Response of Canola advanced lines to delay plantings upon late season drought stress

In order to evaluation of canola advanced lines response to delay plantings under late season drought stress conditions, an experiment was carried out in a factorial split-plot arrangement based on RCBD with three replications during two years (2012-2014) in Karaj of Iran. Treatments were; (1): Planting date in two levels (16 October and 1 November), (2): irrigation, in two levels (I1: normal irrigation as control and I2: restricted irrigation after pod formation stage) as main plots and (3): twelve oilseed rape genotypes as sub plots such as BAL2, BAL1, BAL3, BAL6, BAL8, BAL9, BAL11, BAL15, L72, R15, L109 and Okapi. The interaction effects of planting date, irrigation and genotype on pod number per plant, seed number per pod, 1000-seed weight, seed yield, and oil yield were significant at 1% level probability. The maximum seed yield under planting at the appropriate time (16 October), normal irrigation and drought stress conditions (restricted irrigation after pod formation stage) was observed in Okapi and L109, respectively). Among genotypes, R15 line under delay planting (1 November) and both normal irrigation and drought stress conditions (restricted irrigation after pod formation stage) showed the maximum seed yield.


Introduction
There are different types of climates in different regions of Iran, where approximately 85% of the land is situated in arid, semi-arid or hyper arid regions.There are reports showing the decrease in rainfall in different areas of Iran in recent decades (Modarres and da Silva, 2007).
Among the oilseed crops, Brassica species are most affected by drought, as these crops are mainly grown in arid and semiarid areas.Oilseed rape production significantly reduced due to water deficit stress.This yield reduction can be lessened by multiple approaches of plant management practices including breeding strategies (Zhan et al., 2014).
The breeding for stress tolerance is the real need of the day to develop excellent varieties for cultivation and increasing production (Zhan et al., 2014).
Canola is relatively poorly adapted to drought prone conditions (Wright et al., 1997) and its yield is often decreased if moisture stress occurred, particularly at reproductive stage (Ahmadi and Bahrani, 2009;Shirani Rad and Abbasian, 2011).The reduction in seed yield ranged from 19 -39% compared with well-watered control, when drought stress was imposed at reproductive stage (Gunasekara et al., 2006).Even temporary drought stress caused substantial losses in brassica by affecting growth and yield traits (Pervez et al., 2009).However, genetic variations among genotypes to tolerate drought stress have been reported in wide variety of crops including canola (Kauser et al., 2006).Ul-Haq et al. (2014) reported that three genotypes of Brassica napus showed different response to irrigation treatments for plant height, number of branches, biological yield, number of pod, number of seeds per pod and seed yield.Higher seed yield under drought stress conditions resulted from various morphological and metabolic activities, in tolerant oilseed rape genotypes (Jabbari et al., 2016).Accordingly, the role of root morphology in improving drought tolerance is critical.In addition, having effective root system, maintaining SC and higher pod number are among the most important traits for oilseed rape yield maintenance, especially under drought stress conditions (Jabbari et al., 2016).Mirshekari et al. (2012) indicated that minimum seed yield of safflower was obtained from cutting irrigation in the last sowing date at heads forming, flowering and seed filling stages, respectively.Shirani Rad et al. (2014Rad et al. ( , 2015) ) reported that season of planting was significantly influenced all measured traits of rapeseed.Recently, Nasiri et al. (2017) reported the effect of plant density on yield and physiological characteristics of six canola cultivars.Given that, field studies were conducted using an agronomical practice including irrigation treatments and oilseed rape genotypes.The primary objective of this research is to study of drought tolerance in canola genotypes under late season drought stress conditions, grown in semiarid regions.

Materials and methods
The experiments were conducted during the 2012-2014 growing seasons in the Seed and Plant Improvement Institute, Karaj, Iran (35° 50′ 08′′ N, 51° 00′ 37′′ E).The Karaj region is characterized as cold semi-arid steppe climates (BSk by the Köppen-Geiger classification system) with relatively warm summers and cold winters.The average annual temperature is 14.2 °C and the average annual rainfall is 244 mm in Karaj.Before planting, several soil samples were collected and tested for physico-chemical properties.

Land preparation and introduction of treatments
Oilseed rape was planted following wheat in karaj.The field was prepared by shallow plowing, followed by disking in September and October.Each experimental unit was 4 m long and consisted of 4 rows spaced 0.3 m apart.The main plot treatments were irrigation, which was defined included (1): Planting date in two levels such as 16 October and 1 November, (2): irrigation, in two regimes (I1: normal irrigation as control and I2: restricted irrigation after pod formation stage) as main plots and (3): twelve canola genotypes as sub plots such as BAL2, BAL1, BAL3, BAL6, BAL8, BAL9, BAL11, BAL15, L72, R15, L109 and Okapi.
Standard recommended fertilizers and cultivation practices were followed in the experiment.To determine morphological and agronomical traits (including pod number per plant, seed number per pod, 1000 seed weight) 7 plants from each plot and to measure seed yield, 4.8 m 2 of each plot was hand-harvested at the physiological maturity stage.The oil yield was estimated via multiplying seed oil percentage by seed yield.

Statistical analysis
All data were subjected to an analysis of variance (ANOVA) using the SAS software (SAS Institute, 2003).

Results and discussion
The results demonstrated that the interaction effects of planting date×irrigation×genotype on pod number per plant (PNP), seed number per pod (SNP), 1000-seed weight (SW), seed yield (SY), seed oil yield (OY) were significant at 1% level probability.

