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Fig. 8.4 Effect of phosphate starvation on plant senescence and yield (Adapted from Batten and
Wardlaw 1987 ). Under ample phosphate supply ( solid line ) chlorophyll content, which is a proxy
for photosynthetic activity, decreases slowly for the first 40 days post anthesis (DPA). Grain dry
weight ( dashed line ) increases during this period and reaches a plateau at about 50 DPA. Only in
the latest stages of grain filling does chlorophyll breakdown accelerate, even though complete
degradation is not reached. Under phosphate deficient conditions ( dotted line ) chlorophyll content
declines steeply starting at about 20 DPA and is completely degraded by 30-40 DPA, indicating
that the flag leaf is fully senesced. Due to this early and enhanced senescence, the flag leaf is
unable to provide photosynthates to the developing grain. Grain filling ( dash - dotted line )is
truncated by about 20 days, and final grain dry weight reaches only about 60 % of the weight under
full phosphate supply
quantities of sulfate actually remain in the leaf, though organic sulfur compounds
appear to be exported from the senescing leaf (Watanabe et al. 2013 ). Breeding
efforts to improve nutrient use efficiency need to take into account the inevitable
compromise between nutrient mobilisation and maintenance of leaf functionality,
especially photosynthesis, as already discussed for the stay-green phenotype.
Rather than gross changes, fine-tuning of senescence processes to the actual growth
condition and for each crop are more likely to produce reliable increases in crop
nutrient use efficiency.
Plant Systems Biology Approaches to Nutrient Depletion
Responses
In order to understand and eventually to manipulate nutrient use efficiency in crops
it is necessary to understand the interplay of nutrient availability, nutrient depletion
induced senescence and nutrient use efficiency. As these are not simple causal
relationships but rather dynamic networks of multiple factors which cross-influence
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