Allometric analysis reveals enhanced reproductive allocation in historical set of soybean varieties
Introduction
Seed yield in crops is commonly expressed as the product between shoot biomass and the reproductive allocation through the harvest index (HI; i.e., the ratio of seed to shoot (aboveground) biomass; Donald and Hamblin, 1976) both determined at physiological maturity. These two traits have been commonly used to dissect management and genotypic effects on yield improvement.
Reproductive allocation differs among cultivated species defining physiological strategies between shoot biomass accumulation and its allocation to reproductive organs partially shaped by natural and agronomic selection and plant morphology (Vega et al., 2000, 2001a; Weiner et al., 2009). For example, the relationship between yield and crop or plant growth rate is linear in indeterminate crops such as soybean [Glycine max (L.) Merr.] and chickpea (Cicer arietinum L.) (Lake and Sadras, 2016; Vega et al., 2001a) and non-linear in maize (Zea mays L.) and sunflower (Helianthus annus L.) where morphological constraints impose a ceiling in grain set (Vega et al., 2001a).
In soybean, the contribution of shoot biomass production to seed yield improvement has been large and consistent relative to the influence of HI in retrospective studies comparing historic sets of genotypes (Balboa et al., 2018; Rowntree et al., 2014; Specht et al., 1999). In contrast, the contribution of HI to genetic yield gain has been large and consistent in cereals (Hay, 1995; Slafer, 1994), with the exception of maize in North America, with HI presenting minimal changes over time, one percentage unit per decade (Duvick et al., 2004).
There are two main reasons for inconsistencies in shifts of soybean HI in comparisons of historic varieties. The first one is related to errors and different approaches in the estimation of HI, including intensity and timing of plant sampling (e.g., phenological stages), treatments applied, environmental conditions and genotypic background (Kumudini, 2002; Rowntree et al., 2014). In particular, the loss of leaves in mature soybean is a major source of variation in HI (Hay, 1995; Schapaugh and Wilcox, 1980; Unkovich et al., 2010). The second is that the HI of soybeans is a size-dependent plant trait (Vega et al., 2000), and ratios are a biased measure of reproductive allocation when plant size varies. Thus, allometric analysis is needed to account for size-dependent variation in shoot biomass allocation (Jasienski and Bazzaz, 1999; Niklas, 1994; Pearsall, 1927; Poorter and Sack, 2012). Of special relevance for this paper, Qin et al. (2013) highlighted the bias and instability of HI in comparison to allometric exponents relating grain and vegetative biomass on log-log scales.
Soybean yield improvements are usually associated with seed number rather than seed weight (de Felipe et al., 2016). Seed set occurs between the beginning of flowering (R1) and the onset of seed filling (R5) where the number of reproductive sinks reaches a maximum (Egli and Zhen-wen, 1991). During this period, the rate of reproductive partitioning peaks and the relationship between crop growth rate and seed number is linear (Vega et al., 2001b) but with significant variation among environments and genotypes (Masino et al., 2018; Rotundo et al., 2012). Furthermore, other physiological frameworks include partitioning to reproductive structures and the seed set efficiency at R5 to explain seed number determination (Charles-Edwards, 1984). However, comparison of older and modern soybean varieties have shown similar dynamics in shoot biomass accumulation until R5 where newer cultivars portray greater rates of shoot biomass accumulation during the seed filling period (Shiraiwa and Hashikawa, 1995; Specht et al., 1999).
Historical shifts in biomass allocation patterns in soybean would benefit from an allometric perspective to account for size-dependent allocation. We propose to analyze allocation of biomass to reproductive organs in R5 using allometric analyses in a set of historical soybean varieties. This evaluation will help to better understand the effects of genetic gain in reproductive partitioning as a more accurate alternative to HI at physiological maturity.
Section snippets
Crop husbandry
Field experiments were conducted in Kansas River Valley research station in Rossville, Kansas, United States (39°07′N; 95°55′W). Table 1 shows weather and soil details. Crops were sown after maize on May 12, 2016 and May 18, 2017 with seed inoculated using a commercial strain of Bradyrhizobium japonicum and commercially treated with insecticide and fungicide. Target plant density was 25.5 plants m−2. Crops were rainfed and supplemented with irrigation when needed to avoid water stress. Pest and
Seed yield genetic gain
Experimental seasons showed similar growing conditions for soil characteristics, temperature, water supply, and accumulated evapotranspiration (Table 1). Growing conditions were reflected in similar average seed yield of 2988 ± 100 kg ha−1 in 2016 and 2969 ± 89 kg ha−1 in 2017.
Genetic gain for seed yield was 33.1 kg ha−1 yr−1 (Fig. 1) or 0.74 % yr−1 (Fig. 3) irrespective of N rate. The seed yield vs. year of release regressions were parallel, with an offset of 461 kg ha−1 associated with
Discussion
Seed yield increased linearly for a set of seven commercial varieties released between 1980 and 2013. The overall increase of 33.1 kg ha−1 year−1 in both N conditions of this study is in agreement with those present in the literature (de Felipe et al., 2016; Rowntree et al., 2013; Rincker et al., 2014; Specht et al., 1999). In common with these studies, genetic gain in seed number was the driver of genetic crop yield improvement.
Wilson et al. (2014) found the rate of genetic gain in 57
Conclusion
For our set of varieties and environments, seed yield in soybean increased at a rate of 0.74 % yr−1 between 1980 and 2013 whereas the rate of increase in shoot biomass was only 0.41 % yr−1. New analysis of independent studies reinforced the conclusion that improvement in biomass explains only part of the genetic gain in yield. This means allocation to seed must have increased, but HI did not capture this necessary change in allocation. Allometric analysis at the end of the seed set revealed
Declaration of Competing Interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
CRediT authorship contribution statement
Santiago Tamagno: Writing - original draft, Conceptualization, Visualization, Formal analysis, Investigation. Victor O. Sadras: Writing - review & editing, Conceptualization. Osler A. Ortez: Writing - review & editing, Investigation. Ignacio A. Ciampitti: Writing - review & editing, Supervision, Conceptualization.
Acknowledgments
Authors would like to acknowledge the support of the Fluid Fertilizer Foundation, and the International Plant Nutritional Institute–Global Soybean Project (IPNI GBL-62). This is contribution no. 20-090-J from Kansas Agricultural Extension Station.
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