Journal of Experimental Botany, Vol. 53, No. 378, pp. 2217-2224,
November 1, 2002
© 2002 Oxford University Press
Are there associations between grain-filling rate and photosynthesis in the flag leaves of field-grown rice?
Received 12 March 2002; Accepted 18 June 2002
1 Robert Hill Institute, Department of Molecular Biology and Biotechnology, University of Sheffield, Western Bank, Sheffield S10 2TN, UK
2 Crop, Soil and Water Sciences Division, International Rice Research Institute, PO Box 933, 1099 Manila, Philippines
Abbreviations: 
PSII, quantum efficiency of photosystem II; Chl, chlorophyll; DAF, days after flowering; DW, dry weight; IRRI, International Rice Research Institute; LHCII, light harvesting complex of photosystem II; NPT, new plant type; PPFD, photosynthetic photon flux density; PSI, photosystem I; PSII, photosystem II; RGFP, rapid grain-filling phase; Rubisco, ribulose bisphosphate carboxylase-oxygenase; Pmax, light-saturated rate of net CO2 assimilation.
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Abstract |
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Rate of grain filling in terms of dry mass accumulated per panicle per day was measured in field-grown rice in the dry season in the Philippines and compared to rates of light-saturated photosynthesis per unit leaf area (Pmax) measured at 350 µl l–1 CO2 for 21 d after flowering. Five new plant type (tropical japonica) varieties (NPT) and one indica variety (IR72) were used and these gave some variation in rates and patterns of grain filling. A rapid grain-filling phase (RGFP) occurred approximately 10 d after flowering in most varieties. There was no consistent relationship in any variety between the rate of grain-filling and Pmax and chlorophyll content, both of which remained mostly unchanged throughout grain filling. Significant declines in the amount of total leaf protein and ribulose bisphosphate carboxylase-oxygenase (Rubisco) occurred, but these did not occur at the same time as the RGFP in all varieties. A decrease in the ratio of chlorophyll a/b preceded these changes and a transient rise in chlorophyll content was also observed in four varieties at this time. There was no significant change in leaf non-structural carbohydrate content during or following the RGFP. It is concluded that the decline in Rubisco and protein content in NPT was not reflected in photosynthetic activity. Hence in these field experiments Rubisco accumulated to a level in excess of photosynthetic requirements, serving as a store of nitrogen for grain filling.
Key words: Key words: Grain filling, leaf senescence, photosynthesis, rice, Rubisco.
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Introduction |
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Photosynthesis in rice plants during the grain-filling period contributes 60–100% of the final grain carbon content (Yoshida, 1981). The remainder is made up from remobilized storage carbohydrate in leaf sheaths and culms laid down before anthesis (Yoshida, 1981; Watanabe et al., 1997). To achieve yield potential, metabolic activity within the grain must coincide with maximum activity of source leaves and indeed high yielding cultivars possess leaves which can retain photosynthetic activity well into the grain-filling period (Murchie et al., 1999a). However, an important component (70–90%) of N in rice panicles is derived from remobilized leaf N (Mae and Ohira, 1981; Mae, 1997). Since the majority of leaf N is found in chloroplasts, there is a potential conflict between the maintenance of photosynthate supply and the dismantling of photosynthetic proteins into component amino acids for translocation. There is, therefore, a need to examine the timing of grain filling and photosynthetic capacity more closely.
A recent study (Yang et al., 2000b) identified different patterns of grain filling among rice cultivars according to the rate of development and timing of superior (top of panicle) and inferior (base of panicle) spikelets. There were one or two rapid grain-filling periods (RGFP) depending on whether filling was synchronized between inferior and superior spikelets. Fast, synchronized filling is associated with higher yields and a higher percentage of grains completely filled. Moreover, the rates of grain filling correlated well with cytokinin content of grains and roots. The periods of rapid grain filling can, therefore, influence leaf photosynthesis in two ways. Firstly, the increase in sink activity may permit a greater export of carbohydrate from the leaves and relieve any end-product inhibition of photosynthesis (Winder et al., 1998). Secondly, the hormonal changes associated with the RGFP within the plant may influence photosynthesis by inducing senescence. Chlorophyll loss in leaves has been correlated with leaf cytokinin flux (Soejima et al., 1995) implying that changes in cytokinin content, which are also associated with grain-filling rate (Yang et al., 2000b) may be involved in the onset of leaf senescence.
