Grain yield of irrigated and upland rice as a function of irrigation regime, fertilizer level and inoculation with AMF
To test the impact of inoculation with AMF on rice growth and yield at different inorganic fertilization levels and under different watering regimes, field trials were set up over two successive years in Fanaye (Senegal). For both irrigated and upland ecotypes, inoculation with AMF significantly increased grain yield (Fig. 1). An LME analysis revealed that the effects of irrigation regime, fertilizer level and inoculation with AMF on grain yield were dependent on the ecotype (Fig. 1; Table S2). Alternate wetting and drying (AWD) irrigation resulted in significant reduction in grain yield compared to continuous flooding (CF), with a larger magnitude for irrigated ecotype (29% yield reduction for irrigated vs 10% yield reduction for upland ecotype). Furthermore, the positive effect of inoculation with AMF on grain yield was generally more pronounced under AWD irrigation than under CF, irrespective of the fertilizer level (Fig. 1, Figure S1). The positive effect of inoculation with AMF was also more pronounced at half the recommended dose of NK (F2; 75:00:30 Kg NPK/ha) as compared to other fertilizer doses, irrespective of the irrigation regime (Fig. 1, Figure S2).
The separate in-depth analyses of each rice ecotype revealed that the effect of irrigation regime, fertilizer level and inoculation with AMF were significantly influenced by the rice variety (Fig. 2; Table S3). For irrigated varieties, Sahel 108, Sahel 202 and IR 64 showed the lowest yield (1452, 1488 and 1218 kg/ha, respectively) under the non-inoculated, unfertilized and AWD condition (Fig. 2A). Sahel 108 and Sahel 202 achieved the highest yield (9558 and 8698 kg/ha respectively) under the inoculated, half recommended dose of NK (F2; 75:00:30 kg NPK/ha) and CF condition, while IR 64 had the highest yield (7295 kg/ha) under the inoculated, recommended dose of NPK (F5; 150:60:30 kg NPK/ha) and CF condition. Under CF, grain yield decreased at half recommended dose of NPK (F3; 75:30:30 kg NPK/ha) as compared to the recommended dose of NPK (F5; 150:60:60 kg NPK/ha), while this pattern was not observed under AWD irrigation for all irrigated varieties (Fig. 2A). Moreover, the application of AWD irrigation caused significant yield losses in the non-inoculated plants at half recommended dose of NPK (F3; 75:30:30 Kg NPK/ha) and NK (F2; 75:00:30 Kg NPK/ha), while inoculation with AMF fully offset those AWD-related yield losses, especially for IR64, Sahel 108 and Sahel 202 (Fig. 2A).
For upland varieties, NERICA 4, WAB56_104 and CG14 showed the lowest yield (484, 683 and 1492 kg/ha, respectively) in non-inoculated and unfertilized plants under AWD conditions (Fig. 2B). CG14 had the highest yield (4913 kg/ha) with the inoculated plants under CF at recommended dose of NK (F4; 150:00:60 Kg NPK/ha), whereas WAB56_104 and NERICA 4 displayed the highest yield (4652 and 4130 kg/ha respectively) with the inoculated plants under CF at recommended dose of NPK (F5; 150:60:60 Kg NPK/ha). NERICA 4 showed substantial reduction in grain yield at half recommended dose of NPK (F3; 75:30:30 Kg NPK/ha) as compared to at recommended dose of NPK (F5; 150:60:60 Kg NPK/ha) under both CF and AWD irrigation. The application of half recommended dose of NPK (F3; 75:30:30 Kg NPK/ha) also resulted in yield losses under CF for WAB56_104, whereas it did not significantly affect the yield of CG14 under both CF and AWD irrigation (Fig. 2B). CG14 exhibited yield losses with the non-inoculated plants under AWD irrigation as compared to the non-inoculated plants under CF, however inoculation with AMF compensated for yield losses caused by AWD irrigation whatever the fertilizer level. This pattern was also observed with WAB56_104 at the recommended dose of NPK (F5; 150:60:60 Kg NPK/ha) and half recommended dose of NK (F2; 75:00:30 Kg NPK/ha). In contrast, NERICA 4 did not exhibit significant yield losses under AWD irrigation whatever the fertilizer level and whether inoculated or not (Fig. 2B).
