Impact of high temperatures in maize: Phenology and yield components
Introduction
FAO has reported an improvement in food security in the last two decades, with a global reduction of undernourished people of 216 million in 2015 compared to 1990–92. These figures are especially encouraging in developing regions dropping from 23.3% of the population undernourished in 1990–92 to 12.9% in 2015 (FAO et al., 2015). In spite of these positive data, the already observed and projected impacts of climate change on agriculture (IPCC, 2013) and their implications for the food security of current world population and of the 9 billion people foreseen by 2050 emphasize the urgent need for farmers to adapt to a changing climate (FAO, 2016). In addition, crops with high water requirements cultivated under semi-arid or arid conditions require to be adapted to the new climate conditions to increase water productivity and irrigation water efficiency (Molden et al., 2010) in an elevated temperature environment.
The major staple crops, such as maize (Zea mays L.), the cereal with greatest world production (in the period 2010–2014, average production was 932.7 Million Mg with an average yield of 5.27 Mg/ha, http://www.fao.org/faostat/en/) will need to adapt to the new conditions. Maize is cultivated in a wide range of climate conditions, following the rainy season in tropical regions and as a summer crop in temperate ones, with high irrigation requirements under semi-arid conditions. Maize adaptation should deal not only with changed climate averages, but also with the increased frequency and intensity of extreme events (IPCC, 2012). More specifically, several studies have identified heat stress as a main threat for future maize cultivation in several relevant production regions (e.g. Gourdji et al., 2013).
Kernel number, i.e. the size of the physiological sink of assimilates, is a key yield component to determine final maize grain yield (Fischer and Palmer, 1984, Andrade et al., 2000). In turn, this component is closely related to the source of assimilates during a narrow time window of four or five week period around anthesis (Fischer and Palmer, 1984, Otegui and Bonhomme, 1998, Andrade et al., 1999). No clear dependency of the kernel number on growth rates during the occurrence of heat stress in pre-silking period has been found (Cicchino et al., 2010b). However, heat stress during the period around silking leads to high yield reduction (Cicchino et al., 2010b) affecting both plant sources and sinks. Source capacity is directly affected by a reduced synthesis of carbohydrates (Barnabás et al., 2008), in turn caused by decreased photosynthesis and escalated respiration rates (Rattalino-Edreira and Otegui, 2012, Wahid et al., 2007, Ordóñez et al., 2015). Sink capacity is affected by the disruption of the anthesis-silking synchrony, reduced ovule fertilization and increased kernel abortion. In turn, these effects disturbs pollination and kernel set and can result in severe yield losses (Herrero and Johnson, 1980, Rattalino-Edreira et al., 2011, Ordóñez et al., 2015, Dupuis and Dumas, 1990, Cicchino et al., 2010b). Also, recent studies (Rattalino-Edreira et al., 2011, Ordóñez et al., 2015) have found an important role of the female component of the sinks in the maize response to heat stress.
The upper optimum temperature for maize flowering has been considered to be between 29 and 37.3 °C (Schlenker and Roberts, 2009, Gilmore and Rogers, 1958, Tollenaar et al., 1979, Cicchino et al., 2010b; Sánchez et al., 2014). Some authors have explained partially this wide range by the experimental error coming from considering air temperature instead of canopy temperature (Craufurd et al., 2013, Siebert et al., 2014, Siebert et al., 2017; Webber et al., 2016; Lobell et al., 2008) or plant profile temperature (Rattalino-Edreira and Otegui, 2012). Differences in vapor pressure deficit (VPD) may also affect these responses. On one hand, the difference between those temperatures can be especially large under irrigated conditions (up to 10 °C according to Kimball et al., 2015), but even the smaller differences registered under rainfed conditions (ca. 2 °C) can lead to underestimation of heat stress impact (Webber et al., 2016). Most of the previous experiments introduced modifications in temperature, gas exchange, wind profile and radiation not just in the greenhouse experiments but also in the field ones (e.g. by using polyethylene films as Cicchino et al., 2010a, Cicchino et al., 2010b, Rattalino-Edreira et al., 2011, Ordóñez et al., 2015) to achieve fully or partially controlled heat stress conditions. On the other hand, increases in air temperature under field conditions usually induce higher VPD, enhancing the demand for soil water and the effect of water deficits (Mittler, 2006), which in turn can raise canopy temperature.
The objective of this study was to improve the understanding of the response of maize development, growth and grain production to heat stress conditions. For that reason, our study combines data collection under controlled conditions (greenhouse) with field experiments under natural conditions with unperturbed wind, radiation, humidity, and temperature regimes. Also, data collection on the same hybrid under several field and controlled conditions across all years was crucial to remove the uncertainty linked to genotype variation.
Section snippets
Field treatments
The study was conducted over three years (2014–2016) growing the short-season maize (Zea mays L.) hybrid PR37N01 (FAO-300) in three locations in Spain with a North-South thermal gradient (Candás in Northern Spain, Aranjuez a Central site, and Córdoba in the South, Fig. 1a). The soils of the field experiments (Fig. 1b), were fertilized according to soil analysis recommendations, typically with 250 kg N/ha split in two applications at V4 and V8, to avoid nutrient limitation. Irrigation was applied
Field experiments
The three field experimental locations showed an increasing Southward gradient of temperatures. In our hottest year (2015) and location (South) the range of mean temperatures during the growing season was 12.5–32.4 °C. Maximum temperatures (Tmax) registered in the North site were always below 30 °C, while Tmax was over 35 °C for several days in both Central and South locations, mostly in the middle of the crop cycle, and over 40 °C for some days in the summer of 2015 (Fig. 2 for the 1 st sowing
Crop phenology
The purpose of this study was to assist improving our understanding of the responses of maize crops to heat conditions. Our first concern was the developmental responses. We found one-month difference in crop cycle (emergence-maturity) between our Northern and Southern locations. This is especially significant considering that we worked with a short-season hybrid, and illustrates the adequacy of the sites selected for this study. This also raised a major challenge to correctly calculate thermal
Conclusions
To and Tx are key parameters for estimating plant developmental rate but currently large uncertainty in their estimation under warm conditions affect the quality of the estimations. Thus, additional field experiments in warm conditions, with a wider range of temperatures that assure To being overpassed are needed to reduce this uncertainty. The simplified beta function can take this range of temperatures into account and seems to be a more effective approach than classic thermal time
Acknowledgements
This work was financially supported by project MULCLIVAR, from the Spanish Ministerio de Economía y Competitividad (MINECO, CGL2012-38923-C02-02); by the Spanish National Institute for Agricultural and Food Research and Technology (INIA, MACSUR01-UPM) through FACCE MACSUR – Modelling European Agriculture with Climate Change for Food Security, a FACCE JPI knowledge hub; and through SUSTAg project (INIA, 652915 ERA-NET Cofund FACCE-SURPLUS). The field and greenhouse support by Roman Zurita was
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