Assessment of genetic potential and stress selection indices for important yield related traits in bread wheat ((Triticum aestivum L.)

Current wheat improvement programs focuses on development of new high yielding, disease resistant, stable and climatic resilient genotypes. Late planting is one of the major abiotic stresses, seriously influencing wheat production. In the current study, twenty seven bread wheat genotypes along with one check cultivar (Pirsabak-2008) were evaluated independently under normal (optimal) and late (stress) planting conditions at Cereal Crops Research Institute (CCRI), Pirsabak Nowshera Khyber Pakhtunkhwa Pakistan during 2013-14. Analysis of variance revealed highly significant (P < 0.01) differences among the genotypes, sowing dates and genotype × sowing dates interactions effects for the studied traits. Generally, reduction in plant height (0.41 to 10.91%) and grain yield (0.36 to 53.35%) was observed among the tested genotypes under late planting as compared to normal (optimal) planting. Least % reduction in grain yield was recorded for genotypes BWL-23 (0.36%), BWL-4(0.76%) and BWL-16 (1.22%) as compared to check (Pirsabak-2008). Eight stress selection indices i.e. Mean Productivity (MP), Tolerance (TOL), Geometric Mean Productivity (GMP), Harmonic mean (HM), Stress Selection Indices (SSI), Stress Tolerance Index (STI), Yield Index (YI) and Yield Stability Index (YSI) were determined for each genotype. Correlation analysis revealed that plant height and grain yield under both the planting conditions, had significant positive correlation with stress selection indices i.e. GM, HM, SSI and YI. These selection indices could be effective in identification of genotypes to late planting. Based on MP, GMP, HM, STI and YI genotypes i.e. Pirsabak-2008, BWL-23 and BWL-27 were found late planting stress tolerant and could be sown in both normal and late planting conditions.


Introduction
Wheat (Triticum aestivum L.) belongs to family poaceace and is one of the most important staple food of more than 36% of the world population.Its demand is increasing day by day due to ever-increasing population and its utilization in a variety of ways [1].Its production has been greatly affected by various biotic and abiotic stresses.Among the abiotic stresses, late planting and sudden variable climatic changes (less rainfall, drought, heat stress at sowing time and increase in temperature at the terminal stages of crop) are the major threats and seriously influencing wheat production.
Poor agronomic management practices i.e. unavailability of superior wheat genotypes for marginal environments, inefficient utilization of fertilizers and irrigation water etc are the key factors responsible for low wheat production.In Pakistan wheat being an important cereal crop its production is much better than the past but wheat yield is still behind compared to agricultural developed countries of the world.In Pakistan, late planting is among one of the major reason for low yield.Late planting adversely affects the growth, yield and quality of wheat.A delay of each day in sowing of wheat after 15th November onward decreases grain yield by 1%.Late sowing of wheat can reduce yield by 30 to 40% [2].Wheat cultivation at its proper time is thus of extreme importance to obtain high yield as proper sowing time accounts for 10% of grain variation [1].In the current scenario of climatic change to feed the over increasing population there is dire need to develop new climatic resilient cultivars that produced higher yield under variable climatic conditions especially wheat planted late by farmers.Exploration and identification of diverse climatic resilient wheat genotypes perform better under variable environmental condition (especially late planting or heat stress) is one of the main tasks of wheat breeders.To improve stress tolerance or identify stress tolerant lines in wheat several selection indices have been used by wheat breeders, when genotypes are evaluated under normal and stress conditions [3,5].Tolerance (TOL) and mean productivity (MP), stress susceptibility index (SSI [6], geometric mean productivity (GMP) and stress tolerance index (STI [7] have all been employed under various conditions.In order to identify stable genotype under stress and non-stress conditions indices like yield index (YI), yield stability index (YSI) were proposed by [8,9].Keeping in view the importance of improving wheat production under the current variable climatic conditions this research was undertaken with the objectives to: (i) Screen and identify potential lines that perform better under normal and late planting conditions.(ii) Identify late planting stress tolerant genotypes of wheat based on stress selection indices.(iii) Utilization of the germplasm in future breeding programs especially breeding wheat for late planting.

Experimental site and genetic material
To explore the genetic potential and asses stress tolerant genotypes, the study was carried out at Cereal Crops Research Institute (CCRI), Pirsabak Nowshera Khyber Pakhtunkhwa-Pakistan during crop season of 2013-2014.The experimental site is located at elevation of 288 meters (945 Ft) and located on the intersection of longitude 740 E and latitude 320 N and having silt loam with soil pH of 7.8.Breeding material comprised a set of 28 wheat genotypes (27 lines and one check cultivar).Details of the genotypes used in the study are presented in (Table 1).These genotypes were evaluated as independent experiments under normal and late planting conditions.Both experiments were conducted in Randomized Complete Block Design with three replications.The normal experiment was planted on November 15, 2013 while, late experiment was planted on December 15, 2013.Each genotype was planted in 5 rows plot -1 , with row length of 5 meter and row-torow spacing of 30 cm under each environment.In both the experiments standard cultural practices were adopted including fertilizers and other crop husbandry practices throughout the growing season.Data was recorded on plant height (cm) and grain yield (kg ha -1 ) in both the experiments on all the genotypes.Data were analyzed across the two environments using mixed effect model to ascertain genotype × environment interaction effect for each trait.

Correlation analysis
Simple correlation among the stress selection indices and the studied traits under normal and late planting conditions was carried out using statistical software (Statistix-8.1).

Results and Discussion
Analysis of genotypic variation Analysis of variance revealed highly significant (P≤ 0.01) differences among the genotypes, sowing/planting dates and genotypes × sowing dates for plant height and grain yield (Table 2).Our results are in agreement with those of [13, 14] who found significant variation among the genotypes for plant height and grain yield under optimal and late planting.Significance of genotypes × sowing dates interaction indicated that these genotypes performed differently under normal and late planting conditions and sufficient diversity existed among the evaluated germplasm under both the planting environments.

Figure 1 .Figure 2 .
Figure 1. % reduction in plant height of wheat genotypes under late planting as compared to normal (optimum) planting conditions

Table 1 . List of the 27 bread wheat advanced lines received from CIMMYT-Mexico and local check used in the study S. No.
sown) was considered as non-stress and stress environment, respectively to determine the following selection indices.Let YN= Yield of a given genotype in normal planting (non-