Acta agriculturae Slovenica, 117/4, 1–13, Ljubljana 2021
doi:10.14720/aas.2021.117.4.2176 Original research article / izvirni znanstveni članek
Introduction of the best criterion for evaluation of tolerance to drought 
stress in sorghum’s genotypes
Leyla NAZARI
 1, 2
, Ebrahim DEHGHANIAN
 3
, Afshar ESTAKHR
 1
, Azim KHAZAEI
 4
, Behzad SORKH-
ILALEHLOO
 4
, Mohammad Reza ABBASI
 5 
Received April 15, 2021; accepted December 09, 2021.
Delo je prispelo 15. aprila 2021, sprejeto 9. decembra 2021
1 Crop and Horticultural Science Research Department, Fars Agricultural and Natural Resources Research and Education Center, Agricultural Research, Education and 
Extension Organization (AREEO), Shiraz, Iran
2 Corresponding author, e-mail: l.nazari@areeo.ac.ir
3 Agricultural Engineering Research Department, Fars Agricultural and Natural Resources Research and Education Center, Agricultural Research, Education and 
Extension Organization (AREEO), Shiraz, Iran
4 Seed and Plant Improvement Institute, Agricultural Research, Education and Extension Organization (AREEO), Karaj, Iran
5 Crop and Horticultural Science Research Department, Khorasan Razavi Agricultural and Natural Resources Research and Education Center, Agricultural Research, 
Education and Extension Organization (AREEO), Mashhad, Iran
Introduction of the best criterion for evaluation of tolerance 
to drought stress in sorghum’s genotypes
Abstract: Sorghum (Sorghum bicolor (L.) Moench) is 
the fifth important cereal considered a drought-tolerant crop. 
However, its reduction of grain yield considerably occurs in a 
shortage of water. In the current study, 10 sorghum genotypes 
were assessed for their grain yield under normal irrigation and 
water deficit irrigation. As well, the efficacy of several drought 
indices was evaluated for the selection of high-yield and 
drought-tolerant genotypes. The experiment was conducted 
as a split-plot considering three irrigation levels as main-plot 
and 10 genotypes as sub-plot. Correlation among the indices, 
clustering of the genotypes along with principal component 
analysis was employed. Yield production was significantly and 
positively correlated with indices MP (mean productivity), STI 
(stress tolerance index), GMP (geometric productivity), HM 
(harmonic mean), and YI (yield index) in all the irrigation lev-
els. Therefore, these indices are more effective in the selection 
of high-yielding genotypes under different water conditions. 
Rank means of stress indices for each genotype revealed that 
genotype TN-04-79 in mild deficit irrigation and genotypes 
KGS23 and TN-04-79 in severe deficit irrigation were the most 
tolerant.
Key words: sorgum; drought stress; grain yield; water 
productivity; drought response indices
Uvajanje najboljših kriterijev za ovrednotenje tolerance na 
sušo pri genotipih sirka
Izvleček: Navadni sirek (Sorghum bicolor (L.) Moench) je 
peto najpomembnejše na sušo odporno žito, a se kljub temu 
njegov pridelek zrnja znantno zmanjša ob pomanjkanju vode. 
V tej raziskavi je bilo ocenjenih 10 genotipov navadnega sirka 
glede na pridelek zrnja ob normalnem namakanju in v razme-
rah vodnega deficita. Ocenjeni so bili tudi različni indeksi to-
lerance na sušo pri izboru genotipov z velikimi pridelki zrnja 
in dobre tolerance na sušo. Poskus je bil izveden kot puskus z 
deljenkami, kjer so bila obravnavanja z namakanjem na glavnih 
ploskvah in 10 genotipov na podploskvah. Uporabljene so bile 
korelacije med indeksi in združevanje genotipov glede na glav-
no komponento. Velikost pridelka je bila značilno pozitivno 
povezana z indeksi MP (poprečna produktivnost), STI (indeks 
tolerance na stres), GMP (geometrična produktivnost), HM 
(harmonično poprečje) in YI (indeks pridelka) pri vseh načinih 
namakanja. Ti indeksi so torej bolj učinkoviti pri izboru visoko 
donosnih genotipov v razmerah različne preskrbe z vodo. Po-
prečje rangov stresnih indeksov za vsak genotip je odkrilo, da 
je genotip TN-04-79 najučinkoviteši ob blagem pomanjkanju 
vode, genotipa KGS23 in TN-04-79 pa sta bila najbolj odporna 
na sušo.
Ključne besede: navadni sirek; sušni stres; pridelek zrnja; 
učinkovitost izrabe vode; indeksi odziva na sušni stres

Acta agriculturae Slovenica, 117/4 – 2021 2
L. NAZARI et al.
1 INTRODUCTION
Sorghum (Sorghum bicolor  (L.) Moench) is a C
4
 
