Comparison of productivity between tropical and temperate maize

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1 Soil Science and Plant Nutrition ISSN: (Print) (Online) Journal homepage: Comparison of productivity between tropical and temperate maize Mitsuru Osaki To cite this article: Mitsuru Osaki (1995) Comparison of productivity between tropical and temperate maize, Soil Science and Plant Nutrition, 41:3, , DOI: / To link to this article: Published online: 04 Jan Submit your article to this journal Article views: 311 View related articles Citing articles: 16 View citing articles Full Terms & Conditions of access and use can be found at Download by: [ ] Date: 29 November 2017, At: 01:11

2 Soil Sci. Plant Nutr., 41 (3), 43%450, Comparison of Productivity between Tropical and Temperate Maize I. Leaf Senescence and Productivity in Relation to Nitrogen Nutrition Mitsuru Osaki Faculty of Agriculture, Hokkaido University, Sapporo, 060 Japan Received February 16, 1994; accepted in revised form December 7, 1994 To identify the difference in maize productivity between tropical and temperate regions, maize (Zea mays L.) plants were grown in two regions: in the fields of two CIMMYT Experimental Stations in Mexico and in a field of Hokkaido University in northern Japan. Results obtained were as follows. 1) The grain yield of tropical maize was lower than that of temperate maize. As the harvest index of dry matter of tropical maize was not appreciably low, the biological yield determined the grain yield of tropical maize. 2) Amount of dry matter and amount of nitrogen absorbed in tropical maize did not increase during maturation in spite of adequate amount of nitrogen applied, while in temperate maize the values continued to increase until late maturation. 3) Dry weight and amount of nitrogen in leaves and stems of tropical maize decreased steeply after flowering regardless of the growth conditions and in spite of the adequate amount of nitrogen applied. Especially, leaf senescence of tropical maize became rapid just after flowering. It was impossible to prevent the leaf senescence from proceeding by additional nitrogen application at flowering. Therefore, leaf senescence in tropical maize is considered to be regulated by an autonomous process. 4) When the lower leaves received a sufficient amount of light, tropical maize could absorb nitrogen after flowering, and the decrease in the rate of chlorophyll in leaves became less pronounced, indicating that the higher the activity of lower leaves the higher the root activity. In conclusion, low productivity of tropical maize was mostly caused by rapid leaf senescence after flowering, then partially by the plant architecture. Since, after the rapid decrease of the nitrogen content from leaves, which is another aspect of leaf senescence, nitrogen absorption in plant decreased in spite of the adequate supply of nitrogen nutrient in soils, leaf senescence and root activity of tropical maize were regulated by leaf autonomy. Key Words: carbon-nitrogen interaction, chlorophyll content, leaf autonomy, leaf senescence, plant type. Yield of tropical maize was lower than that of temperate maize, because the grain number per land area and harvest index (HI) of tropical maize were lower than those of

