Chapter 4 Genotypic variation and the effect of N supply on leaf
4.4.1 Effect of N supply and genotypic differences on leaf chemistry,
Table 4.1 shows values of a range of traits related to leaf chemistry, structure and physiology of plants grown on low and high N supply, for all 10 rice genotypes. For several traits (e.g. Ma, Na, Chlorophyll content, gs, area- and mass-based rates of A400, Vcmax, a25, Jmax, a25) there was a significant main effect of
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N supply, whereas for other traits (e.g. N-based rates of A400), N had no overall effect (Table 4.2). The two way ANOVA (Table 4.2) also confirmed that there was no significant N x G interaction term for most parameters except leaf mass per unit area (Ma , p < 0.001) – thus, other than for Ma, the effect of N supply did not statistically differ among the genotypes, when considering all 10 genotypes collectively. Given this, independent sample t-tests were carried out to explore differences between the two N treatments when averaged across all 10 genotypes. Both leaf N per unit area (Na) and chlorophyll (a+b) content significantly (p < 0.001) reduced (by about 40 and 45% respectively) under low N treatment (Tables 4.1 and 4.3) when considering values averaged across all genotypes. There were genotypic differences (p < 0.001) for leaf Na (Table 4.2). Leaf chlorophyll (a+b) content strongly correlated with leaf Na [Leaf chlorophyll (a+b) = 8.27 + 281.527 * Na, r2 = 0.394, p < 0.001] at high N (Fig. 4.4), but not significantly at low N.
Figure 4.4 Leaf chlorophyll (a+b) content is plotted against leaf nitrogen per unit area (Na). Closed and open symbols represent high
and low N supply respectively. Values are means (n=3; ± SE). Relationship between leaf Na and chlorophyll (a+b) content at 2 mM
is indicated by the solid line [Leaf chlorophyll (a+b) = 8.3 + 281.5 *
Na, r2=0.394, p < 0.001] while the non-significant relationship at 0.06
103 Genotype N level mM Leaf Na g m-2 Leaf g m-2Ma Chlorophyll a+b µmol m-2 gs
mol m-2s-1 Cppm i Ci/ Ca µmol mA400, a -2s-1 Anmol g400, m -1 s-1 Aµmol g400, N N-1 s-1 Vcmax, a25 µmol m-2 s-1 Jmax, a 25 µmol m-2 s-1 Jmax, a 25/ V cmax ,a25 Vcmax, N25 µmol gN-1 s-1 RD, a25/ Vcmax, a25 Takanari 2 1.60±0.02 34.2±1.1 496.0±11.7 0.75±0.10 304.8±8.5 0.79±0.02 24.7±2.4 633.5±29.8 13.0±0.4 82.3±4.5 129.5±15.5 1.68±0.19 47.3±2.4 0.008±0.001 0.06 0.88±0.08 35.3±2.9 352.1±26.4 0.63±0.17 302.9±19.1 0.80±0.02 19.9±2.0 569.0±44.9 18.0±0.8 76.0±5.0 120.2±9.1 1.58±0.05 77.9±8.5 0.012±0.002 IR-64 2 1.42±0.07 26.0±0.5 394.9±50.4 0.21±0.04 266.5±15.6 0.67±0.04 17.5±3.2 663.7±130.6 12.7±3.4 76.3±13.7 123.9±5.5 1.93±0.37 53.4±14.0 0.015±0.003 0.06 1.06±0.08 39.8±1.5 226.9±42.9 0.32±0.08 263.5±21.3 0.62±0.08 19.0±2.3 543.0±93.6 15.4±2.5 84.7±13.3 119.3±6.8 1.55±0.20 55.2.