and drought-tolerant crop used for food, feed, and fiber 
(Ludlow et al., 1990). Its tolerance to drought can be at-
tributed to morphological characteristics (e.g. deep root 
system and thick leaf wax), physiological responses (e.g. 
stay green and osmotic adjustment), and adaptive mech-
anisms allowing tolerance under extreme drought con-
ditions (reviewed in Tari et al., 2013). In the dry region 
of Asia and the Middle East, drought is one of the most 
important abiotic stresses, leading to the limitation of 
plant growth and yield productivity (Zhang et al., 2018). 
Therefore, improving yield production per unit of water 
(water productivity) is an efficient strategy in dry regions 
(Ali and Talukder, 2008).
Blum (2005) suggested that the selection of geno-
types should mainly focus on high yield under non-stress 
conditions and secondly under water stress conditions. 
The selection of genotypes that have tolerant genes is 
difficult as drought tolerance is a quantitative trait with 
intricate heritability. Therefore, despite the lack of infor-
mation on drought tolerance mechanisms, researchers 
have proposed the utility of different selection indices to 
screen drought-tolerant genotypes (Anwaar et al., 2019). 
Hence, we have employed the following selection criteria 
for screening drought-tolerant genotypes and introduc-
ing the best indices. 
Several indices based on the yield under control (Y
p
) 
and stress (Y
s
) have been introduced for the selection of 
drought-tolerant genotypes. Among these, the indices 
employed in various stress conditions are stress tolerance 
(TOL) and mean productivity (MP) introduced by Ros-
ielle and Hamblin (1981), Stress susceptibility index (SSI) 
by (Fischer and Maurer, 1978), stress tolerance index 
(STI) and geometric mean productivity (GMP) by Fer-
nandez (1992), Harmonic mean of yield (HM) by Jafari 
et al. (2009), yield index (YI) by Gavuzzi et al. (1997), 
yield stability index (YSI) by Bouslama and Schapaugh 
(1984), yield reduction ratio (YRR) by Golestani-Araghi 
and Assad (1998). Selection of high-yield genotypes in 
both normal and deficit irrigation using a combination of 
these indices is preferred. Therefore, different statistical 
analyses including analysis of variance (ANOV A), corre-
lation, principal component analysis (PCA), and cluster 
analysis were performed. The study aimed to investigate 
the efficiency of the mentioned indices for screening tol-
erant genotypes of sorghum to drought stress.
2 MATERIALS AND METHODS
2.1 EXPERIMENTAL SITE
The experiment was conducted at the Research Farm 
of Fars Agricultural and Natural Resources Research and 
Education Center, Shiraz, Iran (52°42’ E, 29°46’ N, 1.604 
m elevation) with a semi-arid environment (Fig. 1a). It is 
characterized by mean annual precipitation of 345 mm 
copper 
ppm
magnesium 
ppm
zinc 
ppm clay % silt % sand %
potassium 
ppm
phosphorus 
ppm O.C % EC*10
3
pH
soil depths 
cm
0.90 7.60 0.80 33.4 46.2 20.4 434 11.2 0.95 0.97 8.0 0-30
0.96 8.50 0.96 36.8 42.8 20.4 310 4.2 0.81 2.15 7.9 30-60
Table 1: The chemical properties of the soil in the experimental area
EC: electrical conductivity; O.C: organic carbon; extractable phosphorus was measured according to Olson method
Fig. 1: The spatial position of the experimental site captured 
on 07/11/2021 (a); minimum and maximum of temperature 
(b) and ET
0
 (c) during the growth season (2018) conducted at 
Zargan, Iran (52°42’E, 29°46’N)

Acta agriculturae Slovenica, 117/4 – 2021 3
Introduction of the best criterion for evaluation of tolerance to drought stress in sorghum’s genotypes
and an annual temperature of 15.8 °C. Minimum and 
maximum temperature and ET
0
 during growth season 
are presented in Fig. 1b,c. The soil is characterized by 
fine, carbonatic, active, thermic Typic Calcixerepts (soil 
taxonomy, 2014) and Cambic Calcisol (Lomic, Ochric) 
(WRB, 2015). The fertilizers were distributed based on 
soil test results (Table 1). 
2.2 EXPERIMENTAL DESIGN
The plants (10 sorghum genotypes, supplemen-
tary Table 1) were cultivated manually as split-plot in a 
randomized complete block design with three replicates 
on 6 June 2018. Water deficit treatment was considered 
as the main factor and genotype as the sub-factor. Sub-
plots were 12 m
2
 including 4 rows of 5 m long with a row 
distance of 0.6 m. Tinning was performed 4 weeks after 
sowing with a target of 10 plants per linear meter. Weed 
control was performed manually during the season. 
Irrigation treatments were applied to the main 
plots at three levels of normal irrigation, mild and severe 
water-deficit irrigation defined as irrigation when the 
evaporation rates from pan class A exceeded 60, 120, and 
180 mm, respectively. Water stress was started from the 5 
leaves stage and continued during the season.
Irrigation was applied using a tape drip and the irri-
gation volume was recorded by using a volumetric coun-
ter. FAO-CROPWAT 8.0 as a decision support system 
(DSS) was used to calculate the reference crop evapo-
transpiration (ET
o
) (Clarke, 2001) and schedule differ-
ent levels of irrigation. The accuracy of this method was 
demonstrated by comparing it to original crop water re-
quirements (Surendran et al., 2019). Meteorological data 
were taken daily from the Zargan Meteorological Station 
near the experimental field. The irrigation requirement 
was calculated according to Doorenbos and Pruitt (1977) 
(Table 2).
2.3 MEASUREMENTS AND DROUGHT INDICES
Agronomic characteristics including plant height 
(PH), panicle length (PL), stem diameter (SD), and the 
number of leaves per plant (NoL) were recorded for 10 
plants per plot from the middle two-row of each plot. As 
well, 1000 seed mass (1000 SM), dry matter yield (DMY), 
and harvest index (HI) were recorded. Water productiv-
ity (WP) was calculated as Ali and Talukder (2008) (Ta-
ble 2).
Drought tolerance indices were calculated accord-
ing to the equations in Table 2. Ranking of the genotypes Index description equation Reference
Crop Water Requirement
 (mm/day)
K
c
: water requirement coefficient changing with the growth stages of  
sorghum; ET
o
: the reference evapotranspiration of plant under specified  
conditions measured by pan evaporation.
ET
c
 = K
c
 × ET
o Doorenbos and Pruitt, 1977; 
Doorenbos and Kassam, 1986
Water productivity (WP) crop production per unit volume of water; high values are more desirable Ali and Talukder, 2008
Tolerance index (TOL) Low values indicate more stability under deficit irrigation Rosielle and Hamblin (1981)
Mean productivity (MP) High values are more desirable Rosielle and Hamblin (1981)
Stress susceptibility index (SSI) Values < 1 are more tolerant Fischer and Maurer (1978)
Stress tolerance index (STI) High values indicate more tolerant Fernandez (1992)
Geometric productivity (GMP) High values are more desirable Kristin et al. (1997)
Harmonic mean of yield (HM) High values are more desirable Jafari et al. (2009)
Yield index (YI) High values indicate more tolerant Gavuzzi et al. (1997)
Yields stability index (YSI) High values indicate more stability under normal and deficit irrigation Bouslama and Schapaugh (1984)
Yield reduction ratio (YRR) Low values indicate more suitable for deficit irrigation Golestani-Araghi and Assad 
(1998)
Table 2: Description, equation and reference of crop water requirement, water productivity, and drought tolerance indices

