Leaf photosynthesis and associations with grain yield, biomass and nitrogen-use efficiency in landraces, synthetic-derived lines and cultivars in wheat
Introduction
Bread wheat (Triticum aestivum L.) accounts for 30% of the global cereal production and for 20% of the calories in human diets (FAO, 2014). The world population is predicted to reach 9 billion by 2050 and global demand for food to increase at an annual rate of ca. 1.6% (Godfray et al., 2010, Hall and Richards, 2013). The relative rate of global yield increase in wheat is declining and is currently about 1.1% yr−1 (Hall and Richards, 2013). Furthermore, in some countries and regions, a yield plateau or a decrease in rate of genetic gains in grain yield potential is reported in the last decades, e.g. for spring wheat in Mexico (Fischer and Edmeades, 2010), Chile (Matus et al., 2012) and Spain (Acreche et al., 2008) and for winter wheat in the great plains of North America (Graybosch and Peterson, 2010), France (Brisson et al., 2010) and the UK (Mackay et al., 2011). Although there is scope for raising harvest index (HI) in the short to medium term from current values of ca. 0.50–0.55 in winter wheat and ca. 0.45–0.50 in spring wheat (Foulkes et al., 2011) towards the theoretical maximum of ca. 0.64 (Austin et al., 1982, Foulkes et al., 2011), future increases in grain yield will increasingly depend on raising above-ground biomass production. There is some evidence that above-ground biomass has started to increase in recent decades in wheat breeding programmes worldwide (Perry and D’Antuono, 1989, Siddique et al., 1989, Shearman et al., 2005, Lopes et al., 2012, Aisawi et al., 2015) but there is a need to accelerate the rate of genetic gains.
The biomass of a wheat crop has on average 40% carbon by dry weight, and any increase in biomass production will require an increase in photosynthetic carbon fixation (Murchie et al., 2009, Parry et al., 2011). Historically several studies did not shown an association between genetic variation in flag-leaf light-saturated photosynthetic rate (Amax) and grain yield in wheat (Richards, 2000, Calderini et al., 1995). However, associations between leaf photosynthetic rate or associated traits and grain yield progress were reported in the last decades, e.g. in eight spring wheat cultivars in Mexico (Fischer et al., 1998), in 18 winter wheat cultivars in China (Jiang et al., 2003) and in 18 facultative wheat cultivars in China (Zheng et al., 2011). Driever et al. (2014), however, reported that for 64 cultivars grown in the UK flag-leaf photosynthesis in the pre-anthesis phase was not well correlated with above-ground biomass or grain yield. Traits reported to be associated with genetic variation in flag-leaf rate light-saturated photosynthetic rate in cereals are flag leaf N content in wheat (Austin et al., 1982) and leaf Rubisco content in rice (Hubbart et al., 2007).
Increasing biomass of the wheat crop implies an additional requirement for N capture to support photosynthesis. Up to 75% of the reduced N in cereal leaves is located in the mesophyll cells, mainly as Rubisco, and is involved in photosynthetic processes (Evans, 1989). Increased N fertilizer inputs, however, will have economic implications as well as environmental impacts, through nitrate leaching into groundwater and conversion of nitrate by denitrifying soil bacteria into nitrous oxide, a greenhouse gas which contributes to global warming (Brynes, 1990, Goulding, 2004). The development of cultivars with reduced requirements for N fertilizer will therefore be of economic benefit to farmers and will help to reduce environmental contamination associated with inputs of N fertilizers (Foulkes et al., 2009, Hawkesford, 2014). Promising traits for selection by breeders to increase NUE (grain dry matter yield/N available to the crop from soil and/or fertilizer) include deeper roots for increased N uptake (Foulkes et al., 2011), increased leaf photosynthesis rate and the stay-green trait associated with optimized post-anthesis N remobilization (Gaju et al., 2011) and/or late N uptake (Bogard et al., 2011, Gaju et al., 2014).
