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Leaflet orientation and fibrous root trait combination effects on water use characteristics in soybeans (Glycine max) via reciprocal grafts

Richard D. Johnson, Fred L. Allen and Carl E. Sams

Department of Plant Sciences, University of Tennessee, Knoxville, TN USA, www.utk.edu Emails rjohnsoy@utk.edu, allenf@utk.edu, carlsams@utk.edu

Abstract

The objective of this study was to evaluate the combinations of leaflet orientation and root morphology traits in soybean on water use characteristics. Three soybean varieties were chosen: USG 5601T, PI 416.937 and Williams 82 which differ in leaflet orientation and root morphology. Non-grafted plants of each variety, self grafts and reciprocal combinations of scion and rootstocks were made among the three varieties. Water use of each plant was measured on several successive days via a Dynamax Flow 32 Sap Flow Monitor™ when the plants were in the R5 stage of growth. USG 5601T (high leaflet orientation, normal root morphology) improved water use efficiency when used as a scion across the different rootstocks. The low leaflet orienting, fibrous root accession, PI 416.937 decreased water use efficiency when used as a scion across rootstocks, to produce the same seed yield. The intermediate leaflet orienting line, Williams 82 increased water use efficiency when grafted on the PI 416.938 rootstock but decreased water use efficiency when used as a scion on the USG 5601T rootstock. This study provides further evidence of the role of leaflet orientation in water use efficiency and a possible drought tolerance trait in soybeans. The high leaflet orientation phenotype had equal water use efficiencies when grafted onto the fibrous or the normal rootstocks. There was abundant soil moisture through the 2003 growing season in Knoxville. It is not known whether the fibrous root trait would have been beneficial during a moisture stress season.

Media summary

A new study by the University of Tennessee illuminates the role of leaflet orientation in water use efficiency and a potential drought tolerance trait in soybeans.

Key words

Sap flow, drought tolerance, soybean, transpiration, grafting, heliotropism

Introduction

In most years, inadequate moisture during flowering and seed-fill is a yield-limiting factor to soybean production throughout many soybean growing regions of the world. Because of this fact, drought tolerance is a highly sought after trait in soybean varieties. However, drought tolerance is a complex response and is determined by the interaction of several traits (and genes) in the plant. Researchers at North Carolina State University (Pantalone et al. 1999; 1996) have demonstrated that a soybean plant introduction, PI 416.937, has a fibrous root system and possesses some tolerance to drought and Al toxicity. Research conducted at the University of Tennessee has demonstrated that soybean varieties differ in their ability to orient their leaves during the course of the day (Wofford and Allen, 1982). For some varieties, the leaf surface remains very flat relative to the position of the sun, even during the hottest part of the day. Other varieties move their leaves during the course of the day such that the leaves have maximum exposure to the sun in the early and late parts of the day, but during mid-day the leaves are oriented such that the surface of the leaves has a minimum exposure to the sun. In a study of the variety Essex (high leaflet orientation) and Dare (low leaflet orientation), the two varieties produced about equal yields; however Essex used about one-half the amount of water as Dare during the growing season (Paris, 1997). We speculate that leaflet orientation might be a trait that contributes to drought tolerance.

Methods

An experiment was conducted at Knoxville, TN USA (35.96 lat., 83.91 long.) during the 2003 growing season using grafted plants to evaluate various combinations of leaflet orientation and root morphology traits on water use characteristics. Three soybean varieties were chosen for this study: USG 5601T, PI 416.937, and Williams 82. USG 5601T is a high yielding, maturity group 5, determinate cultivar that has high leaflet orientation and normal root morphology. PI 416.937 is a maturity group 6, determinate accession that has low leaflet orientation and fibrous roots. Williams 82 is an older, maturity group 3, indeterminate cultivar that has intermediate leaflet orientation and normal root morphology. Self grafts as well as all possible combinations of scion and rootstocks were made among the three varieties. Non-grafted plants of each variety were also included in the study for a total of 12 treatments. Water use of each plant was measured on several successive days via a Dynamax Flow 32 Sap Flow instrument when the plants were in the active pod filling stage of growth (R5). Leaflet orientation notes, plant height, plant yield, and sap flow rates were recorded in four replications. Because of missing data due to sensor malfunctions on some dates of measurements, we chose to use the complete, representative data set on 15 August 2003 only for presentation in this paper.

