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stalling growth (Wulff-Zottele et al. 2010 ). Mineral nutrient deficiencies also result in
ROS accumulation, which triggers stress and senescence responses such as the
accumulation of anthocyanins. During this initial response phase, resupply or acqui-
sition of nutrients can restore normal cellular functions and rescue the plant, though
yield reductions are likely as biomass accumulation was reduced during the time of
nutrient deprivation.
Continued starvation of one or more mineral nutrients leads to an emergency
response phase. The plant irreversibly switches its developmental programme to
maturation and senescence (NuDIS; Watanabe et al. 2010 ). This transition marks a
point of no return. Resupply of nutrients in this phase does not reverse the
maturation programme leading to flower formation, seed ripening and terminal
leaf senescence. The plant apparently invests all remaining resources in producing
as many seeds as possible. The programme actually resembles that of developmen-
tal senescence (Fig. 8.1 ), which also irreversibly propagates from the vegetative to
the generative phase when the decision for flower formation has been made.
However, mineral nutrient starvation shifts the timing for senescence to an earlier
time point, thus shortening the individual life span.
The effect of NuDIS on yield can be dramatic in crop plants. This has been
demonstrated for example for potassium deficiency (Armengaud et al. 2004 ;
Amtmann and Armengaud 2009 ), phosphate deficiency (Batten and Wardlaw
1987 ) or nitrogen depletion (Parrot et al. 2010 ). For example, potassium exerts
multiple functions in metabolism and is the second most abundant mineral in plants
after nitrogen. Deficiency leads to multiple negative effects on osmoregulation,
photosynthesis, protein synthesis, enzyme activities and results in light-dependent
chlorosis and necrosis of leaf tissues, all of which impair growth and yield. Wheat
grown on insufficient phosphate senesce far more rapidly and have drastically
reduced seed yield (Batten and Wardlaw 1987 ). The flag leaf, which provides 51-
89 % of grain phosphorus (Batten et al. 1986 ;Fig. 8.4 ), starts to rapidly senesce about
20 days post- anthesis (DPA). This early and steep decay in chlorophyll content is
associated with slower grain filling, probably due to inadequate carbon supply from
reduced photosynthetic capacity. About 40 DPA, flag leaves of phosphate-depleted
plants are essentially devoid of chlorophyll and unable to perform photosynthesis for
carbon supply to the grain. Grain filling plateaus at about 60 % of the potential grain
dry weight result in a yield penalty of about 40 %. Under sufficiently high phosphate
supply, chlorophyll decay also starts about 20 DPA, although at a much slower rate
and accelerates only at about 40 DPA when grains have already reached about 80 %
of their final dry weight. Under phosphate-depleted conditions seed filling stops at
about 35 DPA while under phosphate-sufficient conditions grain filling continues for
an additional 20 days. Thus, early senescence in vegetative tissues truncates seed
maturation and reduces yield (Batten and Wardlaw 1987 ; Lynch and White 1992 ;
Lauer et al. 1989 ; Fig. 8.4 ).
Notably, under optimal phosphate supply conditions, approximately 30 % of
wheat flag leaf chlorophyll is intact when grain filling is complete, and therefore a
substantial amount of leaf nitrogen remains immobilised in the leaf (Fig. 8.4 ; Batten
and Wardlaw 1987 ). Similarly, it has been shown in Arabidopsis that N and P are
substantially but incompletely mobilised during leaf senescence, while significant
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