The occurrence and magnitude of the tragedy of the commons were
affected by soil fertility
In low fertility, old Monkhead showed some tragedy of the commons and
modern 92-46 changed the slope of root-total allometic relationships
when confronted with an intra-variety neighbour, whilst in high
fertility all these patterns disappeared and both varieties showed
consistent allocation trajectories between two partition treatments
(Table 3). Therefore, our results provided support for the third
hypothesis that the occurrence and magnitude of the tragedy of the
commons were affected by soil fertility. Different fertilization effects
on the old and modern varieties were likely to be driven by different
resource strategies and sensitivity to competitive environments
(Semchenko and Zobel 2005; Guo et al. 2012; Wang et al. 2014).
Modern varieties which were subjected to human domestication to improve
seed yield often possess a smaller root system, shorter root length and
smaller diameter of the metaxylem vessels of the seminal roots (Li and
Zhang 1999). These characteristics in modern 92-46 make cooperation
between individuals possible when confronted with neighbour’s roots.
Landraces and old varieties were subjected to long-term natural
selection under high-input nutrient conditions (Roucou et al. 2018), and
thus resulted in exploitative resource strategies represented by higher
growth rates, larger root systems, larger fine-root proportion, and
these landraces are likely limited more by light than soil nutrients.
Different resource strategies may result in between-species differences
in plant – plant interactions, with resource-exploitative old landraces
being more plastic in response to light competition compared with
resource-conservative modern varieties. Our results were consistent with
this idea; old Monkhead showed significant changes in relative
allocation to stems and leaves across fertilization rates, whereas no
change was recorded for modern 92-46 (Table 2, Fig. 3c-d). We deduced
that under low fertility rates, plant communications were likely
mediated by root recognition (Dudley and File 2007; Chen et al. 2012),
because of higher relative allocation to root biomass and lower tiller
numbers in wheat varieties which minimized extensive between-individual
communications at the level of above-ground organs. Under high fertility
rates, the lack of differences in allometric relationships between
plants in two partition treatments potentially reflected that
above-ground sensing had dominated plant communications via the
photochrome system (Smith 2000), and that individual plants could
develop extensive communications even in the plastic partition
treatments due to higher relative allocation to stem&leaf biomass and
greater tiller numbers. However, future work is needed to examine
whether the occurrence and magnitude of the tragedy of the commons
varies with soil fertility by experimentally separating plant
communications between above-ground and below-ground components.