The use of synthetic-derived germplasm, introgressing genes from the ancestral wheat diploid species, and landraces has been suggested as a source of variation for biomass and NUE in wheat breeding programmes (Moore, 2015). Wild relatives of bread wheat have been reported to have higher leaf photosynthetic rates than modern cultivars (Evans and Dunstone, 1970, Johnson et al., 1987, Carver et al., 1989) suggesting that wheat breeding may have resulted in lower photosynthetic rate. Austin et al. (1982) reported that rate of net photosynthesis was in general highest in the diploid wheat species, intermediate for the tetraploid species and lowest for hexaploid T. aestivum L. with means of 38, 32 and 28 mg CO2 dm−2 h−1, respectively. Synthetic-derived lines (Triticum durum × Triticum tauschi) developed by CIMMYT have been associated with higher leaf photosynthetic rate (Del Blanco et al., 2000), thousand grain weight (Calderinri and Reynolds, 2000) and grain yield (Ogbonnaya et al., 2007) than recurrent parents under optimal conditions. In addition, primary synthetic spring wheats have been shown to have greater root biomass compared to recurrent parents in Australia (Dreccer et al., 2004) and CIMMYT synthetic-derived wheat lines expressed increased partitioning of root mass to deeper soil profiles and ability to extract moisture from those depths and higher grain yield under drought compared with the recurrent parents in NW Mexico (Reynolds et al., 2007, Lopes and Reynolds, 2011). There is also evidence that root size of landraces is larger compared with that of modern cultivars (Ehdaie et al., 1991, Ehdaie and Waines, 1993, Ehdaie and Waines, 1997, Ehdaie, 1995).
This study aimed to quantify novel genetic variation in hexaploid wheat for grain yield, above-ground biomass and NUE and associated physiological traits under HN and LN conditions and responses to N availability by screening three groups of germplasm: (i) landraces, (ii) synthetic-derived lines and (iii) modern UK cultivars, in field experiments in two seasons in the UK. The hypotheses examined were that: (i) under HN conditions genetic variation in flag-leaf photosynthesis rate is associated with above-ground biomass and grain yield and there is variation in the synthetic-derived lines above the parental hexaploid wheat cv. Paragon and (ii) under LN conditions the synthetic-derived lines and landraces have greater N uptake and biomass than the modern cultivars.
Section snippets
Site, plot management and experiment design and treatments
One field experiment was carried out at University of Nottingham farm, Leicestershire, UK (52°50′N, 1°14′W) in each of 2010–11 and 2011–12. Fifteen bread wheat genotypes comprising five landraces from the AE Watkins collection, five synthetic-derived hexaploid wheat lines and five UK modern cultivars were sown using a split-plot design in which N fertilizer treatment was randomised on main plots and genotype was randomised on the sub-plots in three replicates. The five landraces were from the
Growing conditions in experiments
Temperatures were similar in the two seasons, except during April (i.e. early-to-mid stem extension; GS31/33) when daily mean temperatures were on average 4.3 °C cooler in 2012 than 2011 (Supplementary Table 1). The 2010–11 season was brighter than 2011–12 during grain filling in June and July with a cumulative total solar radiation for these two months of 1103 MJ m−2 in 2011 and 935 MJ m−2 in 2012. Rainfall was higher in 2012 than in 2011 particularly during April (112 and 23 mm, respectively) and
Physiological basis of grain yield, biomass and NUE under high N conditions
Under HN conditions, the modern UK cultivars and the synthetic-derived lines yielded higher than the landraces as expected. Similarly higher above-ground biomass overall was observed for the modern cultivars and synthetic-derived lines compared to the landraces under HN conditions. Previous studies have shown that synthetic-derived wheats have higher biomass compared to recurrent bread wheat parents under optimal conditions (Lopes and Reynolds, 2011). In the present study, the SD lines did not
Acknowledgements
We thank the UK Biotechnology and Biological Sciences Research Council for funding the Wheat Improvement Strategic Programme (WISP) project. We thank John Alcock and Matt Tovey with technical assistance in managing the field experiments at Nottingham University and Simon Orford at John Innes Centre, UK and Phil Howell at National Institute of Agricultural Botany, UK for providing the landraces and synthetic-derived wheat germplasm, respectively.
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