tests. Tolerant rices gave grain yields of nearly 3 t/ha when iron toxicity VARIETAL REACTIONS OF RICE TO IRON

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1 VARIETAL REACTIONS OF RICE TO IRON TOXICITY ON AN ACID SULFATE SOIL F.N. Ponnamperuma and J.L. Solivas The International Rice Research Institute Los Baños, Philippines 1 Summary A total of 420 rices was screened on an acid sulfate soil in a farmer's field, Albay, Philippines, during three seasons. Forty-one were found to have tolerance or iron toxicity. Iron toxicity was severe in the wet season following the dry season apparently because of strong acidification following soil drying. But the tolerant rices generally fared better than the suscep.tible in both seasons. Yield trials confirmed the tolerance of the varieties identified as tolerant in mass screening tests. Tolerant rices gave grain yields of nearly 3 t/ha when iron toxicity was severe and over 6 t/ha when it was mild. Tolerant varieties may be a substitute for lime on moderately toxic acid sulfate soils. 2 Introduction Iron toxicity is a widespread nutritional disorder of wetland rice associated with excess water-soluble iron (Ponnamperuma et al. 1955). It occurs on acid sulfate soils (Sulfaquepts) and strongly acid Oxisols, Ultisols, Hydraquents, Tropaquents, and Histosols (Ponnamperuma 1958, Howeler 1973, Tanaka and Yoshida 1975, Panabokke 1975, van Breemen and Moormann 1978). Iron toxicity is characterized by the appearance of small brown spots on the lower leaves starting from the tips. Later the whole leaf turns brown, purple, yellow, or orange. Growth and tillering are depressed and 29 1

2 the root system is coarse, scanty, and dark brown (Ponnamperuma et al. 1955, Tanaka and Yoshida 1975). Yield reduction may range from IO to 90% depending on soil, variety, and growth stage of the appearance of symptoms (Gunawardena 1979, Virmani 1978). In acid sulfate soils, iron toxicity is an important growth limiting factor (Nhung and Ponnamperuma 1966, Tanaka and Navasero 1966, Ponnamperuma et al. 1973, van Breemen and Pons 1978). Although iron toxicity can be alleviated by liming and drainage, varietal tolerance is a more simple and economical solution (Ponnamperuma 1974, Ikehashi and Ponnamperuma 1978) especially for the small farmers of South and Southeast Asia. Several workers have shown that marked differences exist in rice varieties in their tolerance for excess iron (IRRI 1971, Ponnamperuma and Castro 1972, Gunawardena 1975 and 1979, Virmani 1978). With the initiation of the Genetic Evaluation and Utilization (GEU) Program (IRRI 1974), mass field screening for iron toxicity tolerance was begun. Mass screening was followed by yield trials of promising rices from greenhouse and field tests. 3 Materials and methods The soil at the experimental site (a farmer's field at Malinao, Albay, Philippines) is a Sulfaquept developed in a recent sulfurous alluvial fan sediment of volcanic origin. Chemically and morphologically it is an acid sulfate soil (van Breemen 1978). The chemical characteristics are in Table 1. It was plowed wet and harrowed after broadcasting 50 kg N/ha and 25 kg P/ha. 292

3 Table I. Some chemical characteristics of the soil at the experimental site, Malinao, Albay, Philippines ph of dry soil (1:l H20) 3.5 ECe (mmho/cm) 1.o CEC (meq/100 g) 9.5 Organic C (%> 1.23 Total N (%) O. 15 Active Fe (%) 2.50 Active Mn (%) EX. K+ (meq/100 g> o. 17 EX. Sob2- (%> 0.27 Available (Olsen) P (ppm) Mass screening The test materials for mass screening were drawn largely from the elite lines of the breeding program of the International Rice Research Institute. Three week-old seedlings raised on a normal seedbed were planted in three 3-m rows at a spacing of 20 cm x 20 cm. A randomized complete block design replicated three times was used. A resistant check and a susceptible check were included in the rices tested. The plots were irrigated with water from a nearby stream. The plants were scored for iron toxicity based on foliar symptoms and general appearance on the scale 1 to 9 (1 = nearly normal plant; 9 = nearly dead or dead plant). A score of 3 or less indicated tolerance; 6 or more indicated susceptibility. 3.2 Yield trials Fifteen promising rices from screening tests were tested each season in replicated yield trials at the field screening site. Three-week old seedlings raised on a normal wet seedbed were transplanted in 2 m X 3 m plots at a spacing of 20 cm x 20 cm. The treatments were replicated 3 times. In later tests the plot size was increased to 3 m x 29 3