Understanding the factors that regulate grain filling is important for improving rice yield potential. However, there is disagreement as to whether improving sink size alone in rice crops will also result in an increase in leaf and whole plant photosynthesis (Horton, 2000; Reynolds et al., 2000; Richards, 2000). It is generally recognized that the photosynthetic performance of crop plants needs to be improved in order to increase the rate of biomass production. Because canopy structure has been greatly improved and leaf area index is very high, the increase in assimilation will probably have to arise from the leaf level. Recently, it has been shown that higher yields in wheat were associated with a higher light-saturated rate of leaf photosynthesis (Pmax) (Fischer et al., 1998). In rice there is evidence that panicle removal has no effect upon Pmax (Nakano et al., 1995), but it has not been shown whether timing and variation in panicle activity are related to changes in Pmax.
In this paper, several cultivars of rice have been used which differ according to the rates and patterns of grain filling as identified by Yang et al. (2000b). It was determined whether (a) photosynthetic capacity in the flag leaves is related to the timing and magnitude of RGFP (implying sink regulation of photosynthesis) and (b) whether the onset of senescence is consistent with the timing of the RGFP.
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Materials and methods |
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Growth of plants and sampling method
Rice (Oryza sativa L.) was grown in the field at the IRRI farm in the Philippines as described by Murchie et al. (1999a). Measurements were made in the dry season of 1999. For the period 22 February (5 d before the commencement of measurements) to 16 March (end of the experiment), average radiation was 18.2±1.2 MJ m–2 d–1 and average temperature was 23.08±0.2 °C (minimum) and 29.7±0.3 °C (maximum) (means ± standard error of mean: data supplied by IRRI). Varieties grown were IR72 (indica) and the new plant types (NPT, tropical japonica) IR65998-112-2, IR65600-42-5-2, IR65600-129-1-1-2, and IR68544-29-2-1-3-1-2. For convenience these will be referred to as NPT1, NPT2, NPT3, and NPT4, respectively.
Panicles that headed on the second day from the onset of heading were selected and tagged. The heading period was 4–5 d for an NPT line or cultivar (10–90% panicles headed). A consequence of this was that the panicles sampled were mostly from main stems (approximately 70%) and the remainder from primary tillers (approximately 30%). Previous data showed no significant differences between the main stem and primary tiller in either final panicle weight or the timing or rate of grain filling within a line or cultivar if their panicles headed on the same day (Yang et al., 2000a). Only tagged tillers were used for measurements of photosynthesis, sample collection (1 cm2 leaf discs per sample) and panicle dry weight (DW). Five tagged tillers per variety were used per measurement of protein, ribulose bisphosphate carboxylase-oxygenase (Rubisco), non-structural carbohydrates and chlorophyll (Chl). Due to unavoidable variation in field measurements of photosynthesis and fluorescence parameters up to six tillers per variety were used for the measurements of photosynthesis and 12–15 for PSII. Measurements were started on the day after the onset of flowering (day one) and repeated at intervals of 3 d. Due to variation between varieties with regard to the day of the onset of heading, measurements were staggered: NPT2 and NPT3 flowered on the 27 February, NPT4 and IR72 flowered on the 1 March and NPT1 flowered on the 4 March. On each tiller, one measurement of photosynthesis was made (in the morning) and leaf discs for protein, carbohydrate and Chl analysis were cut from that tiller and frozen in liquid N2 immediately. Three of these panicles were then removed for immediate assay of DW. Panicles were dried at 70 °C and weighed at 2 d intervals until the weight did not change.
Gas exchange and Chl fluorescence
CO2 exchange was measured with a Li-Cor 6400 portable photosynthesis measurement system (Li-Cor, Lincoln, NE) at 350 µl l–1 CO2, flow rate of 500 µmol s–1 and a PPFD of 1800 µmol m–2 s–1. Block temperature was set to 31 °C and ambient humidity was used (approximately 70%). Leaf temperature was typically slightly below that of block temperature. Irradiance was provided by a red LED array (Li-Cor). Some measurements were made using an alternative photosynthesis system: Li-Cor 6200 at ambient temperature and ambient humidity was used, illumination being provided by an external halogen light source and adjusted to give a PPFD of 1800 µmol m–2 s–1. The two systems provided photosynthetic data that closely matched. The light conditions were selected as being representative of that found in the field where typical maximum PPFD was 2000 µmol m–2 s–1. 1800 µmol m–2 s–1 is saturating for photosynthesis in field-grown leaves of IR72 (Murchie et al., 1999a). Leaves were held in the chamber until values of photosynthesis were observed to be as constant as possible, i.e. ‘steady state’, this was generally rapid (2–3 min) due to the similarity of conditions both inside and outside the leaf chamber. Generally, leaves were at steady-state for 1 min before measurements were taken. Chlorophyll fluorescence was measured using a PAM 2000 fluorometer (Walz, Effeltrich, Germany) as described by Murchie et al. (1999a).
