Dry direct-seeded rice (DSR) has emerged as a socio-economically viable and environmentally promising alternative to puddled transplanting to achieve productivity gains with lower water and labor utilization, production costs, and greenhouse gas emissions. Risks of poor and non-uniform crop establishment and higher weed infestations are some of the constraints in the wide-scale adoption of DSR. The rapid and uniform emergence with early vigor can lead to uniform and good crop establishment, which is crucial for attaining full yield potential and suppressing weeds in DSR.
In northwest India, where high yields of rice are quite common, rice is widely grown by the puddled transplanting (PTR) method during the kharif/wet-season (June to October/November). In this method, rice seedlings are first raised in a nursery. At about 25–30 days, seedlings are uprooted and manually transplanted to the puddled (wet-tilled) soil in the main field, and the field is then kept flooded for the majority of the season. This method is preferred because of its several advantages, including assured good crop establishment, weed suppression, and higher nutrient availability under flooded/anaerobic conditions.
Recently, the sustainability of the flooded PTR method in northwest India has been threatened because it consumes a large amount of labor, water, and energy—all of which are becoming increasingly scarce and expensive in the region. All of these factors present foundational challenges to sustaining high levels of rice production.
The puddling operation in PTR also adversely affects the yield of the succeeding wheat crop in rotation by 8% due to its negative impact on soil physical properties. Negative impact of puddling has also been reported on succeeding soybean crop. Moreover, PTR is also associated with emissions of large amounts of greenhouse gas (GHG) in the form of methane, and higher energy consumption. All of these factors demand alternate methods that are labor-, water-, and energy-efficient, cost-effective, and mitigate climate change effects.
Dry direct-seeded rice (DSR) has emerged as a socio-economically viable and environmentally promising alternative to PTR to achieve productivity gains with lower water and labor utilization, production costs, and GHG emissions. In DSR, seeds are directly sown (drill-sown with a machine or manually broadcast) in the main field, instead of nursery raising and transplanting rice seedlings as is the case in PTR. DSR can be established either by (1) sowing in dry soil followed by irrigation (conventional DSR) or by (2) sowing in moist soil after pre-sowing irrigation (locally known as vattar DSR, also known as soil mulch DSR).
Based on field studies conducted in the region, DSR in comparison to PTR provides multiple benefits, including savings in labor (by eliminating the processes of nursery raising, uprooting, and transplanting seedlings), water (18–50%), cost of cultivation (INR 6436–7950 ha−1), and positively impacting succeeding wheat yield (8–10%) in rotation, higher net income, and reduction in global warming potential.
Risks of poor and non-uniform crop establishment and higher weed infestations are some of the constraints in the wide-scale adoption of DSR. The rapid and uniform emergence with early vigor can lead to uniform and good crop establishment, which is crucial for attaining full yield potential and suppressing weeds in DSR.
In DSR, desired crop establishment is constrained by multiple factors, such as soil moisture drying associated with high temperatures, and inundation/flooding caused by monsoon rains during crop emergence/early establishment.
Soil mulch/vattar DSR, an innovative approach, was developed to address the issue of inundation risk because this method reduces the early irrigation requirement for the first 15 to 21 days by conserving soil moisture through the soil mulch effect, and hence facilitates early planting of DSR (i.e., 2 to 3 weeks before the onset of the monsoon) which, in turn, reduces the risk of stand mortality caused by inundating rains during the early phases of crop growth.
However, in this method, the top soil layer (~2 cm) dries up very quickly; hence emergence can be affected by soil moisture depletion if seeds are not placed in the moist zone. In the conventional DSR method, there is a risk of temporary excess moisture stress, especially at places in the field where water stagnates at low-lying places in a non-leveled field or if rainfall occurs during the germination period leading to field inundation.
Sustainable and effective technologies are inevitable to improve the rapid and uniform crop emergence and early growth under DSR. Pre-sowing seed priming is one such technology that suggests that the yield gap in DSR compared to PTR caused by poor crop establishment can be closed with seed priming technology.
Improved seed invigoration techniques, such as seed priming for its positive impacts including rapid and uniform crop establishment, early vigor, and yield gains, have been studied in various crops including mungbean, soybean, sorghum, wheat, and maize, and vegetables such as tomato. Recently, with increased interest in transitioning from PTR to DSR, limited studies have been conducted on rice seed priming.
Seed priming is defined as a pre-sowing treatment that partially hydrates seeds without allowing emergence. Priming often involves soaking the seed in pre-determined amounts of water, called hydropriming. Control of the imbibition rate by osmotic agents such as polyethylene glycol (PEG) is referred to as osmopriming. Similarly, the use of specific salts for priming is called halopriming, and the use of plant growth regulators for priming is known as hormopriming
In rice, although limited studies have been conducted on priming, these studies primarily demonstrated the positive impact on germination/emergence, rate of emergence, root growth, early seedling vigor, and early growth, and a few studies also demonstrated the positive impact on grain yield and quality. Rice seed priming is, therefore, one of the most effective, pragmatic, and short-term approaches for increasing seed vigor and synchronization of germination under different stresses.
Very limited information is available on the impact of seed priming on emergence and seedling growth and yield of DSR under Indian conditions where DSR is established using two methods—conventional and soil mulch DSR. To our knowledge, the impact of seed priming under vattar/soil mulch DSR has not been studied. Therefore, research experiments were conducted with the objective to evaluate the effect of different types and durations of priming on germination, emergence, crop establishment, crop growth, and grain yield of rice under DSR conditions established by two methods.
Our results demonstrated that halopriming with 2% potassium nitrate and hormopriming with 50 ppm gibberellic acid can improve the performance of DSR by improving crop establishment (faster and higher crop emergence), seedling vigor, root density biomass, yield attributes, and grain yield compared to DSR without seed priming.
These results also indicate that halopriming and hormopriming can help overcome the problem of poor and uneven crop establishment in DSR systems, thereby helping attain optimal yield of both conventional DSR and vattar/soil mulch DSR. Osmopriming with PEG increased root mass density and crop growth (higher SPAD value), and reduced spikelet sterility and unfilled grains, but its impact on yield was inconsistent, with a positive impact in 2018 but no effect in 2019.
These findings indicate that halopriming and hormopriming—an easy and affordable technique—should be promoted to make DSR more successful. Further research to quantify the positive impact of priming techniques, especially in farmers’ fields under varying conditions (favorable and stress), would help scaling these seed invigoration techniques in DSR-based systems in India.
Read the study:
Dhillon BS, Kumar V, Sagwal P, Kaur N, Singh Mangat G, Singh S (2021) Seed Priming with Potassium Nitrate and Gibberellic Acid Enhances the Performance of Dry Direct Seeded Rice (Oryza sativa L.) in North-Western India. Agronomy. 11(5): 849.