Abstract

Media summary

Key Words

Water Stress; Root Length; Root Surface; Soybean Yield

Introduction

The rainfall per year ranges from 400 to 700 mm in Heilongjiang province, NE China, and rainfall distribution over twelve months is not balanced. Seasonal droughts often occur but irrigation is nearly impossible for crops in Heilongjian. In China, 59 per cent of soils are low in available P. Low soil concentration of available P is also a major environmental stress for soybean in some regions of Heilongjiang. Boem and Thomas (1998) indicated that the ability of plants to cope with mild water stress was enhanced by adequate P nutrition. Water deficit reduced individual leaf area and, at 0 applied P, reduced the rate of leaf appearance, number of simultaneously expanding leaves, and final number of leaves. Many works focus on the effect of P and soil water availability on the plant’s aboveground growth or the relationship between P and root growth (Rodriguez et al. 1994; Rodriguez and Goudriaan 1995; Wang et al. 2001). Nevertheless, the analysis of the relationships among P shortage, water stress and root characteristics of soybean has had little attention. The purpose of the present study was to identify from some root traits how and by how much P nutrition enhanced the relative tolerance of soybean to water deficit stress at R1 and R4 periods.

Materials and Methods

The study was conducted at Northeast Institute of Geography and Agro-ecology, Chinese Academy of Sciences in 2002. The institute is located at 45º41.8′N, 126º38.1′E and 150 m above sea level. The early maturing cultivar Dongnong 434 is a recently developed soybean cultivar in Heilongjiang province, China. The soil used was slightly alkaline with poor available phosphorus. Total phosphorus and available P of the soil were 0.12 g kg^-1 and 5.1 mg kg^-1 respectively. Soybean plants were grown in pots under a rain-shelter during the growing season. All pots (35 cm high and 28 cm diameter) were on a vehicle that could be easily moved into rain shelter during rain and moved out at other times. Treatments consisted in the combination of two factors: P fertilizer addition (four levels) and soil water availability (three levels).The levels of added P (KH₂PO₄) were 0, 7.3, 14.6 and 29.2 mg P/kg soil, (P0, P1, P2 and P3, respectively). With 192 mg N/kg soil and 103 mg K₂O/kg soil, the fertilizer was thoroughly mixed within the first 20 cm of each pot. Field water capacity (FWC) was determined before sowing. The water deficit treatments were (1) 30-40% of FWC at R1 stage (W1); and (2) 30-40% of FWC at R4 stage (W2). During other stages, the water content in soil was supplied to maintain 65-75% of FWC, the same as the control (W0). The water content was brought to requirement of experimental design by weighing and watering whenever it was needed.

Six seeds with similar size were sown on May 8 and emerged after 8 days. The pots were thinned to three plants per pot on the 15^th day after emergence. The experiment was arranged in a randomized complete block design with five replications per treatment. On June 10, solution with micronutrients was applied: B = 0.1 μg/g soil as H₃BO₄; Mn = 0.1 μg/g soil as MnCl₂·4H₂O; Cu = 0.004 μg/g soil as CuSO₄·5H₂O; Zn = 0.001 μg/g soil as ZnSO₄·7H₂O; Mo = 0.01 μg/g soil as H₂MoO₄·H₂O. The effects of P and water supply on root morphology, including root mass, root surface area and root length, were determined at R5 stage. Shoot mass was also determined at the same time. The entire root system was carefully removed by sliding it from its pot. The stem was cut off and the root system washed. The roots were first completely immersed in a water-filled container and then sprayed with water until it was almost free of soil and sand. A 74 μm mesh sieve was used to prevent loss of fine roots. The method for determining root length followed Newman (1966). An estimate of root length was given by: R =πNA/(2H), where R was the total length of root, N was the number of intersections of between the root and the straight lines, A was the area of the rectangle, and H was the total length of the straight lines. Root surface area (g Ca(NO₃)₂) was determined by immersing air-dried roots for 10 s in a saturated solution held on an analytical balance, suspending them out of solution for 30 s and recording the weight (g) of Ca(NO₃)₂ removed from solution that adhered to the root surface (Carley and Watson 1966). At maturity, plant residues were harvested, seeds were removed from the pods, dried and weighed. Seeds were counted and average weight per seed was calculated.

