Drought is the most restricting factor in agricultural productions in arid and semi-arid regions. This research was conducted to study 19 facultative and winter wheat genotypes under normal irrigation (N), anthesis (S1) and post-anthesis (S2) drought conditions using randomized complete block design with three replications at Karadj, Arak and Jolgehrokh Agricultural Research Stations in Iran in 2008-11 cropping seasons. Based on results, drought stress reduced kernel number per spike, spike weight, thousand kernel weight, grain weight per spike, harvest index, total biomass weight and grain yield. However, remobilization, efficiency of remobilization and pre-anthesis photoassimilate contribution to grain (PAPCG) were increased under drought stress condition. Effect of environment, irrigation and genotype on most of the traits including grain yield, was significant. Correlation coefficients between grain yield and remobilization, efficiency of remobilization and PAPCG were significantly positive under N, S1 and S2 conditions. Based on MP, GMP, STI, SSI and TOL, genotype 6 was determined as the most tolerant genotype. Among the genotypes, genotype 6 had the highest value for remobilization, efficiency of remobilization and PAPCG under drought stress condition, therefore selected as the most stable and tolerant line under mild and severe drought stress.
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Key words: Bread wheat, Grain yield, Drought stress,
Drought stress as an important environmental phenomenon adversely affects performance and yield of cereals (Rang et al., 2011). In Iran, drought stress in irrigated wheat fields due to no adequate access to underground water resources (Mohammadi and Karimpour Reihan, 2008) occurs mainly during terminal growth stages of wheat (Jalal Kamali et al., 2009) which intensify crop failure and economic loss to farmers. According to the Emberger definition, Iran with annual average precipitation of 240 mm is located in the semi-arid and arid areas of the world (Kardavani, 1988). In fact substantial portions of 2.4 million ha of irrigated wheat in Iran suffer from the shortage of irrigation water especially in post-anthesis or through grain filling period (Jalal Kamali et al., 2009). Drought stress occurring in vegetative stages of crop development decreases plant height, leaf area, number of tillers and biomass (Nouri et al., 2011), while reproductive stages of crop development in cereal crops, including wheat (Triticum aestivum L.), are more sensitive to drought stress than vegetative stages (Shpiler and Blum, 1986). Abiotic stresses such as drought at anytime of crop development, decrease leaf chlorophyll and photosynthesis, and hasten senescence (Dulai et al., 2006). Wheat crop failure in temporary drought conditions may be compensated later by some yield components depending mainly on genotype. So, the best option to attain stable production under drought stress condition would be to develop drought tolerant genotypes through physiological approaches which need a deeper understanding of the yield determining traits and processes. But, the multitude of traits involved in wheat response to drought stress makes it difficult to provide a complete test of drought tolerance. Hence, the drought tolerance in wheat can be improved through defining drought problem of the target area, identifying drought tolerance traits and developing selection criteria to propose ideotypes.
Drought stress at anthesis decreases seed set in wheat by inducing pollen sterility (Ji et al., 2010). Drought stress often occurs during the grain filling period of a wheat crop development stage causing severe yield loss in most of the wheat growing areas of the world (Altenbach et al., 2003). The grain filling period is also highly sensitive to drought stress (Yang et. al., 2001). Drought at the grain filling period also decreases individual grain weight but the decrease is often due to decrease in grain filling duration rather than decrease in grain filling rate (Wardlaw and Willenbrink, 2000).
Drought tolerance or susceptibility indices as measures or functions of yield reduction through comparison of genotypes in drought and normal conditions have been used to screen drought tolerant or susceptible genotypes (Mitra, 2001; Fernandez, 1992; Blum, 1996). Hall (1993) defined drought tolerance as the comparative yield of genotypes exposed to similar drought stress. Under drought stress condition, current photosynthesis declines during grain filling period of wheat and consequently demands of remobilization increases (Blum,1998).
