Abstract
High nitrogen (N) supply frequently results in a decreased photosynthetic N-use efficiency (PNUE), which indicates a less efficient use of accumulated Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). Chloroplasts are the location of Rubisco and the endpoint of CO2 diffusion, and they play a vital important role in photosynthesis. However, the effects of chloroplast development on photosynthesis are poorly explored. In the present study, rice seedlings (Oryza sativa L., cv. ‘Shanyou 63’, and ‘Yangdao 6’) were grown hydroponically with three different N levels, morphological characteristics, photosynthetic variables and chloroplast size were measured. In Shanyou 63, a negative relationship between chloroplast size and PNUE was observed across three different N levels. Here, plants with larger chloroplasts had a decreased ratio of mesophyll conductance (gm) to Rubisco content (gm/Rubisco) and a lower Rubisco specific activity. In Yangdao 6, there was no change in chloroplast size and no decline in PNUE or gm/Rubisco ratio under high N supply. It is suggested that large chloroplasts under high N supply is correlated with the decreased Rubisco specific activity and PNUE.
Highlights
The high grain yields of most crops are dependent upon the supply of nitrogen (N) from fertilizers
75% of N is allocated to chloroplasts [1,2] and about 27% of this is in Ribulose–1,5– bisphosphate carboxylase/oxygenase (Rubisco) [3,4], which carries out the primary fixation of CO2 in the Benson-Calvin cycle
There was a decrease in root mass ratio (RMR, = root biomass /whole plant biomass), and an increase in leaf mass ratio (LMR, = leaf biomass /whole plant biomass) in both varieties with increasing N supply
Summary
The high grain yields of most crops are dependent upon the supply of nitrogen (N) from fertilizers. The increasing cost and high energy requirement of such fertilizer, together with the adverse environmental effects of N pollution have stimulated much research activity that aiming towards enhancing the efficiency of its use. An important variable is the intrinsic N-use efficiency (NUE) in plants. A key component of NUE is the photosynthetic N-use efficiency (PNUE), defined as net photosynthetic rate (A) per unit leaf N content. 75% of N is allocated to chloroplasts [1,2] and about 27% of this is in Ribulose–1,5– bisphosphate carboxylase/oxygenase (Rubisco) [3,4], which carries out the primary fixation of CO2 in the Benson-Calvin cycle. Rubisco plays a pivotal role in PNUE as a major repository of N and an enzyme that limits photosynthetic rate under various conditions
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