Physiological mechanisms underlying growth and nitrogen productivity in rice
Date
2016
Authors
Kariyawasam Batuwaththagamage, Buddhima Chathuri
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Abstract
Nitrogen (N) is one of the most important determinants of crop growth and yield. Associated with increasing global population pressures and food demand, N has become one of the most essential, costly inputs in modern crop production and a major environmental pollutant throughout the world. Thus, identifying crop genotypes with better nitrogen use efficiency has become a prioritized research theme to minimize crop dependency on N inputs and reduce the environmental footprint of agriculture. Nitrogen productivity (NP) can be considered as a useful parameter in measuring the efficiency of N use to produce new biomass. NP has been extensively studied in the field of plant eco-physiology; however, less is known about how NP varies among crop genotypes. The overall aim of my PhD research was to evaluate natural variation in whole-plant growth and NP of rice genotypes bred for contrasting habitats under steady state and limited N supply. To explore how rice genotypes interact with N supply, I first established what N concentrations are needed to create phenotypic variation, using a dose-response experiment. Two N treatments were identified as limiting (0.06 mM) and optimum (2 mM) based on growth performances of a single genotype. Next, I assessed genotypic variation in ten rice genotypes for their capacity to grow under N-limiting conditions by performing a functional growth analysis during early vegetative growth. Based on above approach, three rice candidates (Takanari, IR 64 and Milyang 23) were identified for their ability to maintain growth and NP under N limited conditions. Thereafter, I explored what mechanisms account for their improved performance under low N conditions. The key components defining growth were the efficiency of carbon (C) and N use within plant tissues (leading to higher NP) rather than differences in C and N allocation among above and below ground organs. The extent to which changes in photosynthesis and respiration could explain the natural variability in growth and NP was also investigated. There were no statistically significant differences in leaf-level photosynthetic N use efficiency (PNUE) among the ten genotypes and N levels. However, when considering all genotypes, there were strong correlations for PNUE [as indicated by carboxylation capacity and net assimilation rate (N basis)] with whole plant NP at low N. Further, there was tendency for higher PNUE in the three selected genotypes at low N due to maintenance of photosynthetic capacity at low N along with partitioning more N to photosynthesis (particularly Rubisco and electron transport components) under N limited conditions. There was no consistent pattern in the three key performers for the fraction of C loss at whole-plant level. Further work is needed to investigate to what extent variations in leaf PNUE contribute to differences in whole-plant NP in the three genotypes that performed well under N limiting conditions. My results highlight how understanding genotypic variation for shoot PNUE and radiation use efficiency are likely to be important for understanding variations in NP of rice. The results also highlight the need for future work to better understand the genetic and biochemical basis for enhanced NP under low N, as doing so could be beneficial for producing rice varieties that are more efficient under low N environments.
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nitrogen, nitrogen use efficiency, nitrogen productivity, respiration, photosynthesis, carbon balance, carbon economy, nitrogen economy, plant growth, physiological mechanisms
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