Ramasamy et al. - 1997 - Yield formation in rice in response to drainage an

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66 S. Ramasamy et al. / Field Crops Research 51 (1997) 65-82 Kazibwe, 1983; Doi et al., 1988) but not all cases (e.g. Watanabe et al.. 1974; Snitwongse and Jirathana, 1981; Singandhupe and Rajput, 1987; Kogano et al., 1991). One explanation for this apparent inconsistency of crop response may be the method applied to impose drainage conditions experimentally. Manipulation of external drainage is via the depth of the flood water level or the duration of flooding (Upadhya et al., 1973; Veltkamp and Manniesing, 1974; Onikura, 1976; Snitwongse and Jirathana, 1981; Pate1 et al.. 1984; Kogano et al., 1991). Internal drainage is governed by removal of subsurface water via mole drains, tube drains, or open drainage channels, and affects the rate of percolation via a lowering of the subsurface water table (Watanabe et al., 1974; Shirakura et al., 1977; Habibullah et al., 1977; Lee et al., 1979; Khind and Kazibwe. 1983). In addition, the crop development stage at which changes in water management are imposed may determine crop response. Depending on soil characteristics, crop development stage and the type of drainage measures taken, the physico-chemical conditions of the root zone are affected differently by water management practices. Various studies indicate a beneficial effect of relatively high percolation rates of 10 to 25 mm/day. especially for heavy clay soils rich in organic matter (e.g., De Datta, 1981 and Hasehawa et al., 1985). Whereas leaching as a result of increased flooding duration is beneficial in salt-affected soils (Singandhupe and Rajput, 1987), leaching may increase N losses in well-drained soils (Kudeyarov et al., 1989) and thereby reduce yield. Yet, some welldrained soils provided greater N uptake under high than under low percolation (Khind and Kazibwe. 1983). Yamamuro (1983) reported that flooding increased N uptake in soils with good internal drainage. but decreased uptake in poorly drained soil. Nonflooded conditions during the entire grain-filling period increased both N uptake and yield in the study by Miyasaka (1970), whereas negative effects of early drainage on rice growth observed by Kogano et al. (1991) were attributed to enhanced N losses through nitrification-denitrification. High N availability itself may have positive or negative effects on grain yield, depending on cultivar and surface drainage (Rai and Murty, 1979). When the soil surface remains non-flooded for several days, the effects of drought stress might overrule the potential benefits of drainage (Snitwongse and Jirathana, 1981; Singandhupe and Rajput, 1987). Alternatively, draining all surface water to halt tillering, as practiced in China (Doi et al., 19881, may be beneficial for yield formation. Other more detailed analyses (Nho et al., 1975; Trolldenier, 1977; Pate1 et al., 1984; Cheng, 1983; Iida et al., 1990) linked drainage and soil redox condition with root activity and with the uptake of various micro-nutrients (Lee et al., 1979; Pate1 et al., 1979). To develop a rational basis for improving water and N management, better understanding is required of the mechanisms by which drainage interacts with crop growth and N utilization. We conducted a series of field experiments and studied yield formation processes as affected by internal drainage: N uptake; growth; the utilization of radiation and leaf nitrogen to produce dry matter; sink formation; grain filling; and translocation of stored non-structural carbohydrates. Root characteristics were also studied in detail and are related to crop performance. 2. Materials and methods Four field experiments were conducted at Tamil Nadu Agricultural University, Coimbatore (11” N, 77” E), India, in 1990 to 1992. The soils were of loamy clay texture (40% clay, 22% silt, 22% fine sand, 16% coarse sand) with low available N and high P and K contents. The field was under paddy cultivation already for many years prior to the experiment. Experiments 1, 2 and 3 focused on the interaction between internal drainage (groundwater table; percolation rate) and nitrogen application level. The drainage treatments were ‘non-drained’ (0,) - where no provisions were made to change the existing percolation rate of 3.0 mm day-‘; and ‘drained’ (0,) - where open trenches (width and depth 0.60 m) were dug to lower the groundwater level and increase the mean field percolation rate to 12 to 14 mm day-‘. Water was removed from the trenches by pumping. The N levels were 100, 150 and 200 kg N ha-‘. N was applied as prilled urea in three splits with
