Elevated CO2 alters grain quality of two bread wheat cultivars grown under different environmental conditions
Introduction
Increasing carbon dioxide concentration ([CO2]) in the atmosphere together with rising temperature and changes in rainfall amount and patterns are current concerns for agricultural crop production and crop quality in the near future (Miraglia et al., 2009). Under most emission scenarios atmospheric [CO2] (a[CO2]) is expected increase to ∼550 μmol mol−1 by 2050, causing global temperatures to increase by an average of 1.5–4.5 °C with more frequent occurrences of extreme climatic events such as heat waves and/or drought (Carter et al., 2007). Several studies have shown that wheat grain protein and mineral concentrations decrease under elevated [CO2] (e[CO2]) (Kimball et al., 2001, Taub et al., 2008, Högy et al., 2009, Fernando et al., 2012b). As wheat is a staple food crop for almost half the world's human population, and one of the main sources of minerals and protein in most developing countries (Cakmak, 2004), this is of concern for food security and human health. Grain protein concentration is also an important determinant of end product quality, because it influences dough properties and baking quality (Shewry and Halford, 2002).
It has been suggested that reduction of grain protein at e[CO2] is associated with accumulation of excess carbohydrates compared to N acquisition (Loladze, 2002). This phenomenon is referred to as growth dilution or biomass dilution (Taub and Wang, 2008). In a review, Taub and Wang (2008), argued that reduction of grain protein at e[CO2] could not be fully explained by growth dilution. This hypothesis was further supported by the variability in changes in grain mineral nutrient concentration in response to e[CO2]. Several other mechanisms have been suggested to explain the reduction of grain protein concentration under e[CO2] (Taub and Wang, 2008, Bloom et al., 2010). However, the underlying physiological mechanism is still fully understood.
Better understanding how wheat grain quality changes under e[CO2] and its interaction with different growing conditions, is needed to underpin adaptation strategies. The current study was conducted in the AGFACE (Australian grains free air CO2 enrichment) facility within the major wheat production area of Australia (Mollah et al., 2009). The research site receives an average of 250–300 mm rainfall during the growing season making this the driest grain FACE experimental site in the world. Furthermore, high temperature (22–30 °C) often dominates during grain filling and even short period of 1–4 days above 32 °C can have detrimental effects on yield (Fischer, 2011). In the Australian wheat-belt, high temperatures and drought episodes occur frequently during the growing season (Nicolas et al., 1984), and are predicted to increase in the near future (Mpelasoka et al., 2008).
Using two widely grown Australian wheat cultivars (cv. “Yitpi” and “Janz”) classified as bread-wheat quality we tested the following questions: (i) How do bread wheat cultivars respond to e[CO2] in terms of grain physical, nutritional and rheological properties (ii). How do different growing conditions affect grain quality responses to e[CO2]. (iii) Can changes in major grain quality traits be explained by physiological or environmental indices?
Section snippets
Experiment and CO2 exposure
The experiment was conducted under field conditions in the AGFACE facility at Horsham, Victoria, Australia (36°45′07′′S, 142°06′52′′E; 128 m above sea level) during the 2007, 2008 and 2009 growing seasons. The climate in the region is Mediterranean with an average annual rainfall at the experimental site of 427 mm (1981–2010), where around 250–300 mm rainfall is received in the winter crop growing season, between May and November (Australian Bureau of Meteorology, 2012). The soil type of the
Grain physical properties
The effect of e[CO2] on TGW was dependent on the growing environment (Fig. 2a): At 2007-TOS2 (by 5%), 2009-TOS1 (by 5%) and 2009-TOS2 (by 15%) TGW increased under e[CO2], but did not change at other conditions (Table 1, Fig. 2a). Similar to TGW, e[CO2] significantly increased the percentage of >2 mm size grains at 2009-TOS1 (by 3%) and 2009-TOS2 (by 6%) (Fig. 2b). TGW decreased at TOS2 compared to TOS1 for the 2007 and 2008 growing seasons, but increased at TOS2 compared to TOS1 in 2009 (Fig. 3a)
Discussion
Growing conditions of each year (2007-TOS1, 2007-TOS2, 2008-TOS1, 2008-TOS2, 2009-TOS1 and 2009-TOS2) were different based on total water received, average GDD during growing season, grain filling duration and growing season length. Despite their different genetic make-up, the two cultivars used showed similar responses to increasing [CO2] for most physical and chemical quality characteristics of grains. Results from this study demonstrate that some of the grain physical, nutritional and flour
Conclusions
Elevated [CO2] had a variety of effects on grain properties: Most grain mineral concentrations decreased, and rheological properties changed in a way that suggested lower product quality. Effects of e[CO2] on grain physical properties, protein, some minerals (Zn, Mg, Na) and flour rheological properties varied with environmental conditions, but not between the investigated cultivars. We could however not identify a single environmental index that would explain changes of grain quality responses
Acknowledgement
Research at the AGFACE (Australian Grains Free Air CO2 Enrichment) facility is jointly run by the Victorian State Department of Environment and Primary Industries (DEPI) and the University of Melbourne (UM) and supported by the Grains Research and Development Corporation and the Australian Commonwealth Department of Agriculture, Fisheries and Forestry (DAFF; Climate Chage Research Program). Additional support for grain mineral analysis by the International Plant Nutrition Institute is
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