Biochemical assays
Leaf discs (1 cm2 per sample) were excised in the field and frozen immediately in liquid N2 for assays of Chl, carbohydrate, protein, and Rubisco content. Chlorophyll content was determined by extraction and assay in 80% acetone (Porra et al., 1989). For non-structural carbohydrates, leaf discs were taken in the field just before dusk. Extractions were made in perchloric acid (HClO4) and assayed by enzyme-coupled reactions monitored at 340 nm essentially as in Leegood (1993) with modifications as in Murchie et al. (1999b). Rubisco was assayed by SDS-PAGE essentially as described by Makino et al. (1994) except that densitometry was used for the analysis of the gels rather than formamide extraction. Leaf discs were ground to powder in liquid N2 and 1 ml of a buffer was added which contained 100 mM HEPES/HCl (pH 7.6), 1 mM EDTA, 5 mM MgCl2, 5 mM dithiothreitol, 1 mM phenyl methyl sulphonyl fluoride, 0.1% (v/v) Triton-X-100, and 20 mg ml–1 polyvinylpolypyrrolidone. These were centrifuged at 13 000 g for 15 min at 4 °C and the supernatant removed for analysis. Total leaf protein content was assayed by the binding of Coomassie and measurement of absorbance at 595 nm (Bradford, 1976). For this, Bio-Rad protein assay dye reagent was used (Bio-Rad Laboratories, Munich, Germany); 70 µl of undiluted dye reagent was added to 10 µl of sample and total volume made up to 1 ml with purified water. This was left for 15 min and absorbance at 595 nm was measured. For calibration of this assay, purified Rubisco was used (wheat Rubisco, kindly provided by Dr Martin Parry, Rothamsted, UK). Portions of the extract to be assayed were completely denatured by incubating for 3 min at 100 °C after diluting in a ratio of 1:1 with a buffer containing 0.0625 M TRIS/HCl (pH 6.8), 10% glycerol, 5% (w/v) SDS, 5% (v/v) ß-mercaptoethanol, and 0.1% (w/v) bromophenol blue. The amount of Rubisco was assayed by SDS-PAGE with a Bio-Rad mini Protean II apparatus. Gel dimension was 100x80x1 mm. A sample volume corresponding to 7 µg of total protein was resolved on a 10% polyacrylamide gel alongside 5.6 µg purified Rubisco. Gels were stained with Coomassie brilliant blue R-250, dried and scanned on a flatbed scanner (Hewlett Packard Scanjet 4c, Hewlett Packard, California, USA), and images analysed using the software package Optimas (version 5.2) with the profile 1 gel densitometer application (version 1.1) (Optimas Corporation, Washington, USA). Gaussian transformations were performed for the peaks in each gel lane, and the area under each peak compared to that of the purified standards. When tests were carried out using varying amounts of either purified Rubisco or crude extract, a linear relationship was seen between amount per lane and area under each Rubisco peak for the range 0.5–15 µg total protein applied to gel. A sample of pellets following the initial centrifugation were tested for the presence of membrane-bound Rubisco: pellets were washed in extraction buffer and then incubated in the presence of 0.1% Triton X-100 for 5 min, re-centrifuged and the supernatant analysed for the presence of Rubisco. Contamination of the insoluble fraction was negligible (data not shown).
Statistical analysis
Where shown, single factor analysis of variance was performed using Microsoft Excel.
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Results |
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Due to the sensitivity of photosynthetic parameters to temperature, irradiance and humidity, the data were carefully checked against environmental data provided by the IRRI weather station. There was no correlation between a sustained change in weather conditions and changes in Pmax, protein content, Chl content or grain-filling rates.
