Chapter 5: THE ENHANCEMENT OF FUNDAMENTAL PHOTOSYNTHETIC PROPERTIES, GROWTH AND YIELD IN MR219 AND MR263 RICE VARIETIES VIA EARLY-STAGE CO2 ENRICHMENT TO THE SEEDLINGS BEFORE TRANSPLANTING (THESIS)

SUMMARY OF FINDINGS, RECOMMENDATION AND GENERAL CONCLUSION

The present research employs a novel strategy by briefly enriching CO₂ (eCO₂) in the rice plant briefly during its early seedling stage before being transplanted into a conventional farming area, rather than continuously supplying CO₂ as commonly practised. This study has proven that a CO₂ source derived naturally from baker's yeast (Saccharomyces cerevisiae) fermentation is a useful carbon source in performing the CO₂ enrichment routine for rice. Both older (MR219, released in 2001) and newer (MR263, released in 2011) varieties respond largely as anticipated when they receive nearly double the concentration of CO₂ (~800ppm) than regular ambient (aCO₂) during early seedlings growth.  The effects gained at subsequent active vegetative and mature phases are to a lesser extent since CO₂ enrichment is halted right before the seedlings’ transplant. The observed or measured responses in biomass partitioning and yields result from changes in the leaves' structures and biochemical reactions in the mesophyll cells.

In the first experiment, the eCO₂ seedlings consistently result in a significant increment in overall biomass accumulation, particularly root mass. Plant height and leaf properties such as number, length, width, and thickness also increase considerably (above 10%) in eCO₂ seedlings, while leaf thickness and total chlorophyll content change minimally. In general, eCO₂ produces leaves with larger stomatal dimensions encompassing complexes’ length, width, and the size of the aperture, but with reduced stomatal density.  There is a mix of responsiveness between the two varieties physiologically, but in essence, eCO₂ results in leaf with an enhanced rate of electron transport, PSII efficiency, CO₂ assimilation capacity and efficiency, as well as intrinsic water use efficiency but a marked reduction in terms of stomatal conductance rate and efficiency of using absorbed photons to assimilate CO₂. These findings show that eCO₂ treatment enhances the early vigour of rice seedlings which is one of the traits required for high-yielding crops where the seedlings can develop rapidly, faster crop canopy establishment and increase competitiveness.

In the second experiment, the eCO₂-experienced rice plants do not have any notable physical flag leaf properties changes, including thickness, length, and width for both varieties. Even though there is a significant tendency for aCO₂ plants to be higher physically, biomass accumulation in any organs does not change nor differ. Interestingly, even after not receiving high CO₂ treatment for about three months, the number of tillers, panicles number, spikelets number, percentage of filled spikelets, 1000 grain weight and potential yield per area could increase in both varieties. Early-stage CO₂ enrichment not only increased rice seedlings' growth and produced robust seedlings to transplant to the field, but its also increased rice plant establishment, photosynthetic performance, and grain yield.

Some further steps and recommendations could be employed to boost the current results, such as:

i. Applying a higher amount of nitrogenous nutrients such as urea to stimulate the plants to produce more leaf biomass to utilise a higher amount of carbon available for fixation, further increasing the growth and establishment of the rice seedlings.

ii.The use of mycelium bags to supply a source of CO₂ to replace the priming technique using yeast is also suggested for improvement. This is because, in addition to being able to produce a more stable source of CO₂.

In conclusion, the eCO₂ priming technique can help to increase rice yields and improve crop productivity more practically and sustainably. This method is recommended for rice farmers to use along with regular rice cultivation practices which are easy, cheap, and applicable. It has the potential for enhancing the efficiency of photosynthesis and improve growth and yield in rice plants, thus may contribute to the development of more sustainable and efficient agricultural practices that can meet the growing demand for food.