Acta agriculturae Slovenica, 117/4 – 2021 4
L. NAZARI et al.
based on the indices was performed according to the 
method of Mickky et al. (2019).
The means of grain yield and the indices were ranked 
considering that indices with higher values are more de-
sirable except TOL, SSI, and YRR. Afterward, rank mean 
(R’) and standard deviation of rank (SDR) were calcu-
lated. Rank mean is defined as the average of ranking val-
ues across all drought tolerance indices of each genotype. 
Rank sum (RS) of each genotype was then determined by 
the addition of rank mean (R’) and standard deviation of 
rank (SDR). 
2.4 STATISTICAL ANALYSIS
Analysis of variance (ANOVA) and mean compari-
son were performed using SAS release 9.2 (SAS Institute, 
Cary, NC, USA). Before doing ANOVA, normality tests 
were conducted. Provided that F-values were significant, 
a mean comparison was done (Duncan’s test, p ≤ 0.05). 
Drought stress indices, principal component analysis 
(PCA) and Pearson’s correlation between the indices 
were performed using iPASTIC that is an online tool kit 
for the estimation of plant abiotic stress indices (Khalili 
et al., 2016). Genotypes were clustered using Ward’s hier-
archical clustering.
Treatments
PH 
cm
PL 
cm
SD 
mm NoL
1000 SM  
g
 Yield 
kg ha
-1
DMY 
kg m
-3
HI 
%
WP 
kg m
-3
Irrigation level
normal irrigation 175.2
a
29.8
a
26.4
a
16.9
a
29.4
a
5847.2
a
27874.1
a
23.2
a
0.65
a
mild deficit irrigation 152.0
b
27.6
ab
24.5
b
14.0
a
28.5
ab
4026.4
b
22260.9
b
21.3
b
0.58
b
severe deficit irrigation 138.7
c
25.3
b
23.6
c
14.6
a
26.5
b
2759.2
c
21480.3
b
16.5
c
0.43
c
Genotype
MGS2 105.3
g
30.7
c
25.9
c
12.8
bc
2.9
f
2910.1
f
16188.9
e
17.7
e
0.37
e
KGS23 94.0
h
18.9
c
24.0
d
10.9
bc
4.7
bc
4683
bc
16466.6
e
29.2
b
0.63
b
TN-04-78 120.0
f
24.0
d
30.1
b
16.7
b
3.5
e
3491.1
e
24276.2
c
14.4
e
0.45
d
TN-04-79 221.1
a
10.0
g
24.9
cd
15.2
bc
6.5
a
6517.6
a
30906.7
b
21.3
c
0.88
a
TN-04-129 91.9
h
22.3
d
33.4
a
13.4
bc
4.3
c
4304.2
cd
20498.8
d
20.8
d
0.57
c
TN-04-134 201.6
b
14.6
f
21.5
e
22.7
a
6.2
b
4974.3
b
28825.6
b
16.7
e
0.64
b
TN-04-142 227.0
a
7.7
g
21.7
e
28.4
a
1.6
g
1558.7
g
48770.0
a
3.2
f
0.19
f
TN-04-59 159.3
d
53.4
a
24.5
cd
10.4
bc
5.0
b
5047.8
b
18799.6
de
26.8
b
0.67
b
TN-04-86 151.2
e
40.9
b
21.2
e
9.8
c
4.7
bc
4685.4
bc
15641.6
e
30.3
a
0.63
b
TN-04-90 180.6
c
53.2
a
21.1
e
11.4
bc
3.9
d
3937.2
d
18343.6
de
22.6
c
0.538
c
Table 3: The main effects of irrigation level and genotype on morphological traits, yield, and water productivity of 10 sorghum 
genotypes
PH: plant height; PL: panicle length; SD: stem diameter; NoL: number of leaves; 1000 SM: 1000 seed mass; DMY: dry matter yield; HI: harvest index; 
WP: water productivity.
Means followed by the same letter in a column do not differ by Duncan’s test at 5 % probability
3 RESULTS AND DISCUSSION
Here, we evaluated 10 sorghum genotypes for 
drought tolerance collected from different parts of Iran 
and kept at The National Plant Gene-Bank of Iran, SPII. 
Natural genetic diversity may play an important role 
in food security through pre-breeding programs or the 
introduction of important traits or genes into existing 
cultivars (Priyanka et al., 2021). As well, the efficacy of 
drought stress indices for screening of these genotypes 
was scrutinized. Using the yield values of Y
p
 and Y
s
, vari-
ous indices were calculated (Table 4) and the genotypes 
were ranked for each index (Table 5). 
3.1 MORPHOLOGICAL TRAITS
Significant differences were observed between ir-
rigation regimes for all traits (p < 0.01 for PH, SD, and 
DMY; p < 0.05 for PL and 1000 SM) except NoL. There 
were significant differences between genotypes for all 
traits (p < 0.01) indicating significant variation among 
the genotypes. The interaction effect of deficit irrigation 
× genotype was significant (p < 0.01) except for NoL and 
1000 SM.