Pod number per plant (PNP)
The comparison means of Planting date×irrigation×variety showed that the maximum pod number (PNP) under Planting at the appropriate time (16 October), normal irrigation and drought stress conditions (restricted irrigation from pod formation) related to R15 and L109 (167 and 153, respectively).In addition, the maximum pod number (PNP) under delay planting (1 November), normal irrigation and drought stress conditions (restricted irrigation from pod formation) related to R15, (135 and 121, respectively).The reduction in pod number under drought stress conditions is mainly due to an increase in pod abortion and shattering which is possibly a consequence of reduced photosynthate supply.It has been reported that the reduction in pod number under stress conditions is related to flower and pod abortion rather than reduction in flower production (Faraji et al., 2009).

Seed number per pod (SNP)
The comparison means of planting date×irrigation×genotype effect showed that L109 had the maximum seed number in pod (SNP) under planting at the appropriate time (16 October), normal irrigation and drought stress conditions (restricted irrigation from pod formation) (Table 1).In contrast, R15 showed the highest seed number in pod (SNP), as 24 and 23, respectively, in delay planting (1 November), normal irrigation and drought stress conditions (restricted irrigation from pod formation) (Table 1).
Interaction of cultivar × irrigation has been shown significant for number of seeds per pod by Darjani et al. (2013).Similarly, the number of seeds in each pod was affected by stress in early stages of pod filling (Din et al., 2011).
In general, supplying enough water at the flowering and pod-setting stages increases the number of seed per pod.In Brassicaceae, number of seeds per plant is related to number of pod per plant and number of seed per pod, therefore drought stress at this stage reduced the seed number per plant (Wright et al., 1995).Shahsavari et al. (2014) reported that water stress decreases the number of pod in canola, and also that water stress, at the flowering stage, significantly decreases the number of pod in each plant, while postponed stress leads to a decrease in the number of seeds per pod.Thomas et al. (2004) reported that the number of pod is more sensitive to water stress than other yield components.Moreover, Sinaki et al. (2007) pointed out that complete irrigation at first, at the pod-setting stage, had an important role in final yield, and that water stress, at this stage, decreased the length of pod and seed yield.

1000-seed weight (SW)
The comparison means of planting date×irrigation×genotype effect showed that the maximum 1000-seed weight (4.99 g) was achieved from R15 in Planting at the appropriate time (16 October), and drought stress irrigation (restricted irrigation from pod formation stage) treatment (Table 1).The interaction effect of planting date, irrigation and cultivar on SW showed that the maximum SW under planting at the appropriate time (16 October), normal irrigation and drought stress conditions (restricted irrigation from pod formation stage) was observed in R15 and L109 as 4.99 g and 4.93 g, respectively (Table 1).Also, in delay planting (1 November), normal irrigation and drought stress conditions (restricted irrigation from pod formation) the maximum SW was observed in R15, 4.75 g and 4.15 g, respectively (Table 1).In other words, reduction of seed weight in delay planting and late season drought stress conditions (restricted irrigation from pod formation stage) is more likely due to increased leaf senescence, reduced leaf area duration and disruption in assimilate transfer into the seeds.The current results mirror those reported by Bitarafan and Shirani Rad (2012) who studied seed weight changes in response to restricted irrigation in oilseed rape.

Seed yield (SY)
In this study, the comparison means of planting date×irrigation×genotype effect showed that maximum seed yield (SY) under Planting at the appropriate time (16 October), normal irrigation and drought stress conditions (restricted irrigation from pod formation) was observed in Okapi and L109 (5062 kg ha -1 and 3426 kg ha -1 , respectively).Among genotypes, R15 line under delay planting (16 October) and both normal irrigation and drought stress conditions (restricted irrigation from pod formation) showed the maximum SY (2818 kg ha -1 and 1986 kg ha -1 , respectively) (Table 2).
In this study, although R15 and L109 were relatively late maturing genotypes, was able to escape from drought periods by completing its life cycle before the harshest conditions and was obtained the maximum SY.In addition, higher SY under delay planting (1 November) in R15 was driven by more pod number, seed number and weight (Table 1).It should be kept in mind that in indeterminate plants like oilseed rape, there is a competition between vegetative and reproductive organs for produced assimilates, and drought stress whether during flowering stage or pod formation stage would affect photosynthates allocation between vegetative and reproductive organs (Sinaki et al., 2007).

Oil yield (OY)
The comparison means of planting date×irrigation×genotype effect showed that the maximum OY under planting at the appropriate time (16 October), normal irrigation and drought stress conditions (restricted irrigation from pod formation) related to Okapi and L109 as 2193 and 1417 kg ha -1 respectively (Table 2).While in delay planting (1 November), response of genotypes to irrigation treatments was different (Table 2).For example, in both normal irrigation and drought stress conditions (restricted irrigation from pod formation) the maximum OY was observed in R15 as 1145 and 782 kg ha -1 , respectively (Table 2).In other words, OY was considerably affected by drought stress in both Planting date and it is a function of genotype, seed yield and slightly oil percentage generally.

Conclusion
The present study indicated that the interaction effects of planting date, irrigation and genotype on pod number per plant, seed number per pod, 1000-seed weight, seed yield, and oil yield were significant.The maximum seed yield under planting at the appropriate time (16 October), normal irrigation and drought stress conditions (restricted irrigation after pod formation stage) was observed in Okapi and L109, respectively.Among genotypes, R15 line under delay planting (1 November) and both normal irrigation and drought stress conditions (restricted irrigation after pod formation stage) showed the maximum seed yield.
SY= Seed yield (kg ha -1 ), OY= Oil yield (kg ha -1 )* **Mean values of the same category followed by different letters are significant at p≤0.05 level.