S. Rarnrrsamy et al. /Field Crops Resrarch 51 (19971 65-82 67 50% at transplanting (basal). 25% topdressed 15 or 20 days after transplanting (DAT), and 25% topdressed 30 or 40 DAT, depending on the cultivar. Other nutrients applied were 22 kg P (as Pz05), 40 kg K (as K 20), 10 kg Zn and 5 kg S (as ZnSO,) per ha. Green manure (Sesbania rosrruta) was applied uniformly and incorporated before transplanting at 12.5 t ha-’ (fresh weight, corresponding to 75 kg N ha- ’ >. Seedlings of the short-stature, short-duration rice cv. IR50, 25 d old, were transplanted at 15 X 10 cm spacing on 13 July 1990 in Expt. 1; and 19 June 1991 in Expt. 3. In Expt. 2 (late season), the medium-duration, medium-stature cv. IR20 was transplanted on 3 November 1990. with 20 X 10 cm spacing. The field remained fallow for 15 days after the harvest of the first crop, was then flooded for 10 days (field preparation) and was planted to the second crop. A detailed root study (Expt. 4) was conducted to collect information on roots under the two drainage treatments as imposed in Expts. 1 to 3. The treatments comprised drained and undrained plots in combination with 120 kg N ha- ’ (60 kg basal at transplanting; 30 kg at 20 DAT and 30 kg at 40 DAT) or 150 kg N ha- ’ (60 kg basal at transplanting, 30 kg at 20 DAT, 30 kg at 40 DAT and 30 kg N ha-’ at heading). Nutrients other than N and green manure were applied as in Expts. l-3. Seedlings of rice cv. IR20, 24 d old, were transplanted on December 24 at 20 X 10 cm spacing. A factorial split-plot design with four replications (blocks) was used in Expts. 1-3. Plots were strips of 20 X 5 m. Each block consisted of one D,. and one D, plot, randomly chosen, separated by one buffer plot with plastered walls to minimize border effects arising from the trenches surrounding D, plots. Each plot was subdivided into two subplots (20 X 2.5 m) for planting density, each with three sub-subplots (6 X 2.5 m) for N levels. The results of only one planting density are presented in this paper, as no major differences were found between the two density treatments. A randomized complete block design with five replications was used in Expt. 4. Adequate plant protection measures were taken in all experiments and irrigation was provided daily to maintain a ponding depth of approximately 5 cm until 7 days before harvest. In Experiments 1-3, observations of biomass of green leaves, senescent leaves, stems, roots and panicles were made at 10 d intervals, from the tillering stage onward. Five plants from sample rows were removed carefully along with the roots and plant parts were separated (green leaves, ‘dead’ leaves, stems plus leaf sheaths, roots, panicles) and ovendried at 80 C for 72 h. Dry weight of the biomass of all organs was determined and N contents were measured by the micro-Kjeldahl method (Humphries, 1956). The amount of N translocated (N,) from leaves and stems was assessed as the difference between the highest value (A!,,,,,) of N accumulation recorded in those organs at any time and the amount of N retained at harvest. Grain yield was determined from entire plots, excluding border rows and rows used for periodical sampling. In recording grain yields, panicles were threshed, grain moisture was determined and grain yields were calculated by correction to 14% moisture content. The number of productive tillers per hill was determined at harvest from 10 hills. Filled and non-filled grains were counted after threshing individual panicles. lOOO- Grain weight was determined for each treatment. Roots were separated into active (white), moderately active (brown) and less active (black), as suggested by Cheng (1983). The proportion of differently colored roots was determined at various growth stages and was expressed as percentage of the total number of roots. In Expt. 4, the roots were carefully rinsed and detached from their nodal bases. Their volume was measured by the displacement method, after removing adhering surface moisture. The length and weight of 10 randomly selected roots were measured. Total length per plant was derived from this sub-sample and total root weight. The diameters of roots from the third. fifth and seventh nodes of the culm were measured, at 3.0 cm from the base. The root CEC was measured as suggested by Crooke (1964). The oxidizing activity of the roots was determined by measuring oxidation of alpha-naphthylamine ( (Y- NA) as proposed by Ota (1970). One gram of fresh roots was transferred into a 150 ml flask containing 50 ml of 20 ppm a-NA. The flasks were incubated for 2 h at room temperature in an end-over-end shaker. After inoculation, the aliquots were filtered and 2 ml of aliquot was mixed with 1 ml NaNO,
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