Acta agriculturae Slovenica, 117/4 – 2021 5
Introduction of the best criterion for evaluation of tolerance to drought stress in sorghum’s genotypes
The average PH reduced 13.2 % and 20.8 % under 
mild and severe deficit irrigation, respectively (Table 3). 
The reduction ratios for PL were 7 % and 15 % under 
mild and severe deficit irrigation, respectively (Table 3). 
Ashraf and Foolad (2007) indicated a reduction in tur-
gidity and cell growth and development under water 
shortage observed as a reduction in PH or panicle size. 
A significant effect of drought stress on the PH of for-
age sorghum has also been demonstrated (Mutava et al., 
2011).
Stem diameter (SD) decreased 7.2 % and 10.6 % 
under mild and severe stress, respectively (Table 3). Our 
results were in line with Almodares et al. (2013) dem-
onstrating that the stem diameter of sorghum decreased 
proportionally to water deficit intensity. The reduction 
of SD in sugarcane under water deficit has been proven 
(Silva et al., 2008). Controversial results were reported 
by other studies pointing out no reduction in SD under 
drought stress (Almodares et al., 2013; Fracasso et al., 
2016; Ottman et al., 2001). The 1000 SM reduction was 
9.8 % from normal irrigation to severe deficit irrigation 
(Table 3). Deficit irrigation resulted in a notable fall in 
DMY equal to 79.9 % and 77.1 % under mild and severe 
stress, respectively (Table 3). 
Genotypes TN-04-79 and TN-04-142 exhibited the 
highest PH value and genotypes KGS23 and TN-04-129 
were the lowest in PH (Table 3). Generally, taller sor-
ghum genotypes are favored for small-scale farms that 
mechanical harvests are not employed (Devnarain et al., 
2016). Genotypes TN-04-90 and TN-04-59 had the high-
est PL and genotypes TN-04-79 and TN-04-142 ranked 
the last (Table 3). Different values of SD were obtained 
with the highest value for the genotype TN-04-129 (Ta-
ble 3). The highest NoL belonged to TN-04-134 and TN-
04-142. Genotypes TN-04-79 and TN-04-90 ranked the 
highest and the lowest 1000 SM, respectively (Table 3). 
The genotype TN-04-142 produced the highest DMY. 
There were no significant differences between MGS2, 
KGS23, TN-04-59, TN-04-86, and TN-04-90 in DMY as 
the lowest rank (Table 3).
3.2 GRAIN YIELD, HARVEST INDEX, AND W A-
TER PRODUCTIVITY (WP)
The effects of deficit irrigation, genotype, and their 
interaction on yield, HI, and WP were significant (p < 
0.01). Grain yield and HI decreased significantly in re-
sponse to water deficit, resulting in lower values equal to 
52.8 % and 28.9 %, respectively (Table 3). The mean of 
WP under severe deficit irrigation was reduced by 31.7 % 
compared to normal irrigation (Table 3). Chimonyo et 
al. (2016) reported no significant reduction in sorghum 
yield under deficit irrigation in comparison to full irriga-
tion (3160 kg ha
-1
 vs. 3240 kg ha
-1
), indicating sorghum 
as drought tolerant, which is suitable for marginal lands. 
However, our results noted that sub-optimal irrigation 
resulted in sub-optimal WP. Hence, an important point 
to farmers is the benefit of irrigating sorghum consider-
ing the water supply. 
The highest grain yield and WP under normal irri-
gation belonged to genotype TN-04-79 followed by TN-
04-134 (Fig. 2). Under mild deficit irrigation, genotype 
TN-04-79 had the highest grain yield and WP, because 
WP of this genotype under mild deficit irrigation was 
slightly higher (not statistically significant) than that val-
ue under normal irrigation. The highest values of grain 
yield and WP, when severe deficit irrigation was im-
posed, were related to genotype KGS23 followed by TN-
04-79 and TN-04-86. Moreover, the highest WP obtained 
for genotype TN-04-79 under mild deficit irrigation and 
the lowest value of WP belonged to genotype TN-04-142 
under severe stress (Fig. 2). 
It has been reported that sorghum WP was in a 
range of 1.24-1.34 kg m
-3
 in Nebraska under normal ir-
rigation (Maman et al. 2003). Grain WP in the trial of 
Hadebe et al. (2020) was relatively lower in a range of 
0.75-1.1 kg m
-3 
for three different genotypes. Moreover, 
they attributed high WP under irrigation to high yield 
proportional to water applied in the field. The effect of 
genotype, duration, and extent of water stress may ac-
count for the variation of results in this study with those 
of other studies.
3.3 V ALUES AND RANKS OF DROUGHT INDICES 
Mean comparison of ranking values (R), ranking 
mean values (R’) and rank sum (RS) under mild deficit 
irrigation showed that genotype TN-04-79 performed 
superiorly except for TOL (ranked 3) (Table 5). The su-
perior genotype based on the TOL index was genotype 
TN-04-129. On the other hand, genotype TN-04-142 
performed inferiorly based on Y
p
, Y
s
,
 
