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Variation in shoot and root growth in primary synthetic wheats - implications for overcoming water deficits in marginal environments

Fernanda Dreccer, Francis Ogbonnaya, Gabriela Borgognone and Jayne Wilson

Department of Primary Industries Victoria, PMB 260, Horsham, Vic 3401 www.dpi.vic.gov.au

Abstract

We evaluated the shoot and root growth performance of primary synthetic wheats compared to commercial bread wheat cultivars during early vegetative stages. Variation in total biomass and leaf area at the 5-6 leaf stage was present in primary synthetics, with many lines producing significantly more than the bread wheat cultivars. In the primary synthetics, higher seed weight and relative growth rate were associated with higher growth rate. Their net assimilation rate was on average higher than that of the bread wheats and was accompanied by thicker leaves. Primary synthetic wheats and bread wheat cultivars did not differ in the root shoot ratio. Thus, genotypes with higher total biomass had higher root mass, often accompanied by longer root length. In summary, in the primary synthetics there are lines with high above and below ground growth that could be useful in developing germplasm for drought-prone environments. Additionally, the roots of primary synthetic wheats tended to be thicker than those of bread wheat cultivars. The adaptive significance of this characteristic is yet to be explored.

Media summary

Primary synthetic wheats are a source of genetic variability for early biomass production and root characteristics that could be exploited in developing drought tolerant wheats.

Key Words

Primary synthetic wheats, growth, leaf area, roots, hydroponics

Introduction

This study assesses the variability in shoot and root growth in primary synthetic wheats, focusing on constitutive traits that might confer a comparative advantage under drought. The use of synthetic hexaploids, resulting from the cross between bread wheat's progenitors (Triticum durum/Aegilops tauschii), is an attempt to redress the limited genetic variability of commercial bread wheat breeding programs. In the natural synthesis of wheat, only a small percentage of the genetic variability present in the gene pool of the wild progenitors was captured (Appels and Lagudah 1990). The plants under study were evaluated in a hydroponic system using a combination of non-destructive and destructive measurements to estimate growth related parameters under potential conditions (Lambers and Poorter 1992, Villar et al. 1998).

Methods

Experimental set-up and statistical analysis

A continuous flow hydroponic system with three trays (75 (w) x 110 (l) x 17 (d) cm) was used to grow plants for four weeks in two independent experiments. A different set of primary synthetics (A: 47 lines and B: 34 lines) and the same set of commercial bread wheats (8) were evaluated each time. Germinated seeds, vernalised (at 4C for 2 weeks) and of known seed weight, were transplanted to the system on 29th July 2002 (Experiment 1) and 7th October 2002 (Experiment 2). In order to recover roots intact, individual plants were grown in separate pots (7.5 (w) x 15 (d) cm) and were held vertical by a flexible rubber top. Average day/night temperatures were 18/9C and 25/12C in Experiments 1 and 2 respectively. Plants were grown in full strength Hoagland nutrient solution renewed every 10 days. The pH of the nutrient solution was adjusted to 5.5 on a daily basis.

A row-column design with three replications was used; there were two consecutive plants per genotype per replicate. The relations between growth rate and its components were analysed using the correlation procedure in GenStat (2002).

Measurements

Fresh weight and Haun stage (Haun 1973) of two plants per genotype per replicate were recorded once each week, after blotting the roots with tissue paper. Plants were then returned to the hydroponics. The relative growth rate was calculated from the log transformed fresh weight data of whole plants against accumulated thermal time from sowing. Plants were destructively harvested after four weeks in both experiments. In Experiment 2 there was also a harvest after the first week. Plants were dissected into leaf blades, stems+sheaths and roots. Leaf area was measured with a LI-3100 area meter (LI-COR Inc, Lincoln, NE, USA) and dry mass determined after oven-drying for 72 h at 70C. The relative growth rate was partitioned into its components: leaf area ratio and net assimilation rate.

A comparison between the estimates of relative growth rate calculated on fresh weight data via destructive vs. non-destructive sampling was carried out in both experiments using an independent set of plants consisting of three genotypes differing in seed weight.

Results

Variation in total biomass at the 2-leaf stage (Exp. 2) and 5-6 leaf stage (Exps. 1 and 2) was present in the primary synthetics, with many lines producing significantly more biomass than commercial cultivars (Fig. 1).