4 4 m, and the susceptible check was planted in two rows between plots throughout the field to monitor soil differences. Fifty kg N/ha as urea and 20 kg P/ha as ordinary superphosphate were broadcast and incorporated just before transplanting. The plants were scored at 4 and 8 weeks after transplanting for iron toxicity on the scale 1 to 9. Grain yield was measured at maturity. 4 Results and discussions 4.1 Mass screening Table 2 summarizes the results of three seasons mass screening. The scores at 4 weeks after transplanting were used for rating tolerance because the differences were more marked and the coefficient of variation of the scores was less than at 8 weeks after transplanting. Table 3 lists the names of the tolerant rices. Table 2. Scoring results of field mass screening of rices for tolerance to iron toxicity in an acid sulfate soil, Malinao, Albay, Philippines. Figures show number of varieties rated on the scale 1 to 9 (1 = nearly normal growth, 9 = most plants nearly dead or dead). Varieties tested Season Total IRON Dry O O O O GEU Elite 1979 Wet O O O O GEU Elite 1980 Dry O O O O Total O O O 420 IRON: International Rice Observational Nursery GEU: Genetic Evaluation and Utilization Problems encountered included soil heterogeneity and seasonal differences in expression of toxicity symptoms, but varietal differences were clearly discernible (Table 4). Seasonal differences are evident from the exclusion of IR36, the resistant check, as a tolerant rice in the 1979 wet season (Table 3), although during this season any rice that scored 4 or less was considered tolerant. The severity of iron toxicity in the 1979 wet season was probably due, to the prolonged drying and acidifica- 294

5 tion of the field during the dry months immediately preceding the wet season. Table 3. Varietiesflines found tolerant of iron toxicity in field mass screening on an acid sulfate soil, Malinao, Albay, Philippines 1979 dry season 1979 wet season 1980 dry season BPI RI-2 IR IR36 IR36 IR (IR52) IR O IR IR IRl3423-l7-3-2-l IR IR IR IR IR IET 1444 IR IET 1444 IR B2149-PN Table 4. Analysis of variance for scores for iron toxicity at 4 weeks after transplanting in 3 seasons Mean squares Sources of Season variation 1979 dry 1979 wet 1980 dry Replication 9.319** ** 2.26" Var ie ty I. 083** 2.709** I. 276** Error cv z ** Significant at the 1% level * Significant at the 5% level 4.2 Grain yield The 1979 dry season grain yields of 15 selected rices and iron toxicity scores are in Table 5. IR gave the highest yield (4.5 tfha) and the lowest score (2.5); IR26, the susceptible check, gave the lowest yield (1.7 tfha) and the 2nd highest toxicity score (4.8). Apart from that, there was no correlation between symptoms at 4 weeks after trans- 295

6 planting and grain yield. Seven of the 15 rices yielded significantly more than IR36, the resistant check. Table 5. Iron toxicity tolerance scores and grain yield of selected rices on an acid sulfate soil in Malinao, Albay, Philippines, 1979 dry season Variety/line Score at Grain yield 4 WAT^ (tlha) IR d 4.5 a IR bcd 3.7 b IR abc 3.6 b IR ab 3.4 b BHR bcd 3.4 b BHR abc 3.3 bc IR abc 3.2 bc BHR bcd 2.8 cd BHR abc 2.5 de IR36 (resistant check) 3.3 bcd 2.5 de BHR a 2.3 de BHR abc 2.3 de BHR bcd 2.2 def IR abc 2.1 ef IR26 (susceptible check) 4.8 ab 1.7 f WAT: weeks after transplanting Note: In a column values followed by the same letter are not significantly different at the 5% level. In the 1979 wet season iron toxicity was severe because of soil drying and acidification in the dry months preceding the wet season. The toxicity scores were high and the yields were low (Table 6). 296

7 Table 6. Iron toxicity tolerance scores and grain yield of selected rices on an acid sulfate soil in Malinao, Albay, Philippines, 1979 wet season Varietylline Score at Grain yield 4 WAT' (t/ha) IR bc 2.7 a B2 149-PN bc 2.6 a IR c 2.6 a IR bc 2.5 a IR c 2.4 ab IR bc 2.4 ab BPI R ab 2.2 ab IR bc 2.1 abc IR ab 2.1 abc IR abc 1.8 bc IR ab 1.8 bc IR abc 1.6 cd IR bc 1.0 d IR abc - IR a - Note: WAT: weeks after transplanting In a column values followed by the same letter are not significantly different at the 5% level. But IR , IR and IR46, which were among the top yielders in the dry season experiment, were among the best in the wet season as well. IR26, the susceptible check and IR42 suffered severely from iron toxicity and had their maturity delayed so long that they could not be harvested with the' rest. During the 1980.dry season iron toxicity was not severe (due apparently to the soil being wet during the months before planting) and toxicity scores were low and the yield was high (Table 7). IR which had given low scores and high yields in the two earlier tests was the highest yielder with 6.7 t/ha. IR46 also showed its tolerance for excess iron both in the score and in yield. 297