and all drought tol-
erance indices except for TOL ranked 8 (Table 5). A dif-
ferent trend in the response of the genotypes to severe 
deficit irrigation was observed. While genotype TN-04-
79 performed superiorly based on Y
p
, MP , STI, GMP , and 
HM, genotype KGS23 was superior when considering Y
s
, 
TOL, SSI, YSI, YRR, and YI indices (Table 5). 
 It could be concluded that different drought-tol-
erance indices presented herein introduced different 
genotypes as drought tolerant. Similar results have been 
reported for the screening of drought-tolerant genotypes 
based on various indices (Nikneshan et al., 2019; Abd El-
Mohsen et al. 2015). Therefore, the selection of tolerant 

Acta agriculturae Slovenica, 117/4 – 2021 6
L. NAZARI et al.
genotypes was adopted by the ranking method based on 
ranking mean values (R’), standard deviation of ranks 
(SDR), and rank sum (RS). 
According to values of R’ and RS calculated based 
on the yield under normal irrigation and mild deficit 
irrigation, genotype TN-04-79 exhibited the first mean 
rank value and sum rank value followed by genotype TN-
04-129 indicating that these genotypes can be primarily 
categorized as the most tolerant to mild deficit irrigation. 
Whilst, genotype TN-04-142, MGS2, and TN-04-78 ex-
hibited the worst mean rank and rank sum, respectively, 
that can be considered as the most susceptible to mild 
water deficit irrigation (Table 5). On the other hand, R’ 
and RS calculated based on yield in normal irrigation 
and severe deficit irrigation presented different results. 
The first mean rank and sum rank value belonged to gen-
Figure 2: Grain yield and water productivity (WP) of 10 grain sorghum genotypes under normal irrigation, mild and severe defi-
cit irrigation. Means followed by the same letter are not significantly different in each level of irrigation treatment (Duncan’s test, p 
< 0.05)
otype KGS23, while genotype TN-04-142 was inferior in 
R’ and RS (Table 5).
3.4 CORRELATIONS AMONG DROUGHT INDI-
CES
Pearson’s correlation coefficients (r) between Y
p
, Y
s
, 
and the indices were determined to select the best indices 
for the screening of drought-tolerant genotypes (Fig. 3). 
A positive significant correlation between Y
p
 and Y
s
 un-
der mild and severe deficit irrigation was recorded (Fig. 
3). This may imply that high yielding potential under 
normal irrigation is necessarily accompanied by reasona-
ble yield under mild and severe deficit irrigation. Similar 

Acta agriculturae Slovenica, 117/4 – 2021 7
Introduction of the best criterion for evaluation of tolerance to drought stress in sorghum’s genotypes
genotype
Calculated based on yield under normal and mild deficit irrigation
Y
p
Y
S
TOL MP SSI STI GMP HM YSI YRR YI
MGS2 4.39±0.36 2.67±0.28 1.71±0.32 3.53±0.28 0.74±0.10 0.34±0.06 3.42±0.28 3.32±0.29 0.61±0.06 0.39±0.06 0.97±0.10
KGS23 5.58±0.27 4.28±0.54 1.31±0.28 4.93±0.40 0.45±0.12 0.70±0.12 4.88±0.43 4.84±0.45 0.76±0.06 0.24±0.06 1.55±0.20
TN-04-78 6.07±0.69 2.70±0.5 3.38±0.73 4.38±0.48 1.04±0.15 0.48±0.12 4.03±0.49 3.72±0.53 0.45±0.08 0.55±0.08 0.98±0.18
TN-04-79 8.18±0.57 7.30±0.32 0.88±0.39 7.74±0.42 0.20±0.08 1.75±0.18 7.73±0.41 7.71±0.41 0.89±0.04 0.11±0.04 2.65±0.12
TN-04-129 5.98±0.45 5.23±0.36 0.75±0.78 5.60±0.12 0.23±0.23 0.91±0.03 5.58±0.11 5.56±0.10 0.88±0.12 0.12±0.12 1.89±0.13
TN-04-134 7.91±0.23 4.06±0.62 3.85±0.47 5.99±0.40 0.92±0.13 0.94±0.17 5.66±0.49 5.35±0.58 0.51±0.07 0.49±0.07 1.47±0.22
TN-04-142 3.47±0.60 0.72±0.11 2.76±0.50 2.10±0.35 1.50±0.03 0.07±0.02 1.58±0.25 1.19±0.19 0.21±0.01 0.79±0.01 0.26±0.04
TN-04-59 6.68±0.29 4.91±0.19 1.78±0.42 5.79±0.12 0.50±0.10 0.96±0.04 5.72±0.12 5.65±0.12 0.74±0.05 0.26±0.05 1.78±0.07
TN-04-86 5.45±0.63 4.69±0.43 0.76±0.23 5.07±0.52 0.26±0.05 0.75±0.16 5.06±0.52 5.04±0.51 0.86±0.03 0.14±0.03 1.70±0.16
TN-04-90 4.75±0.11 3.71±0.37 1.03±0.28 4.23±0.23 0.41±0.12 0.52±0.06 4.20±0.25 4.16±0.26 0.78±0.06 0.22±0.06 1.35±0.13
genotype
Calculated based on yield under normal and severe deficit irrigation 
Y
p
Y
s
TOL MP SSI STI GMP HM YSI YRR YI
MGS2 4.39±0.36 1.67±0.19 2.72±0.55 3.03±0.09 1.16±0.15 0.21±0.01 2.70±0.05 2.41±0.14 0.39±0.08 0.61±0.08 0.61±0.07
KGS23 5.58±0.27 4.19±0.37 1.39±0.52 4.89±0.19 0.47±0.16 0.68±0.06 4.83±0.21 4.78±0.24 0.75±0.08 0.25±0.08 1.52±0.13
TN-04-78 6.07±0.69 1.70±0.28 4.37±0.72 3.89±0.39 1.35±0.10 0.30±0.07 3.21±0.34 2.65±0.36 0.28±0.05 0.72±0.05 0.62±0.10
TN-04-79 8.18±0.57 4.07±0.47 4.11±0.62 6.13±0.42 0.95±0.11 0.97±0.15 5.76±0.44 5.42±0.48 0.50±0.06 0.50±0.06 1.47±0.17
TN-04-129 5.98±0.45 1.71±0.18 4.27±0.61 3.84±0.16 1.34±0.09 0.30±0.02 3.19±0.09 2.65±0.18 0.29±0.05 0.71±0.05 0.62±0.06
TN-04-134 7.91±0.23 2.95±0.10 4.96±0.23 5.43±0.13 1.18±0.03 0.68±0.03 4.83±0.12 4.30±0.12 0.37±0.01 0.63±0.01 1.07±0.04
TN-04-142 3.47±0.60 0.48±0.02 2.99±0.59 1.98±0.31 1.62±0.04 0.05±0.01 1.29±0.13 0.85±0.04 0.14±0.02 0.86±0.02 0.17±0.01
TN-04-59 6.68±0.29 3.55±0.28 3.13±0.50 5.12±0.13 0.88±0.11 0.69±0.05 4.87±0.16 4.63±0.21 0.53±0.06 0.47±0.06 1.29±0.10
TN-04-86 5.45±0.63 3.91±0.31 1.54±0.66 4.68±0.37 0.52±0.18 0.62±0.09 4.61±0.34 4.54±0.32 0.72±0.10 0.28±0.10 1.42±0.11
TN-04-90 4.75±0.11 3.35±0.22 1.39±0.30 4.05±0.09 0.55±0.11 0.46±0.03 3.99±0.11 3.93±0.13 0.71±0.06 0.29±0.06 1.21±0.08
Table 4: Mean values ± standard deviation of grain yield (ton/ha) and drought tolerance indices of ten sorghum genotypes under normal irrigation, mild and severe water deficit 
irrigation
Yp = grain yield under normal irrigation, Ys = grain yield under deficit irrigation, TOL = tolerance index, MP = mean productivity, SSI = stress susceptibility index, STI = stress tolerance index, GMP = 
geometric productivity, HM = harmonic mean of yield, YSI = yield stability index, YRR = yield reduction ratio, YI = yield index

