Dear  Fariya Abubakari

I think the best stage for determining P use efficiency in grain legumes is physiological maturity (end of seed filling phase), at which maximum dry biomass was achieved. At this stage you can calculate P use efficiency for plant biomass as well as for grain yield by measuring P content of vegetative tissues and grains.

Regards

Kazem Ghassemi-Golezani 

Dear Fariya Abubakari

For nutrition requirement of plants, booting stage is considered the best. For nutrient use efficiency, economic yield (grain yield, fodder yield etc) is used for which physiological maturity of crops is essential.

Very interesting question Fariya. My friend , Dr Ghafoor has already provided you some intriguing pints to consider. I also agree with points of Kazem about the P-requirement at seed filling stage , since seed formation consumes maximum metabolic P to be used effectively . At the time of reproductive growth, remobilization of P from senescing vegetative tissues is typically the main source of P for sink tissues, as P uptake by roots decreases, as a result of declining root growth and depletion of available soil P. In crops, a very large fraction of the P present in vegetative parts is remobilized to the grain, and soil P availability at this stage has a relatively small effect on grain yield (Römer & Schilling, 1986; Rose et al., 2008).The whole-plant changes in P economy during crop development highlight the fact that, besides an efficient use of P in processes that lead to accumulation of biomass, optimal allocation and efficient reallocation of P will also contribute to high PUE. In this review we explore opportunities for improvement of PUE by analysing the size and identity of P pools in plant tissues and their functional importance, as well as their dynamics, including re-use elsewhere in the plant.

Here is excerpts of some interesting work in on PUE:

Limitation of grain crop productivity by phosphorus (P) is widespread and will probably increase in the future. Enhanced P efficiency can be achieved by improved uptake of phosphate from soil (P-acquisition efficiency) and by improved productivity per unit P taken up (P-use efficiency). This review focuses on improved P-use efficiency, which can be achieved by plants that have overall lower P concentrations, and by optimal distribution and redistribution of P in the plant allowing maximum growth and biomass allocation to harvestable plant parts. Significant decreases in plant P pools may be possible, for example, through reductions of superfluous ribosomal RNA and replacement of phospholipids by sulfolipids and galactolipids. Improvements in P distribution within the plant may be possible by increased remobilization from tissues that no longer need it (e.g. senescing leaves) and reduced partitioning of P to developing grains. Such changes would prolong and enhance the productive use of P in photosynthesis and have nutritional and environmental benefits. Research considering physiological, metabolic, molecular biological, genetic and phylogenetic aspects of P-use efficiency is urgently needed to allow significant progress to be made in our understanding of this complex trait. , published in New Phytologists, 2012, DOI: 10.1111/j.1469-8137.2012.04190.x

Dear  scientist,

thanks all for the interesting answers

P is better to determined in legume seeds at maturation and dry seeds. Since, phytate  is fulfilled during late mature seeds 

Dear Fariya Abubakari,

As Caser Ghaafar Abdel points out that the main reservoir of Phosphorus is grain legume seeds is the mature seed as phytate. Phytate is notable for immobilizing Zinc and Iron needed for human nutrition but is also an energy reservoir needed for high seed vigor. 

The phenological distribution of P uptake is different that either Nitrogen or Potassium. Much of the critical uptake is in the seedling stages rather than the Nitrogen vegetation and Potassium more during the flowering reproductive. For optimizing legumes this Phosphorus must be available early. The other issue is soil application can be largely immobilized so I suggest you concentrate on seed starter P for your studies. 

For this reason I would suspect identifying what is going on in the first 5 weeks of development would be of critical interest. When looking at Phosphorus efficiency they say only 15% of the Phosphorus applied as fertilizer is absorbed by the plant. When looking at this mycorrhizae can completely change the efficiency of scarce soil Phosphorus use being up to several times more efficient for improving Phosphorus absorption that mycorrhizae unaided root systems.

It is demonstrated that mycorrhizae also favor the critical Biological Nitrogen Fixation process. 

If I was looking to get critical information in this area I would concentrate on the time zone of 4 to 5 weeks considering the critical importance of this element at that stage for aggressive rooting and at maturity if resources permit. 

I believe the most efficient P usage is as seed starter application and not applying to try to remediate the whole soil situation. Ideally the starter could be combined with optimized Rhizobial inoculant a helping dose of mature compost and some mycorrhizal inoculant. 

One of the biggest problems with getting efficiency in a legume system is micronutrient issues since the Rhizobial system is more demanding in that area. Luckily the use of specially designed seed treatment can assure the small amounts of micronutrients needed for the Legume system. 

To review some of these issues like phenology and seed treatment and starter nutrient  I suggest the book Modern Soybean production.

Mycorrhizae are important for efficiency of P use studies suggest to optimize crop production a nonmycorrhizae plant may need 7 times the P supplement compared to a fully inoculated mycorrhizal plant. 

Good luck on your studies I would like to hear more as you get more results and if I can I will aid my thought which I hope will be useful for your professional growth. 

Sincerely Paul Reed Hepperly