±8.8 0.011±0.002 Milyang 23 2 1.43±0.04 28.4±0.6 539.4±81.1 0.26±0.07 280.3±15.7 0.70±0.05 15.1±3.8 539.0±140.1 12.1±2.8 62.0±15.9 137.5±4.4 1.79±0.21 49.8±11.2 0.010±0.003 0.06 0.77±0.08 30.4±1.3 271.5±58.8 0.22±0.04 281.7±7.3 0.70±0.02 16.0±2.6 519.0±68.1 16.1±1.5 63.4±9.0 109.0±13.6 1.67±0.03 66.6±6.6 0.011±0.001 Opus 2 1.69±0.05 35.8±0.4 578.3±47.0 0.33±0.06 281.5±15.6 0.70±0.04 17.5±2.6 487.7±72.9 10.3±2.0 72.6±11.7 141.3±10.5 1.81±0.18 43.1±8.7 0.015±0.004 0.06 0.97±0.04 38.8±3.4 259.0±16.8 0.27±0.06 311.5±4.9 0.76±0.02 13.3±1.7 349.4±39.2 11.2±1.0 50.0±4.5 98.0±7.9 1.84±0.13 47.7±5.3 0.016±0.004 Dular 2 1.45±0.05 28.8±1.1 440.0±73.0 0.26±0.04 256.9±9.3 0.64±0.02 17.3±1.8 560.7±43.4 11.3±0.5 75.1±6.5 129.1±6.4 1.75±0.09 50.4±2.0 0.012±0.002 0.06 0.86±0.05 32.8±1.5 194.8±22.0 0.21±0.04 292.8±14.0 0.73±0.03 13.7±2.4 342.9±34.6 12.2±0.9 54.6±8.0 92.4±8.1 1.71±0.04 50.1±1.7 0.016±0.001 Bg 34-8 2 1.67±0.14 35.5±0.9 424.3±7.3 0.79±0.02 307.5±7.8 0.77±0.02 24.8±3.5 785.4±42.2 17.1±1.3 94.8±11.3 149.2±13.2 1.48±0.01 55.2±8.4 0.008±0.001 0.06 1.08±0.09 43.9±2.7 221.5±7.0 0.46±0.10 310.0±8.5 0.77±0.02 17.1±2.1 392.9±49.5 13.1±0.8 63.8±8.0 112.8±11.5 1.80±0.09 48.4±1.8 0.014±0.001 Koshihikari 2 1.71±0.08 35.4±1.5 471.8±36.9 0.39±0.13 303.3±18.1 0.76±0.05 14.7±3.6 405.6±103.1 9.0±2.4 56.9±13.0 120.3±3.0 1.91±0.32 34.8±8.8 0.015±0.003 0.06 0.80±0.05 33.6±2.7 264.0±30.5 0.21±0.05 297.3±9.3 0.74±0.02 10.1±1.5 309.6±44.5 10.0±1.7 40.5±5.5 68.7±0.0 1.97±0.11 41.1±6.7 0.024±0.004 Akihikari 2 1.61±0.10 32.3±1.2 407.9±26.0 0.47±0.08 275.7±10.7 0.69±0.03 24.3±1.0 750.5±58.7 15.3±1.2 98.5±7.5 151.4±9.7 1.56±0.08 61.2±3.9 0.008±0.001 0.06 0.84±0.01 38.4±0.6 264.4±3.4 0.33±0.10 295.3±10.8 0.74±0.03 13.9±3.0 369.9±75.8 11.3±2.1 52.8±9.8 88.0±0.0 1.63±0.08 45.4±5.9 0.014±0.003 Azucena 2 1.80±0.13 33.2±0.9 506.8±90.5 0.35±0.04 271.4±2.8 0.68±0.01 20.3±1.0 592.8±42.0 10.6±0.2 84.4±4.3 128.7±9.5 1.52±0.06 44.3±2.1 0.010±0.001 0.06 1.18±0.13 46.1±1.6 289.6±36.1 0.35±0.12 265.5±21.7 0.66±0.05 16.6±3.4 367.2±80.9 11.2±3.6 68.9±9.2 113.8±12.9 1.59±0.02 50.7±10.4 0.015±0.002 Nipponbare 2 1.37±0.11 30.2±0.6 442.7±31.0 0.30±0.05 271.5±5.1 0.71±0.02 22.0±3.7 738.3±133.5 17.7±4.2 88.1±13.0 123.3±26.0 1.21±0.20 69.2±15.7 0.010±0.001 0.06 0.83±0.07 36.8±1.2 222.6±47.4 0.31±0.11 303.1±11.3 0.76±0.03 13.1±2.4 384.9±67.2 11.8±1.9 54.2±10.2 98.4±0.0 1.95±0.14 46.9±10.2 0.012±0.002 Mean of all 10 G 2 1.57±0.05 32.0±1.1 470.2±18.8 0.41±0.06 281.9±5.5 0.71±0.01 19.8±1.2 615.7±38.7 12.9±0.9 79.1±4.2 133.4±3.5 1.66±0.07 50.9±3.1 0.011±0.001 0.06 0.93±0.04 37.6±1.6 256.6±14.0 0.33±0.04 292.4±5.4 0.73±0.02 15.3±0.9 414.8±29.3 13.0±0.8 60.9±4.2 102.1±5.1 1.73±0.05 53.0±3.5 0.014±0.001 Mean of three G 2 1.48±0.06 29.5±2.5 476.8±42.8 0.41±0.17 283.9±11.2 0.72±0.04 19.1±2.9 612.1±37.6 12.6±0.3 73.5±6.0 130.3±4.0 1.80±0.07 50.2±1.8 0.011±0.002
