Abstract
Darwinian agriculture (also called Evolutionary agroecology) which
focuses on the trade-off between individual fitness and community
performance, can provide an efficient approach to enhance crop
production. However, evolutionary or ecological processes, in particular
the tragedy of the commons in crops, remain poorly understood. We used a
pot experiment to examine whether wheat plants fell into this ‘tragedy’
when confronted with roots of an intra-variety neighbour and how the
magnitude was influenced by soil fertility. Two varieties (old landrace
Monkhead and modern variety 92-46) were selected for this study. Root
competition scenarios were set by growing two plants of the same variety
with a mesh or plastic partition. Soil fertility gradients were set by
adding nutrient solutions with low or high application frequency.
Biomass were tested and allometric relationship were analyzed to reveal
resource allocation pattern. Old Monkhead had similar relative
allocation to root biomass with modern 92-46, but had greater relative
allocation to stem&leaf biomass and lower relative allocation to seed
biomass. The presence of neighbour affected allometric relationships
only in 4 out of 24 allometic comparisons, which all appeared in low
fertility treatments. Fertilizer addition increased stem&leaf biomass
allocation of Monkhead and decreased root and seed biomass allocation at
the same time. By contrast, stem&leaf and seed allocation of 92-46 was
not changed by fertilizer addition although root allocation was
decreased. Our results demonstrated that there was a trade-off between
non-reproductive and reproductive allocation for wheat varieties. There
was not notable tragedy of the commons in wheat varieties, although a
small degree was observed in Monkhead. Moreover, the occurrence and
magnitude of the tragedy was dependent on resource availability and
variety resource-use strategy. Our results underlie the importance of
understanding the mechanisms behind the tragedy of the commons in crop
varieties with different ecological strategies, and provide insights
into the role of Darwinian agriculture in global food production.
Keywords: Root competition; Resource allocation; Growth
redundancy; The tragedy of the commons; Trade-offsIntroduction
In recent years, researchers or plant breeders have applied evolutionary
or ecological theories to improve crop yields while maintaining or
increasing agricultural sustainability, an approach called Darwinian
agriculture (Denison et al. 2003; Denison 2012) or Evolutionary
agroecology (Weiner et al. 2010; 2017). This approach can provide an
efficient and correct solution to global crop risks through group
selection for crop genotypes that have low individual fitness and high
population yields (Zhang et al. 1999; Denison et al. 2003; Denison 2012;
Weiner 2019).
In crop stands, genotypes of high competitiveness allocate more
photosynthetic products to vegetative organs for capturing limiting
resources, suppress weak competitors (e.g. Donald’s ideotypes), and thus
can be favoured by natural selection (Donald 1968). However, high
individual competitive ability can have negative effects on other
individuals and results in declines in population yield (Zhang et al.
1999; Weiner et al. 2017). It has been recognized that breeders should
practice group selection to maximize the collective performance of crops
(Weiner et al. 2010; Denison 2012). It has long been argued that
high-yielding ideotypes should be weak competitors (Donald 1968). For
example, the dwarf cereal varieties which contributed greatly to the
green revolution (Borlaug 2007), are less competitive for light compared
with taller varieties (Jennings and de Jesus 1968).
When competing for a common pool of resources crop plants increase
growth of vegetative organs at the expense of reproductive growth (Zhang
et al. 1999; Gersani et al. 2001; Maina et al. 2002; Padilla et al.
2013), resulting in a phenomenon called ‘tragedy of the commons’ (Hardin
1968). We expect that crop plants of higher competitiveness and growth
redundancy have greater expression of the tragedy of the commons.
However, direct empirical studies testing sensitivity to the tragedy of
the commons in crop varieties remain rare.
The tragedy of the commons can arise if plants can detect and
discriminate between their own roots and roots of their neighbours (i.e.
self/non-self discrimination). The magnitude of the tragedy of the
commons caused by root competition in crop plants may decline in fertile
soil, because high soil fertility can decrease nutrient limitation for
plant growth and alleviate competition below-ground between individual
plants (O’Brien et al. 2005; Schenk 2006; de Kroon et al. 2012). On the
global scale, plant species can be put along a continuum with
exploitative and conservative growth strategies as two ends according to
their resource environment they adapt to (Reich 2004; Wright et al.
2004). Exploitative species are characterized by traits related to
resource-rich habitats including high resource acquisition and
individual competitiveness, while conservative species have the opposite
characteristic combination (Maire et al. 2009; Wooliver et al. 2016). We
expect that crop varieties would possess different sensitivities to
competition with neighbour plants if they are selected artificially
under different resource (water and fertilization) conditions (Zhu and
Zhang 2013; Weiner et al. 2017). However, little is known on how the
magnitude of the tragedy of the commons varies in relation to resource
availability.
Previous studies on the tragedy of the commons often involve treatments
that provide a constant rooting space and amount of nutrients per plant
(Gersani et al. 2001; Maina et al. 2002; O’Brien et al. 2005). However,
this results in plants that share rooting space with a neighbour
potentially having access to twice as much volume as plants grown alone
(Schenk 2006; Semchenko et al. 2007; Semchenko et al. 2017). Thus,
effects of detection of neighbours may be confounded with effects of
size of rooting volume for each competitor (Hess and de Kroon 2007;
Semchenko et al. 2007). Previous studies have shown that plant
production increases in response to the increase in the volume to which
a plant has access (McConnaughay and Bazzaz 1991; Hess and de Kroon
2007). To solve this problem, Semchenko et al. (2007) used a ‘mesh
partition’ to separate two plants in a pot which halved the volume but
allowed competition, in which each plant had access to the same volume
as plants separated by a ‘plastic partition’.
In this research, we adopted the method of Semchenko et al. to set up
mesh partition and plastic partition (Semchenko et al. 2007), examining
the responses of two varieties from different eras to the presence of
intra-variety neighbours and changes in soil fertility. Old Monkhead was
the local landrace and modern 92-46 was released in 2000. Both were
widely grown in semi-arid areas in Gansu Province; modern 92-46 has a
smaller root system but higher grain yield than old Monkhead (Zhu and
Zhang 2013). We make the following predictions: (1) modern 92-46 has
more reproductive allocation and less allocation to non-reproductive
components than old Monkhead; (2) old Monkhead has a greater expression
of the tragedy of the commons than modern 92-46; (3) the occurrence and
magnitude of the tragedy of the commons is dependent on soil fertility.