3 440 M. OSAKI temperate maize (Goldsworthy et al. 1974; Fisher and Palmer 1983), suggesting that the sink capacity may limit the yield of tropical maize. In general, as maize is characterized by a C4 pathway, which is adapted to high temperature, it is assumed that maize can attain high dry matter production in the tropics (Evans 1975). On the other hand, Tollenaar (1977) mentioned in the report summarizing data on the source-sink relationship of maize that as the grain number of maize was determined by 1) the CGR at the critical stage of approximately flowering, 2) the distribution of photosynthates into ears, and 3) fertility of spikeiets, the yield of maize was determined not only by the sink but also by the source. To achieve a high yield, a sufficient amount of nitrogen must be absorbed (Osaki et al. 1991a, 1992, 1994). Fifty to 60% of nitrogen in grains was derived from the nitrogen accumulated in leaves and stems before flowering (Crawford et al. 1982; Osaki et al. 1991b). On the other hand, since the photosynthetic rate is closely related to the nitrogen content of leaves (Makino et al. 1988), it is possible to reduce the dry matter production by intensive nitrogen retranslocation from leaves to ears. Thus, although nitrogen is assumed to be the most important factor which determines the crop yield, few studies on nitrogen nutrition have been carried out in tropical maize. Therefore, maize was cultivated in tropical and temperate regions, and the difference in the physiological characteristics related to nitrogen nutrition between tropical and temperate maize plants was examined to analyze the reasons for the low productivity of tropical maize. MATERIALS AND METHODS Experimental sites and seasons. In the tropical region of Mexico, experiments were carried out at the CIMMYT experimental stations at Poza Rica (latitude N 19"31 ', altitude 60 m) and at Tlaltizapan (latitude N 18~ ', altitude 940 m) in both the winter season (from November 1982 to May 1983) and summer season (from June to October 1983). In the temperate region of Japan, experiments were conducted in the field of Hokkaido University in Sapporo (latitude N 43~ ', altitude 50 m) in the summer season (from May to September 1984, 1986, 1989). Varieties and cultivation. For tropical maize, Poza Rica 7822 (PR7822, openpollinated variety of white dent, late maturity, and tall height) was cultivated at the density of 5.33 plants m -2 (75cmX25cm). Planting was performed on November 23, 1982 at Tlaltizapan, November 24, 1982 at Poza Rica, June 22, 1983 at Tlaltizapan, and June 30, 1983 at Poza Rica with three replications. For tropical maize, nitrogen (gnm -2) was applied at the rate of 0 (0 N plot), 15 (15 N plot, basal application), and 30 (30 N plot, 15 basal application + 10 side dressing and covering with soil at primordia formation stage + 5 side dressing and covering with soils at flowering stage) as ammonium sulfate, and phosphorus was supplied as basal application at the rate of 8 g P205 m -~ as superphosphate. Water was supplied adequately. For temperate maize, Wisconsin hybrid No. 110 (WHll0, yellow dent, late maturity, and tall height) in 1984, Wisconsin hybrid No. 90 (WH90, yellow dent, mid-maturity, and tall height) in 1986, and Pioneer 3540 (P3540, yellow dent, late maturity, and tall height) in 1989 were cultivated at the density of 6.67 (60 cm X 25 cm), (30 cm 30 cm), and 8.33 plants m -2 (40 cm x 30 cm), respectively. In 1984, 15 g m -2 of N, P2Os, and K20 (ammonium sulfate, superphosphate, and potassium sulfate, respectively, basal application) were applied, and additional 5 g N m -2 as ammonium sulfate was top-dressed at flowering. In 1986, basal application of nitrogen (g N m -2) was conducted at three levels, 0, 10, and 30 (0 N plot, i0

