil sample was extracted by Olsen method with 0.5 M NaHCO3 as outlined by Huq and Alam (2005). Zn in the soil sample was measured by an atomic absorption spectrophotometer (AAS) after extracting with DTPA (Soltanpour and Workman 1979).
Phosphorus determination in soil and plant tissue
The amounts of P in root, shoot and soil were determined. After digestion in a mixture of concentrated nitric and percloric acids (4:1), the concentration of P in root and shoot materials were determined using the vanadomolybdate method after digestion in a mixture of concentrated nitric and perchloric acids (4:1) (Zheng et al. 2005). Colorimetric method for the determination of phosphorous concentrations in digest solutions was used. This method is called the molydovanado-phosphate method (AOAC 1975). Briefly, phosphorous was assayed using the molydovanado-phosphate method adding 3-ml digested solution, 2-ml reagent and 5-ml DI water. The absorbance reading was used at 470 nm (Iqbal et al. 2010).
Shoot parameters were analysed by two-way ANOVA (Treatment × Variety) and root parameters were analysed by three-way ANOVA (Treatment × Variety × compartment), total P uptake as well as distribution of P in different plant parts were determined by one-way ANOVA using Genstat 11th edition for Windows (Lawes Agricultural Trust, UK).
Significance levels for the main and interactive effect of P and varieties on seedlings growth
Source of variation
Shoot dry weight
P concentration in shoot
Root dry weight
P uptake in root
T × V
T × C
C × V
T × V × C
Effect on plant height
Shoot dry weight
Root dry weight
Shoot P concentration
Root P concentration
Total P uptake and P distribution
Growth response of wheat plant in split root system
Total plant biomass, total shoot and root biomass in different plant parts of the split-root system and distribution of biomass in shoot and two separate compartments
Total plant biomass (g/pot)
Bari GOM 25
Bari GOM 26
Total biomass (g/pot) in different plant parts of the split-root system
Bari GOM 25
Bari GOM 26
The distribution of biomass (%) in shoot and roots grown in two separate soil compartments (I and II)
Bari GOM 25
Bari GOM 26
Root biomass, shoot biomass, and root/shoot ratio of two wheat varieties across different P applications
P rate (mg/kg)
Biomass production (mg/plant)
Bari GOM 25
Bari GOM 26
P distribution and translocation in wheat plant within split-root system
Total P uptake in different plant parts of the split-root system and distribution of P in shoot and root two separate compartments
Total P uptake (g/kg)
Bari GOM 25
Bari GOM 26
Total P uptake (g/kg) in different plant parts of the split-root system
Bari GOM 25
Bari GOM 26
The distribution of P (%) in shoot and roots grown in two separate soil compartments (I and II)
Bari GOM 25
Bari GOM 26
Mimura et al. (1996) and Jeschke et al. (1997) described a picture of patterns of inorganic P movement in whole plants. In P-sufficient plants most of the inorganic P absorbed by the roots is transported through the xylem to the younger leaves. Concentrations of inorganic P in the xylem range from 1 mm in inorganic P-starved plants to 7 mm in plants grown in solutions containing 125 µm inorganic P (Mimura et al. 1996). There is also significant retrained location of inorganic P in the phloem from older leaves to the growing shoots and from the shoots to the roots. In inorganic P-deficient plants the restricted supply of P to the shoots from the roots via the xylem is supplemented by increased mobilization of stored P in the older leaves and retranslocation to both the younger leaves and growing roots. This process involves both the depletion of inorganic P stores and the breakdown of organic P in the older leaves. A curious feature of P-starved plants is that approximately one-half of the inorganic P translocated from the shoots to the roots in the phloem is then transferred to the xylem and recycled back to the shoots (Jeschke et al. 1997).
Increase of the external P supply to split root from 0 to 200 mg P/kg significantly increased the P concentration in those roots, and in shoots, but had no significant effect on the P concentration of the controlled roots. This lack of response of controlled roots has been demonstrated in other split-root studies with, e.g. barley (Drew and Saker 1984), subterranean clover (Scott and Robson 1991), tomato (Burleigh and Harrison 1999) and Hakea prostrata (Proteaceae) (Shane et al. 2003). In contrast with the results of split-root plants, the results of our wheat plant split-root study and those of others using foliar spray (e.g. Marschner et al. 1987) demonstrate that P retranslocated in the phloem sap can result in increased root P concentrations. In our plants, very high P supplies (200 mg P/kg KH2PO4) to just one crown root of wheat plants significantly increased the P concentration of compartment-I roots in respect of Treatment B compartment II. It was expected that in Treatment C, plants would be able to translocates P from the roots in compartment I to those compartment II. Studies with barley (Greenway and Gunn 1966; Clarkson and Scattergood 1982) indicated that P-stressed leaves absorb P more rapidly than control leaves do, and they export much larger amounts to the roots. Higher P concentrations in the shoot of our wheat plants probably provided more P for shoot unloading of P and for P assimilation in the controlled roots, resulting in increased P concentrations in the roots of wheat plants. In contrast, the split-root technique in alkaline soil probably provides a more stable supply of P at a lower concentration.
Considering that P is an essential and often limiting nutrient for plant growth, it is surprising that many aspects of P uptake and transport in plants are not thoroughly understood. This study reveals that P uptake and P translocation in split root system of the wheat plant in alkaline soil. These findings indicate that the added soluble P increases the absorption of nutrients from the soil solution. However, added P is efficient both for increasing shoot development and root growth. Moreover, no varietal difference is found in various experiments. Again, elevated P concentrations in the shoot of wheat plants probably provided more P for shoot unloading of P and for P assimilation in the controlled roots, resulting in increased P concentrations in the roots of wheat plants in split root system in alkaline soil. Perhaps the next important leap in our conceptual understanding in this area will come from the integration of these techniques to provide a comprehensive picture of the function of phosphate transporters and how they control their spatial and temporal expression to allow the plant to cope with changing environmental conditions.
RS carried out the whole research and wrote the manuscript; TI supervised the whole research work by providing necessary logistical support and guidance; RS conducted all the experiments, recorded all the data, collected all the samples and conducted lab analysis. Both authors read and approved the final manuscript.
The authors are thankful to the Institute of Biological Sciences, Rajshahi University, Rajshahi for providing Post-graduate Research opportunity and Wheat research Centre, Shampur, Rajshahi for providing wheat plants and Soil Resources Development Institute, Shampur, Rajshahi for Testing soil and plants. The authors are also grateful to the Department of Agronomy and Agricultural Extension, Rajshahi University, Rajshahi.
The authors declare that they have no competing interests.
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