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PHYSIOLOGICAL ASPECTS OF DROUGHT TOLERANCE IN Brassica napus and B. juncea.

Sharon R Niknam1 and David W Turner1

1Plant Sciences, Faculty of Agriculture, The University of Western Australia,
Nedlands, WA, 6907

ABSTRACT

The work reported in this paper examines osmotic adjustment under drought in two Brassica species. Glasshouse and field experiments were carried out to establish evidence of osmotic adjustment in genotypes of B. napus and B. juncea species. The glasshouse experiment showed that in B. napus there was evidence of osmotic adjustment in the post-anthesis stage but not earlier in the plant's development. However, in B. juncea osmotic adjustment appeared much earlier at the elongation and anthesis as well as the post-anthesis stage. In field experiments, there was evidence of genetic variation among two B. juncea lines and five B. napus cultivars. There was a significant negative correlation between percentage yield depression and osmotic adjustment.

KEYWORD: Canola, Osmotic adjustment, yield, Western Australia

Introduction

In Western Australia, canola is grown during winter months under rain-fed conditions. The low rainfall zone (300-450 mm) in the Western Australian wheat belt comprises a vast area of approximately 12 million hectares of arable land. Canola, as a valuable rotation crop, is becoming increasingly more popular. For example, in 1998, 517,000 hectares of canola were cultivated in WA compared with 11,000 hectares in 1992. However, in the low rainfall regions, its low yield and poor performance, in comparison with cereals, has limited its adoption by farmers. There is a need for development of cultivars of canola adapted to lower rainfall areas that will greatly benefit farmers in those regions.

There are some traits that are responsible for tolerance of plants to drought. One of the physiological traits is osmotic adjustment. Plant cells contain many dissolved substances called solutes. Under drought, the cells and tissues of some plants can adjust osmotically. That is, they increase their solute concentration before there is much water lost. This maintains turgor because water is drawn back into the cell, rather than being lost. This is called osmotic adjustment. This character might enhance the capacity of the crop to cope better with declining soil water reserves during the later stages of seed filling, as they clearly do with wheat (Morgan, 1983). In oilseed Brassicas, for example, higher yield is closely associated with greater post-anthesis growth which, in turn, is correlated with a capacity for osmotic adjustment to drought (Lewis and Thurling, 1994). Studying two grain sorghum lines exhibiting contrasting drought tolerance, Premachandra et al., (1995) found that the tolerant line maintained significantly higher osmotic adjustment than the drought susceptible line.

The aim of this study was to establish evidence of osmotic adjustment among genotypes of B. napus and B. juncea species as well as to define an association between the ability to adjust osmotically and final yield.

Methods

B. napus cultivar Monty and B. juncea line 397-23-2-3-3 were used in a glasshouse experiment. It was conducted during the summer of 1997-8 in a naturally lit phytotron with maximum and minimum temperatures of 18C (day) and 13C (night). Plants were subjected to drought at juvenile, stem elongation, anthesis and post-anthesis stages. Plant water relations, growth and development were assessed before and after each drought period. A set of plants was left to mature and yield components and plant partitioning of assimilates were measured. For each development stage, two treatments were imposed: a control and drought for 14, 9, 9, and 8 days for the four development stages respectively.

The field experiment was conducted at Merredin research station (Agriculture Western Australia) where annual rainfall for 1998 was 280 mm. Five B. napus cultivars, Hyola 42, Monty, Karoo, Rainbow and Narendra and two B. juncea lines, PI-81792 and 347-6-3-1-3 were sown in two blocks in a split plot design. Three treatments of rainout shelter, rain-fed and irrigation were imposed. Plant water relations including water potential, osmotic potential and relative water content were measured twice, once after anthesis when plants were not under water stress and different treatments were not yet imposed and once during pod filling stage when the different treatments were well in place.

For both glasshouse and field experiments, water potential of leaves was measured with a pressure bomb and osmotic potential was measured with a 1/10 Fisk Osmometer. Osmotic adjustment was estimated from measurement of the relative water contents and osmotic potentials of leaves. For each genotype, log 10 x 100 of these parameters were regressed and osmotic adjustment was assessed as the slope of the line (b) minus the passive adjustment (slope of –1). Regression was fitted with π as independent variable (log10ξ = a – b log10π). In the field experiment, % yield depression was calculated from yield values of genotypes under rain-fed and irrigated treatments.

Results:

The glasshouse experiment showed that in B. napus there was evidence of osmotic adjustment in the post-anthesis stage but not earlier in the plant's development. However, in B. juncea osmotic adjustment appeared much earlier at elongation and anthesis, as well as the post-anthesis stage.

In the field experiment, there was a positive correlation between osmotic adjustment and % yield depression for the seven genotypes (Figure 1). The seven cultivars were clustered into two groups for their ability to osmotically adjust (Figure 1). The group with no osmotic adjustment (Hyola 42, Monty and Line 347-6-3-1-3) had higher % yield depression. On the other hand, the group with significant osmotic adjustment (Karoo, Rainbow, Narendra and line PI-81792) had a lower decrease in yield. The Indian B. juncea line PI-81792 showed higher yield in the droughted treatment than the irrigated treatment (Table 1).