4 Comparison of Productivity between Tropical and Temperate Maize I 441 N plot, and 30 N plot) as ammonium sulfate, while basal application of phosphorus and potassium was conducted at the rates of 10 g P205 m -2 and 10 g K20 m -2 as superphosphate and potassium sulfate, respectively. In 1989, 10 g N m -2 (ammonium sulfate), 15 g N m -2 (coated urea), 10 g P2Os m -2 (superphosphate), and 10 g K20 m -2 (potassium sulfate) were supplied as basal application. Planting was carried out on May 16, 1984, 1986, and 1989 with two replications. Sample preparation and analysis. Maize plants grown at the CIMMYT stations were sampled at the 8th leaf stage (8 plants), 50% flowering stage (8 plants), maturation stage (8 plants), and final harvest (40 plants) in 1982, and at 2-week intervals from 6 July (6 plants at each sampling time and 30 plants at final harvest only) in Maize plants grown in Hokkaido were sampled at 2-week intervals in 1984, at 2-week intervals in 1986, and 1-week intervals in Samples were separated into leaves, stems, ears, and grains (at final harvest only), dried at 80~ in air-forced oven for 2 d, weighed, and ground. Nitrogen content was analyzed by the Kjeldahl method. Chlorophyll content was measured using a chlorophyll meter (Minolta, SPAD-501), and the chlorophyll content was calculated from the regression line between the value read on the chlorophyll meter and the value of chlorophyll extracted according to the method of Osaki et al. (1993b). Leaf area was determined using an automatic leaf area meter, and the data were expressed as leaf area index (LAI). Light transmission ratio (LTR) was determined as the ratio of the light intensity at the ground level to that on top of the crop canopy using an illuminometer. The k-value was calculated by the following equation. k = - log(ltr/100)/lai. Weather data. The weather data were recorded at the Experimental Stations of CIMMYT (1982 and 1983), the Sapporo District Meteorological Observatory (1984), and the Experimental Station of the Faculty of Agriculture, Hokkaido University (1986 and 1989). RESULTS 1. Difference in physiological characteristics between tropical maize and temperate maize grown with standard nitrogen application rate It is appropriate to compare PR7822 (tropical maize) and WHll0 (temperate maize) because the fertilizer application design and plant type were similar in the two varieties (Table I). in P3540 (temperate maize), as a very high economic yield such (1,140 g m -2 on the dry weight basis) was achieved (Table l), data of P3540 were used in the model for the simulation of higher yield in tropical maize. Climate. Mean temperature values at the CIMMYT experimental stations during the vegetative growth stage (from sowing to silking) were 27.8, 25.1, 18.7, and 19.3*C at Poza Rica in summer (PR.S), Tlaltizapan in summer (TL.S), Poza Rica in winter (PR.W), and Tlaltizapan in winter (TL.W), respectively, and those during maturation (from silking to harvest) were 29.0, 24.7, 22.9, and 21.2~ at PR.S, TL-S, PR-W, and TL-W, respectively. Mean temperature at Sapporo during the vegetative growth stage was in the range of *C in 1984 and 1989, and that during ripening was in the range of "C in 1984 and 1989, indicating that the mean temperature at Sapporo was not different between the 2 years, and lower than that at the CIMMYT experimental stations. Mean radiation values (MJ m -2 d -1) at the CIMMYT experimental stations during the vegetative growth stage was 1.40, 1.73, 0.93, and 1.54, and those during ripening were 1.96,

5 442 M. OSAKI 2.71, 1.39, and 2.07 at PR.S, TL-S, PR-W, and TL-W, respectively. Mean radiation at Sapporo was in the range of during the vegetative growth stage, and during ripening in 1984 and Amount of mean radiation (MJ m -2 d -l) was almost the same at Tlaltizapan and at Sapporo during the vegetative growth stage, while the amount of mean radiation was higher at the CIMMYT experimental stations than at Sapporo during maturation. Growth phase. Grain filling period, 50% flowering stage and 50% black layer formation stage occurred earlier at PR-S and TL-S than at other locations, indicating that the differences between tropical maize in winter and temperate maize were negligible (Table 