Agriculture Reference
In-Depth Information
(Benlloch-Gonzalez et al.
2014
). The basic development of a root system is given
by its genetic predisposition but conditions such as, for example, soil nutrient
availability or soil moisture content will greatly affect root growth as soon as
growth commences (Rich and Watt
2013
).
All C in roots comes from the re-allocation of photosynthates from
photosynthesising above ground biomass with up to 50 % of all the C fixed being
transported below ground during early development (Gregory et al.
1996
). Consid-
ering that C3 crops increase their light-saturated CO
2
uptake rates on average by
~13 % when grown under CO
2
enrichment (Ainsworth and Rogers
2007
), the
amount of fixed C allocated to roots increases in plants growing under high CO
2
and this can lead to changes in root architectural traits (Pritchard and Rogers
2000
).
For example, high [CO
2
] significantly increases the root biomass and roots often
undergo the greatest relative dry weight gain among all plant organs (recently
reviewed by Madhu and Hatfield
2013
). CO
2
enrichment also changes the vertical
distribution of roots in a way that more roots accumulate in the top soil layers as
compared to the deeper layers in the soil. This leads to a shallower root system with
greater root density in the top layers (Pritchard and Rogers
2000
; Pritchard
et al.
2006
; VanVuuren et al.
1997
). Although this may increase access to nutrients
which are generally concentrated in topsoils (e.g. Ho et al.
2005
), it may also have
undesirable consequences where rooting access to subsoil water is important (Lilley
and Kirkegaard
2011
).
Changes in root architectural traits will affect the spatial patterns of soil nutrient
exploitation. A dense root system with many lateral branches and root hairs is
considered to be better for nutrient acquisition as compared to plants with a sparse
root system because of their greater root surface (Kong et al.
2013
). Total nutrient
uptake should therefore benefit from increased root length and mass under CO
2
enrichment. However, it has been reported that root physiological activity often is
reduced in crops grown under CO
2
enrichment (Fitter
1996
; Bassirirad et al.
1996
),
with potential repercussions on specific uptake capacity of roots. Uptake systems
for nutrients such as N in the form of nitrate and ammonium (Miller and Chapman
2011
), or S in the form of sulphate (Hawkesford and De Kok
2006
), are now well
characterised and the specific transport systems are subject to physiological regu-
lation. Whilst it is likely that changes in the carbon to nutrient balance in plants
affect such regulation, it is unknown how growth under elevated [CO
2
] affects
nutrient transporters and their regulation.
Decreased Nitrate Reduction
Another hypothesis regarding the decrease in (specifically) plant N nutrition was
recently put forward by Bloom et al. (
2010
). Their paper showed a significant
decrease of nitrate reduction under elevated [CO
2
] in wheat. The hypothesis was
that the decrease in photorespiration affected nitrate assimilation whereby a number
of mechanisms could contribute (Bloom et al.
2010
). Yong et al. (
2007
) reported