8 Table 7. Iron toxicity tolerance scores and grain yield of 15 selected rices on an acid sulfate soil, Malinao, Albay, Philippines, 1980 dry season Varietylline Score at Grain yield 4 WAT (tlha) IR c 6.7 a IR abc 6.4 ab IR abc 6.3 abc IR abc 6.2 abc IR abc 6.0 abc IR abc 6.0 abc IR ab 5.9 abc IR c 5.7 abcd IR a 5.6 abcd IR abc 5.4 bcde IR abc 5.2 cde IR abc 4.6 def IR abc 4.6 def IET abc 4.4 ef IR bc 3.8 f WAT: weeks af ter transplanting Note: In a column, values followed by the same letter are not significantly different at the 5%. level. 5...Conclusions Marked varietal differences exist in the tolerance of rice varieties for excess iron. The visual score according to foliar symptoms and general appearance separates tolerant from the susceptible in mass screening tests but is not a good index of tolerance as measured by grain yield within varieties having a similar degree of tolerance. There were marked seasonal differences in the performance of rices on the acid sulfate soil. The differences appeared to be associated with the water regime before planting. Tolerant rices gave nearly 3 t/ha of 298

9 grain when iron toxicity was severe and over 6 t/ha when Costly amendments such as liming may be avoided by using varieties. it was mild. to 1 er ant

10 References Breemen, N. van Landscape, hydrology and chemical aspects of some problem soils in the Philippines and in Sri Lanka. A terminal report submitted to the International Rice Research Institute, Los Baños, Philippines. pp Breemen, N. van and F.R. Moormann Iron toxic soils. In: International Rice Research Institute. Soils and rice. Los Baños, Laguna, Philippines. p Breemen, N. van and L.J. Pons Acid sulfate soils and rice. In: International Rice Research Institute. Soils and rice. Los Baños, Laguna, Philippines. p Gunawardena, I.W Screening for resistance to salinity and iron toxicity in Sri Lanka. Paper presented at the International Rice Research Conference, April 21-24, 1975, Los Baños, Philippines. 3 p. Gunawardena, I.W Field screening for tolerance to iron toxicity and phosphorus deficiency. Paper presented at the International Rice Research Conference, April 16-20, 1979, Los Baños, Philippines. 2 p. Howeler, R.H Iron-induced oranging disease in relation to physicochemical changes in a flooded Oxisol. Soil Science. Soc. her. Proc. 37: Ikehashi, H. and F.N. Ponnamperuma Varietal tolerance of rice for adverse soils. In: International Rice Research Institute. Soils and rice. Los Baños, Philippines. p International Rice Research Institute Annual report for Los Baños, Philippines. 265 p. International Rice Research Institute Annual report for Los Baños, Philippines. International Rice Research Institute Annual report for Los Baños, Philippines. 478 p. Nhung, M.M. and F.N. Ponnamperuma Effects of calcium carbonate, manganese dioxide, ferric hydroxide, and prolonged flooding on chemical and electrochemical changes and growth of rice in a flooded acid sulfate soil. Soil Science 102: Panabokke, C.R Problem rice soils in Sri Lanka. Paper presented at the International Rice Research Conference, April 21-24, 1975, Los Baños, Philippines. 5 p. I 300

11 Ponnamperuma, F.N Lime as a remedy for physiological disease of rice associated with excess iron. International Rice Comm. Newsletter 7(1): Ponnamperuma, F.N Problem rice soils. Paper presented at the International Rice Research Conference, April 22-25, 1974, Los Baños, Philippines. 11 p. Ponnamperuma, F.N., T. Attanandana and G. Beye Amelioration of three acid sulfate soils for lowland rice. In: Dost, H. (ed.) Acid Sulphate Soils. International Institute for Land Reclamation and Improvement, Wageningen. Publ. 18, Vol. 2: Ponnamperuma, F.N., R. Bradfield and M. Peech Physiological disease of rice attributable to Fe toxicity. Nature 175: 265. Ponnamperuma, F.N. and R.U. Castro Varietal differences in resistance to adverse soil conditions. In: International Rice Research Institute, Rice Breeding. Los Baños, Philippines. p Tanaka, A. and S.A. Navasero Growth of the rice plant on acid sulfate soils. Soil Science and P1. Nutr. 12: Tanaka, A. and S. Yoshida Nutritional disorders of rice in Asia. International Rice Research Institute Technical Bulletin p. Virmani, S.S Varietal tolerance of rice to iron toxicity. IRRN 2(1): 4. Virmani, S.S Some results of varietal screening for tolerance to iron toxicity in Liberia. Paper presented at the International Rice Research Conference, April 17-21, Los Baños, Philippines. 9 p. 30 I