Abstract
The biological improvement of fertilizer nitrogen use efficiency (NUE) is hampered by the poor characterization of the phenotype and genotype for crop N response and NUE. In an attempt to identify phenotypic traits for N-response and NUE in the earliest stages of plant growth, we analyzed the N-responsive germination, respiration, urease activities, and root/shoot growth of 21 Indica genotypes of rice (Oryza sativa var. indica). We found that N delays germination from 0 to 12 h in a genotype-dependent and source-dependent manner, especially with urea and nitrate. We identified contrasting groups of fast germinating genotypes such as Aditya, Nidhi, and Swarnadhan, which were also least delayed by N and slow germinating genotypes such as Panvel 1, Triguna, and Vikramarya, which were also most delayed by N. Oxygen uptake measurements in the seeds of contrasting genotypes revealed that they were affected by N source in accordance with germination rates, especially with urea. Germinating seeds were found to have endogenous urease activity, indicating the need to explore genotypic differences in the effective urea uptake and metabolism, which remain unexplored so far. Urea was found to significantly inhibit early root growth in all genotypes but not shoot growth. Field evaluation of 15 of the above genotypes clearly showed that germination rates, crop duration, and yield are linked to NUE. Slow germinating genotypes had longer crop duration and higher yield even at lower N, indicating their higher NUE, relative to fast germinating or short duration genotypes. Moreover, longer duration genotypes suffered lesser yield losses at reduced N levels as compared to short duration genotypes, which is also a measure of their NUE. Together, these results indicate the potential of germination rates, crop duration, urea utilization and its effect on root growth in the development of novel phenotypic traits for screening genotypes and crop improvement for NUE, at least in rice.
Highlights
Nitrogen is quantitatively the most important nutrient input for intensive crop production, and the improvement of Nitrogen use efficiency (NUE) is an important economic and environmental goal (Hakeem et al, 2011; Sutton and Bleeker, 2013)
The germination rate is defined as the time taken for 50% seeds to germinate (t1/2)
A comparison of all the genotypes at the t1/2 time point is shown in Table 1, which highlights the variation in the germination rates between the genotypes ranging between 22 to 55 h in distilled water, 21 to 53 h in media without N, 21 to 55 h in media with ammonium chloride, 18 to 58 h with ammonium nitrate, 22 to 62 h with potassium and calcium nitrate and 22 to 67 h with urea. These data clearly show that the germination rate of each genotype varied uniquely in response to each form of N in the media, which was best resolved and quantified in terms of the delay in the time taken for 50% seeds to germinate (t1/2)
Summary
Nitrogen is quantitatively the most important nutrient input for intensive crop production, and the improvement of Nitrogen use efficiency (NUE) is an important economic and environmental goal (Hakeem et al, 2011; Sutton and Bleeker, 2013) While this is generally true for all crops, rice is important, because it is the third most produced and consumed food grain in the world, and because of its lowest NUE among the cereals (Norton et al, 2015), consuming most of the cultivated land in India and about half of all N fertilizer used in Indian agriculture (Abrol et al, 2017). By osmotic titrations of the growth stages, it was shown that mitochondrial energetics via oxygen consumption regulate vegetative and reproductive branching, which determine the life span and yield in Arabidopsis (Sitaramam and Atre, 2007)
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