Rosielle and Hamblin (1981) presented stress tolerance (TOL) as the differences in yield between the stress (YS) and supplementary irrigation (YP) environments, and mean productivity (MP) as the average yield of YS and YP. Fischer and Maurer (1978) suggested stress susceptibility index (SSI) for wheat cultivars. Fernandez (1992) defined a new advanced index of stress tolerance index (STI) to identify high yielding genotypes under both stress and supplementary irrigation conditions. Geometric mean productivity (GMP) and mean productivity (MP) are two other yield based measures of drought tolerance. Breeders which often use GMP, are interested in relative performance of a genotype due to the variation in drought stress severity in field environment over years (Ramirez and Kelly, 1998). Clarke et al. (1992) used SSI to evaluate wheat genotypes for drought tolerance and found year-to-year variation in SSI for genotypes and their ranking pattern.
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The objective of this study was to evaluate effect of drought stress on grain yield and some morpho-physiological traits and select the superior genotypes.
Materials and Methods
To evaluate post-anthesis drought stresses tolerance in wheat genotypes, this research was conducted at Karadj, Arak and Jolgehrokh agricultural research stations in Iran in 2008-11 cropping seasons. The experimental design was split-plot based on RCB with three replications in which irrigation treatments [normal irrigation (N), irrigation was cut-off during post-anthesis (S1) and irrigation was cut off 20 days after anthesis (S2)] were as main-plots and 19 winter and facultative wheat genotypes as sub-plots. The pedigrees of the wheat genotypes are given in Table 1. The plot size for each genotype was 7m2 (1.2 m x 6 m) which sown by an experimental seed planter (WintersteigerTM) using 450 seeds m-2 with the final harvest area of 6m2 (1.2 m x 5 m ) by an experimental plot combine harvester (WintersteigerTM). The experimental field was under two years cereal-fallow rotation and the soil preparations included stubble mulch fall tillage and next spring tillage with moldboard plow, disking, two times perpendicular land leveling, fertilizer spreading and making beds as furrows. Application of fertilizers were according to the soil test recommendations with the NPK formula of 120-90-50 kg ha-1 applying urea (equally base and top-dressed), potassium sulfate and di-ammonium phosphate sources. The irrigation treatments in three years of experiments were the same for all the main plots as one time irrigation in fall after planting but different in spring times i.e. 6, 2 and 4 irrigations for N, S1 and S2 treatments, respectively. All the necessary field managements and data collections were done throughout the growing seasons. To evaluate and study the morpho-physiological traits in genotypes, in three growth stages i.e. at anthesis, 20 days after anthesis and physiological maturity, plant samples were randomly taken from the plots including 20 complete stems (consisting leaves and the spike). The samples were dried in a forced air oven for 72 hours at 70 Â° C and then the traits such as total dry matter weight, spike weight, peduncle unit length weight, kernel number per spike, thousands kernel weight, kernel weight per spike and harvest index were measured. The traits of remobilization from stem to the grains and efficiency of remobilization were estimated as follows:
(Ehdaie, 1998; Kobata et al., 1992) (1)
SaGR: Stem assimilate to grain remobilization.
SdWA: Stem dry weight in post-anthesis.
SdWM: Stem dry weight in maturity.
(Palta et al., 1994)
SGRE: Stem to grain reserve remobilization efficiency
CGR: Culm to grain remobilization.
SWA: Stem dry weight in post-anthesis
(Niu et al., 1998)
Pre-anthesis photoassimilate contribution to grain = (PAPCG)
CGR: Culm to grain remobilization
GWM: Grain weight at maturity.
After data collection, the combined ANOVA for three conditions (N, S1 and S2) was carried out to determine main effects of irrigation, genotype and their interaction on the studied traits. To evaluate drought tolerance of the genotypes, the indices of tolerance (TOL), mean productivity (MP) (Rossielle and Hamblin, 1981), stress susceptibility index (SSI) (Fisher and Maurer, 1978), stress tolerance index (STI) and geometric mean productivity (GMP) (Fernandez, 1992) were used. The correlation coefficients between the traits in normal (N) and drought stress conditions (S1 and S2) were also calculated.
Table 1. Name or pedigree of genotypes.
