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.