Figure 1 shows the pattern of DW accumulation in panicles of each variety. A lag phase and an exponential phase were clearly seen in NPT2, NPT4 and IR72. These were less distinct in NPT1 and NPT3. The duration of measurements (18–20 d) was sufficient to reveal rapid grain-filling phases (Yang et al., 2000a, b) although as Fig. 1 shows this did not extend to the maturation phase (Yoshida, 1981). However, due to the biphasic nature of grain filling in NPT varieties, the superior spikelets will have been completely filled during this period (Yang et al., 2000b).
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Figure 2 shows the daily rate at which panicles accumulated DW during the grain-filling period in the five rice varieties and compares this to parameters of photosynthesis and leaf composition. Spikelets were not categorized during weighing and so it was not possible to distinguish filling of inferior and superior spikelets, although some biphasic behaviour could be seen (IR72). The most important parameters derived from these data are (a) the maximum rate of grain filling and (b) the time point at which this occurred. The RGFPs of the panicle were clearly identified in all varieties except NPT1 and allowed good comparison with leaf biochemical parameters. NPT1 showed the lowest overall rate of grain filling, and this variety showed a gradual increase in grain-filling rate, rather than clear maxima. The RGFP occurred earlier in IR72 (6 DAF) than the NPT varieties (10–13 DAF). NPT2, NPT3, and NPT4 had higher maximum rates of grain filling, peaking at around 0.3 g DW panicle–1 d–1 while IR72 peaked at 0.2, although only one of these was statistically significant (NPT3; P <0.10). These data are similar to those of Yang et al. (2000b) who used three of the same varieties (IR72, NPT1, NPT2).
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Pmax was measured and results are shown in Fig. 2. There were only small changes over the 3 week period. NPT1, NPT3 and NPT4 showed virtually no change. IR72 showed the greatest decline, from 32 to 27 µmol CO2 m–2 s–1. There was no correlation between Pmax of each variety and the maximum grain-filling rates. Measurements on leaves penultimate to the flag leaf were also carried out concurrently with flag leaf measurements and showed similar patterns although rates of Pmax were lower (data not shown). Chlorophyll fluorescence yield, measured under ambient conditions and natural sunlight confirmed the absence of significant changes in flag leaf photosynthesis (Fig. 3). The efficiency of photosystem II (
PSII) decreased by less than 20% between 1 and 16 DAF.
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Changes in total leaf protein content are shown in Fig. 2. Despite the relatively constant rates of photosynthesis, there were significant reductions in protein content of the leaf during the grain-filling period in all cultivars. Over the time period measured, NPT2, NPT3 and NPT4 lost 50% or more of their total leaf protein. The changes in the content of Rubisco were broadly similar to those for total protein content, with some differences: whilst the total protein over the first 7 DAF either declined or remained unchanged, the Rubisco content was maintained at the same level or increased slightly during this period. The reductions in Rubisco content were greater than those for total protein. In NPT2 and NPT3 approximately 70–80% Rubisco was lost by 19 DAF. The onset of loss of total protein content occurred at 10 DAF in all varieties except IR72.
There were only small changes in Chl content of the leaves during grain filling. There was a rise in Chl at 7–10 DAF in all varieties except NPT1, and this was followed by a decline a few days later. The rise in Chl occurred at the same time as a decrease in Chl a/b ratio, indicating the preferential synthesis of chlorophyll b. This could have arisen from an increase in the number of light-harvesting complexes. In principle, the changes in Chl a/b are sufficient to account for the increase in total Chl if the shift in Chl a/b was entirely caused by a synthesis of LHCII. In NPT2 and NPT3 there were indications that the increase in Chl was accompanied by an increase in protein content. The decline in Chl a/b occurred at the start of the RGFP. In the case of NPT1, the decline occurred much earlier.
