A number of scientists, administrators, and politicians have gravitated to the idea of developing C4 rice  due to the maximum photosynthesis rate of C4 species being higher than that of C3 species. Some have also gravitated to this goal due to the potential increased in water use efficiency that C4 rice might afford, which might allow rice to achieve higher yields under upland management and allow increased acreage with the same amount of water used in conventional flood rice management. The C4 trait has independently evolved in a number of plant species separately and is found in some aquatic species (See Sage et al. (2011). It is feasible for C4 rice to eventually be developed; however, the complexity of capturing necessary agronomic, pest management, and grain quality traits will be far from trivial.

I fully support the answer of Dr.Wilson.The gain from C4 rice  is to get rid of photo respiration which is the problem with C3 rice and increasing water use efficiency and hence yield.

Thank you Lloyd and Muayed

I am not arguing against the efficiency of C4 photosynthesis against C3, but rather I am raising a question as of: will C4 rice be as productive as projected, in the light of the fact that the majority of C4 plants are terrestrial and very few aquatic plants are C4. Does not this imply that the aquatic environment (where rice grows) is not preferred by C4 photosynthesis or in other words C4 rice may not be as productive as we thought?

First of all, I am very much doubtful whether a C4  rice plant can be created  with full genetic complement.  Just changing the mode of carbon fixation alone will not do.  There are many genes that contribute to grain development.  They should function in unison with the switching over to C4 mode.  In rice we have only C3 and not even C3- C4 intermedites Moreover conversion of C3 to C4 or C3 is only  a wild imagination.  It is all like creating sterile root nodules on rice root.  Therefore let us turn around to see the agronomy part of it more seriously.  The photo respiratory -nitrogen cycle may confer certain advantages to C3 plants.  It may not be a wasteful phenomenon.  Nature does not preserve anything unwanted.

Like in Human Genome Project, the so called huge junk genome has been found to be contributory for the functioning of genome as a hole

There are examples of C4 and CAM plants in aquatic environments. In water, especially when there is not a lot of bicarbonate (situation in which biophysical CO2 concentrating mechanisms are not very effective), C4 metabolism may be extremely useful to concentrate/pump CO2 towards rubisco. In fact, lacustrine environments may have been the original habitats of plants capable of such metabolic processes.Some general information on this topic can be found in Giordano et al 2005 Annual Review of Plant Biology and in numerous publications by John A. Raven & co.

Probably the reason why the examples of aquatic plants capable of C4 and CAM photosynthesis is not that there are few such plants, but that there are not many researchers working on such scientific question (who is going to fund basic research anymore?).

A well documented example of an aquatic plant with C4 metabolism (without Kranz anatomy) is Hydrilla verticillata, discovered in the early eighties by George Bowes and co-workers (Holiday, Salvucci, Reiskind). Unfortunately, this work is often forgotten when C4 photosynthesis is discussed.

All this , of course, has little to do with the opportunity of working on C4 rice, which is mostly an ethical and feasibility problem. It is however a good example of how much we need basic research to really understand how things work: read the messages above to see how many things are unclear about C4 photosynthesis and how rudimental is our understanding of the complexity of metabolic interactions within a C4 plant. Only after knowledge of first principles is acquired we can usefully think of ways to use it to fully exploit the physiological potential of crops (this can be extended to any field of science). Basic research first; application later. Unfortunately, funding is for applied, empirical research and not for basic research. It remains a mystery what is going to be applied, when basic research will have been murdered and buried.

Anyway... I do think that a C4 rice plant would most likely be more productive, surely for the decrease in photorespiratory losses and also because C4 metabolism increases, typically, N-, Fe-, light-use efficiency. By how much would productivity increase, I do not know.  Hopefully, there are simulation/hypotheses on this.

Making C4 rice using the classical C4 model seems rather complicated.  The matter would perhaps be more approachable with an inducible, Hydrilla-like C4 metabolism, in which all enzymes are in the same cell and there is no morphological differentiation of tissues with different functions.

The main advantage of C4 photosynthesis and C4 plants - it saves carbon dioxide ... unnecessarily. These plants closed their stomata long and carbon dioxide can not enter the plant. In aquatic plants (such as rice) there is also a lack of carbon dioxide - it is poorly soluble in water - worse than in air .. In this reason the presence of C4 photosynthesis in rice is quite purposeful

 It has been over a year since the C3/C4 discussion was initiated by Dr. Abogadallah, and I thought I would respond to one aspect of what Dr. Giordano stated, that being the following:

"Only after knowledge of first principles is acquired we can usefully think of ways to use it to fully exploit the physiological potential of crops (this can be extended to any field of science). Basic research first; application later. Unfortunately, funding is for applied, empirical research and not for basic research." 

My experience with basic or reductionist research is the it has the greatest impact when it is conducted hand-in-hand with adaptive or applied research. 

A case in point where applied knowledge has not been consistently used to provide an appropriate focus to basic research is the use of QTL type analyses for "yield component traits". Yield component traits are too often secondary in nature and as such are relatively distant from the underlying primary traits that are controlled by genes. As a result QTL analyses for yield component traits almost always end up producing complex and inconsistent correlations. I think Granem et al. (2015), Trends in Plant Science, March 2015, Vol. 20, No. 3, were correct when they stated 

"Future progress in crop breeding requires a new emphasis in plant physiological phenotyping for specific, well defined traits", 

which basically suggests to me that mis-focused reductionist research is unlikely to produce needed breakthroughs.

This issue is highly relevant for the C3/C4 discussion in that a reductionist approach to addressing rice photosynthesis is more likely to succeed if the question or solution to the problem is formulated jointly with scientists who have a greater understanding of how rice photosynthesis is related to crop growth, development, maturation, and yield. Some argue that the best way to improve rice yield performance is to increase the efficiency of the photosystem. But it might be more productive to increase photosynthesis by increasing the efficiency of light interception by changing the canopy structure. It might also be more efficient to increase the early season rate of phytomer production thereby reducing feedback inhibition, which otherwise prevents rice from achieving potential starch and sugar assimilation rates. identifying how to improve rice yield performance requires a detailed understanding and accompanying analysis of the relative importance of each rate limiting step and a detailed understanding of how the current or future modified rice photosystem controls plant grows and develops. Such knowledge is primarily obtained through integration and not via reductionist research.