Inoculation with AMF reduces yield losses due to AWD irrigation and low fertilizer inputs
We then determined the relative yield losses (RYL) of irrigated and upland rice varieties inoculated or not that were caused by reduced watering and fertilization by using the yield of non-inoculated plants under CF at the recommended dose of NPK (F5; 150-60-60) as the yield reference.
At ecotype level, substantial RYL were induced by AWD irrigation at all fertilizer levels for irrigated rice, whether inoculated or not. For upland rice, RYL caused by AWD irrigation were observed at all fertilizer levels in the non-inoculated plants, whereas inoculation with AMF fully offset those AWD-related RYL at half recommended dose of NPK (F3; 75-00-60) and at recommended doses of NK and NPK (F4 and F5; Fig. 3).
For both ecotypes, reduction in fertilizer levels under CF caused significant yield declines in the non-inoculated plants, except for the recommended dose of NK (F4). However, inoculation with AMF compensated for yield declines, especially under half recommended dose of NK (F2). Under CF, the yield of inoculated irrigated and upland rice exceeded yield reference at recommended doses of NPK (F5) and NK (F4), while inoculation with AMF was not sufficient to offset yield declines at zero NPK application (F1) for all irrigation regimes (Fig. 3).
For irrigated varieties, the AWD-related RYL in the non-inoculated plants varied from 31 to 83% for IR 64, from 32 to 84% for Sahel 108, and from 24 to 81% for Sahel 202; the greatest loss being observed at zero NPK application (F1; Fig. 4). However, inoculation with AMF reduced the AWD-related RYL for all irrigated varieties whatever the fertilizer level (Fig. 4, Table S4). Hence, the yield loss reduction after inoculation with AMF (RYLRInoAMF) under AWD irrigation ranged from 7% at zero NPK application (F1) to 11% at recommended dose of NK (F4) for IR 64, from 9% at recommended dose of NPK (F5) to 22% at half recommended dose of NK (F2) for Sahel 108, and from 10% at recommended dose of NK (F4) to 36% at half recommended dose of NK (F2) for Sahel 202 (Fig. 4, Table S4). Under CF, the yield of Sahel 202 exceeded yield reference by 17, 15, and 12% for inoculated plants at half recommended dose of NK (F2), recommended dose of NK (F4), and recommended dose of NPK (F5), respectively. Similarly, the yield of Sahel 108 surpassed yield reference by 8, 4 and 5% with the inoculated plants at half recommended dose of NK (F2), recommended dose of NK (F4), and recommended dose of NPK (F5), respectively (Fig. 4).
For upland varieties, the AWD-related RYL in the non-inoculated plants varied from 5 to 65% for CG14, from 30 to 87% for NERICA 4, from 20 to 83% for WAB56_104 (Fig. 5). The greatest RYLRInoAMF under AWD irrigation was obtained at half recommended dose of NK (F2; 39%) for NERICA 4, at recommended dose of NPK (F5; 31%) for WAB56_104, and at half recommended dose of NPK (F3; 24%) for CG 14 (Fig. 5, Table S4). Interestingly, the yield of inoculated plants under AWD surpassed yield reference by 1 to 8% for CG 14, and by 7 to 14% for NERICA 4, depending on the dose of NPK fertilizers used. Under CF, the yield of inoculated plants exceeded yield reference by 2 to 15% for CG 14, by 7 to 14% for NERICA 4, and by 2 to 12% for WAB56_104 (Fig. 5).
Mycorrhizal inoculation effect on yield-related traits of irrigated and upland rice as a function of irrigation regime and fertilizer level
We next determined the mycorrhizal inoculation effect (MIE) on yield-related traits of irrigated and upland rice, and then assessed significant sources of variation in those MIE. Irrigation regime, fertilizer level and ecotype were identified as significant sources of variation in MIE for total biomass (Table 1), while their interactions were not. The irrigated and upland rice showed positive response to inoculation with AMF for total biomass, with higher MIE for rice plants subjected to AWD irrigation as compared to those under CF (Figure S3). Furthermore, irrigated ecotypes had greater total biomass response to inoculation with AMF. Considering the fertilizer level, the highest MIE (0.18) was obtained at zero NPK application (F1), whereas the lowest MIE (0.04) was observed at recommended dose of NPK (F5; Table 1, Figure S3).