Measurements were made to determine whether the changes in sink activity were reflected in altered leaf carbohydrate content (Fig. 4). Sucrose, starch, glucose, and fructose were measured. The major carbohydrates were sucrose and starch and these are shown with values for total non-structural carbohydrates (Sucrose+starch+glucose+fructose). Three key time points were chosen: 1, 4 and 13 DAF. Samples were taken at the end of day when accumulation of sucrose and starch is at its highest in rice leaves (P Horton and EH Murchie, unpublished data). Changes in sucrose for all varieties were not significant (P 
0.10) over this time period for any variety. Sucrose levels in NPT1 were higher (P 
0.10) than all varieties (except NPT3) at all times. Starch levels declined dramatically in all NPT varieties between 1 and 4 DAF (although this was not significantly different in IR72, NPT1 and NPT2 (P 0.10)) and between 4 and 13 DAF in IR72. The accumulation of starch at pre-heading and its loss following anthesis is consistent with previous observations (Yoshida, 1981). Starch levels in IR72 at 1 DAF were two to three times lower than all other varieties. Although total NSC showed a decline between 1 and 4 DAF in all NPTs it was only statistically significant (P 0.10) in NPT1. IR72 had a lower total NSC than NPT1 and NPT3 at 1 DAF and lower than all varieties at 13 DAF. NPT1 had a higher total NSC content (P 0.10) than all varieties at all time points except NPT3 and NPT4 at 13 DAF, which was due mostly to the higher sucrose content of these leaves. This may be linked to the especially poor grain-filling rate of NPT1. The conclusion from these data is that there were no major changes in leaf carbohydrate profile between 4 and 13 DAF over which time RGFP was under way in most varieties.
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The appearance of the rapid phase of grain filling in NPTs did not coincide with a change in Pmax in the flag leaf. Notably, despite the large fluctuations in the rate of grain filling, Pmax was almost unchanged throughout the grain-filling period in these varieties. Between 4 and 13 DAF the levels of end-of-day leaf sucrose and starch were also unchanged, consistent with the suggestion that there was no control over leaf photosynthesis or leaf carbohydrate reserves by the RGFP. A drop in levels of leaf starch at the onset of grain filling was the only detectable change in photosynthetic carbon metabolism. This, however, did not coincide with a change in Pmax, except in NPT2. The implications of this are that panicle sink strength was not an immediate factor in determining the photosynthetic productivity of source leaves.
Although the majority of grain carbon derives from photosynthesis operating during grain filling (Yoshida, 1981), source–sink interactions between leaf and grain are complicated by the existence of an intermediate store of starch within the leaf sheath and culm (Watanabe et al., 1997). This starch reserve is laid down during panicle initiation and exported during the early stages of grain filling. This may have the effect of de-coupling photosynthesis from grain filling.
Despite the fact that grain-filling rate of NPT varieties was equal to (or in the case of NPT3 greater than) IR72, all of the NPTs possessed a lower Pmax. The failure of photosynthesis in NPT to respond to a large sink demand may have been caused by an inherent limitation at the source leaf or an inability to transport large quantities of assimilates. It is argued that the latter is unlikely since there was little evidence of carbohydrate accumulation in the leaf during grain filling. NPT1 had a higher sucrose content than other varieties at 1 and 4 DAF, but at this point the grain-filling rate was low among all the plants tested including IR72. Additionally, Pmax was lower in NPT at all points during development, suggesting an intrinsic limitation on photosynthesis.
Any correlation between the RGFP and the decrease in Rubisco and total leaf protein (symptomatic of leaf senescence) was inconsistent. In NPT2 there was a clear decline in protein and Rubisco content at the point of the RGFP (10 DAF). In NPT3 these changes occurred before the RGFP and in IR72 the drop in protein coincided with the RGFP whilst that of Rubisco occurred 3 d later. In NPT1, which had no defined RGFP, there was still a decline in total protein content at 10 DAF, as for NPT2, NPT3 and NPT4. If RGFP is causally related to leaf senescence then the timing of events is dependent on the variety used. In conclusion, grain sink strength itself did not appear to exert control over the onset of senescence in the flag leaf in all the varieties tested. The similarity in the timing of the decline in total leaf protein is remarkable however (it occurred at day 10 in four of the varieties) and is speculated to be under hormonal regulation (Yang et al., 2000b; Soejima et al., 1995).