Acta agriculturae Slovenica, 117/4 – 2021 8
L. NAZARI et al.
Mild deficit irrigation
R
R’ SDR RS
Genotype Y
p
Y
s
TOL MP SSI STI GMP HM YSI YRR YI
MGS2 9 9 6 9 7 9 9 9 7 7 9 8.18 1.17 9.35
KGS23 6 5 5 6 5 6 6 6 5 5 5 5.45 0.52 5.98
TN-04-78 4 8 9 7 9 8 8 8 9 9 8 7.91 1.45 9.36
TN-04-79 1 1 3 1 1 1 1 1 1 1 1 1.27 0.65 1.92
TN-04-129 5 2 1 4 2 4 4 3 2 2 2 2.55 1.29 3.84
TN-04-134 2 6 10 2 8 3 3 4 8 8 6 5.55 2.73 8.28
TN-04-142 10 10 8 10 10 10 10 10 10 10 10 9.82 0.60 10.42
TN-04-59 3 3 7 3 6 2 2 2 6 6 3 4.00 1.84 5.84
TN-04-86 7 4 2 5 3 5 5 5 3 3 4 4.18 1.40 5.58
TN-04-90 8 7 4 8 4 7 7 7 4 4 7 6.09 1.70 7.79
Severe deficit irrigation
R
R’ SDR RS
Genotype Y
p
Y
s
TOL MP SSI STI GMP HM YSI YRR YI
MGS2 9 9 4 9 6 9 9 9 6 6 9 7.73 1.85 9.58
KGS23 6 1 1 4 1 3 4 2 1 1 1 2.27 1.74 4.01
TN-04-78 4 8 9 7 9 7 7 7 9 9 8 7.64 1.50 9.14
TN-04-79 1 2 7 1 5 1 1 1 5 5 2 2.82 2.23 5.05
TN-04-129 5 7 8 8 8 8 8 8 8 8 7 7.55 0.93 8.48
TN-04-134 2 6 10 2 7 4 3 5 7 7 6 5.36 2.46 7.82
TN-04-142 10 10 5 10 10 10 10 10 10 10 10 9.55 1.51 11.05
TN-04-59 3 4 6 3 4 2 2 3 4 4 4 3.55 1.13 4.67
TN-04-86 7 3 3 5 2 5 5 4 2 2 3 3.73 1.62 5.35
TN-04-90 8 5 2 6 3 6 6 6 3 3 5 4.82 1.83 6.65
Table 5: Ranking values (R), ranking mean values (R’), standard deviation of ranks (SDR) and rank sum (RS) of grain yield of ten sorghum genotypes under normal irrigation 
and water deficit irrigation after 120 mm evaporation from Pan class A
Yp = grain yield under normal irrigation, Ys = grain yield under deficit irrigation, TOL = tolerance index, MP = mean productivity, SSI = stress susceptibility index, STI = stress 
tolerance index, GMP = geometric productivity, HM = harmonic mean of yield, YSI = yield stability index, YRR = yield reduction ratio, YI = yield index

