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Variability in structure and function of sorghum root systems

Vijaya Singh, Graeme Hammer, Erik van Oosterom

Agricultural Production Systems Research Unit, School of Land, Crop and Food Sciences, The University of Queensland, Brisbane, Qld 4072, Australia. Email: v.singh@uq.edu.au

Abstract

The design and degree of yield advantage of skip-row sorghum over solid-planted crops in water-limited situations could be increased if germplasm was identified that more effectively occupied the available soil volume. The aim of this research was to screen for genetic variation in structure and function of sorghum root systems during early growth, and to assess the functional implications of such variation in terms of rooting effectiveness and occupancy, which represent the rate and amount of water extraction. To achieve this, we grew plants in purpose built rhizotrons. Preliminary experiments, involving four hybrids, indicated that sorghum has only one seminal root, and that the first nodal roots appear at the five-leaf stage. Importantly, genotypic differences in the encompassing angle (relative to the vertical) of nodal roots were observed. Variation in root angle has been associated with architecture of the mature plant root system in wheat. Based on these initial results, a large-scale screening experiment was designed, in which >70 inbred lines and hybrids, relevant to sorghum breeding in Australia, were grown in small rhizotrons with perspex sheets on both sides. Plants were harvested after the first flush of nodal roots had appeared. Root angle ranged from 15 to 50 and because genetically related germplasm had similar root angle, these differences appeared repeatable. A small number of lines have been selected for trials in large chambers to study if these differences in root angle affect the rooting effectiveness and occupancy.

Key Words

Water extraction, roots, genetic variation

Introduction

Planting row configuration in sorghum is known to affect the timing and extent of soil water extraction with consequent effects on crop growth and yield (McLean et al., 2003; Whish et al., 2005). Plant roots take longer to explore the entire soil profile in wide row systems and may not fully explore it (Broad and Hammer, 2004). However, as accessing small quantities of additional water, especially after anthesis, can increase grain yield considerably (Manschadi et al., 2006; Hammer, 2006), variation in the structure of root systems to facilitate this would be advantageous. Studies in wheat highlighted an association of drought adaptation and root system architecture with variation in seminal root angle (Manschadi et al., 2006, 2008). Lines with more vertical seminal roots had enhanced water extraction at depth. Recent studies in maize also support the importance of root system architecture in yield advance in that crop (Hammer et al., 2008).

There is little known about genetic variation in root system structure and function in sorghum. It is known that nodal roots have a major influence on mature plant root system architecture but development of the root system has been little studied. Further, there have been no studies attempting to link this variation to patterns of water extraction. The aim of this study was to develop and apply a rapid phenotyping system that captures the major attributes of the sorghum root system in young plants. To achieve this, a preliminary experiment aimed at characterising the development of seminal and nodal roots in young sorghum plants was conducted using four sorghum hybrids varying in drought tolerance. This resulted in understanding the development of the sorghum root system to inform design of a chamber used for rapid phenotyping and screening of >70 genotypes for root attributes.

Root System Development in Sorghum

Experimental Methods and Measurements

The seeds of four sorghum hybrids (A35/RQL12, A35/RQL36, AQL39/RQL12, and AQL39/RQL36), known to vary in drought adaptation, were grown in purpose built rhizotrons. Each chamber was 60 cm high, 40 cm wide and 5 cm thick, with transparent perspex (6 mm thick) on both sides. They were filled with air dried soil (a mixture of 70% vertosol soil and 30% sand) and one pre-germinated seed was planted in each chamber. The experiment consisted of three replications with 5 plants per hybrid in each replication.

At the full emergence of leaf 2 and each subsequent leaf until leaf 6, one plant per hybrid in each replication was harvested. The perspex on one side of the chamber was removed, and the intact root system was transferred on to a specially designed pinboard with nails in a 2x2 cm2 grid. Soil was carefully washed with water spray without disturbing the root system, and a digital image of each intact root system was taken. Observations recorded included the number, length and thickness of seminal and nodal roots, the encompassing angles for the first and second flush of nodal roots, leaf area, and shoot and root dry weight.

Results

Figure 1 shows the intact root system of A35/RQL12 from leaf 2 to leaf 6. There was only one seminal root, the length and branching of which increased with development. The first flush of nodal roots appeared around the 5 leaf stage, by which time the seminal root had reached the bottom of the chamber. At the 6 leaf stage, the second flush of nodal roots had appeared, the first flush had extended to the bottom of the chamber, and seminal roots had branched to fill the entire chamber.

Figure 1. Intact root system of A35/RQL12 at the full emergence of Leaf 2 up to Leaf 6.

The significant differences in root mass (Table 1) were associated with a difference in leaf appearance rate, that caused differences in harvest dates. However, differences in root number and length were absent or small, supporting the hypothesis that root appearance is linked to leaf appearance. RQL36 hybrids consistently had a wider basal nodal root diameter than RQL12 hybrids. Hybrids A35/RQL12 and A35/RQL36 had the widest and narrowest root angles, respectively, for both flushes of nodal roots.