(Takanari, IR 64 and Milyang 23) 0.06 0.90±0.08 35.2±2.7 283.5±36.7 0.39±0.12 282.7±11.4 0.71±0.05 18.3±1.2 543.7±14.4 16.5±0.8 74.7±6.2 116.1±3.6 1.60±0.03 66.5±6.5 0.011±0.001 Mean of other seven G 2 1.61±0.06 33.0±1.0 467.4±22.2 0.41±0.07 281.1±6.9 0.71±0.02 20.1±1.4 617.3±54.8 13.1±1.3 81.5±5.4 134.8±4.7 1.61±0.09 51.2±4.4 0.011±0.001 0.06 0.94±0.06 38.6±1.9 245.1±12.4 0.30±0.03 296.5±5.8 0.74±0.01 14.0±0.9 359.6±10.7 11.5±0.4 55.0±3.5 96.0±5.8 1.79±0.06 47.2±1.2 0.016±0.001 Abbreviation: leaf Na =leaf N per unit area, leaf Ma = leaf mass per unit leaf area, gs= stomatal conductance for CO2 diffusion in the leaf measured at 400 µmol mol-1 atmospheric [CO2]
per unit area, Ci=intercellular CO2 partial pressure measured at 400 µmol mol-1 atmospheric [CO2] per unit area, Ci/ Ca = the ratio between intercellular and atmospheric (400 ppm)
partial pressures, A400, a = light-saturated net photosynthesis measured at 400 µmol mol-1 atmospheric [CO2] per unit area, A400, m = light-saturated net photosynthesis measured at 400
µmol mol-1 atmospheric [CO
2] per unit mass, A400, N = light-saturated net photosynthesis measured at 400 µmol mol-1 atmospheric [CO2] per unit leaf N, Vcmax, a25= maximum
carboxylation velocity of Rubisco per unit area normalised to 25°C, Jmax, a25= maximum rate of electron transport per unit area normalised to 25°C, Jmax, a25/ Vcmax, a25 = ratio of maximum
rate of electron transport over maximum carboxylation velocity of Rubisco, both normalised to 25°C, Vcmax, N25 = ratio of maximum carboxylation velocity of Rubisco normalised to 25°C
per unit leaf N, RD, a25/ Vcmax, a25= ratio of leaf dark respiration per unit area normalised to 25°C (RD, a25) to Vcmax, a25. Values are mean (n=3, 4, 5 or 6) ± SE. A two-way ANOVA was carried
out to test any interaction term between N levels and genotypes (see table 4.2). Genotypic differences for A400, a, A400, m, A400, N and Vcmax, a25, Jmax, a25, Jmax, a25/ Vcmax, a25 are further
illustrated in Figures 4.5 and 4.6 respectively.
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Although not statistically significant, there was about 20% reduction in stomatal conductance (gs) under low N compared to high N grown plants (Tables 4.1 and 4.3). Significant (p < 0.001) differences were also found among genotypes for gs (Table 4.2). Despite these differences in gs, neither Ci nor the ratio between Ci and atmospheric partial pressures (Ci/Ca) was influenced by N supply (Table 4.3), suggesting that declines in gs were matched by a decline in photosynthetic capacity.
Table 4.2Results of two-way ANOVA for variables presented in Tables 4.1 and 4.4. F-statistics, degrees of freedom (df) and significance of each factor and their interactions are presented.
