**
EVALUATING DROUGHT RESISTANCE IN TERMS OF YIELD – THE DROUGHT RESISTANCE
INDEX (DRI)**

Yield under stress is an important criterion
and selection index in breeding for water-limited environments. However, because
of the effect of potential yield on yield under stress the latter is not a
solid estimate of drought resistance in terms of yield. The rate of yield or
biomass reduction by stress (*e.g.* yield under stress as percent of yield
under non-stress) is often used as an estimate of resistance in terms of plant
production, in addition to absolute yield under stress. This ratio requires
tests under both stress and fully irrigated conditions. Two more accurate
estimates of drought resistance in terms of yield were developed and are being
used by breeders.

**The drought resistance index according to Fisher and Maurer, 1978 *
**

This index is an improvement over the simple
expression of yield under stress as percent of yield under non-stress
conditions. Here, DRI is ratio of yield reduction due to stress in a given
genotype as compared to the mean reduction over all genotypes in a given test.
Originally they proposed a drought susceptibility index. Breeders prefer to use
the calculation as a Drought Resistance Index.

DRI
= (Ys/Yn)/(Ms/Mn),

Where, Ys and Yn are
the genotype yields (or biomass) under stress and non-stress respectively and
Ms and Mn are the mean yields (or biomass) over all
genotypes in the given test under stress and non-stress respectively.

Sometimes claims were made that this index was
not a true representation of drought resistance in terms of yield because it is
biased by yield potential (yield under non-stress), namely genotypes of higher
yield potential tend to express a lower DRI. This is true, but it is not
necessarily a mathematical or a statistical bias. This bias has real biological
grounds as discussed by Blum (1995).

**The drought resistance index according to Bidinger
et al. (1982) ****

Yield potential and drought escape (short
growth duration) can sustain yield under stress. These are not true drought
resistance traits even though they can support yield under stress. There is
interest in identifying drought resistance of a cultivar beyond its capacity
for potential yield and drought escape. With this analysis the drought (or stress)
resistance index (DRI) is identified as variation in grain yield under stress
which is not explained by yield potential and drought escape. It is relevant
for stress conditions that ascribe an advantage to early flowering, such as
terminal stress (stress at final growth stages). Actual yield under stress
which is higher than that estimated due to its yield potential or drought
escape is an indication of drought resistance.

For this analysis all cultivars must be tested
together for yield under stress and non-stress conditions. The procedure for
DRI is done first by regressing stress yield on potential (non-stress) yield
and drought escape (days to heading measured under non-stress conditions) over
all cultivars (“initial regression”).
Where this regression is significant, the estimated stress yield for
each cultivar by this regression is then calculated. The actual measured yield
under stress is then regressed on this estimated yield across all cultivars
(“second regression”) (Table 1; Fig. 1). Any cultivar that expresses a positive
deviation from this regression line can be defined as drought resistant,
independently of the effect of potential yield or growth duration on its yield
under stress. This deviation, whether positive (resistant) or negative (susceptible)
can be calculated as depicted in Table 1 or it can be calculated by the studentized residuals from the regression for each
cultivar, as available in most statistical packages.

Table 1. The regression of actual stress yield on estimated yield (“second
regression”) and the calculation of the stress index.

The magnitude of the index indicates the degree
of the response. Because the residual includes experimental error as well as
specific cultivar by stress interaction (Table 1), most attention should be
given to those cultivars with largest indices, typically 1.3 or more.

In the breeding program DRI is considered
together with absolute yield under stress and non-stress conditions as well as
with other selection criteria such as biotic stress resistance or product
quality.

Fig.1. Example of the “second regression” to identify stress
resistant and susceptible cultivars.

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**(*)** According to:

Fischer, R.A. and R. Maurer, 1978. Drought resistance in spring wheat cultivars. I. Grain yield
response. Aust. J. Agric. Res., 29: 897–907.

**(**)** According to:

Bidinger F.R., Mahalakshmi Y., Talukdar B.S.,
and Alagarswamy G. 1982. Improvement of drought resistance in pearl millet. In IRRI (Ed) Drought
Resistance in Crops with Emphasis on Rice. IRRI, Los Banos,
Laguna Philippines. pp. 357-375.