1). Biological yield (Yb), economic yield (Ye), and yield components. The biological yield ( Yb; g m -2) of PR7822 was 1,260 at PR.S, 1,270 at TL-S, 1,080 at PR.W, 1,610 at TL- W, and that of WIll 10 and P3540 was 2,070 and 2,240, respectively. Thus, the Yb of tropical maize was lower than that of temperate maize. Harvest index of dry matter (HIoM) was low at PR-S and TL-W, while HIoM in tropical maize was not appreciably low compared to temperate maize (Table 1). Therefore, low HInM is not an inherent characteristic of tropical maize as reported for old varieties by Golds- Table 1. Yield and yield components of tropical and temperate maize. Ear Grain number 1,000 Grain 50% 50% black Variety Yield number grains filling flowering layer for- Region and plot (g m -a) weight HIoM period stage mation stage b (No. m -2) (No. ear -a) (No. m -2) (g) (d) (d) (d) Tropical PR , (PR.S) a PR , (TL-S) a PR , (PR.W) ~ PR , (TL-W)" Temperate WH , P3540 1, , a PR7822 (PR-S): Poza Rica 7822 grown at Poza Rica in summer; PR7822 (TL.S): Posa Rica 7822 grown at Tlaltizapan in summer; PR7822 (PR-W): Poza Rica 7822 grown at Poza Rica in winter; and PR7822 (TL. W): Posa Rica 7822 grown at Tlaltizapan in winter, b Fifty percent black layer formation stage corresponds to the physiological maturation stage, which therefore indicates the growth duration. Table 2. Characteristics of tropical and temperate maize at flowering stage. Chlorophyll Region Variety Plant height Leaf number LAI k-value content and plot (cm) (g chl m -2 LA) Tropical PR7822 (PR-S) PR7822 (TL.S) PR7822 (PR-W) PR7822 (TL-W) Temperate WHI P

6 Comparison of Productivity between Tropical and Temperate Maize I 443 worthy et al. (1974). Economic yield ( Ye, on a dry weight basis) of tropical maize ( g m -2) regardless of growth conditions (seasons and locations) was lower than that of temperate maize (910-1,140 g m -2) (Table 1). Under typical tropical conditions such as those of PR.S, the lie value was smallest due to the small Yb and small HIDM values. Ear number and grain number (number per ear and number per m 2) in tropical maize at TL.W were not appreciably lower than in temperate maize, but the 1,000 grains weight in tropical maize regardless of growth conditions was clearly lower than that in temperate maize (Table 1). Plant type and leaf growth. Plant height, leaf number, LAI (corresponding to maximum LAI), k-value, and chlorophyll content of ear leaf at flowering were not appreciably different between tropical and temperate maize (Table 2). The k-value of P3540 was low, indicating that the plant type of this variety was improved with upright leaf. On the other hand, the plant type of PR7822 estimated by the k-value was similar to that of WH110 ZO I //-- / ; " s, ~ m /a t~, o.=" =" /!'~u/t"," P" 0 5'O IO0 150 Days after planting Fig. 1. Days ofemergence (e and []) and senescence (9 and []) of leaves at each position in tropical maize (Poza Rica 7822) at Poza Rica in summer and temperate maize (WHII0) at Sapporo. $ and 9 Poza Rica 7822; [] and [], WHII0. 2 7a22('VL. W) 5O IO0 150 Days after planting Fig, 2. Changes of LAI of tropical maize (PR7822) at Poza Rica in summer and at Tlaltizapan in winter and of temperate maize (P3540 and WHII0) at Sapporo at successive growth stages o ;o. :/:.: i o 9 J 0.2 O Z O o'., ~ o." o~ o o o Chlorophyll content (gchl m-=la) B loo Days after planting I 150 Fig. 3. Chlorophyll content of leaves at each position at flowering (A) and changes in chlorophyll content at successive growth stages (B) in tropical maize (PR7822) at Poza Rica in summer and temperate maize (WHll0) at Sapporo. e, Poza Rica 7822; 9 Poza Rica 7822 in which plants on one side of row were cut at flowering for improvement of light distribution to lower leaves; m, WH110.

