Agriculture Reference
In-Depth Information
Reduced Mass Flow
Transpiration is the evaporation of water into the atmosphere from the leaves and
stems of plants. It is a passive process mainly governed by the vapour pressure
deficit of the atmosphere and the soil moisture content, but mediated by the variable
resistance of the stomata. CO
2
enrichment decreases stomatal conductance leading
to decreased transpiration in plants (Ainsworth and Rogers
2007
). Transpiration has
several functions, and one of its functions is to drive the mass flow of nutrients in
the soil to the rhizosphere and root surfaces (Cramer et al.
2009
). Mass flow driven
by transpiration contributes significantly to the delivery of nutrients to the plant,
and then the translocation within the plant. For example, Barber (
1995
) reported
that mass flow contributed by more than 70 % for N, S, Mg and Ca to the nutrient
concentrations in
Zea mays
with the remaining percentages taken up by diffusion
and interception of roots. The effectiveness of mass flow depends on factors that
influence transpiration. Decreased transpiration rates under CO
2
enrichment will
reduce the mass flow of nutrients in the rhizosphere and hence decrease nutrient
availability for plant uptake, as well as decrease the delivery rate of nutrients to the
above ground plant parts. It was hypothesised early that this decreases mineral
nutrition of plants (Conroy and Hocking
1993
; McGrath and Lobell
2013
).
Evidence that reduced mass flow is partially responsible for decreases in nutrient
concentrations can be provided by comparing the extent to which individual
nutrients decrease under high [CO
2
]. For example, the largest decrease in concen-
tration was found for mobile nutrients such as N, Mg or Ca that are supplied to the
rhizosphere by mass flow. Conversely, the concentration of less mobile nutrients,
e.g. P, and those that are mainly dependent on physiological uptake processes was
decreased to a lesser extent by CO
2
enrichment (H¨gy and Fangmeier
2008
;H¨gy
et al.
2013
; Taub and Wang
2008
). A first conclusion might be that biomass dilution
leads to a decrease in concentration of all nutrients while concentrations of mobile
nutrients are further decreased via decreased transpiration mass flow. A recent
analysis aimed at testing the mass flow hypothesis on synthesised literature data
had to resort to pairing mass flow and nutrient uptake data from different studies,
because “No published studies that simultaneously examined mass flow and nutri-
ent concentration for plants grown in elevated [CO
2
] were available” (McGrath and
Lobell
2013
), thus pointing out a major gap in experimental verification.
Root Architecture and Function
Roots are the first plant organs receiving nutrients from the soil and root morphol-
ogy and architectural traits such as length, depth, branching and curving determine
a plant
s access to nutrients (and other resources such as water) in the soil. Despite
their importance to plant growth, most studies on impacts of climate change vari-
ables such as elevated [CO
2
] have focused on above ground traits rather than roots
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