Researchers questioned whether current improved rice varieties are suitable for organic agriculture. Through an experiment focused on nitrogen use efficiency (organic and inorganic sources) and root system architecture, they concluded that varieties bred for high-nitrogen inputs may not be suitable for organic agriculture — reinforcing the need for varieties to be bred specifically for organic agricultural systems. Here the researchers present their work:
The production and extensive application of N fertilizer to crops worldwide has contributed to major environmental problems due to soil leaching and greenhouse gas emissions that play a large role in ozone depletion. Sustainable agriculture aims to conserve natural resources with the mitigation of climate change, and there is increasing interest to move toward organic agriculture. An important issue regarding the acceptance of organic agriculture is the question of productivity. In addition to readily available ammonium and nitrate ions, the soil of organic agriculture can contain a wide range of organic nitrogen compounds such as peptides, proteins, free amino acids, amino sugars and nitrogen heterocyclic compounds.
Nitrogen use efficiency has been defined in many ways. However, in general, it can be divided into N utilization efficiency and N acquision (uptake) efficiency. Uptake activity and root architecture are the major determinants of acquisition efficiency. The former is facilitated by alterations in growth and development in responses to local and systemic N signals. The role of root traits in nitrogen use efficiency is not well understood, as our basic knowledge of root biology in a soil context is limited due to difficulties in characterizing root morphology and functionally in the field.
Root system exploitation and modification in crops may enable plants to make more efficient use of existing soil nutrients and increase stress tolerance, improving yields while decreasing the need for heavy fertilizer application. The root system architecture (RSA) features are of utmost importance for increasing nitrogen use efficiency of future climate-resilient varieties. From a fundamental point of view, the influence of nitrogen on root development is still poorly understood. Modifying root architecture is a strategy that aims at developing crops that capture nutrients more efficiently and are thus suitable for sustainable agriculture with fewer fertilizer inputs. Needless to say, the ability of plants to quickly and efficiently modulating its root architecture may determine its comparative success and productivity in nitrogen-limiting environments.
Our experiment aimed to study the responses of IR-28 root system architecture to the availability of different forms of nitrogen, including organic at seedling stage, so that we can understand up to what extent our current rice varieties, which are exclusively breed for intensive agriculture, are suitable for organic agriculture. Four different levels of nitrogen nutrition — 50, 75, 100 and 125 percent were imposed using three different forms (NO3-, NH4+ and organic nitrogen (mixture of amino acids)) on rice seedlings. Root architectural responses were measured with the help of 2-D imaging software (Ez-Rhizo).
Root System Architecture
Significant differences were observed due to nitrogen forms and their levels on seminal root length, main root angle and straightness of main root. Among these parameters all were increased with increasing levels of nitrogen except seminal root length. Effects of nitrate and ammonia were similar for seminal root length, and at the 100 percent level they remained at par with Yoshida Solution. However, at the 125 percent level of ammonia, nitrate showed inhibitory effect on seminal root length. The organic form of nitrogen recorded maximum root length, and it was at par with no nitrogen treatment (Yoshida-Nitrogen). Lateral root number, lateral root density and mean lateral root length increased with the availability of nitrogen.
Significantly highest mean values were obtained with the nitrate and ammonia forms though remained significantly lower than 100 percent-N Yoshida solution. The organic form of nitrogen recorded significantly lower values for all traits except mean lateral root length where its effect is similar to nitrate and ammonical treatments. Total root system size was significantly decreased with sole source of nitrate and ammonia as compared to control-I (Yoshida Solution) where both forms were available. The organic form of nitrogen exhibited similar effects to ammonia for root system size. All three forms of nitrogen significantly reduced shoot and root biomass as compared to control-I. The organic form of nitrogen recorded significantly lower root and shoot biomass among all three forms. Lateral root density was found to be the most phenotypic plastic trait due to various nitrogenous treatments while total root system size was identified as the least fluctuating trait.
Our study demonstrated that different forms of nitrogen (ammonia, nitrate and organic (mixture of amino acids)), have remarkable and contrasting effects on root system architecture in rice. To the best of our knowledge, ours is a first-of-its-kind report where organic nitrogen nutrition is compared with inorganic nitrogenous sources on root system architecture. The results indicate that combined nitrogen nutrition through nitrate and ammonia is most suitable for root system and seedling growth of rice as compared to a sole source.
Organic nitrogen nutrition was found least suitable. Even at higher concentration where ammonia and nitrate showed inhibitory/toxic effects, root system of IR-28 in organic nitrogen treatment showed phenotype of nitrogen starvation. The results of the study support the view that we need to breed varieties suited for organic agriculture, and varieties such as IR-28, which is selected and bred for high-nitrogen input intensive agriculture, may not be suitable for organic agriculture. However, to confirm responses of root system architectural traits to organic nitrogen, the study should be replicated with wild and/or landraces of rice, which may have high uptake and assimilation efficiency for organic nitrogen.
Article by Jayesh A. Bhabhor, Kirti Bardhan and D. P. Patel; Navsari Agricultural University, Navsari (Gujarat).