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«Crop Science: Posted 7 Mar. 2013; doi: 10.2135/cropsci2012.09.0545 Identification of drought, heat, and combined drought and heat tolerant donors in ...»

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Crop Science: Posted 7 Mar. 2013; doi: 10.2135/cropsci2012.09.0545

Identification of drought, heat, and combined drought and heat tolerant donors in maize (Zea

mays L.)

Jill E Cairns*, Jose Crossa, PH Zaidi, Pichet Grudloyma, Ciro Sanchez, Jose Luis Araus, Suriphat

Thaitad, Dan Makumbi, Cosmos Magorokosho, Marianne Bänziger, Abebe Menkir, Sarah

Hearne, Gary N Atlin

ABSTRACT

Low maize yields and the impacts of climate change on maize production highlight the need to

improve yields in eastern and southern Africa. Climate projections suggest higher temperatures within drought-prone areas. Research in model species suggests that tolerance to combined drought and heat stress is genetically distinct from tolerance to either stress alone, but this has not been confirmed in maize. In this study we evaluated 300 maize inbred lines testcrossed to CML539. Experiments were conducted under optimal conditions, reproductive stage drought stress, heat stress and combined drought and heat stress. Lines with high levels of tolerance to drought and combined drought and heat stress were identified. Significant genotype x trial interaction and very large plot residuals were observed; consequently, the repeatability of individual managed stress trials was low. Tolerance to combined drought and heat stress in maize was genetically distinct from tolerance to individual stresses, and tolerance to either stress alone did not confer tolerance to combined drought and heat stress. This finding has major implications for maize drought breeding. Many current drought donors and key inbreds used in widely-grown African hybrids were susceptible to drought stress at elevated temperatures. Several donors tolerant to drought and combined drought and heat stress, Crop Science: Posted 7 Mar. 2013; doi: 10.2135/cropsci2012.09.0545 notably La Posta Sequia C7-F64-2-6-2-2 and DTPYC9-F46-1-2-1-2, need to be incorporated into maize breeding pipelines.

J.E. Cairns and C. Magorokosho, International Maize and Wheat Improvement Centre (CIMMYT), PO Box MP163, Harare, Zimbabwe; J. Crossa, C. Sanchez, J.L. Araus, M. Bänziger, CIMMYT, Km. 35 Carr. Mexico-Veracruz, Texcoco, Edo de Mexico, DF, Mexico; P.H. Zaidi, CIMMYT, Patancheru 502 324, India; P. Grudloyma, Nakhon Sawan Field Crops Research Center (NSFCRC), Thailand; D. Makumbi, CIMMYT, United Nations Avenue, Gigiri PO Box 1041, Village Market-00621, Nairobi, Kenya; A. Menkir, S. Hearne, International Institute of Tropical Agriculture (IITA), Carolyn House, 26 Dingwall Road, Croydon, UK; G.N. Atlin, Bill & Melinda Gates Foundation, PO Box 23350, Seattle, WA 98102, USA*corresponding author Keywords: maize, climate change, drought, heat, combined drought and heat, grain yield, genetic correlations Abbreviations: ASI, anthesis-silking interval; CIMMYT, International Maize and Wheat Improvement Center; ESA, eastern and southern Africa; H, broad sense heritability, IITA, International Institute of Tropical Agriculture, PH, plant height; SSA, sub-Saharan Africa; VPD, vapour pressure deficit.

Crop Science: Posted 7 Mar. 2013; doi: 10.2135/cropsci2012.09.0545

INTRODUCTION

In eastern and southern Africa (ESA), maize is the most important crop, accounting for up to 40 to 50% of both calories and protein consumed in Malawi, Zimbabwe, and Zambia, the most maize-dependent countries in the region (FAOSTAT, 2010). However, maize yields in this region remain low averaging 1.4 t ha-1, or one-sixth the average yields in the USA (FAOStat, 2010).