A striking feature of these data is that the alterations in Rubisco content occurred with no concurrent decline in Pmax, implying that more Rubisco was present than needed for in situ photosynthesis. This is in contrast to previous studies which concluded that Rubisco limits Pmax at ambient CO2 levels in rice plants during leaf development (Makino et al., 1985, 1997). Moreover, two of the Rubisco amounts given in Fig. 1 (NPT1 and NPT2 at 9 DAF) were particularly high at around 6 g m–2: equivalent to those seen in a previous study (Makino et al., 1994). Additionally, previous work shows a higher N content (approximately 15%) and greater leaf thickness (approximarely 20%) in an NPT variety compared to IR72 (Peng et al., 1998). These data show that the Rubisco content of NPT was higher than that of IR72 in young leaves of NPT1 and NPT2. Similar results have been seen in the laboratory (P Horton, EH Murchie, S Hubbart, unpublished data). Leaves of rice plants in the vegetative stage can show a decline in Rubisco content after full leaf expansion (Hidema et al., 1991, 1992), but this was associated with a decline in Pmax. It is likely that the high leaf protein content observed in the present studies allows remobilization to proceed whilst still permitting high carbon assimilation rates. This interpretation is consistent with the suggestion that an important function of leaves within rice canopies is to be stores of N for grain development (Sinclair and Sheehy, 1999). Thus flag leaf senescence may be specifically adapted such that the leaf sustains, for long periods, the supply of C and N needed for grain development. Leaves with a high specific leaf N content are known to retain chlorophyll and protein content into the grain filling period (Borrell et al., 2001). Such an interpretation has consequences for the desirability of delaying senescence in rice leaves. Gan and Amasino (1995) manipulated leaf cytokinin levels in tobacco and reported plants with delayed leaf senescence and greater biomass and seed production. Since the rice leaves studied here already possess leaves which are photosynthetically productive for the majority of the grain-filling process, delaying senescence may have limited or no effect on grain yield but, importantly, could interfere with the process of N-allocation to developing grains. This would have to be tested by a dynamic study of N movement during grain filling. Delaying leaf senescence may have more value under conditions of stress where premature senescence is more likely or in conditions where leaf N content is lower.
The decline in Chl a/b occurred before that of total protein and Rubisco (Fig. 1). A similar decline in Chl a/b ratio has been observed before, during leaf senescence in rice (Jiang et al., 1999). It indicates a significant alteration in thylakoid composition, with either a net synthesis of LHCII and/or a net degradation of PSII and/or PSI. It is likely to have arisen from an increase in amounts of LHCII. The purpose of such a change in Chl organization is unclear. It is interesting to note that the responses of both Rubisco and chlorophyll organization at 9–12 DAF are, in fact, similar to those which have been seen in leaves during acclimation to low irradiance (Walters et al., 1999). These responses are known to be controlled by the redox state of the chloroplast. It is possible that catabolism of leaf protein during grain filling is associated with altered redox signalling within the thylakoid membrane, for example, the demand for ATP relative to NADPH may increase during this period. It has been suggested that this signal transduction pathway interacts with both hormonal and carbohydrate sensing processes (Walters et al., 1999).
Based upon the data obtained here, it is suggested that there are two main stages of senescence in rice flag leaves. Firstly, a rapid decline in protein content which is initiated around the time of RGFP, possibly by a hormonal signal. The first indications of this response appear to be an increase in level of LHCII. This phase is not normally associated with a decline in photosynthetic rate because of the accumulation of ‘excess’ leaf protein. Secondly, a major loss of photosynthetic capacity when grain filling is complete (not shown in his study). Therefore mechanisms governing protein composition, photosynthesis and Chl content of leaves can be ‘uncoupled’ in order that the separate roles of N remobilization and carbon assimilation can be efficiently carried out.
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The regulation of photosynthesis during the grain-filling period in rice plants is poorly understood. A detailed comparison of photosynthetic characteristics and rate of grain filling in several rice cultivars that possessed variation in characteristics of the RGFP has been carried out. In one variety (IR72) the onset of the RGFP coincided with the onset of photosynthetic decline. No relationships were found between photosynthetic capacity and the rate or magnitude of RGFP in any NPT variety. Additionally, the timing of the RGFP was not consistent with the onset of decline in leaf protein or Rubisco content in most varieties. In the four NPT varieties tested, no changes in photosynthetic capacity occurred despite the fact that substantial decline in protein and Rubisco was observed. More work is needed to establish the cause of the difference in photosynthetic characteristics of IR72 and NPT varieties.
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Acknowledgements |
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This work was supported by grants from the UK Department for International Development and the UK Biotechnology and Biological Sciences Research Council. We are extremely grateful for help provided by staff at IRRI, particularly members of Dr Shaobing Peng’s laboratory and to Dr Gurdev Khush for use of his experimental plots.
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