Stress and disease tolerance

(ii) Yield loss due to disruption of spikelet fertility when severe stress occurs around flowering

Drought is especially damaging when it occurs immediately before and during flowering (Ekanayake et al., 1990; Garrity and O’Toole, 1994).  This is particularly true in upland rice, where brief periods of drought around flowering can result in near-complete spikelet sterility.  For this reason, much research on drought tolerance has focused on tolerance to stress at the flowering stage.  Genetic variation exists within O. sativa for the trait (e.g. Atlin et al. 2006).  

Some varieties have a high degree of tolerance to short periods of stress around flowering, whereas others experience markedly reduced seedset and harvest index.  For example, a set of varieties was evaluated at IRRI under rainfed upland conditions in the wet seasons of 2004 and 2005.  In both seasons, drought at flowering resulted in severe stress between panicle initiation and anthesis.  

For a subset of lines with similar days to flower under non-stress conditions, mean yield and harvest index are presented in Table 1.

Table 1.  Mean yield and harvest index (HI) of rice cultivars exposed to severe reproductive-stage stress under upland conditions at IRRI during the wet seasons of 2004-2005

2. Practical approaches to screening for drought tolerance in a cultivar development program

Screening for drought tolerance is screening for yield under stress

For a farmer, a drought-tolerant cultivar is one that produces a relatively high yield under stress. Screening for drought tolerance is therefore a matter of screening for yield under conditions as similar as possible to those experienced by farmers.  The hydrology of the screening environment should mimic that of a drought-prone field.  

The drought screen itself is a yield trial that should be managed in the way that effective yield trials are usually managed, i.e., it must be adequately replicated and uniformly managed.  Because drought screening is yield screening under stress, and replicated trials are needed, drought tolerance screening cannot be effectively applied at very early stages in a pedigree breeding program.  It is appropriate to begin drought screening at the initial replicated yield testing stage.

Possible rice drought tolerance screens…

Developing varieties combining high yield potential with tolerance to drought

Farmers do not usually want cultivars that are drought tolerant but low in yield potential in favorable years.  They want cultivars that respond to favorable conditions but that “protect” an economically useful yield under drought conditions.  Therefore, breeding lines should be screened under both stress and non-stress conditions.  

Mean yield of stress trials should be 20-40% of the yield in non-stress trials.  At these yield levels, cultivars with genes for tolerance can be distinguished from those that simply have high yield potential.  The aim of the stress screening protocol should thus be to reduce mean yield by 60-80% relative to a fully irrigated control trial.  Selections should be made on the basis of yield under both stress and non-stress conditions.  It is fortunate that most studies show that there is a low but positive correlation between yield under stress and yield potential.  It is therefore possible to identify varieties with both high yield potential and relatively high yield under stress.

Screening for tolerance to intermittent stress bracketing the flowering stage.

Because flowering is the stage when rice is most susceptible to drought damage some researchers devote a lot of effort to trying to ensure that each cultivar in the trial is subjected to water stress at the same phenological stage.  This can be done by planting each variety in a separate basin or irrigating each variety separately using drip irrigation equipment, permitting irrigation to be discontinued for individual varieties at the optimum moment.  These methods are not practical in a breeding program which must screen hundreds of lines.  

The IRRI breeding program screens for drought tolerance using protocols (described below), where stress is imposed before the first genotypes in the trial flower.  Lafitte and Courtois (2004) reported that variety means in screens of this type are highly correlated with means from trials in which stress is precisely applied to target the sensitive flowering process.

Screening for tolerance to lowland drought stress

Lowland fields regularly affected by drought are usually upper fields with light soil texture.  These fields are usually without standing water for most of the growing season, and may dry out repeatedly.  If this type of field is the target environment, then screening should mimic these conditions.  

The following protocol may be used:

1. Lowland drought screening trials should be conducted in a level, well-drained  field at the top of the toposequence.  There should be no irrigated or flooded trial planted above the drought screening site.

2. A ground-water tube 1 m deep should be installed in each replicate.

3. Lines should be screened in trials with at least 3 replicates.  Plots should be at least 2 rows.

4. Trials should be transplanted into puddled soil.  The field should be  that drained about one week after transplanting.

5. The field should be allowed to dry until soil cracks and/or the surface is completely dry.  The field should not be irrigated again until the local check variety is wilting, and the water table is at least 1 m below the surface.  If tensiometers are installed, the field should be irrigated when soil water tension = -40 kPA at a depth of 20 cm.

6. One day after re-irrigation, the field should be drained again.

7. Steps 5 and 6 should be repeated until harvest.

8. Yield and harvest index should be determined.

Screening for tolerance to upland stress

This screen may be used in either the dry or wet season.  Upland varieties are usually photoperiod-insensitive, so if temperatures permit, dry season screening is the preferred option for reliably imposing stress.