N levels Genotypes N levels*Genotypes p value for N levels * Genotypes interaction df 1 9 9 Leaf Na (g m-2) 259.479*** 4.005*** 1.372 0.220 Leaf Ma (g m-2) 52.081*** 7.974*** 4.190*** 0.000
Chlorophyll a+b (µmol m-2) 115.991*** 2.026 0.767 0.647
gs (mol m-2s-1) 4.208* 5.833*** 0.847 0.575 Ci (ppm) 2.776 2.363* 0.723 0.686 Ci/ Ca 2.812 2.901** 0.582 0.809 A400, a (µmol m-2s-1) 14.342*** 2.273* 0.997 0.448 A400, m (nmol g-1 s-1) 31.782*** 2.236* 1.460 0.175 A400, N (µmol gN-1 s-1) 0.017 1.620 1.302 0.254 Vcmax, a25 (µmol m-2 s-1) 16.142*** 2.104* 1.383 0.207 Jmax, a25 (µmol m-2 s-1) 38.879*** 1.459 1.424 0.189 Jmax, a25/ Vcmax, a25 0.799 1.081 1.583 0.133 Vcmax, N25 (µmol gN-1 s-1) 2.073 1.466 0.563 0.822 RD, a25/ Vcmax, a25 2.552 1.897 1.236 0.284 nP 10.797** 1.361 1.210 0.316 nR 1.716 1.515 0.497 0.871 nE 5.761* 1.521 1.208 0.305
Abbreviation: leaf Na = leaf N per unit area, leaf Ma = leaf mass per unit leaf area, gs= stomatal
conductance for CO2 diffusion in the leaf measured at 400 µmol mol-1 atmospheric [CO2] per unit area,
Ci=intercellular CO2 partial pressure measured at 400 µmol mol-1 atmospheric [CO2] per unit area, Ci/ Ca
= the ratio between intercellular and atmospheric (400 ppm) partial pressures, A400, a = light-saturated net
photosynthesis measured at 400 µmol mol-1 atmospheric [CO
2] per unit area, A400, m = light-saturated net
photosynthesis measured at 400 µmol mol-1 atmospheric [CO
2] per unit mass, A400, N = light-saturated net
photosynthesis measured at 400 µmol mol-1 atmospheric [CO
2] per unit leaf N, Vcmax, a25= maximum
carboxylation velocity of Rubisco per unit area normalised to 25°C, Jmax, a25= maximum rate of electron
transport per unit area normalised to 25°C, Jmax, a25/ Vcmax, a25 = ratio of maximum rate of electron
transport over maximum carboxylation velocity of Rubisco, both normalised to 25°C, Vcmax, N25 = ratio of
maximum carboxylation velocity of Rubisco normalised to 25°C per unit leaf N, RD, a25/ Vcmax, a25= ratio of
leaf dark respiration per unit area normalised to 25°C (RD, a25) to Vcmax, a25, nP = fraction of leaf N in
pigment-protein complexes, nR = fraction of leaf N in Rubisco and nE = fraction of leaf N in electron
transport. *p < 0.05, **p < 0.01, ***p < 0.001. Statistics were not performed for nA as that was calculated
using mean values of nP, nR and nE. If above N x G interaction was non-significant, an independent
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There were genotypic variations (Table 4.2) in Ci (p < 0.05) and Ci/Ca (p < 0.01). When considering individual genotypes or averaged across all 10 genotypes, A400, a and A400, m declined at low N (Fig. 4.5A, B and Tables 4.1 and 4.3). Genotypic differences (Table 4.2) were also found for A400, a, (p < 0.05) and A400, m, (p < 0.05). Averaged across all 10 genotypes, A400, N was unaffected by N supply (Fig. 4.5C, Tables 4.1 and 4.3). No correlation was found between A400, N and leaf Na (Fig. 4.7E). Taken together, these observations suggest that, when
Figure 4.5 Bar graphs showing genotypic variation in light-saturated net photosynthesis measured at 400 µmol mol-1 atmospheric [CO
2] (A) per
unit area (A400, a); (B) per unit mass (A400, m); (C) per unit N (A400, N) under
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averaged across all 10 genotypes, there were relatively consistent responses to low N supply.
Table 4.3 An independent samples t-test was carried out to determine if there were significant differences for chemical, structural and gas exchange parameters among N levels at each genotype group [i.e. all 10G, the average of three genotypes (3G) that maintained growth and NP at low N (Chapter three) and other seven genotypes (7G)] and among 3G and 7G at each N treatment.