7 444 M. OSAKI (Table 2). Maximum LAI (LAI at flowering) of temperate maize was higher than that of tropical maize (Fig. 2 and Table 2). As, the leaf senescence of tropical maize was faster than that of temperate maize, leaves of tropical maize were almost dead at harvest, while those of temperate maize were partially alive at harvest, especially in P3540 (Figs. 1 and 2). Leaf formation rate was 0.44 leaf d -1 in PR7822 at Poza Rica in summer and 0.28 leaf d -1 in WHll0, indicating that the leaf formation rate was faster in PR7822 (Fig. I). Chlorophyll content of ear leaf at flowering was not different between tropical and temperate maize (Table 2). Chlorophyll content of leaves at each position at flowering was higher in tropical maize at Poza Rica in summer than in temperate maize (Fig. 3). Chlorophyll content of ear leaf after flowering decreased rapidly with growth in tropical maize, but remained almost constant until the late maturation stage, then decreased in temperate maize (Fig. 3). Nitrogen accumulation and distribution. Nitrogen content of leaves and stems at flowering and of grains at harvest was not appreciably lower in tropical maize than in temperate maize, while the nitrogen content of leaves and stems at harvest showed only a small difference between tropical and temperate maize (Table.3). The amount of nitrogen absorbed by tropical maize was g N m -2, a value lower than that of temperate maize (21-27 g N m -2) (Table 4). HIN ot" tropical maize was not appreciably lower than that of temperate maize (Table 4). N-redistribution rate, which is estimated from the equation: (amount of nitrogen in leaves and stems at flowering-amount of nitrogen in leaves and stems at harvest)/(amount Table 3. Nitrogen content (g kg -t) of leaves, stems, and grains of tropical and temperate maize. Region Variety and plot Flowering Harvest Leaf Stem Leaf Stem Grain Tropical PR7822 (PR- S) PR7822 (TL. S) PR7822 (PR.W) PR7822 (TL.W) Temperate WHll P Table 4. Amount of nitrogen absorbed, HIN, N-redistribution rate, and N-contribution rate of Region Variety and plot tropical and temperate maize. N absorbed ~ N-redistribution N-contribution Harvest Maturation HIN rate rate (gn m -2) (gn m -2) Tropical PR7822 (PR- S) PR7822 (TL- S) PR7822 (PR. W) PR7822 (TL- W) Temperate WH P I aharvest: from planting to harvest, Maturation: from flowering to harvest.

8 Comparison of Productivity between Tropical and Temperate Maize I 445 of nitrogen in leaves and stems at flowering), was not different between tropical and temperate maize (Table 4). N-contribution rate, which is estimated from the equation: (amount of nitrogen in leaves and stems at flowering--amount of nitrogen in leaves and stems at harvest)/(amount of nitrogen in harvesting organs at harvest), was higher for tropical maize than for temperate maize, indicating that the large amount of nitrogen in the grains of tropical maize was retranslocated from leaves and stems, and that the large amount of nitrogen in grains in temperate maize was absorbed by roots during maturation (Table 4). Dry matter increase and nitrogen absorption ceased at the early maturation stage in PR7822(PR,S) m7822ttl, W) O E,r ~I oo oo0 50r PR7822(PR'S) $1m 5o loo 15o PRT822(~.,W) Days after planting ~o Ioo 15o "~: '~ 25 0 WHllO 5o Le~a~ 1oo Iso o s'o I~o ~o Days after planting Fig. 4. Changes in dry weight and amount of nitrogen of leaves, stems, and ears of tropical maize (PR7822) at Poza Rica in summer and at Tlaltizapan in winter, and of temperate maize (WH110 and P3540) at Sapporo at successive growth stages. Table 5. Effect of nitrogen application rate on Yb, Ye, amount of nitrogen absorbed N-redistribution rate, and N-contribution rate in tropical maize at Poza Rica in summer and temperate maize. N absorbed ~ N application rate lib Y~ N-redistribu- N-contribu- Variety (g m_2) (g m_2) (g m_2) Harvest Maturation tion rate tion rate (g N m -2) (gn m-') PR7822 WH90 a same as in Table , , , l , ,

9 446 M. OSAKI tropical maize, but continued until the late maturation stage in temperate maize (Fig. 4). Dry weight and the amount of nitrogen in leaves decreased abruptly just after flowering in tropical maize. In temperate maize, the amount of nitrogen of leaves also decreased, but it started to decrease after the mid-maturation stage, and the final amount of nitrogen in leaves at harvest was larger in temperate maize than in tropical maize. In temperate maize, the dry weight of leaves remained constant after flowering (Fig. 4). 