The following protocol may be used:

1. Upland drought screening trials should be conducted in an unbunded, well-drained  field at the top of the toposequence.  There should be no irrigated or flooded trial planted above the drought screening site.

2. A ground-water tube 1 m deep should be installed in each replicate.

3. Lines should be screened in trials with at least 3 replicates.  Plots should be at least 2 rows.

4. Trials should be direct-sown into dry soil.  The field should be irrigated to maintain soil at field capacity or above until canopy closure, or for about 30 DAS.

5. At 30 DAS, the frequency of irrigation should be reduced.  Fields should be allowed to dry until the surface is completely dry.  The field should not be irrigated again until the local check variety is severely wilted, and the water table is at least 1 m below the surface.  If tensiometers are installed, the field should be irrigated when soil water tension = -50 kPA at a depth of 30 cm.

6. When the target level of soil dryness and plant stress are reached, the field should be liberally irrigated.  Enough water should be applied to saturate the root zone.  This is likely to require 60-80 mm of water.

7. Steps 5 and 6 should be repeated until harvest.

8. Yield and harvest index should be determined.

3. Evidence that managed-stress screening is predictive of cultivar performance under natural stress in the TPE.

There is some evidence that differences in drought tolerance measured in the screens described above predict differences observed under natural stress in the target population of environments.

For example, at IRRI, 30 varieties were screened under severe upland stress artificially imposed at IRRI in DS 2005.   These same varieties were screened under rainfed upland conditions at IRRI in WS 2004 and WS 2005.  In both of these years, severe drought stress occurred at flowering.  The correlation between variety means for grain yield in the dry season stress screen and under natural stress in the wet season was 0.87, indicating that the ability of the artificial drought screen to predict performance under natural stress was high.

Severe upland drought screening (stress around flowering) at IRRI

Selection of random breeding lines under artificial stress has been shown to result in gains under natural stress in the wet season.  

Venuprasad et al. screened several hundred lines from the crosses Apo/IR 64 and Vandana/IR64 in the dry season of 2003.  

The lines were evaluated for grain yield under both severe upland stress and irrigated control conditions.  Selected lines from both the stress and the irrigated control screens were then evaluated under natural stress at IRRI in the wet seasons of 2004 and 2005.  Yield gains under natural stress were greater in the subset of lines selected under artificial stress than under fully irrigated conditions.  

Selection under stress gave no gains under non-stress conditions (Table 2).

Table 2. Yield (g m-2) of stress-selected, non-stress-selected, and random lines and checks in Apo/IR64 and Vandana/IR64 populations in selection response trials: IRRI, 2004 dry and wet season.

* significant difference of a selected set from random set, p=0.05 level

Drought tolerance of some widely-grown varieties and potential donors

Below is a list of some varieties and their relative drought tolerance as determined in testing over several seasons at IRRI or in countries of origin.  These levels of tolerance are approximate only.  Lines with high and low levels of tolerance should be included as checks in drought screening trials.

Table 3. Relative levels of reproductive and vegetative drought tolerance of selected lowland- and upland-adapted genotypes.

Aerobic rice cultivars should be ...

Aerobic rice cultivars (middle) have vigorous seedlings,

rapid biomass development, deep roots and erect leaves.

Aerobic rices (left) are highly weed-competitive due to vegetative vigor

Aerobic rices are input-responsive and lodging-resistant +

They have a high harvest index, even under moderate stress

Let's conclude

Summary

• Breeding cultivars with improved drought tolerance is possible by selecting for grain yield under artificially-imposed stress.

• Yield under stress is a quantitative trait with H similar to yield under non-stress conditions.  It must be measured in replicated trials.

• Drought tolerance screening should be initiated after pedigree line development, at the replicated yield evaluation stage.

• Large sets of breeding lines can be evaluated in screens where drought stress is imposed intermittently from transplanting (lowland) or maximum tillering (upland) through harvest by irrigating only when soil is dry (below field capacity) in the root zone. Yields in the stress trial should be reduced to 20-40% of yields under non-stress conditions.

• Screening using repeated intermittent stress bracketing the flowering stage gives results that are similar to screens targeted precisely at flowering.  Repeated intermittent stress is therefore an appropriate method for screening large breeding populations with a range of flowering dates.  However, an effort should be made to group lines according to similar flowering date in drought screens.

• Yield under managed stress is predictive of yield under natural stress.  Gains from direct selection from yield under managed stress have been demonstrated.

Click here to see some selected references (not all have been cited above, but all are useful)