Results and Discussion

Root dry mass varied among the applied P rates under water stress conditions at different development stages (R1 and R4) and was greater at P3 than at either of the other P fertilization rates (Fig.1). At P3 fertilization rate, root mass was 1.45-, 1.34- and 1.41-fold greater than P0 for W0, W1 and W2 treatments respectively. The positive effect of P on root dry mass has been previously reported (Liao and Yan 2000). But root/shoot ratio did not show great difference among P fertilization rates except for W2 (Fig.1).

Total root mass alone cannot adequately describe many root functions involved in plant-soil relationship. However, total root length, surface area, and branching patterns have been shown to influence nutrient uptake (Raper et al. 1978). And estimates of root length per unit of soil are sometimes used in quantitative studies of water and nutrient uptake (Cowan 1965). In our study, because the volume of the containers in which soybean were grown was the same, root length could represent root density. The responses of root traits across all of P treatments to water deficit at R1 or R4 were significant. Total root length of W0 was 1.7 and 1.6-fold higher than those of W1 and W2, and 1.4- and 1.2-fold higher for root surface area (Fig.2). Root length of P-limiting conditions was obviously lower than when P was added, for both W1 and W2 treatments. At R5 stage, root length of P3 water-stressed at R1 or R4 stage was 2.50- and 2.05-fold higher, respectively, than those of P0. But, root length of P3 without water stress was just 1.67-fold higher than that of P0. Root surface area was also higher when the P supply was increased (Fig.2). Of course, many of the root characteristics, such as length, surface area, and mass, had been used to assess the quantity of roots and the functional size of the root system (Fitter 1996). Root traits are important determinants of soil water extraction. High P fertilizer, in our work, made root length, root surface area and root dry mass greater than either no-P fertilizer or low P fertilizer This could enhance resistance capability to drought stress. Increased soybean root growth by P may alleviate plant drought stress, especially in soil of low available P. Greater root surface may increase the available volume of soil to be exploited, so enhanced rooting may be an important mechanism for improving water-use efficiency in soybean (Hudak and Patterson 1995; Carter 1989). Yield data from this study support this conclusion.

Table 1. Yield and yield components at final harvest. Different letters indicated significant differences (p < 0.05) by a LSD test.

Water stress at R1 or R4 reduced seed number but yield was only reduced by water stress at R4 (Table 1). The reduction in seed number was related to the amount of P added. P nutrient statistically increased seed number and yield per plant whether water stress was applied or not. The yield reduction by water stress at R4 was much greater than that at R1 and P addition reduced yield loss by drought stress more effectively at R4. P nutrient also increased seed size significantly except for water stress at R1. Plants stressed at R1 had fewer seeds than for W0, but the seed size was greater for plants stressed at R1 than for W0. Generally, P application rates enhanced the yielding ability of soybean to water stress at different reproductive stages by improving some morphology traits. It was worthy to mention that although yield loss by water stress was greater at R4 than at R1, the root traits of water stress were similar at R4 and R1 or even better at R4, so there would be other reasons for this phenomenon, such as root function, root activity to absorb nutrients and photosynthesis etc.

Conclusions

Phosphorus nutrient affected root characteristics of soybean. Compared with either the absence of fertilizer P or P at low rates, greater root mass, root length and root surface area were obtained at high rates of applied P when water deficit occurred at R1 or R4 stage. Overall, P nutrient improved root traits to enhance the relative tolerance to water stress at two reproductive periods, by reducing yield loss of soybean resulting from this stress.

Acknowledgements

This paper is a part of Innovation Project granted by Chinese Academy of Sciences, grant number: KZCX2-SW-416-3

References

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