Table 1. Root parameters of sorghum hybrids measured at 6th leaf developmental stage

Sorghum hybrids

Root parameters

A35/RQL12

A35/RQL36

AQL39/RQL12

AQL39/RQL36

P

Lsd(.05)

 

Manual measurements

Total root dry mass (g)

0.287

0.407

0.257

0.867

0.023*

0.3751

Mean nodal root diam. (mm)

0.80

1.06

0.95

1.27

0.024*

0.296

Branching angle 1st flush ()

52.0

37.5

38.2

44.0

0.008**

8.10

Branching angle 2nd flush ()

31.0

13.2

27.2

27.2

0.043*

12.50

No of seminal roots

1

1

1

1

1

ns

No of nodal roots

4.2

4.4

3.9

4.7

0.961

ns

Total nodal length (cm)

205

290

217

304

0.091

ns

             

The results of this preliminary experiment indicated that screening of the nodal root system of sorghum requires that plants be grown until the 6-leaf stage, as nodal roots start appearing at 4 and 5 leaf stage. In addition, the rhizotrons need to be at least 60x40 cm2 in order to accommodate the root system. The differences in genotypes for branching root angle (1st flush) was highly significant (Table 1), indicating a desirable character for root screening at an early growth stage for sorghum.

Screening for Genetic Variation in Root System Structure

Experimental Methods and Measurements

Pre-germinated seeds of 74 sorghum genotypes (including inbred lines and hybrids, relevant to the sorghum breeding program of the Queensland Department of Primary Industries and Fisheries) were grown in specifically designed root chambers, 60 cm high, 45 cm wide, and 3 mm thick. The chambers were constructed with perspex on both sides for viewing and scanning of roots. The chambers were filled with a black coarse sandy soil, sourced from a location close to Bribie Island, Qld. A single plant was grown in each chamber, and the experiment had three replications. At the 6–leaf stage, roots in intact chambers were scanned with a flatbed scanner and plants were harvested. The intact root system was carefully washed with water spray without disturbing the root system, and a digital image was taken.

Figure 2. Encompassing root angle of nodal roots (relative to the vertical) measured at 6th leaf stage for 74 sorghum genotypes (including inbreds and hybrids).

Results

There was large variation in encompassing root angle (15 to >50) for the 1st flush of nodal roots among the sorghum genotypes (Fig. 2). At both extremes, there was considerable genetic commonality, indicating that the observed differences in root angle were repeatable.

Discussion

This study offers potential for large-scale selection for drought adaptation in young plants, if the root architecture measured in young plants can be connected to rooting effectiveness and occupancy of the soil (and hence water uptake) of mature plants, as reported for wheat (Manschadi et al., 2008). This is the focus of on-going experiments, in which lines with the most extreme root angle (Fig. 2) are being grown in large root chambers. Identification of QTL’s associated with root architecture (in particular root angle) would subsequently offer the potential for marker-assisted selection. This could greatly improve the efficiency of identifying genotypes with adaptation to specific management*environments combinations, such as the skip row management systems that are popular in rainfed sorghum production.

References

Broad I, Hammer GL (2004) Soil exploration by sorghum in wide row cropping systems. Contributed Paper in New directions for a diverse planet, Proceedings of the 4th International Crop Science Congress, 26 Sep – 1 Oct 2004, Brisbane, Australia. www.cropscience.org.au

Hammer G (2006) Pathways to prosperity: Breaking the yield barrier in sorghum. Agricultural Science 19(2), 16-22. The Journal of the Australian Institute of Agricultural Science and Technology.

Hammer GL, Dong Z, McLean G, Doherty A, Messina C, Schusler J, Zinselmeier C, Paszkiewicz S, Cooper M (2008) Can changes in canopy and/or root system architecture explain historical maize yield trends in the US corn belt? Crop Science, in press.

Manschadi AM, Christopher J, deVoil P, Hammer GL (2006) The role of root architectural traits in adaptation of wheat to water-limited environments. Functional Plant Biology 33, 823-837.

Manschadi AM, Hammer GL, Christopher JT, and deVoil P (2008) Genotypic variation in seedling root architectural traits and implications for drought adaptation in wheat (Triticum aestivum L.). Plant and Soil 303, 115-129.

McLean G, Whish J, Routley R, Broad I, Hammer G (2003) The effect of row configuration on yield reliability in grain sorghum: II. Modelling the effects of row configuration. Proceedings of the Eleventh Australian Agronomy Conference, Geelong, Jan 2003.

Whish J, Butle G, Castor M, Cawthray S, Broad I, Carberry P, Hammer G, McLean G, Routley R, Yeates S (2005) Modelling the effects of row configuration on sorghum yield in north-eastern Australia. Australian Journal of Agricultural Research 56, 11-23.

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