In the irrigated treatment, there was no yield advantage of B. juncea over B. napus cultivars (Table 1). However, in the rain-fed treatment, B. juncea yielded about 17 % more than B. napus non-triazine tolerance cultivars and 50 % higher than TT cultivar Karoo (Table 1).

Conclusion

There was a positive correlation between osmotic adjustment and % yield depression for the seven genotypes (Figure 1). Studying osmotic adjustment in 2 Brassica species, Kumar et al., (1984) reported a positive relationship between osmotic adjustment and grain yield of two genotypes. In a study on chickpea breeding lines, Morgan et al., (1991) found a positive association between controlled-environment measurements of osmotic adjustment and grain yield in field experiments. Earlier, Morgan, et al., (1986) reported a similar association for bread and durum wheat. In our experiment, although there was a significant correlation between % yield depression and degree of

Figure 1. The relationships between % yield depression and osmotic adjustment for 7 genotypes of B. napus and B. juncea in the field.

Table 1. Yield (tonnes/hectare) standard errors of the seven genotypes of B. napus and B. juncea for different treatments in the field.

Cultivars

Rainout shelter

Rain-fed

Irrigated

B. juncea

     

Line 347-6-3-1-3-3-1-3

1,00 0,00

1,37 0,04

1,59 0,30

PI-81792

1,12 0,19

1,56 0,12

1,45 0,04

B. napus

 

 

Karoo (TT)

0,51 0,09

0,98 0,22

1,11 0,19

Hyola 42

1,32 0,03

1,09 0,12

1,77 0,18

Rainbow

0,76 0,23

1,46 0,08

1,49 0,05

Narendra

1,04 0,00

1,20 0,04

1,37 0,06

Monty

0,94 0,09

1,28 0,04

1,66 0,37

osmotic adjustment among the seven cultivars, these cultivars were not similar in development, they were similar in maturity.

The correlation between % yield depression and osmotic adjustment was only between the rain-fed and irrigated treatment. There was no such correlation when the two treatments of rainout shelter and irrigated treatment were considered. At some instances, the erection of rainout shelter produced a glasshouse environment for the plants while the radiation was prevented. These effects are evident in higher water potential values for such periods while the final yield values are in general 25 % lower than the rain-fed treatment.

The Indian B. juncea line PI-81792 showed higher yield in the droughted treatment than the irrigated treatment (Table 1). Higher yield of B. juncea in low deficit conditions compared with B. napus was also observed by Wright el al., (1995).

In breeding programs for increase in drought resistance, a selection criterion such as osmotic adjustment is most useful. It has been reported that a single recessive gene controls osmotic adjustment in wheat and that the gene is located on chromosome 7A (Morgan, 1991). However, an unequivocal proof that shows correlation between yield and osmotic adjustment can only be found if near isogenic lines are utilised (Wright et al., 1997).

Acknowledgement

This project is funded by the Grains Research and Development Corporation. We thank Agriculture Western Australia for access to their facilities at the Merredin Dryland Research Institutes. We also thank Mr. Graham Walton and Dr. Rex Oram for supply of seeds.

REFERECES

1. Kumar, A., Singh, P., Singh, D. P., Singh, H. and Sharma, H. C. (1984) Differences in osmoregulation in Brassica species. Annals of Botany 54, 537-541.

2. Lewis, G. L. and Turling, N. (1994) Growth, development, and yield of three oilseed Brassica species in a water-limited environment. Australian Journal of Experimental Agriculture 34, 93-103.

3. Morgan, J. M. (1983) Osmoregulation as selection criterion for drought tolerance in wheat. Australian Journal Agriculture Research 34, 607-614.

4. Morgan, J. M., Hare, R.A. and Fletcher, R.J. (1986) Genetic variation in osmoregulation in bread and durum wheats and its relationship to gain yield in a range of field environments. Australian Journal Agriculture Research 37:5, 449-457.

5. Morgan, J. M. (1991) A gene controlling differences in omoregulation in wheat. Australian Journal of Plant Physiology 18:3, 249-257.

6. Morgan, J. M., Rodriguez-Maribona, B. and Knights, E.J. (1991) Adaptation to water-deficit in chickpea breeding lines by osmoregulation: relationship to grain-yields in the field. Field Crops Research 27, 61-70.

7. Premachandra, G. S., Hahn, D. T., Rhodes, D. and Joly, R. J. (1995) Leaf water relations and solute accumulation in two grain sorghum lines exhibiting contrasting drought tolerance. Journal of. Experimental Botany 46, 1833-1841.

8. Wright, P. R., Morgan, J. M., Jessop, R. S. and Cass, A. (1995) Comparative adaptation of canola (Brassica napus) and Indian mustard (B. juncea) to soil water deficits: yield and yield components. Field Crops Research 42, 1-13.

9. Wright, P. R., Morgan, J. M., Jessop, R. S. (1997) Turgor maintenance by osmoregulation in Brassica napus and B. juncea under field conditions. Annals of Botany 80, 313-319.

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