Modern 92-46 had greater relative seed production by reducing stem&leaf allocation
Previous studies suggest that modern crop varieties have higher population yields, but lower competitive ability and individual fitness (Zhang et al. 1999; Zhai et al. 2016; Weiner et al. 2017). Our first prediction was based on these findings and was supported by our results in this research, which showed that old Monkhead had significantly greater stem&leaf and root biomass than modern 92-46, but the two wheat varieties had similar seed production per individual plant (Fig. 2). Static allometric analysis suggested that there were no significant differences in root-total allometric relationship between Monkhead and 92-46 regardless of fertility levels (Table 2, Fig. 3a-b). However, Monkhead had greater relative allocation to stem&leaf biomass than modern 92-46 (i.e. greater intercepts of the stem&leaf biomass – total biomass regression, Table 2, Fig. 3c-d), and lower relative allocation to seed biomass (i.e. lower intercepts of the seed biomass – total biomass regression, Table 2, Fig. 3e-f). This is to say, modern 92-46 consumed less resources / biomass to produce the same amount of seeds compared to old Monkhead, reflecting more efficient conversion of vegetative biomass to reproductive biomass (Weiner 2004).
Our results provided evidence that there was a trade-off between non-reproductive and reproductive allocation for these wheat varieties (Zhang et al. 1999). However, our findings were not in line with previous studies that found root redundancy in old grain plant varieties (Siddique et al. 1990; Semchenko et al. 2007), and did not confirm our previous experimental finding that old Monkhead had more root redundancy than modern 92-46 (Zhu and Zhang 2013). To be caution, however, these previous studies tested root redundancy or allocation patterns using absolute root biomass or biomass ratio perspectives, and indeed these analysis approaches have been criticized that they confound effects of plant size on resource allocations because plant growth is allometric (Weiner et al. 2009; Qin et al. 2012; Du et al. 2020). It is possible that the magnitude of root redundancy of old Monkhead was over-estimated in our previous study (Zhu and Zhang 2013). The presence of stem&leaf redundancy in old Monkhead in the present study may simply reflect greater tiller number in individual plants (Fig. 2a), and an increased cost of stem&leaf biomass in producing unit seed biomass. Moreover, light competition is likely to be more intense than nutrient competition in our system because plants tend to allocate more resources to stems and leaves to obtain size-asymmetric advantages over neighbouring plants (Schwinning and Weiner 1998; Zhang et al. 1999), resulting in a greater risk of stem&leaf redundancy.