Parameter All 10 G 3G Other 7 G 2 mM 0.06 mM differences among 2 and 0.06 mM differences among 2 and 0.06 mM differences among 2 and 0.06 mM differences among 3G and other 7G differences among 3G and other 7G Leaf Na (g m-2) *** ** *** n.s. n.s. Chlorophyll a+b (µmol m-2) *** * *** n.s. n.s. gs (mol m-2s-1) n.s. n.s. n.s. n.s. n.s. Ci (ppm) n.s. n.s. n.s. n.s. n.s. Ci/ Ca n.s. n.s. n.s. n.s. n.s. A400, a (µmol m-2s-1) ** n.s. ** n.s. * A400, m (nmol g-1 s-1) *** n.s. ** n.s. *** A400, N (µmol gN-1 s-1) n.s. ** n.s. n.s. *** Vcmax, a25 (µmol m-2 s-1) ** n.s. *** n.s. * Jmax, a25 (µmol m-2 s-1) *** n.s. (0.057) *** n.s. n.s. (0.066) Jmax, a25/ Vcmax, a25 n.s. n.s. (0.069) n.s. n.s. n.s. (0.078) Vcmax, N25 (µmol gN-1 s-1) n.s. n.s. (0.073) n.s. n.s. ** RD, a25/ Vcmax, a25 n.s. n.s. * n.s. n.s. (0.055) nP * n.s. * n.s. n.s. nR n.s. n.s. (0.091) n.s. n.s. ** nE n.s. * n.s. n.s. ** nA n.s. n.s. n.s. n.s. *
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Abbreviation: leaf Na =leaf N per unit area, leaf Ma = leaf mass per unit leaf area, gs= stomatal conductance for
CO2 diffusion in the leaf measured at 400 µmol mol-1 atmospheric [CO2] per unit area, Ci=intercellular CO2
partial pressure measured at 400 µmol mol-1 atmospheric [CO
2] per unit area, Ci/ Ca = the ratio between
intercellular and atmospheric (400 ppm) partial pressures, A400, a = light-saturated net photosynthesis measured
at 400 µmol mol-1 atmospheric [CO
2] per unit area, A400, m = light-saturated net photosynthesis measured at
400 µmol mol-1 atmospheric [CO
2] per unit mass, A400, N = light-saturated net photosynthesis measured at 400
µmol mol-1 atmospheric [CO
2] per unit leaf N, Vcmax, a25= maximum carboxylation velocity of Rubisco per unit
area normalised to 25°C, Jmax, a25= maximum rate of electron transport per unit area normalised to 25°C, Jmax, a25/ Vcmax, a25 = ratio of maximum rate of electron transport over maximum carboxylation velocity of Rubisco,
both normalised to 25°C, Vcmax, N25 = ratio of maximum carboxylation velocity of Rubisco normalised to 25°C
per unit leaf N, RD, a25/ Vcmax, a25= ratio of leaf dark respiration per unit area normalised to 25°C (RD, a25) to Vcmax, a25, nP = fraction of leaf N in pigment-protein complexes, nR = fraction of leaf N in Rubisco, nE = fraction of leaf
N in electron transport and nA = total fraction of leaf N invested in photosynthetic metabolism. *p < 0.05, **p
< 0.01, ***p < 0.001, n.s. - non-significant. p value is shown within brackets in situations where the significance is marginal.
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Given the above findings, how do biochemical activities underpinning A change in response to low N and across genotypes? There was genotypic variation for maximum carboxylation velocity of Rubisco per unit area normalised to 25°C (Vcmax, a25, p < 0.05); however, no significant differences were found for the maximum rate of electron transport per unit area normalised to Table 4.4Average values for the fractions of N partitioned to photosynthesis in 10 genotypes of rice under high and low N supply