Results

Comparing the rates and efficiencies of the non-grafted and self grafted plants, it is evident that the grafting per se reduced the sap flow rates (Figure 1 and Table 1). This was expected and was the reason the self grafted plants were part of this study. The self grafted plants were therefore used as the basis for comparisons of different scions effects on water use efficiency. There were differences in the start, peak, and ending times of sap flow for the different scion / rootstock combinations (Table 1 and Figure 1). USG 5601T started sap flow earlier, peaked later and ended at the same time when compared to the other two varieties. R square values for the peak flow, total flow and water use efficiency analyses were 0.91, 0.95, and 0.95, respectively. Average peak sap flow rates and total accumulated flow for USG 5601T and Williams 82 were very similar while the PI 416.937 had a significantly higher peak flow rate and total accumulated sap flow. As expected the USG 5601T had the highest seed yield, while the older variety (Williams 82) yielded less, and the plant introduction (PI 416.937) was lowest in seed yield. The high leaflet orienting line USG 5601T had the highest water use efficiency measurement of 10.5g of water in a 24 h period to produce one gram of seed. This is compared to 19.3g for PI 916.937 and 13.4g for Williams 82. PI 416.937 used nearly twice as much water to produce the same amount of seed as the high leaflet orienting line USG 5601T (Table 2). The PI 416.937 self grafted plants had a water use efficiency of 11.4g (Table 3). When the high leaflet orientation line USG 5601T was used for a scion on that rootstock it used 3.8g less water per day to produce the same amount of seed. Similarly when the intermediate leaflet orientation line Williams 82 was used as the scion it used 3.6g less water per day per gram of seed yield. This demonstrates that when a higher leaflet orienting line was used for the scion the overall effect was an improved water use efficiency. The USG 5601T self grafted plants had a water use efficiency of 11.7g. When the low leaflet orienting line PI 416.937 was used as a scion on the USG 5601T rootstock it used 1.6g more water per day to produce the same seed yield. The Williams 82 self grafted plants had an intermediate leaflet orientation and a water use efficiency of 11.5g. When USG 5601T was used as the scion on the Williams 82 rootstock the plant used 3.9g less water per day to produce the same amount of seed. When PI 416.937 was used as the scion the plant used 20.4g more water per day to produce the same yield.

Conclusion

We demonstrate in this study the differences in sap flow characteristics of lines with differing leaflet orientation and root morphology traits. The line used in this study with a high leaflet orientation, USG 5601T increased water use efficiency when used as a scion across the different rootstocks. The low leaflet orienting line PI 416.937 decreased water use efficiency when used as a scion across rootstocks. This increased the amount of water extracted from the soil to produce the same seed yield. The intermediate leaflet orienting line Williams 82 increased water use efficiency when grafted on PI416.937 (low leaflet orienting phenotype) rootstock but decreased the water use efficiency when used as a scion on USG 5601T (high leaflet orienting phenotype) rootstock. This study provides further evidence of the role of leaflet orientation in water use efficiency and a possible drought tolerance trait in soybeans. The high leaflet orientation phenotype had equal water use efficiencies when grafted onto the fibrous or the normal rootstocks (Table 3). There was abundant soil moisture through the 2003 growing season in Knoxville. It is not known whether the fibrous root trait would have been beneficial during a moisture stress season. A population of isogenic lines is being developed from a cross of USG 5601T x PI416.937.

References

Pantalone VR Rebetzke GJ Burton JW Carter TE, Jr. and Israel DW (1999). PI 416937 root system contributes to biomass accumulation in soybean reciprocal grafts. Agron. J. 91:840-844.

Pantalone VR Burton JW and Carter TE Jr. (1996) Soybean fibrous root heritability and genotypic correlations with agronomic and seed quality traits. Crop Sci. 36:1120-1125.

Paris RL 1997. Water use efficiency of two soybean cultivars differing in leaflet orientation. M.S. Thesis, Univ. of Tennessee.

Wofford TJ and Allen FL (1982). Variation in leaflet orientation among soybean cultivars. Crop Sci. 22:999-1004.

Figure 1. Sap Flow measurements over a 24 hour period for reciprocal grafts among three soybean varieties.

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