Acta agriculturae Slovenica, 117/4 – 2021 9
Introduction of the best criterion for evaluation of tolerance to drought stress in sorghum’s genotypes
results of the wheat response to drought were previously 
recorded by Abebe et al. (2020).
While there was a significant correlation between 
Y
s
 (mild and severe deficit irrigation) and all the indi-
ces, there was no correlation between Y
p
 and SSI, YSI, 
and YRR. There was no correlation between Y
p
 and TOL 
under mild deficit irrigation. On the other hand, a posi-
tive significant correlation was obtained between Y
p
 and 
TOL calculated based on yield in severe deficit irrigation 
suggesting that selection based on the low score of TOL 
may lead to enhanced yield under severe deficit irriga-
tion but reduced yield under normal irrigation (Fig. 3). 
Also, yield in all irrigation treatments was significantly 
and positively correlated with MP, STI, GMP, HM, and 
YI (Fig. 3). Thus, it can be concluded that these indices 
were more efficient in the selection of genotypes with 
high yield potential under different water conditions. 
Indices being significantly correlated with grain 
yield under both normal irrigation and water deficit ir-
rigation are suitable for the screening of genotypes (Mi-
tra, 2001). Therefore, indices MP , STI, GMP , HM, and YI 
which were positively correlated with both Y
p
 and Y
s
 at p 
≤ 0.01 (Fig. 3) may be considered as better predictors of 
yield in different irrigation. As well, sorghum genotypes 
with high values of MP, STI, GMP, HM, and YI can be 
thus regarded as drought tolerant. Our results are some-
what in agreement with those findings of Nouri et al. 
(2011) and Golabadi et al. (2006) who found a correla-
tion between either Y
s
 or Y
p
 and MP , GMP , and STI.
A perfect positive correlation (r = 1) was noted be-
tween Y
s
 and YI and between SSI and YRR. On the other 
hand, a perfect negative correlation (r = -1) was noted 
between SSI and YSI, YSI, and YRR in both mild and se-
vere water deficit irrigation (Fig. 3). A similar finding was 
recorded by Mickky et al. (2019) who evaluated 10 wheat 
cultivars based on drought tolerance indices under nor-
mal irrigation (Y
p
) and deficit irrigation (Y
s
).
3.5 CLUSTER ANALYSIS
Classification of genotypes according to Y
p
, Y
s
, and 
various indices under normal irrigation and mild defi-
cit irrigation categorized 10 sorghum into three groups; 
group 1 including MGS2, TN-04-90, TN-04-78 and TN-
04-14; group 2 including KGS23, TN-04-86, TN-04-12, 
TN-04-99 and TN-04-13; and group 3 including geno-
type TN-04-79 (Fig. 4a). Clustering based on yield and 
drought tolerance indices under normal irrigation and 
mild deficit irrigation grouped the genotypes into toler-
ant, semi-tolerant/susceptible, and susceptible. The first 
group with the lowest value of R’ and RS (TN-04-79) can 
be distinguished as tolerant to mild deficit irrigation. The 
second group had mean values of R’ (2.75-5.67) and RS 
(3.97-8.44) considered as semi-sensitive/tolerant and the 
third group with higher R’ and RS was the most suscepti-
ble genotypes to mild deficit irrigation (Fig. 4 a).
Fig. 3: Heat map based on the actual values of indices (Pearson’s correlation analysis) across 10 sorghum genotypes produced us-
ing iPASTIC online tool kit. Y
p
, yield under normal irrigation; Y
s
, yield under mild deficit irrigation for (a) and under severe deficit 
irrigation for (b); TOL, tolerance index; MP , mean productivity, GMP , geometric mean probability; HM, Harmonic mean; SSI, 
stress susceptibility index; STI, stress tolerance index; YI, yield index; YSI, yield stability index; RSI, relative stress index

Acta agriculturae Slovenica, 117/4 – 2021 10
L. NAZARI et al.
Fig. 4: Dendrograms of the cluster analysis and similarity coefficients among 10 sorghum genotypes based on Y
p
, Y
s
, and the 
drought tolerance indices under normal irrigation and mild deficit irrigation (a) and under normal irrigation and severe deficit 
irrigation (b)
On the other hand, three different clusters were 
observed based on Y
p
, Y
s
, and drought tolerance indices 
under normal irrigation and severe deficit irrigation (Fig. 
4b). Genotypes MGS2, TN-04-14, TN-04-78, and TN-
04-12 were classified into group 1; KGS23, TN-04-86, 
and TN-04-90 into group 2; and TN-04-79, TN-04-13, 
and TN-04-59 into group 3. The first and second groups 
included the genotypes with the lowest to medium values 
of R’ and RS and thus were considered to be tolerant or 
semi-tolerant. The genotypes of the third group had the 
highest values of R’ and RS indicating the most suscepti-
ble to severe deficit irrigation. Cluster analysis has been 
extensively employed for the determination of genetic 
diversity and classification of genotypes under various 
abiotic stresses (Golabadi et al. 2006; Mohammadi et al. 
2011).
3.6 PRINCIPAL COMPONENT ANALYSIS (PCA) 
AND BIPLOT 
 The PCA results revealed that the first two prin-
cipal components accounted for 98.51 % (PC1:81.03 %, 
PC2:4.63 %) of the total variation in yield performance 
and nine yield-based indices calculated under normal ir-
rigation and mild deficit irrigation. Biplot showed that 
the PC
1
 was positively correlated with yield (Y
p
 and Y
s 
under mild stress) and all indices except TOL and SSI, 
whereas PC
2
 was positively correlated with yield (Y
p
 and 
Y
s
 under mild deficit irrigation) and all indices excluding 
RSI and YSI (Fig. 5a).
On the other hand, the PCA biplot for yield (Y
p
 and 
Y
s
 under severe deficit irrigation) and drought tolerance 
indices of sorghum genotypes was reflecting 99.28 % 
(PC
1
:74.6 %, PC
2
:24.63 %) of the total variability in data 
(Fig. 5b). The biplot categorized the indices into three 
groups (Fig. 5). The first group was those with high PC
1
 
