Heat Adaption Strategies
A number of traits have been studied in wheat for adaptation under high temperature stress. One of the strategies being followed in CIMMYT is breeding for early maturing high yielding heat tolerant wheat varieties. Earliness or early maturity is an adaptation strategy where early heading lines complete the initial seed setting and grain filling under favorable temperatures and avoid the late incidence of heat stress. Earliness has been suggested as a good approach for wheat breeding in the eastern Gangetic plains that suffers from high temperature stress during grain filling (Joshi et al. 2007). Tewolde et al. (2006) evaluated a diverse set of US wheat
Fig. 26.1 Maximum (a) and minimum (b) temperatures during the wheat crop growing cycle (November, 2012–April, 2013) across ME1 and ME5 locations in South Asia
cultivars for their adaption to high temperature stress and concluded that early maturity was the single effective trait that defined adaptation under post heading high temperature stress. Al-Karaki (2011) observed that the length of pre heading period in durum wheat affects grain yield under heat stress conditions. They reported that the early maturing high yielding durum lines were suitable for the heat stress affected areas in semi-arid environment. Thus the early maturing high yielding CIMMYT wheat lines developed in Mexico were evaluated for grain yield and adaptation to high temperature stress in South Asia.
Performance of Early Maturing Wheat Lines
High yielding early maturing wheat lines were developed and tested in CIMMYT research station, Cd. Obregon, Sonora, Mexico. The best performing lines were selected for evaluation in South Asia and Mexico. Each trial had 28 entries, with one CIMMYT check 'Baj' and a local check which was the best locally adapted variety at each location. The trials were grown in 9, 13 and 15 locations across South Asia in 2009–2010, 2010–2011 and 2011–2012 respectively. Each year the trials were also planted at Cd. Obregon, Mexico in two environments, normal sowing in November and late sowing in February for heat stress. The locations were grouped in to mega environments, ME1 and ME5.
In 2009–2010, 20 % of the lines had grain yield 1–4 % higher (p < 0.05) than the local check across all locations (Fig. 26.2). In the next two cycles (2010–2011 and 2011–2012) nearly 50 % of the lines yielded 1–20 % higher (p < 0.05) than the local checks. A similar trend was seen when compared to the CIMMYT early maturing check line 'Baj'. In 2009–2010 a few lines were performing above 'Baj', but a gradual increase was seen in 2010–2011 and 2011–2012. Thus early lines bred in Mexico were able to outperform the locally adapted checks in South Asia. The performance of the lines in the individual MEs was also interesting. The mean grain yield of the trial had significant reduction in the warm ME5 compared to cooler ME1. When the entries that performed above local check in both MEs in each year were grouped together, it was observed that each year these entries had around 5–10 % yield advantage (p < 0.05) over the local checks in each ME (Fig. 26.3). Thus the early maturing high yielding lines were able to adapt under both terminal and continual high temperature stresses.
High temperatures affect both grain filling duration and grain filling rate. Studies have reported an increase in grain filling rate and reduction in grain filling duration due to high temperatures (Farooq et al. 2011). But the increase in grain filling rate has been reported to not compensate for the shorter grain filling duration (Wardlaw et al. 1980; Stone et al. 1995). The results from this study show otherwise. The early maturing short duration wheat lines were able to outperform the best locally adapted varieties. A comparison of grain accumulation rate based on grain filling duration of the five top high yielding stable lines in each year with the local checks showed that the early maturing lines had higher grain accumulation rates as well as higher grain yields (Fig. 26.4).
With high temperature becoming a major issue in wheat producing areas, there is an enhanced focus on developing heat tolerant high yielding wheat varieties. The results show that simultaneous enhancement of yield potential and heat tolerance is possible and that the CIMMYT strategy to develop high yielding early maturing wheat lines is promising for South Asia. The early maturing lines were able to
Fig. 26.2 Mean grain yield of the 30 lines across South Asia in 2009–2010, 2010–2011 and 2011–2012. Grain yield is expressed as % above (a) local check and (b) CIMMYT check line BAJ
outperform the locally adapted high yielding lines under both terminal and continual high temperature stress. Thus early maturity while maintaining yield superiority is beneficial for South Asia in adapting to high temperature stress and enhancing productivity. The identified heat tolerant, high yielding and early maturing Lines are available for the national wheat improvement programs in South Asia for testing and further development of superior heat tolerant wheat varieties.
Fig. 26.3 Grain yield of the top five lines and local checks in (a) ME1 and (b) ME5 of South Asia in 2009–2010, 2010–2011 and 2011–2012
Fig. 26.4 Grain yield accumulation rate (t/ha/day) of the top five lines and local checks in South Asia in years 2009–2010, 2010–2011 and 2011–2012
Acknowledgments We would like to thank all our co-operators and National Agricultural Research Station partners in India, Bangladesh, Nepal and Pakistan for conducting trials at the respective locations and donor organizations Bill & Melinda Gates Foundation and USAID for providing financial support through the CSISA project.