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Results and Discussion
Results of combined ANOVA showed that main effects of environment, genotype and irrigation were significant on grain yield and most of the traits (Table 2). However among the interactions, only interaction effect of genotype x environment was found significant on grain yield. The mean grain yield of 19 genotypes under normal irrigation (N), and anthesis (S1) and post-anthesis (S2) drought stress conditions were 5936, 4139 and 5162 kg ha-1, respectively (Table ?). As the results show, genotypes significantly produced less grain yield under anthesis (S1) drought stress than post-anthesis (S2) drought stress and normal irrigation (N) conditions. These findings are not in agreement with the results of Calhoun et al. (1994) and van Ginkel et al. (1998) who reported a higher grain yield under early drought than late drought stress conditions. The reason for lower grain yield under anthesis rather than post-anthesis drought stress conditions was mainly due to a reduction in 1000 kernel weight which determined after anthesis and grain number per spike which determined before and after anthesis. Thousand kernel weight under N, S1 and S2 drought stress conditions was 37.8, 29.8 and 34.5; and number of grain per spike was 40.8, 37.7 and 38.9, respectively. These results were consistent with results of Inness et al. (1981) and Plaut et al. (2004). Plaut et al. (2004) also reported that 1000 kernel weight was more severely decreased by water deficit than by heat stress in wheat varieties, i.e. the rate of dry matter accumulation by kernels was considerably decreased by water deficit. Genotype no. 6 significantly had the highest grain yield in both stress conditions with 4737 and 5713 kg ha-1, respectively. However, genotype no. 15 produced the highest grain yield (6265 kg ha-1) in normal irrigation condition.
The drought stress intensities were 0.30 and 0.13 under anthesis (S1) and post-anthesis (S2) drought stress conditions, respectively, i.e. applied drought stress at anthesis was more severe than in post-anthesis. Mean of all the traits were reduced in stressed conditions (S1 and S2) compared to normal irrigation. Moreover, the reduction was higher in S1 than S2 for all traits except for PWA and PWM which the means remained the same in both stressed conditions and for BWA which the mean value was slightly and non-significantly higher in S1 than S2.
Pre-anthesis photoassimilate contribution to grain (PAPCG) in N, S1 and S2 was estimated 14.3, 25 and 19.1%, respectively which indicated higher remobilization in severe stress condition (S1). In all three conditions, genotype no. 6 had the highest PAPCG.
Results of susceptibility and tolerance indices at anthesis (S1) and post-anthesis (S2) drought stress conditions are presented in table . Genotype 6 was the highest yielding at S1 and S2 conditions with 4737 and 5713 Kg ha-1, respectively. This genotype had the best and absolute value of rank for all indices at S1 and S2 conditions, so that its mean rank () and standard deviation of rank (SDR) for MP, GMP, STI, SSI and TOL at both stress conditions (S1 and S2) were 1 and 0, respectively. Drought stress conditions resulted higher SaGR, SGRE and PAPCG than normal condition (Table ) in which S1 (severe stress) had higher value than S2 (mild stress). Among genotypes, the highest value of SaGR, SGRE and PAPCG in N, S1 and S2 conditions were belonged to genotype 6, so that 42.2, 30.1 and 21% of its grain yield resulted from SaGR, SGRE and PAPCG, respectively (Table ). However, Shahryar variety (no.1) was the lowest yielding and weakest in all susceptibility and tolerance indices at both S1 and S2 conditions (Table ).
The mean grain yield and some morpho-physiological traits of wheat genotypes under three conditions of anthesis, post-anthesis drought and normal (table ), genotype no.6 with grain yield of 5517 kg per hectare showed highest performance and explained by its highest scores in harvest index (46.3%), remobilization (0.38 g), remobilization efficiency (19.65%) and PAPCG (31.14%). There was a significant difference among genotypes in grain yield (Table ). Drought stress reduced grain yield so that the grain yield for normal, anthesis and post-anthesis stress were 5936, 5162 and 4139 kg ha-1, respectively. Grain yield reduction due to post-anthesis drought has been previously reported (Gooding et al, 2004; Ozturk and Aydin, 2003). Grain yield increment observed in wheat line no.6 seems to be related to its highest remobilization, PAPCG and harvest index in drought stress condition. Drought stress generally reduced harvest index (Ehdaei, 1993), while they were 43.35%, 38.8% and 44.8% in anthesis, post anthesis and normal conditions, respectively (Table ). The results of reduction in grain yield due to drought stress comparing to normal condition (Gooding et al., 2003; Ozturk and Aydin, 1993) was the same for all the measured characteristics of thousand grain weight (37.86, 34.48 and 29.8 g); total plant weight (3.44, 3.12 and 2.89 g); spike weight (2.2, 2 and 1.7 g); peduncle weight (0.27, 0.26 and 0.26 g); number of seeds per spike (40.8, 38.9 and 37.7) and seeds weight per spike (1.54, 1.34 and 1.13 g) in normal, anthesis and post-anthesis drought conditions, respectively.
