Does Chloroplast Size Influence Photosynthetic Nitrogen Use Efficiency?

Citation: Li Y, Ren B, Ding L, Shen Q, Peng S, et al. ( Does Chloroplast Size Influence Photosynthetic Nitrogen Use Efficiency? Yong Li 0 Binbin Ren 0 Lei Ding 0 Qirong Shen 0 Shaobing Peng 0 Shiwei Guo 0 Ive De Smet, University of Nottingham, United Kingdom 0 1 College of Resources and Environmental Sciences, Nanjing Agricultural University , Nanjing, Jiangsu , China , 2 National Key Laboratory of Crop Genetic Improvement, MOA Key Laboratory of Crop Ecophysiology and Farming System in the Middle Reaches of the Yangtze River, College of Plant Science and Technology, Huazhong Agricultural University , Wuhan, Hubei , China 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. - 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. Approximately 75% of N is allocated to chloroplasts [1,2] and about 27% of this is in Ribulose1,5 bisphosphate carboxylase/oxygenase (Rubisco) [3,4], which carries out the primary fixation of CO2 in the Benson-Calvin cycle. Thus, Rubisco plays a pivotal role in PNUE as a major repository of N and an enzyme that limits photosynthetic rate under various conditions. Due to the low concentration of CO2 in the atmosphere and the low affinity for CO2, the catalytic effectiveness of Rubisco is poor under ambient conditions [47]. Rubisco may operate significantly below its potential catalytic capacity in C3 plants, suggesting that under high N supply or in high N content leaves, there is excess Rubisco protein serving only as a N storage and not contributing to photosynthesis [812], especially under limiting light. The lower relative Rubisco activity in high N content leaves may thus contribute to a decreased PNUE in such leaves. In full sunlight, photosynthesis in C3 plants is mainly limited by Rubisco activity [11,13,14]. Rubisco activity is related to CO2 concentration in chloroplasts [15], and therefore it has been suggested that the decreased Rubisco activity in high N content leaves is due to an insufficient supply of CO2 [16]. In the diffusion pathway from atmosphere to chloroplasts, CO2 diffuses across a boundary layer above the leaf surface, and then through the stomata into the substomatal cavity. In the substomatal cavity, CO2 dissolves in the water-filled pores of the cell wall and then diffuses through the cell wall, the plasma membrane, the cytosol, and the chloroplast envelope to enter the chloroplast. The rate of CO2 diffusion from the intercellular spaces to the carboxylation sites in chloroplasts is referred to as the mesophyll conductance, gm. It has been demonstrated that gm markedly limits chloroplast CO2 concentration relative to intercellular CO2 concentration (Ci) [1720]. It is thought that chloroplast size would probably affect gm [21]. The conductance in the liquid phase in mesophyll cells is the dominant component of gm [17,22], especially the conductance through the inner chloroplast envelope membrane, which constitutes about one half of total internal resistance [23]. Thus, gm depends upon the conductance per unit of chloroplast surface area and the surface area of chloroplasts facing the intercellular air spaces [22]. Larger chloroplasts are usually correlated with higher N content [24] and would potentially increase gm [25]. A larger chloroplast would also store more leaf N and Rubisco. However, it is not clear whether the extent of the increase in gm is sufficient to provide enough CO2 for activating the increased amount of Rubisco, and thus whether an imbalance between the increases in gm and in Rubisco content contributes to the decrease in PNUE observed in high N leaves. Few studies have specifically investigated the relationship between chloroplast ultrastructure and PNUE that aiming at testing whether larger chloroplasts are related to lowered Rubisco activity and PNUE. We have studied the responses of two rice varieties that respond differently to N supply and provide evidence that links changes in chloroplast size with a deficiency in gm that can explain reduced PNUE and Rubisco activity. Hence, we propose a novel explanation for decreased PNUE under high N supply, and suggest an approach to plant breeding to increase N productivity. Growth response to N supply The response of both Shanyou 63 and Yangdao 6 to the N supply was as predicted (Table 1). Increases in plant biomass were observed at high N in both cases. 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. Leaf sheath and culm mass ratio (SCMR, = leaf sheath and culm biomass/ whole plant biomass) was unresponsive to N supply, except for a decrease under high N supply in Yangdao 6. SLW was also unresponsive to N supply, indicating no alterations in leaf thickness. Photosynthetic variables In both rice cultivars, A, N, NO32 and relative Rubisco content were higher under high N supply compared with low N supply (Table 2). However, the responses of these varieties differed markedly when other variables were measured. Most importantly, PNUE (calculated as A/N) decreased with increasing N supply in Shanyou 63, but did not change significantly in Yangdao 6. The same trends (decrease in Shanyou 63 and no change in Yangdao 6) were observed in A/Rubisco. This phenomenon can also be observed from the relationsh (...truncated)