Sorghum dwarfing genes can affect radiation capture and radiation use efficiency
Introduction
A positive correlation between plant height and grain yield is often reported for sorghum (Graham and Lessman, 1966, Campbell and Casady, 1969, Doggett, 1988, Morgan and Finlayson, 2000, Jordan et al., 2003). Partly this is due to a direct negative effect of the presence of the sorghum dwarfing gene dw3 on grain yield (Hadley et al., 1965, Casady, 1967, Campbell and Casady, 1969, Campbell et al., 1975). The two most widely used dwarfing genes in wheat, Rht-B1b and Rht-D1b, on the other hand, are generally associated with higher grain yield per plant (Austin et al., 1980, Gale and Youssefian, 1985, Borner et al., 1993, Flintham et al., 1997). This has been attributed to only moderate reduction in overall plant biomass and reduced competition from the stem leading to improved floret fertility and consequently increased grain number in the presence of either of these dwarfing genes (Gale and Youssefian, 1985, Miralles et al., 1998). More recently, some of the gibberellin-responsive wheat dwarfing genes Rht4, Rht12 and Rht13 have also been found to have positive effects on grain number (Rebetzke et al., 1999).
In a recent sorghum study, reduced competition from the stem due to the presence of the dw3 dwarfing gene had no positive effect on grain number (George-Jaeggli et al., 2011), most likely because this was off set by a reduction in crop growth rate, resulting in little effect on panicle growth rate and hence grain number (van Oosterom and Hammer, 2008). Rather, the presence of dw3 was associated with reduced grain size, which negatively affected grain yield in some genotypes and environments (George-Jaeggli et al., 2011). More importantly, in some cases the effects of dw3 on overall plant biomass were rather large, mainly due to large reductions in stem mass, but to a lesser extent also related to a negative effect of dw3 on tiller mass (George-Jaeggli et al., 2011). Under well-watered conditions, biomass accumulation is the product of light interception (LI) and radiation use efficiency (RUE). LI is a function of leaf area index (LAI) and canopy light extinction coefficient (k) (Beer–Lambert–Boguer Law).
Earlier studies found no difference in light interception between tall 2-dwarf sorghum genotypes and that of shorter 3-dwarf types (Graham and Lessman, 1966). Interestingly though, a very tall, 1-dwarf sorghum hybrid was reported to have significantly greater radiation use efficiency (RUE, above-ground biomass produced per unit light intercepted) than standard 3-dwarf hybrids (Hammer et al., 2010). In the absence of stress, RUE is mainly dependent on maximum leaf photosynthetic rate and generally shows very little within-species variation (Sinclair and Muchow, 1999). The RUE calculated for the 1-dwarf sorghum hybrid was closer to the value of 1.6–1.7 g MJ−1 (based on total solar radiation) usually reported for maize (Sinclair and Muchow, 1999) and therefore more than 20% greater than the values of 1.2–1.3 g MJ−1 normally reported for grain sorghum (Muchow and Davis, 1988, Muchow and Sinclair, 1994, Sinclair and Muchow, 1999, Gilbert et al., 2003). RUE values reported for sugarcane, another C4 species, were also similar to the values found in maize (Sinclair and Muchow, 1999). Commercial grain sorghum hybrids are generally much shorter than maize or sugarcane crops, but also much shorter than the material they originated from. Up to four dwarfing genes are combined in countries where machine harvesting prevails, reducing plant height from up to 4 m down to around 60 cm (Doggett, 1988). It is curious therefore that the 1-dwarf sorghum, which was of similar height as a maize crop, had a 20% greater RUE than grain sorghum crops of standard height. This raises the question whether plant height might affect RUE. To study the effects of height separate from the effects of other genes, we used isogenic lines that differed in just one allele (dw3).
The objective of this study was to determine whether a reduction in shoot biomass in response to dw3 was associated with reduced RUE or reduced radiation capture (LAI, LI or k).
Section snippets
Plant material
Standard sorghum grown in industrialised countries is usually recessive at three of four known dwarfing loci reducing plant heights of up to 4 m to a more manageable 90 cm (Doggett, 1988). Because dwarfing genes are loss-of-function mutations (Quinby and Karper, 1954), but one of the dwarfing alleles (dw3) is unstable and frequently reverts back to its tall wildtype (Multani et al., 2003), we were able to develop isogenic pairs that differed just by one dwarfing gene from three different inbred
Effects of stature on LAI, LI and k
Tall (2-dwarf) plants intercepted significantly more light up to anthesis than their short (3-dwarf) comparisons (P < 0.001, Table 1, Table 2). However, the effects of stature on light interception differed between experiments as indicated by a significant E × S interaction (P = 0.047, Table 2). Differences in light interception between tall and short canopies were much more pronounced in Exp1 and Exp2 than in Exp3 (Fig. 1). This was mainly due to greater LI by the short canopies (especially the
Discussion
Increased plant height due to the absence of the dw3 dwarfing gene has been associated with increased shoot biomass at anthesis (George-Jaeggli et al., 2011). As differences in shoot biomass were largely due to differences in stem biomass, 2-dwarfs had more stem reserves per grain available for translocation which was beneficial for grain size and (dependent on environmental and genetic background effects on grain number) led to increased grain yield in some cases (George-Jaeggli et al., 2011).
Conclusion
dw3 negatively affected sorghum shoot biomass mainly via a reduction in stem mass. This was previously shown to have negative effects on seed size which in some backgrounds and environments led to reduced grain yield, but not in others. The factors leading to a biomass reduction also strongly depended on genetic background and environment and were not exclusively due to either amount of light intercepted, canopy light extinction, or radiation use efficiency. While 2-dwarf plants intercepted
Role of the funding source
This research was financially supported by the Grains Research and Development Corporation (GRDC) and the Queensland Government. Neither organisation had any input in experimental design, data collection and interpretation of results, nor in the decision to submit this paper for publication.
Acknowledgement
We gratefully thank Scott Chapman for his input into the conception of these experiments and Colleen Hunt and Greg McLean for their advice on data analysis.
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