These yield levels are barely enough to ensure food security, and often fall short. The need to increase maize yields for food security in ESA is heightened by both population growth and climate change. The population of sub-Saharan Africa is predicted to double by 2045 (World Population Prospects, median variant; United Nations, 2009), while climate projections for ESA show decreasing precipitation, increasing temperatures, and a higher frequency of extreme events (IPCC, 2007). A comparison of different cropping systems across regions identified maize production in southern Africa as one of three climate risk hotspots (Lobell et al. 2008). Drought has long been recognised as a major constraint to maize yields in this region (Heisey and Edmeades, 1999), however, heat stress both alone and in combination with drought stress is likely to become an increasing constraint to maize production (Cairns et al. 2012a). Lobell and Burke (2010) showed that an increase in temperature of 2 °C would result in a greater reduction in maize yields than a decrease in precipitation of 20 %. Similarly, a recent study in Tanzania also indicated that increasing temperatures would result in a greater reduction in maize yields than increased intra-seasonal variability in precipitation (Rowhani et al. 2011). In this study a projected increase in temperature of 2 °C reduced maize yields by 13 %, while a 20 % increase in intra-seasonal variability reduced maize yields by only 4.2 %. These studies Crop Science: Posted 7 Mar. 2013; doi: 10.2135/cropsci2012.09.0545 highlight the need to incorporate heat tolerance, as well as increased drought tolerance, into African maize germplasm to offset predicted yield losses.

Substantial progress has been made in drought breeding in subtropical and tropical maize. In the 1970s CIMMYT initiated a drought breeding programme for maize using the elite lowland tropical maize population “Tuxpeño Sequia” (Bolaños and Edmeades, 1993a,b; Bolaños et al., 1993). Over eight cycles of full-sib recurrent selection for grain yield and increased flowering synchronisation (reduced anthesis-silking interval (ASI)) resulted in gains of up to 144 kg ha-1 yr-1 under drought stress (Edmeades et al. 1999). In the late 1990’s CIMMYT initiated a product-orientated maize breeding programme in southern Africa (Bänziger et al. 2006). Maize varieties were simultaneously selected for performance under optimal, low nitrogen and managed drought stress conditions. CIMMYT hybrids yielded more than commercial checks at all yield levels. Under severe stress CIMMYT hybrids had a 40 % yield advantage compared to commercially available hybrids. Recent on-farm trials in ESA of new hybrids showed a 35 % and 25 % yield advantage against farmers own varieties under low ( 3 t ha-1) and high yield conditions, respectively (Setimela et al. 2012). The best hybrid (CZH0616) out-yielded the most popular commercial check, which was released approximately 15 years ago, by 36 % and 26 % under high and low yield conditions, respectively, indicating that gains from selection for both yield potential and stress tolerance have been large.





In contrast to drought research, relatively less effort has been devoted to breeding specifically for heat stress tolerance in maize. Earlier studies highlighted the negative impact of Crop Science: Posted 7 Mar. 2013; doi: 10.2135/cropsci2012.09.0545 increased growing season temperatures on temperate maize yields. Thomson (1966) showed that an increase in temperature from 22 °C to 28 °C during the grain filling period in the US Corn Belt resulted in a 10 % yield loss, while Badu-Apraku et al. (1983) showed a 42 % yield reduction when mean daily temperatures were increased by 6 °C. A recent analysis of more than 20,000 historical maize trial yields in southern Africa showed that maize production linearly decreased with every accumulated degree day above 30 °C (Lobell et al. 2011). Heat stress in maize is associated with shortened life cycle (Muchow et al. 1990), reduced light interception (Stone, 2001), increased respiration, reduced photosynthesis (Crafts-Brander and Salvucci, 2002) and pollen sterility (Schoper et al. 1987a, b). A comparison of the response of male and female reproductive tissues to heat stress demonstrated that female tissues have greater tolerance (Dupis and Durnas, 1990), with pollen production and/or viability highlighted as major factors controlling reduced fertilisation under high temperatures. However the period between silk pollination and ovary fertilisation in the female reproductive tissues has also recently been highlighted as a critical period controlling grain yield under heat stress (Cicchino et al. 2011).

Given the diversity of ecosystems in which maize can be grown, it is highly likely that there is genetic variability in the tolerance of tropical and sub-tropical maize to heat stress. However, tropical maize has been shown to have stable yields across a narrower range of temperatures than temperate maize (Lafitte et al. 1997). Climate predictions show an increase in growing season temperatures within drought prone regions (Cairns et al. 2012a). While drought stress is often a combination of water and temperature stress as a result of reduced transpirational cooling under limited water conditions, there is evidence to suggest that the response to drought stress at elevated ambient temperatures is unique and cannot be extrapolated from Crop Science: Posted 7 Mar. 2013; doi: 10.2135/cropsci2012.09.0545 the sum of the effects of both stresses (drought and heat) (Rizhsky et al. 2002; 2004; Barnabás et al. 2008).