2. Effect of nitrogen application rate on growth To analyze the effect of nitrogen nutrition on leaf growth, nitrogen distribution and production, nitrogen was applied at 3 levels: 0 N, 15 N, and 30 N. As the effect of the nitrogen application rate on amount of nitrogen absorbed during maturation was negligible in tropical maize, it is assumed that tropical maize cannot readily absorb nitrogen after flowering even if nitrogen nutrition was adequate in soils due to the additional nitrogen application at 30 N (Table 5 and Fig. 5). The N-redistribution rate and N-contribution rate in tropical maize were similar regardless of the nitrogen application rate and growth conditions, indicating that the mechanisms of nitrogen distribution are stable (Table 5). Amount of nitrogen in leaves and stems decreased abruptly after silking, and nitrogen absorption in tropical maize almost ceased after the amount of nitrogen in leaves and stems decreased regardless of the nitrogen application rate (Fig. 5). As a sufficient amount of nitrogen had been applied at 30 N, the low activity of nitrogen absorption was caused by the inherent characteristics of tropical maize, and not by nitrogen deficiency in soils. Only at Poza Rica in summer, an additional experiment (15 g N m -2 was applied to the 0 N, 15 N, and 30 N treatment at flowering by side dressing covered with soils) was carried out for determining whether nitrogen deficiency occurred in soils. The effect of this additional nitrogen application on Yb, Y~, and amount of nitrogen absorbed during maturation was WHgO wp WP =a PR7822 WH90 WP aooc PF~ ~Sp 2ooc TaDC.--~ laoc J O,.' ts ~,..-. WP la LS 6 sot,, LS B LS so, IOO i Inn, o Io loo 15o "'J,, i Days after planting Days after piantklg Fig. 5. Effect of nitrogen application rate on the changes of dry weight and amount of nitrogen of whole plant (WP) and leaves and stems (LS) in tropical maize (Poza Rica 7822) at Poza Rica in summer and temperate maize (WH 90) at successive growth stages , 0 N; --, 10 N (WH90) or 15 N (Poza Rica 7822);, 30N.

10 Comparison of Productivity between Tropical and Temperate Maize found to be negligible even in the 0 N plot (data not shown). In contrast to tropical maize, nitrogen in temperate maize was actively absorbed after flowering regardless of the nitrogen application rate. 3. Relationship between nitrogen absorption and light conditions To study the relationship between nitrogen absorption and carbon assimilation (dry matter increase), one treatment was added to an experiment at Poza Rica in summer at flowering, in which plants on one side of a row were cut for the improving light distribution to lower leaves. In the ON plot, the effect of the treatment was negligible (Fig. 6). In the 15 N plot and 30 N plot, the dry weight of stems and grains, and amount of nitrogen of stems and grains in the treatments were larger than those of the control. In these experiments, plants in the treatments received 1.5 times more solar radiation and nitrogen nutrients in soils compared to the control plants. Therefore, the amount of nitrogen in the 15 N plot in the treatment corresponded to only 22.5 g N m -a in the calculation, while the plants in the 15 N plot in the treatments absorbed a large amount of nitrogen than the plants in the 30 N plot in the control. Thus, if the light distribution in the canopy could be improved after flowering, tropical maize could absorb a large amount of nitrogen. In other words, tropical maize can not absorb nitrogen after flowering in spite of the presence of nitrogen in soils under canopy conditions. The values for grain number in 0 N, 15 N, and 30 N were 1,244, 1,782, and 1,636 in the control, and 1,230, 1,791, and 1,796 in the treatments, respectively. The 1,000 grains weight (g) in 0 N, 15 N, and 30 N was 266, 270, and 266 in the control, and 295, 302, and 349 in the treatments, respectively. Thus, the 1,000 grains weight was markedly changed by the growth conditions after flowering. DISCUSSION High yield values (g m -z) of temperate maize were 2,200 at Michigan, U.S.A. (Fisher and Palmer 1983), 1,900 (Cuany et al. 1970), 1,490 (Flannery 1982), 1,600 (155 g kg -1 water content, Menzies 1978), 1,790 (155 g kg -1 water content, Vinande 1984). High yield values (g m -2) of maize in tropical highlands were 1,200 (Wilson and Allison 1978) at Salisbury, Zimbabwe, 1,060 (Cooper 1979) at Elonga, Kenya. High yield values (gm -2) of maize in tropical lowlands were 980 (155 g kg -1 water content, Baynes 1972), 980 (155 g kg -1 water content, Spain et al. 1982), and 960 (Muleba et al. 1983) at Kaniama, Zaire. Based on the reports, the high yield values (g m -2, 150 g kg -1 water content) ranged from 1,500-2,000 in Fig. 6. Effect of improvement of light distribution to lower leaves on the increase of dry weight and amount of nitrogen in tropical maize (Poza Rica 7822) at Poza Rica in summer. C, control; T, treatment in which plants on one side of the row were cut at flowering. ~, leaves; I I, stems; I-[l~, ears.