Figure 1. Total biomass per plant in Experiment 1 and at two different stages in Experiment 2 (see inset with average Haun score). Vertical bars are standard errors. Please note different ranges of y-axes.

Leaf area per plant followed a similar pattern to total biomass (Fig. 2). The higher levels of biomass and leaf area observed in Experiment 2 are probably related to the higher level of radiation at the later sowing date.

Figure 2. Leaf area per plant in Experiment 1 and at two different stages in Experiment 2. Vertical bars are standard errors. Please note different ranges of y-axes.

Genotypic differences in biomass production could arise from factors such as seed size, relative growth rate and its components. Average seed weight was 49 to 58% higher in the primary synthetics compared to the bread wheat cultivars in Experiments 1 and 2 respectively. The range in individual grain weight (mg/grain) was 43.9-83.6 for primary synthetics and 36.1-47.2 for bread wheats in Experiment 1, and 48.6-79.0 for primary synthetics and 30.9-48.1 for bread wheats in Experiment 2. The range in relative growth rates (mg/g/dd) was 5.8-7.9 for primary synthetics and 5.4-6.3 for bread wheats in Experiment 1, and 6.9-8.1 for the primary synthetics and 7.6-8.4 for bread wheats in Experiment 2. The relative growth rate calculated by non-destructive samplings was 4.4 to 5.4% lower than that determined destructively on average for three lines differing in seed weight (data not shown).

In the primary synthetics, plants with higher growth rate tended to have higher seed weight as well as relative growth rate (Table1). There was no consistent trend in the bread wheat cultivars, which highlights the need for further experimentation under different environmental conditions.

Table 1. Correlation between growth rate and components.

F-probability value: *P<0.05, **P<0.01,***P<0.001

 

Experiment 1

 

Experiment 2

Growth rate (mg/dd)

Primary
Synthetics A
(n=47)

Bread Wheats
(n=8)

 

Primary
Synthetics B
(n=34)

Bread Wheats
(n=8)

Seed weight(mg)

+0.31**

+0.85**

 

+0.41**

+0.49

Relative growth rate (mg/g/dd)

+0.29**

-0.77**

 

+0.44***

+0.77**

Variation in relative growth rate can be related to changes in the leaf mass ratio (leaf weight/plant weight), the specific leaf area (leaf area/leaf weight) and/or the net assimilation rate (rate of increase in biomass/leaf area). While leaf mass ratio and specific leaf area are a reflection of biomass partitioning and plant morphology, the net assimilation rate covers characteristics such as photosynthesis rate and respiration. The leaf mass ratio and leaf area ratio did not differ among genotypes, primary synthetics and wheat cultivars, in the different experiments (data not shown). However, the net assimilation rate tended to be higher in the primary synthetics than the bread wheats, particularly in Experiment 1, and was accompanied by leaves with low specific leaf area (Table 2) and very high chlorophyll readings (data not shown).

Table 2. Mean, range and coefficient of variation of net assimilation rate and specific leaf area.

       

Net assimilation rate
(mg/cm2/dd)

 

Specific leaf area
(cm2/g)

   

n

 

Mean

Range

CV%

 

Mean

Range

CV%

Exp. 1

Primary Synthetics A

47

 

0.040

0.033-0.048

9.2

 

327

291-371

5.7

 

Bread Wheats

8

 

0.029

0.023-0.034

12.3

 

354

316-409

9.0

Exp. 2

Primary Synthetics B

34

 

0.040

0.036-0.044

5.4

 

364

322-393

4.8

 

Bread Wheats

8

 

0.035

0.031-0.041

9.1

 

408

378-429

4.3

Table 3. Mean, range and coefficient of variation of root length and specific root length for the final harvests of Experiments 1 and 2 (primary+nodal roots) and the primary root system (Haun=1.98) in Experiment 2.