Genotype N level (mM) nA nP nR nE Takanari 2 0.36 0.15 ± 0.01 0.15 ± 0.01 0.05 ± 0.00 0.06 0.50 0.16 ± 0.02 0.24 ± 0.03 0.09 ± 0.01 IR-64 2 0.39 0.15 ± 0.03 0.17 ± 0.04 0.07 ± 0.00 0.06 0.35 0.10 ± 0.02 0.17 ± 0.03 0.08 ± 0.01 Milyang 23 2 0.42 0.19 ± 0.02 0.16 ± 0.04 0.07 ± 0.00 0.06 0.45 0.15 ± 0.03 0.21 ± 0.02 0.09 ± 0.01 Opus 2 0.37 0.18 ± 0.01 0.14 ± 0.03 0.06 ± 0.00 0.06 0.34 0.12 ± 0.00 0.15 ± 0.02 0.07 ± 0.00 Dular 2 0.37 0.14 ± 0.01 0.16 ± 0.01 0.07 ± 0.00 0.06 0.35 0.12 ± 0.01 0.16 ± 0.01 0.07 ± 0.01 Bg 34-8 2 0.40 0.16 ± 0.00 0.17 ± 0.03 0.07 ± 0.01 0.06 0.35 0.13 ± 0.01 0.15 ± 0.01 0.07 ± 0.01 Koshihikari 2 0.33 0.16 ± 0.00 0.11 ± 0.03 0.06 ± 0.00 0.06 0.32 0.13 ± 0.02 0.13 ± 0.02 0.06 ± 0.01 Akihikari 2 0.39 0.12 ± 0.02 0.19 ± 0.01 0.08 ± 0.01 0.06 0.35 0.14 ± 0.01 0.14 ± 0.02 0.07 ± 0.01 Azucena 2 0.35 0.16 ± 0.02 0.14 ± 0.01 0.05 ± 0.00 0.06 0.39 0.16 ± 0.02 0.16 ± 0.03 0.07 ± 0.01 Nipponbare 2 0.47 0.17 ± 0.01 0.22 ± 0.05 0.08 ± 0.02 0.06 0.36 0.13 ± 0.03 0.15 ± 0.03 0.08 ± 0.01 Mean of all 10 G 2 0.38 ± 0.01 0.16 ± 0.01 0.16 ± 0.01 0.07 ± 0.00 0.06 0.38 ± 0.02 0.13 ± 0.01 0.17 ± 0.01 0.07 ± 0.00 Mean of three G 2 0.39 ± 0.02 0.16 ± 0.01 0.16 ± 0.01 0.07 ± 0.01
(Takanari, IR 64 and Milyang 23) 0.06 0.43 ± 0.04 0.13 ± 0.02 0.21 ± 0.02 0.09 ± 0.00
Mean of other seven G 2 0.38 ± 0.02 0.16 ± 0.01 0.16 ± 0.01 0.07 ± 0.00
0.06 0.35 ± 0.01 0.13 ± 0.01 0.15 ± 0.00 0.07 ± 0.00
Abbreviation: nA = total fraction of leaf N invested in photosynthetic metabolism, nP = fraction of leaf N
in pigment-protein complexes, nR = fraction of leaf N in Rubisco, nE = fraction of leaf N in electron
transport. Values are mean (n=3, 4, 5 or 6) ± SE. A two-way ANOVA was carried out to test any interaction term between N levels and genotypes (see table 4.2).
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25°C (Jmax, a25) (Fig. 4.6 A and B, Table 4.2). Averaged across all 10 genotypes, Jmax, a25 and Vcmax, a25 were significantly (p < 0.001 and p < 0.01 respectively) reduced by N deficiency (Table 4.3). Jmax, a25 and Vcmax, a25 strongly correlated with leaf Na where relationships were: Jmax, a25 = 40.1 + 56.2 * Na (r2 = 0.513, p < 0.001) and Vcmax, a25 = 24.1 + 32.1 * Na (r2 = 0.207, p < 0.001) (Fig. 4.7A and B). Importantly, these relationships were held only when data of both N treatments were pooled together. The lack of correlation between Vcmax, a25 and leaf Na within a given N treatment suggests that the efficiency of N use in carboxylation varies among genotypes within a given N treatment. Despite this, a two-way ANOVA found no significant differences among genotypes or N levels in the maximum carboxylation velocity of Rubisco per unit N normalised to 25°C (Vcmax, N25) (Tables 4.1, 4.2 and 4.3, Fig. 4.6D, 4.7D and 4.8). The ratio of Jmax, a25/Vcmax, a25 remained largely constant across genotypes and N treatments (Fig. 4.6C, Tables 4.1, 4.2 and 4.3) and no correlation was found with leaf Na (Fig. 4.7 C). The ratio of leaf dark respiration per unit area normalised to 25°C (RD, a25) to
Vcmax, a25 (i.e. RD, a25/ Vcmax, a25) remained largely constant across genotypes and N
treatments (Tables 4.1, 4.2 and 4.3).
Taken together, the above results suggest stomata partially closed in response to low N when average across all genotypes. Yet, constancy of Ci/Ca
was maintained by matching ~20% reduction in gs with proportionally a similar reduction in Vcmax, a25 (Tables 4.1 and 4.3).This indicates a synchrony between reduced CO2 supply and the demand i.e. the reduced photosynthetic capacity under low N. Although the percentage change was not identical, there was a concomitant reduction in Vcmax, a25 and Jmax, a25 along with leaf Na. Accordingly; these results indicate that A and its underlying components were down- regulated to a similar extent at low N.
