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Penetration of hardpans by roots of wheat under drought

Tina Botwright and Len Wade

The University of Western Australia, School of Plant Biology M084, 35 Stirling Highway, Crawley, WA 6009 www.uwa.edu.au Email tbotwrig@agric.uwa.edu.au


Abstract

Around a third of arable land in Western Australia is in the low rainfall zone and grain yield of wheat in this zone averages 1 t/ha. Clay duplex soils and those compacted by heavy farm machinery restrict water infiltration and root penetration as roots cannot grow through soil pores narrower than their diameter, and are instead deflected and thicken. Although there is little to no information on genetic diversity in root traits in Australian wheat cultivars, there has been success in selecting for root traits beneficial to drought avoidance and hardpan penetration in rice, using a glasshouse technique where a synthetic wax-layer was used to simulate a hardpan. A glasshouse experiment was conducted to validate this method for wheat by comparing root biomass and plant water use in four wheat cultivars in drought or well-watered conditions, with or without a hardpan. Water use with a hardpan differed among treatments and the four wheat cultivars. Halberd and V18 used more soil water in total and had the fastest rate of water use when grown without a hardpan, but ran out of water several days before pots with a hardpan, compared to Cranbrook and CM18. Total water uptake was similar in V18 and Halberd, yet the rate of water uptake per unit root mass was greater in V18, with a smaller root DM. The technique will be used to evaluate a wider range of wheat cultivars and breeding lines for root penetration of hardpans and water-use under drought, but requires further validation to ensure that the identified traits are effective against real hardpans in the field.

Media summary

A method for evaluating hardpan and drought effects on root growth of wheat was validated, and showed genetic diversity

Key Words

Wheat, hardpans, drought, root traits, water use.

Introduction

Around 30% of arable land in Western Australia receives an annual rainfall of 180 mm or less and wheat yields of 1 t/ha are three-fold less than in the higher rainfall areas. Clay duplex soils and those compacted by heavy farm machinery restrict water infiltration and root penetration to soil fissures and biopores (Hamblin et al. 1982; Tennant et al. 1992; Dracup et al. 1993). Roots cannot grow through soil pores narrower than their diameter and are instead deflected and thicken, unless the soil is compressible and can be deformed to make a pore large enough for root elongation (Atwell 1990). Thus, hardpans restrict root growth to the soil surface layer where rapid depletion of soil nitrogen and water during drought reduces shoot growth and grain yield (Barraclough and Weir 1988). There is less resistance to root growth and water infiltration in sandy compared to clay soils, but sandy soils are prone to losses of water and nutrients by deep drainage, unless a hardpan develops from compaction. Root traits, such as deeper or longer roots that can penetrate a hardpan, may alleviate the detrimental effects of hostile subsoils on crop growth, water use and yield.

There is little to no information on genetic diversity in root traits in Australian wheat cultivars, but there has been success overseas in selecting for root traits beneficial to drought avoidance in durum wheat and rice, which can be used as models for future research in Australia. Canadian research by Hurd and others identified a durum wheat cultivar with prolific roots (ie. root length density) that improved access to soil water to avoid drought (Hurd 1964). A subsequent breeding program that selected for root traits lead to the release of two new cultivars, Wascana and Wakooma, with improved drought tolerance. More recently, research at IRRI in the Philippines in the 1990s by Wade and others identified rice cultivars that differed in root traits and/or hardpan penetration to maintain plant water status under drought in rainfed conditions in a series of glasshouse and field experiments (Azhiri-Sigari et al. 2000; Kamoshita et al. 2000; Wade et al. 2000). Of particular interest is a glasshouse technique that used a synthetic wax-layer to simulate a hardpan. The technique was used to test root penetration ability during vegetative growth under controlled conditions (Yu et al. 1995; Clark et al. 2002). Results with this technique were confirmed in field trials (Samson et al. 2002). The wax-layer technique has potential to be used in Australia to screen for hardpan penetration by roots of cereals. In rice, rate of root growth and maximum rooting depth are associated with rate of leaf area development (Wade et al. 2000) and it is possible there is a similar relationship in wheat. There exists genetic diversity for early leaf`area growth in wheat (Rebetzke and Richards 1999) which has been shown to confer a small yield advantage in medium rainfall environments in WA (Botwright et al. 2002), but the potential for this germplasm to grow roots capable of penetrating hardpans has not been investigated. Accordingly a glasshouse experiment was conducted to validate the wax-layer method and to compare root biomass and water use in four wheat cultivars in drought or well-watered conditions, with or without a hardpan.

Methods

Wheat cultivars Halberd, Cranbrook, Chuan Mai 18 (CM18) and Vigour 18 (V18) were grown in pots 85 cm tall and 15 cm diameter in a Karrakatta sand amended with the appropriate nutrients. A 3 or 60% paraffin wax: petroleum jelly hardpan (HP), 5 mm thick, was inserted at a depth of 15 cm below the soil surface. Pots were well-watered (WW) until 18 days after sowing (DAS), after which half of the pots received no further irrigation (drought stressed, DS). The experimental design was a factorial with three replicates. Water use was measured by weighing pots every two to three days until harvest at 48 DAS. Shoots were harvested at the soil surface and the number of tillers counted. Pots were split in two and roots were removed by washing soil from above and below the hardpan. Roots and shoots were dried at 60 C for 48 hours and weighed. The proportion of root DM above and below the hardpan was used as an indicator of root penetration ability. Data were analysed using a split-plot design with the ANOVA procedure in GenStat v 6.1 (GenStat 2002).