11 448 M. OSAKI the temperate zone, 1,200-1,400 in tropical highlands, 900-1,100 in tropical lowlands. Thus, it is assumed that the difference in maize yield at high levels between tropical and temperate plants amounted to 500-1,000 g m -2. On the other hand, tho~agh high yield was expected in tropical highlands because of the high solar radiation and relatively low temperature, yield in the tropical highlands was lower than that in the temperate region. Therefore, in this report, to analyze the limiting factors determining the yield of tropical maize, the difference in growth and productivity in relation to nitrogen accumulation between tropical and temperate maize was examined. Grain number and 1,000 grains weight of maize are mainly determined by the photosynthetic activity before silking (Nishikawa and Kudo 1973) and after silking (Fisher and Palmer 1983), respectively. In comparison with temperate maize, the low Ye of tropical maize is caused mainly by the small 1,000 grains weight because the photosynthetic activity is limited after silking. Based on the results of the improvement of the light distribution to lower leaves by cutting plants on one side of the row, Ye increased through the increase of the 1,000 grains weight, indicating that the photosynthetic activity in lower leaves after flowering is important for achieving a high Ire. In tropical maize, at first, the amount of nitrogen in leaves and stems decreased just after flowering regardless of the growth conditions, then the nitrogen absorption rate decreased (Fig. 4). In temperate maize, the amount of nitrogen in leaves and stems decreased from the mid-maturation stage, then the nitrogen absorption rate decreased. Leaf senescence of tropical maize is assumed to be regulated by the leaf autonomy because the process of leaf senescence was not basically disrupted by the growth conditions (Poza Rica or Tlaltizapan in summer or winter), by nitrogen application at flowering, by the change of the light distribution to lower leaves in the canopy (in this treatment, nitrogen absorption and carbon assimilation were stimulated, while leaf senescence was only slightly depressed), and by varietal differences (leaf senescence and the decrease of the nitrogen content in leaves of short plants such as Pirsabak (1) 7930 and Mexico 8049 started abruptly also after flowering as in the case of tall plants such as Poza Rica 7822 reported here and Poza Rica 7843 (data not shown)). Consequently, in tropical maize, it is assumed that the leaf senescence were enhanced as follows: 1) nitrogen of leaves (especially lower leaves) rapidly retranslocated into ears, 2) photosynthetic activity decreased with the decrease of the nitrogen content in leaves, 3) few photosynthates translocated into roots because when the activity of the lower leaves decreased, the supply of carbohydrates to roots was limited according to the results of ~4C distribution from each leaf (Palmer et al. 1973), 4) root activity was reduced, 5) nitrogen could not be absorbed. When nitrogen absorption ceased, the decomposition of the nitrogen compounds of leaves increased, and the photosynthetic rate was increasingly depressed. This vicious cycle accelerates with ear growth because of the ear-requirement for carbohydrates and nitrogen compounds. The important point is that this phenological process of leaf senescence of tropical maize is autonomous because leaf senescence is not appreciably affected by the environmental conditions and plant type. In the 0 N plot, although the nitrogen of leaves and stems of tropical maize retranslocated into ears after flowering, the nitrogen content of leaves and stems of tropical maize remained constant after flowering (Fig. 5). In the 0 N plot of tropical maize, even when additional nitrogen (15 g N m -2) was applied at flowering, lib, Ye, and amount of nitrogen absorbed during maturation did not increase. Therefore, it is concluded that the mechanisms of nitrogen redistribution from leaves and stems are very stable (autonomous) in tropical maize, which results in a high