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Figure 4.6 Bar graphs presenting (A) maximum rate of electron transport per unit area normalised to 25°C ( Jmax, a25); (B) maximum carboxylation
velocity of Rubisco per unit area normalised to 25°C (Vcmax, a25); (C) Jmax, a25:Vcmax, a25 ratio (both normalised to 25°C) and (D) maximum carboxylation
velocity of Rubisco per unit leaf N normalised to 25°C (Vcmax, N25) for 10
genotypes of rice under high and low N supply (n=3-6; ± SE). The values are given in Table 4.1.
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Figure 4.7 Relationships between N per unit leaf area (Na) and (A) maximum
rate of electron transport normalised to 25°C on area basis ( Jmax, a25) ; (B)
maximum carboxylation velocity of Rubisco on area basis normalised to 25°C (Vcmax, a25) ; (C) Jmax, a25:Vcmax, a25 ratio (both normalised to 25°C); (D) maximum
carboxylation velocity of Rubisco on leaf N basis normalised to 25°C (Vcmax, N25)
and (E) light-saturated net photosynthesis measured at 400 µmol mol-1
atmospheric [CO2] on N basis (A400, N). Closed and open symbols represent high
and low N supply respectively. Colour codes for genotypes are as given in Figure 4.4 (n=3-6; ± SE). Relationships between leaf Na and Jmax, a25 [Jmax, a25 = 40.095 +
56.191 * Na, r2=0.513, p < 0.001] and leaf Na and Vcmax, a25 [Vcmax, a25 = 24.081 +
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The absence of any significant N supply-mediated changes in Vcmax, N25 was a surprise, as the reduction in Vcmax, a25 (~20%) was markedly less than the decrease in leaf Na (~40%) at low N (Fig. 4.7B, Tables 4.2 and 4.3). Thus, while N-mediated changes in Vcmax, N25 werenot significant, the possibility remains that N supply did affect N allocation. For example, plants might reallocate N to maintain photosynthetic capacity at low N treatment. This could have been achieved by investing relatively more N in key components of photosynthesis (e.g. Rubisco). Given that, I explored whether the fraction of N in each photosynthetic component [i.e. Rubisco (nR), electron transport (nE) and pigment-protein complexes (nP)] vary across N treatments and genotypes. According to a two-way ANOVA, there was no G x N interaction or statistically significant genotypic differences for any of above fractions (Table 4.2). N allocation patterns of 10 genotypes on average did not significantly change across N treatments (Table 4.3), except the reduction (p < 0.01) in the fraction of N invested in pigment protein complexes (Table 4.3). There was also no difference among high and low N grown plants in the total fraction of leaf N allocated to photosynthesis (nA; Fig. 4.8B, Table 4.4) across all 10 genotypes. Collectively, these results suggest there was no significant difference among 10 genotypes for their patterns of N partitioning to different components of photosynthesis except the reduction of the fraction of N invested in pigment protein complexes at low N.
Finally, the data from the present study was in agreement with the global relationship identified based on Glopnet database (Hikosaka, 2004, Wright et al., 2004) for photosynthetic rate per unit leaf N against Ma as indicated by the solid line in Figure 4.8C. Importantly, N supply did not significantly change the relationship between Vcmax, N25 and Ma.
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Figure 4.8 The relationships between leaf mass per unit leaf area, Ma and (A) leaf N
per unit area, Na . The solid lines in Figure 4.9A indicate the relationship between leaf
Ma and leaf Na (0.06 mM) [Leaf Na = 0.444 + 0.013 * Ma, r2=0.251, p < 0.001] at low N
and [Leaf Na = 0.120 + 0.045 * Ma, r2=0.539, p < 0.001] at high N; (B) total fraction of
leaf N invested in photosynthetic metabolism, nA (C) maximum carboxylation velocity
of Rubisco on leaf N basis normalised to 25°C (Vcmax, N25). The solid line in Figure 4.9C
represents the global relationship between photosynthetic rate per unit leaf N and Ma
(Hikosaka, 2004, Wright et al., 2004), where the relationship for Vcmax, N25 at any given
Ma calculated based on the equations given in Harrison et al. (2009). Colour codes for
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4.4.2 Rapid estimation of Rubisco via Western blotting using