       

Root length
(cm/pl)

 

Specific root length
(km/g)

   

n

 

Mean

Range

CV%

 

Mean

Range

CV%

Exp. 1

Primary Synthetics A

47

 

1826.7

1048.0-2879.0

22.1

 

0.22

0.15-0.33

18.6

 

Bread Wheats

8

 

1349.8

925.0-1700.0

21.1

 

0.32

0.27-0.38

12.6

Exp. 2

Primary Synthetics B

34

 

55.0

39.1-79.4

17.1

 

0.05

0.04-0.07

14.8

(Haun =1.98)

Bread Wheats

8

 

72.1

43.9-102.2

29.3

 

0.08

0.05-0.09

18.9

Exp. 2

Primary Synthetics B

34

 

2033.8

1317.0-2714.0

17.4

 

0.20

0.12-0.37

24.3

(Haun =6.3)

Bread Wheats

8

 

2269.8

1698.0-3302.0

26.5

 

0.27

0.23-0.34

14.1

The root:shoot ratio was similar among entries in both experiments. Thus, genotypes with higher total biomass had higher root mass, often accompanied by higher root length. The roots of primary synthetic wheats tended to be thicker (low specific root length) than those of bread wheat cultivars either when only the primary root system was evaluated or when both the primary and nodal roots were evaluated (Table 3).

Conclusion

Improved growth under drought is not only related to above-ground performance but is also dependent on the crop's ability to access and extract soil water. Genetic variation in above and below-ground growth characteristics is present in primary synthetic wheats. The high growth rates were explained by higher seed mass and higher relative growth rate. Rapid soil cover by wheat plants has been proposed as an aid to increase the crop water use efficiency in semiarid environments, by reducing direct evaporation from the soil and potentially diverting more water to transpiration when vapour pressure deficit is low (Richards and Lukacs 2002, and references therein). Soil cover has more impact on reducing evaporation when the soil surface is wet (Ritchie 1981). Therefore, early vigorous growth is likely to be an advantage when wheat growth depends on within-season rainfall rather than stored sub-soil moisture (see Condon et al. 2002). Early above-ground vigorous growth has been obtained by breeding for, among other factors, rapid leaf area development through thinner leaves (high specific leaf area) (Rebetzke and Richards 1999). Interestingly, the primary synthetics had thicker leaves, maintained a high net assimilation rate, and were still able to produce a large leaf area. Assays will be repeated in different environments to appraise the extent of the genotype x environment interaction on these growth components.

While so much emphasis has been put on early above-ground performance, much less is known about genetic variation in root growth during early vegetative stages. High root growth in the primary synthetics was generally a reflection of vigorous above-ground growth, since there were no differences in root:shoot ratio. Crops with higher root length in deeper profiles might have increased water use over the entire growing season. A recent evaluation of synthetic Ae. tauschii -derived bread wheats at CIMMYT's program for marginal environments suggests that their enhanced performance under drought is linked to more vigorous root systems (R. Trethowan, pers.comm.), in line with some of our results on primary synthetics. The implication of thicker roots in the synthetic hexaploids for root penetration ability will be assessed in future studies using soil.

References

Appels R and Lagudah E (1990). Manipulation of the chromosomal segments from wild wheat for the improvement of bread wheat. Australian Journal of Plant Physiology 17, 253-266.

Condon AG, Richards RA, Rebetzke GJ, Farqhuar GS (2002). Improving intrinsic water-use efficiency and crop yield. Crop Science 42, 122-131.

Payne RW et al, for the Lawes Agricultural Trust (Rothamsted Experimental Station). 2002. GenStat, Release 6.1. VSN International Ltd, Oxford.

Haun JR (1973) Visual quantification of wheat development. Agronomy Journal 65: 116-119.

Lambers H and Poorter H (1992). Inherent variation in growth rate between higher plants: a search for physiological causes and ecological consequences. Advances in Ecological Research 23, 187-261.

Rebetzke GJ and Richards RA (1999). Genetic improvement of early vigour in wheat. Australian Journal of Agricultural Research 50, 291-301.

Richards RA and Lukacs Z (2002). Seedling vigour in wheat - sources of variation for genetic and agronomic improvement. Australian Journal of Agricultural Research 53, 41-50.

Ritchie JT (1981). Water dynamics in the soil-plant atmosphere system. Plant and Soil 58, 81-96.

Villar R, Veneklaas EJ, Jordano P, Lambers H (1998) Relative growth rate and biomass allocation in 20 Aegilops (Poaceae) species. New Phytologist 140, 425-437.

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