Agriculture Reference
In-Depth Information
phosphorylated proteins and low relative molecular mass metabolites (Veneklaas
et al.
2012
). In the nucleic acid pool, RNA is usually the largest with 40-60 % of this
P pool (Bieleski
1968
) with ribosomal RNA (rRNA) having the biggest share,
adjusting with growing pattern (Suzuki et al.
2010
; Hensel et al.
1993
; Kanda
et al.
1994
). Nucleic acid and protein turnover and repair has a large P cost
(Raven
2012
). Hence, plants cannot dispense DNA or RNA without affecting
growth (Raven
2008
); a target for use efficiency would be the optimising of the
ribosomal pool size, protein biosynthesis and especially protein turnover
(Veneklaas et al.
2012
).
Phospholipids in cell membranes fulfil structural roles, serve as substrates for
biochemical signals and are required in abundance by photosynthetic tissues and
cell-expanding/-dividing tissues. The replacement of phospholipids by glycolipids
(galactolipids, sulfolipids) in plastids, as result of P deficiency are known (Wasaki
et al.
2003
; Andersson et al.
2003
; Essigmann et al.
1998
; Oono et al.
2011
), in
order to economise the use of P, as the total area of membranes can hardly be
decreased (Veneklaas et al.
2012
). In cyanobacteria and algae their replacement can
be complete (Van Mooy et al.
2006
,
2009
) or partial in rhizobial symbionts (Gaude
et al.
2004
), but in plants consequences of such a replacement remain speculative
(Veneklaas et al.
2012
).
Phosphate uptake seems to be most critical during vegetative growth whereas
a shift occurs in the late vegetative or early reproductive stage and remobilisation
and optimal allocation becomes another resource of P (Rose et al.
2007
;R¨mer
and Schilling
1986
; Veneklaas et al.
2012
). There are rankings in the literature
focusing on early growth (Liao et al.
2008
) or contrastingly, on grain yield (Jones
et al.
1992
). P concentrations in the grain of crops are usually much higher than in
vegetative tissues (Veneklaas et al.
2012
), and seed P reserves occur predomi-
nately as phytate (Lott et al.
2009
; Raboy
2009
; White and Veneklass
2012
),
which are the salts of phytic acid with high affinity to Zn and Fe (Michael
et al.
1980
). Seed P content as well as phytate content differs between wheat
genotypes (Batten
1992
) and declines with decreasing P supply (Mengel and
Kirkby
2001
). High P grain content, especially phytate, is not particularly desir-
able, as it acts as an anti-nutrient in animal and human diet which aggravates the
global problem of mineral malnutrition (White et al.
2012
) and causes environ-
mental problems in the form of phosphate-rich manure or sewage (Raboy
2009
)
once it is removed with the grain as the harvested product from the cropping area.
However, whilst low seed P content or concentration could be appropriate selec-
tion criteria for improved PUE, it is controversial. Grain or seed P reserves
support initial seedling growth until it is supplemented through P
i
uptake by the
developing root system (White and Veneklaas
2012
) and correlated with the
initial root biomass (Zhu et al.
2005a
,
b
). It is questionable if seed coating or P
fertiliser placement could compensate low grain P (Rebafka et al.
1993
), even if
lower root development due to lower seed P reserves can be overcome due to
mycorrhizal infection (Zhu and Smith
2001
).