Both conventional and molecular breeding approaches rely on genetic variability for the trait of interest. The aim of this study was therefore to identify lines with tolerance to drought, heat and combined heat and drought stress for use within maize breeding programs, to serve both as donors of drought and heat tolerance in pedigree breeding and as potential sources of alleles with large effects detectable through genome-wide association mapping (to be reported in a subsequent paper). We evaluated a diverse set of 300 inbred lines from CIMMYT and IITA’s tropical and subtropical breeding programmes as testcrosses to the broadly-adapted line CML539 in drought- and heat-stressed field trials in Kenya, Zimbabwe, Mexico, and Thailand.

Additional aims of this study were to establish the relationship between drought, heat and combined drought and heat tolerance in maize, and to assess the magnitude of genotype x trial plot residual variances in managed drought and heat stress trials with respect to their effects on the repeatability of field phenotyping for these stresses.

MATERIALS AND METHODS

Plant material A collection of 300 inbred lines was assembled, representing the genetic diversity within the CIMMYT and IITA tropical and subtropical maize improvement programmes (Wen et al. 2011).

Briefly, lines were assembled from nine CIMMYT and IITA breeding programmes based in Latin America and Africa focussing on yield potential and abiotic and biotic stress tolerance (Table 1).

Crop Science: Posted 7 Mar. 2013; doi: 10.2135/cropsci2012.09.0545 Information on the pedigree and adaptation zones of all lines is presented in Table S1. Single cross hybrids were generated by crossing lines with the tropical tester CML-539, a broadlyadapted inbred that is tolerant to maize streak virus, a disease prevalent only in Africa but not elsewhere.

Trial management Trials were conducted at CIMMYT maize experimental stations in Tlaltizapán, México (18°41’N, 99°07’W, 940 masl), Kiboko, Kenya (2°21’S, 37°72’E, 975 masl) and Chiredzi, Zimbabwe (21°01’S, 31°34’E, 430 masl), at the Nakhonsawan Field Crops Research Center in Takfa, Thailand (15°21’N, 100°30’E, 87 masl), and at the ICRISAT experimental station in Hyderabad, India (7°53’N, 78°27’E, 545 masl). The soil of the experimental field in Mexico is a clay loam with a pH of 7.6 and classified as an Isothermic Udic Pellustert. In Kenya the soil is sandy clay with a pH of 7.9 and classified as an Acro-Rhodic Ferrasol. In Zimbabwe the soil is a clay loam with a pH of 5.5 and classified as a Typic Rhodustalf. In Thailand the soil is a clay loam.

In India the soil is 50 % clay with a pH of 8.5 and classified as a Typic Pellustert.

In the experiment conducted in Mexico in 2009, hybrids were separated into four maturity groups; early (50 entries), early-intermediate (100 entries), intermediate late (100 entries) and late (50 entries). In 2010 and 2011 phenology groups were redefined into two maturity groups, early (150 entries) and late (150 entries) in all locations. Experiments were planted during the dry season in all locations with the exception of India to allow drought stress to be imposed at the anthesis stage (Table 2). Experiments were planted either in two-row plots Crop Science: Posted 7 Mar. 2013; doi: 10.2135/cropsci2012.09.0545 (Mexico and Thailand) or one-row plots (Kenya and Zimbabwe), with a final plant density of

6.67 plants m-2 (Mexico, Kenya, India and Zimbabwe) or 5.33 plants m-2 (Thailand). At all locations two seeds per hill were sown, then thinned to one after emergence. An alpha-lattice design was used, replicated three times in 2009 and two times in 2010 and 2011. All plots received 80 kg N ha-1 (as urea), 80 kg P ha-1 (as triple calcium superphosphate; Ca (H 2 PO 4 ) 2 H 2 0) at sowing. A second application of N (80 kg N ha-1) was applied 5 weeks after sowing (V6 stage, Ritchie et al., 1993). Recommended plant, weed, and insect control measures were used.



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