12 Comparison of Productivity between Tropical and Temperate Maize nitrogen use efficiency for grains (Ye/amount of nitrogen absorbed during maturation) in soils with a small amount of nitrogen nutrient. In contrast to tropical maize, in temperate maize, it is assumed that the high productivity is due to the following factors: I) a large amount of photosynthates was translocated into roots because high nitrogen and chlorophyll contents of leaves persisted until the midmaturation stage, 2) nitrogen could be absorbed during maturation, then 3) nitrogen of leaves retranslocated into ears after the mid-maturation stage. In high-yielding trials at Sapporo (Osaki et al. 1993a), the productivity of several field crops was determined by the leaf area duration (LAD), and not by net assimilation rate (NAR). High-yielding crops could absorb actively nitrogen and assimilate carbon dioxide during maturation. Thus, to achieve a high yield, leaf and root activity must be preserved during maturation. If the light distribution in the canopy of tropical maize could be improved by cutting the plants of an adjacent row at flowering, nitrogen could be absorbed by roots (Fig. 6). However the decreasing rate of nitrogen and chlorophyll contents was only slightly reduced (Figs. 3 and 6). Therefore, high leaf senescence rate is an inherent characteristic of tropical maize because the plant type and plant height of tropical maize had already been improved as in the case of temperate maize. Thus, even if the soils contain a large amount of nitrogen, tropical maize under canopy conditions can not absorb nitrogen, assuming that an adequate amount of carbohydrates was not supplied to roots during maturation. Recently it has been reported in rice plants in which lower leaves were shaded, that carbohydrates assimilated by upper leaves are strictly restricted to the transport from such leaves, and that carbohydrates were not supplied to roots any more (Osaki et al. 1995). Therefore, it is interesting to study the relationship between mutual shading and root activity under canopy conditions, because carbohydrate transport to roots is also restricted by mutual shading. Consequently, improvement of the plant type may enable to overcome this shortcoming. Since plant type and plant height of tropical maize had already been improved as in the case of temperate maize, the severe decrease in productivity after flowering has been attributed to leaf senescence, which is a physiological character, and is not associated with the plant architecture. Consequently, leaf senescence of tropical maize is assumed to be regulated by leaf autonomy, because the rapid senescence can not be prevented by nitrogen application or improvement of light distribution to lower leaves. Acknowledgments. I would like to express my gratitude to Dr. S. Pandey (CIMMYT) for his helpful suggestions and discussion, and to Mr. Y. Fujisaki, Mrs. M. Matsumoto, Mr. K. Morikawa, Mr. T. Shinano, Mr. M. Urayama, and Mr. A. Yoshimura for collecting the data. This research was supported in part by a grant from JICA. REFERENCES Baynes, R.A. 1972: Yields of maize (Zea mays L.) in four Caribbean islands as influenced by variety and plant density. Trop. Agric. (Trinidad), 49, Cooper, P.J.M. 1979: The association between altitude, environmental variables, maize growth and yield in Kenya. 3. Agric. Sci. Camb., 93, Crawford, T.W.J., Rending, V.V., and Broadbent, F.E. 1982: Sources, fluxes, and sinks of nitrogen during early reproductive growth of maize (Zea mays L.). Plant Physiol., 70, Cuany, R.L., Swink, J.E., and Sharer, S.L. 1970: Performance test of corn hybrids in various regions of Colorado in Colo. State Univ. Exp. Stn. Gen. Ser., 904, 31 Evans, L.T. 1975: The physiology basis of crop yield. In Crop Physiology--Some Case Histories, Ed. L.T.

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