For tiller numbers, an ANOVA revealed that only irrigation regime and ecotype had significant influence on MIE (Table 1). Inoculation with AMF resulted in substantial increase of tiller numbers in rice plants under AWD irrigation as compared to those under CF, as shown by their respective MIE (0.124 vs 0.039). At ecotype level, irrigated rice had higher MIE than upland rice. On the other hand, fertilizer level was the only source of variation in MIE for several yield-related traits including panicle number, spikelet fertility and maturity dates. For the latter, a negative MIE was obtained at zero NPK application (F1), while positive MIE was observed at the other fertilizer levels, meaning that the combination of inoculation with AMF and fertilizer application delayed maturity as compared to inoculation with AMF alone (Figure S3). For thousand grain weight (TGW), the assessed factors and their interaction were not significant sources of variation in MIE (Table 1).
Table 1
Results from ANOVA testing the effects of fertilizer level, irrigation regime and rice ecotype on mycorrhizal inoculation effect (MIE) for yield-related traits (total biomass (Biomass), tiller number (Tiller), panicle number (Pan_Num), panicle weight (Pan_Weight), spikelet fertility (Fertility), thousand grain weight (TWG) harvest index (HI), plant height (Height), and maturity dates (Mat_Days)). Different letters in the pairwise comparisons indicate significant differences at p < 0.05.
Factors | Biomass | Tiller | Pan_Num | Pan_Weight | Fertility | TWG | HI | Height | Mat_Days |
Irrigation | F-value | 9.092 | 7.375 | 2.173 | 0.000 | 0.361 | 0.279 | 8.686 | 7.331 | 0.123 |
P-value | 0.003 | 0.008 | 0.143 | 0.999 | 0.549 | 0.598 | 0.004 | 0.007 | 0.727 |
Fertilizer | F-value | 4.640 | 1.923 | 2.800 | 0.312 | 3.685 | 0.897 | 2.118 | 2.501 | 3.412 |
P-value | 0.002 | 0.111 | 0.029 | 0.870 | 0.007 | 0.469 | 0.083 | 0.046 | 0.011 |
Ecotype | F-value | 5.089 | 7.240 | 0.023 | 0.416 | 3.743 | 0.015 | 1.471 | 3.254 | 1.281 |
P-value | 0.026 | 0.008 | 0.880 | 0.520 | 0.056 | 0.901 | 0.228 | 0.074 | 0.260 |
Pairwise Comparisons |
Irrigation | AWD | 0.135 a | 0.124 a | 0.088 a | 0.041 a | 0.034 a | -0.019 a | -0.026 b | 0.033 a | 0.007 a |
CF | 0.068 b | 0.039 b | 0.039 a | 0.041 a | 0.020 a | -0.009 a | 0.053 a | 0.001 b | 0.006 a |
Fertilizer | F1 | 0.183 a | 0.111 a | 0.156 a | 0.002 a | 0.116 a | -0.021 a | 0.036 a | 0.052 a | -0.005 b |
F2 | 0.099 ab | 0.147 a | 0.016 ab | 0.003 a | 0.025 ab | -0.034 a | 0.066 a | -0.005 b | 0.007 ab |
F3 | 0.115 ab | 0.025 a | 0.094 ab | 0.086 a | -0.002 b | -0.013 a | -0.007 a | 0.011 ab | 0.013 a |
F4 | 0.07112b | 0.0781a | 0.055 ab | 0.089 a | -0.007 b | 0.020 a | -0.049 a | 0.011 ab | 0.009 ab |
F5 | 0.0398b | 0.0475a | -0.002 b | 0.023 a | 0.005 b | -0.023 a | 0.022 a | 0.015 ab | 0.007 ab |
Ecotype | Irrigated | 0.12684a | 0.124 a | 0.066 a | 0.018 a | 0.004 a | -0.015 a | -0.003 a | 0.027 a | 0.008 a |
Upland | 0.07649b | 0.039 b | 0.061 a | 0.063 a | 0.050 a | -0.013 a | 0.030 a | 0.006 a | 0.005 a |