and PC
2
 (Y
p
, MP , STI, GMP , HM, YI, and Y
S
 in Fig. 5a and 
Y
p
, MP , STI, GMP , and HM in Fig 5b). The second group 
was indices with low PC
1
 but high PC
2 
(SSI and TOL) 
(Fig. 5a,b) and the third group were those with high PC
1
 
but low PC
2
 including RSI (Fig. 5a,b) and Y
s
 and YI (Fig. 
5b). 
The cosine of the angle between the vectors of any 
two indices in a biplot is an indicator of the correlation 
coefficient. Therefore, we can note that those indices 
whose vector has been placed between the vectors of Y
p
 
and Y
s
 are appropriate for the selection of drought-toler-
ant genotypes. It can be implied that MP , GMP , STI, HM, 
and YI allocating between Y
p
 and Y
s
 are the best indices to 
distinguish tolerant from susceptible genotypes. Herein, 
the results obtained from PCA (Fig. 5) confirmed those 
obtained from correlation coefficients (Fig. 3).
The results of our study showed that TN-04-79 and 
TN-04-59 are tolerant genotypes with the highest values 
for the MP , GMP , STI, and HM indices, while genotypes 
KGS23, TN-04-129, TN-04-86, and TN-04-90 under 
mild stress and genotypes KGS23, TN-04-86, TN-04-90 
with the highest values for YSI and RSI were the most 

Acta agriculturae Slovenica, 117/4 – 2021 11
Introduction of the best criterion for evaluation of tolerance to drought stress in sorghum’s genotypes
stable genotypes (Table 4). Introduction of these tolerant 
genotypes into the sorghum breeding programs may be 
suggested to policymakers to release new cultivars toler-
ant to drought stress. It has been noted that increasing 
harvest index can improve yield stability (Kashiwagi et 
al., 2015). The reduction of grain yield under deficit irri-
gation could lead to a lower harvest index. We also found 
that indices including MP, GMP, HM, STI, YI, and YSI 
are strongly correlated with sorghum yield. Thus, these 
drought-tolerant indices should benefit the breeders in 
breeding programs. 
4 CONCLUSIONS
In the current study, eight sorghum genotypes col-
lected from different parts of Iran along with two prom-
ising lines reported as drought-tolerant were compared 
in terms of response to deficit irrigation. The grain yield 
and water productivity of the genotypes were signifi-
cantly influenced by water deficit irrigation. The relative 
efficacy of selection indices could be an advantage using 
two or more traits simultaneously than using single traits 
independently. Thus, indices including MP, STI, GMP, 
HM, and YI, highly correlated with Y
p
 and Y
s
, may be 
more suitable for the selection of drought-tolerant geno-
types. Screening of tolerant genotypes to water deficit ir-
rigation using the ranking method and cluster analysis 
discriminated genotypes as the most tolerant, semi-tol-
erant/sensitive, and susceptible. Therefore they are rec-
ommended to be used in breeding programs as parents 
for improvement of drought tolerance in commercial 
cultivars. Further evaluation of these genotypes based 
on drought indices across multiple locations and years 
is still demanded to validate their stability for developing 
sorghum cultivars.
5 ACKNOWLEDGMENTS
The present work was supported by Agricultural 
Organization of Fars [Project number 24-50-03-332-
961709].
6 ABBREVIATIONS 
ANOV A: Analysis of variance; PCA: principal com-
ponent analysis; PH: plant height; PL: panicle length; SD: 
stem diameter; NoL: number of leaves per plant; 1000 
SM: 1000 seed mass; DMY: dry matter yield; HI: harvest 
index; WP: Water productivity; Y
p
: grain yield under 
normal irrigation; Y
s
: grain yield under deficit irrigation; 
MP: mean productivity; TOL: tolerance index; SSI: stress 
susceptibility index; STI: stress tolerance index; GMP: 
geometric productivity; HM: harmonic mean of yield; 
Fig. 5: Principal components (PC) analysis based on the correlation matrix of yield under normal irrigation (Y
p
) and yield under 
mild deficit irrigation (a) and severe deficit irrigation (b) (Y
s
) and nine tolerance and susceptibility indices calculated using 
iPASTIC online tool kit. Y
p
, yield under normal irrigation; Y
s
, yield under mild deficit irrigation for (a) and under severe deficit ir-
rigation for (b); TOL, tolerance index; MP , mean productivity, GMP , geometric mean probability; HM, Harmonic mean; SSI, stress 
susceptibility index; STI, stress tolerance index; YI, yield index; YSI, yield stability index; RSI, relative stress index.

Acta agriculturae Slovenica, 117/4 – 2021 12
L. NAZARI et al.
YSI: yield stability index; YRR: yield reduction ratio; YI: 
yield index.
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