Results

Cultivars CM18 and V18 had early maturity but V18 and Halberd were 95% taller, on average, than CM18 and Cranbrook through the expression of the tall (rht) height gene (Table 1). Above-ground dry mass (DM) was, on average, 25% greater in V18 and Halberd, which tillered profusely, than in CM18 and Cranbrook. Drought stress reduced plant height by 35%, above-ground DM by 65% and total root DM by 50%, across cultivars (Table 1). Under well-watered conditions, a hardpan increased total root DM by 17% across cultivars, and was mainly attributed to the large root mass of Halberd, which was between one to three-fold greater than CM18, Cranbrook or V18. While 41% of CM18 total root DM was in the upper 15 cm of the pot, root DM of Halberd and V18 was more evenly distributed above and below the hardpan. On average, drought stress increased the proportion of roots beneath the hardpan by 13% compared to well-watered treatments.

Table 1. Effect of hardpan water regime and cultivar on plant height, biomass partitioning to shoots and roots and the proportion of roots at depths greater than 15 cm (ie. below the hardpan) at 48 DAS. Abbreviations: AGDM above-ground dry mass, DAS days after sowing, DS drought stress, HP hardpan, WW well-watered. The lsd is at P = 0.05.

 

Plant height
(cm)

AGDM
(g)

Root
DM
(g)

Root DM
beyond 15 cm
(%)

Hardpan Water regime

     

WW

55

999

648

63

WW+HP

53

1092

784

60

DS

33

445

355

73

DS+HP

37

421

348

76

lsd

11

200

83

5

         

Cultivar

       

Cranbrook

29

674

509

66

Halberd

68

891

770

75

V18

73

791

372

71

CM18

44

601

485

59

lsd

5

112

57

4

Without a hardpan, Halberd used water more quickly as drought stress intensified from 10 to 22 days after the last irrigation (Fig. 1a). Water use of Halberd then declined rapidly in drought-stressed pots, especially in the absence of a hardpan, where all of the plant available water was used 5 days earlier than in pots with hardpans (Fig. 1a). In contrast, the presence of a hardpan had no effect on the pattern of water use of Cranbrook under drought stress (Fig. 1b). Halberd extracted 5% more soil water on average than Cranbrook (data not shown). V18 and CM18 had similar patterns of water use to Halberd and Cranbrook, respectively (data not shown).

Figure 1. Water use of wheat cultivars Halberd and Cranbrook under drought stress, with and without hardpans. Symbols represent two hardpan impedances: ˜) low, 3:97 and; ™) high, 60:40 paraffin wax: petroleum jelly. Bars represent the SE.

Discussion

V18 and CM 18 had early maturity and reached anthesis in well-watered pots by harvest. Drought effects on reducing stem height were thus most evident in V18 and CM18. Halberd had exceptional root DM production under well-watered conditions compared to the other entries. Nodal roots are produced sequentially with the development of the main stem and tillers (Klepper et al. 1984) and while both Halberd and Cranbrook tillered profusely under well-watered conditions, increased branching of roots, root length or diameter may have contributed to the large root mass of Halberd compared to Cranbrook. Interestingly, total water uptake was similar in V18 and Halberd. Thus, the rate of water uptake per unit root mass was greater in V18, with a smaller root DM. A critical root length density can be identified, but depends on the evaporative demand and soil conditions, especially in the field if roots are constrained to biopores, and additional roots do not exploit more resources. Consequently, root traits should be interpreted according to their benefits for resource acquisition.

Water use under vegetative drought with and without a hardpan differed among the four wheat cultivars. Halberd and V18 used more soil water in total and had the fastest rate of water use when grown without a hardpan, but ran out of water several days before pots with a hardpan, compared to Cranbrook and CM18. The even distribution of root DM of Halberd and V18 above and below the hardpan contributed to the relatively fast rate of water uptake in these cultivars, compared to CM18, which had a large proportion of total root DM in the upper 15 cm of the pot. The ability to penetrate a hardpan may be beneficial, if additional resources are captured that otherwise would not be available, and if the resulting additional biomass is able to be partitioned to yield. The partitioning of DM is influenced by crop phenology and plant size prior to the onset of stress, and the progress in water use is in turn dependant on evaporative demand, the amount of stored water available, and the likelihood of its replenishment. Consequently, the ideal strategy is dependant not only on root traits per se, but also the characteristics of the target environment, and modifying traits influencing plant size and crop phenology, as recognised by Passioura (1977).

Conclusion

This paper provided initial evidence of genotypic variation in hardpan penetration ability in wheat, and of a suitable technique for its study. A synthetic hardpan was able to restrict root growth and water uptake and showed a difference in these traits. In this experiment, Halberd had the greatest root penetration ability and water use of the four wheat cultivars. The technique will be used to evaluate a wider range of wheat cultivars and breeding lines for root penetration of hardpans and water-use under drought, but requires validation to ensure that the identified traits are effective against real hardpans in the field.

References

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