According to the results there was a positive correlation between grain yield and most studied traits such as remobilization, efficiency and partitioning of the remobilized stem reserves to the grain in all three conditions. Papakosta and Gagianas (1991) also reported the same results of positive correlation between remobilized stem reserves and grain yield. The remobilized assimilates were 0.2, 0.24 and 0.26 g in each studied stem for normal, anthesis and post-anthesis drought conditions, respectively. Drought stress increased remobilization efficiency from 9.58% in normal conditions to 11.34% and 12.03% in post-anthesis and anthesis drought stress conditions which are in accordance with the results reported by Ehdaei et al. (2006). This increment due to the effect of drought stress was the same for partitioning of remobilized assimilates (14.29, 19.07 and 24.99%) in normal, anthesis and post-anthesis drought conditions, respectively and has been ported by Yang et al. (2000) and Yang and Zhang, 2006). Based on the above-mentioned findings, the highest values measured in studied characteristics in wheat line no.6 comparing with other genotypes resulted in less grain yield reduction in this genotype. Another advantage for the referred wheat line was more reliance on stem reserves for grain filling period especially in anthesis and post-anthesis drought conditions. Generally, drought stress causes less photosynthesis and more remobilization in grain filling period. So, efficient varieties in remobilization may have less grain loss in drought affected environments and more drought resistance (Niu et al., 1998; Yang et. al., 2000).
According to results of drought resistance and susceptibility indices, wheat line no.6 was the most drought resistant line, while its remobilization rate, efficiency and partitioning was highest amongst studied genotypes. It seems the stability and drought resistance observed in line no.6 can be attributed to its rate, efficiency and partitioning of remobilization.
Hatim and Majidian (2012) reported that grain yield was mainly influenced by TGW in studied wheat genotypes in both normal irrigation and water stress conditions, while HI was not significantly different in genotypes that may be due to the stated reasons of all being ideal genotypes with high HI and with minimum variation and significant differences in grain yield only in water stress condition.
Gupta et al., (2011) stated that mobilized dry matter and mobilization efficiency were higher in the internodes of tolerant cultivar, both under control and stress conditions, which boosted translocation of stem reserves to the grains. It is generally accepted that stem reserve mobilization or percentage of stem reserves in grains is affected by sink size, environment and cultivar (Blum, 1998). In other words the high amount of stem reserve in a variety does not necessarily mean that variety has a good remobilization in drought environment so the sink activity and demand for stem reserves are very much important characteristics in drought tolerant varieties. The mentioned parameters are indirectly observed and related to higher TGW, total plant weight (pre-anthesis biomass), spike, peduncle and seeds weight per spike) which are totally help decrease grain yield loss in wheat line no.6. It may be due to the capability of this genotype to synthesize and store higher concentration of soluble carbohydrates in the stems prior to anthesis (Conocono, 2002).
Zhang et al., (2013) believe that water deficit increase WSC accumulation and remobilization, remobilization efficiency, and contribution to grain yield in non-leaf organs.
Lopes et al., (2012) results suggested that if drought and heat adaptive traits are brought together in one genotype, yields can be further increased particularly in low yielding environments.
Results of calculated indices of MP, GMP, STI, SSI and TOL in the studied genotypes showed that line no.6 was the most drought resistant line, because the highest rate, efficiency and partitioning of remobilization belonged to this line. Thus, the stability and drought resistance of line no.6 may be associated to high values in its rate, efficiency and partitioning of remobilization. We concluded that wheat line no.6 (Alvd/Aldan/Ias58*2/3/Gaspard) as the most drought tolerant genotype with higher grain yield in both normal and drought stressed conditions and recommended to the farmers for onfarm experiments in both normal and anthesis and post anthesis drought stress environments.