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a USDA-ARS Sugarcane Field Stn., 12990 U.S. Hwy. 441, Canal Point, FL 33438
b Everglades Res. and Educ. Cent., Univ. of Florida, 3200 East Palm Beach Rd., Belle Glade, FL 33430
* Corresponding author (bglaz{at}saa.ars.usda.gov).
Received for publication October 28, 2003.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: EAA, Everglades Agricultural Area • TRS, theoretical recoverable sugar
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Several factors have resulted in increased exposure of EAA sugarcane to extended periods of higher-than-desired water tables and to floods for as long as 7 d. Soil subsidence caused loss of depth in EAA Histosols at the rate of about 2.5 cm yr–1 before 1978 (Shih et al., 1978). From 1978 until the most recent survey in 1997, the rate of soil loss declined to 1.4 cm yr–1 (Shih et al., 1998). Some EAA fields had as much as 300 cm of soil above the limestone bedrock when they were first drained and used for agriculture. Depth of soil to bedrock varies, but a small number of sugarcane fields now have less than 40 cm of soil (Shih et al., 1998). Second, for every centimeter of rainfall, the water in the soil profile of EAA Histosols rises about 10 cm (Glaz et al., 2002). Finally, there are regulated and voluntary limits on pumping from farm ditches to public canals as a means of reducing P discharge to the natural Everglades.
The issues of soil subsidence and P discharge also provide incentives to maintain yields under high water tables and periodic flooding. The primary cause of subsidence in the EAA is microbial oxidation (Tate, 1980). The factor that most influences the rate of microbial oxidation is depth of water table in the soil profile. Therefore, the rates of oxidation and subsidence are directly proportional to the depth of the water table. Halving the distance between the water table and the soil surface has been shown to halve the rate of subsidence (Snyder et al., 1978).
Best management practices to reduce P discharge from the EAA often include strategies to reduce quantities and rates of pumping water from agricultural fields (Rice et al., 2002). Therefore, P export to the Everglades could be reduced by allowing high water tables and floods in sugarcane to descend more by evapotranspiration and less by pumping. Developing strategies that allow water tables closer to the soil surface along with increased flood durations at which sugarcane maintains optimum yields would help conserve soil and reduce P discharge.
Previous research indicates inconsistent sugarcane responses to water tables. Carter and Floyd (1971) reported that maintaining four constant water tables between depths of 61 and 122 cm during the active growth phase of sugarcane did not affect cane or sugar yields in Louisiana. Carter and Floyd (1975) maintained water tables at 30, 76, and 122 cm throughout the year in the second- and third-ratoon crops of the plantings reported in their 1971 study. There were no significant differences in sugar yield in the second-ratoon crop, but in the third-ratoon crop, sugar yields decreased as water table rose.
In a field study conducted in Florida, Kang et al. (1986) compared sugar concentration and cane yields of 16 clones of sugarcane (Saccharum spp.), one of S. robustum, one of S. officinarum, and one of Ripidium spp., at water table depths of 30 and 56 cm. Overall mean sugar concentration yields were 15.7 and 17.6% higher in the 30-cm water table depth in the plant-cane and first-ratoon crops, respectively. Overall mean cane yields were 27.5 and 25.3% higher in the 30-cm water table depth in the plant-cane and first-ratoon crops, respectively. Gascho and Shih (1979) maintained water table depths in lysimeters at 32, 61, and 84 cm. They reported that yields were optimum at 61 cm, but two of six cultivars had similar yields at all three water tables. Glaz et al. (2002) maintained, in the field, summer water table depths of <15 cm and between 15 and 38 cm for plant-cane and first-ratoon crops. Sugar yields at the water table maintained at <15 cm from the soil surface were 91.7% of those at the deeper water table. However, yield of one cultivar was reduced by 25% by the shallow water table, and yields of two of nine cultivars were not affected by water table.
Mafizur Rahman et al. (1986) reported that flooding in pots for one month in Louisiana reduced stalk growth rates among genotypes by 40 to 88%. In Barbados, Webster and Eavis (1972) flooded sugarcane in lysimeters for 1, 4, 14, or 30 d at 1- and 3-mo age. During the flooding, tiller formation and shoot growth were decreased, but increased growth after drainage relative to the nonflooded lysimeters resulted in similar yields for all treatments at 5-mo age. Although root weight was similar for all treatments at 5 mo, the flooded sugarcane had fewer and larger roots than the drained sugarcane. In a study conducted outdoors in large pots, Ray and Sinclair (personal communication, 2003) observed that continuous flooding reduced sugarcane yields and that a continuous water table depth of 15 cm resulted in neutral or beneficial yield responses for all three cultivars tested. Deren et al. (1991b) reported that 5-mo flooding reduced yields of 160 sugarcane genotypes by 30 to 100%.
In summary, previous research shows that sugarcane suffers moderate to total yield losses due to long-duration flooding. Optimum yields have sometimes been identified at water table depths of 
30 cm, but optimum yields have generally been reported at water tables substantially deeper than 30 cm. Also, clear reasons to explain the response of sugarcane to different water table depths have not been reported. For EAA sugarcane growers, water table management is more complex than managing only for extended flood or only for high water table duration. Instead, sugarcane is exposed to periodic floods, usually not for durations longer than 1 wk, and when drained, it is often difficult to drain to the desired depth of about 50 cm. The purpose of this study was to evaluate the effects of periodic flooding, followed by drainage to different water table depths, on morphological characteristics and cane and sugar yields of two sugarcane genotypes.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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A pump connected to a ball float was installed in each lysimeter to remove excess water. About 40 L of well water flowed into each lysimeter daily from a hose placed inside a perforated pipe that extended from one corner of the lysimeter above the soil surface to the diagonal corner at the bottom of the lysimeter. A solenoid valve installed on each lysimeter opened automatically each morning for 2 min to permit this water flow. This volume of water was sufficient to return lysimeters to desired water tables each morning if water was lost the previous day. Water levels in each lysimeter were measured before the scheduled opening of the solenoid valve manually 5 d wk–1 in 2000 and daily by automatic recorders in 2001 and 2002 (Table 1). Soil samples were taken from the 0- to 15-cm depth and analyzed for pH (water) and water extractable P and K (Sanchez, 1990). Based on soil test recommendations (Sanchez, 1990), nutrients were banded near the planted sugarcane each year at rates of 25 and 139 kg ha–1 of P and K, respectively, and at rates of 0.1, 0.1, 0.7, 0.3, 0.1, and 0.3 kg ha–1 of B, Cu, Fe, Mn, Mo, and Zn, respectively.
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Each year, all lysimeters were maintained at water table depths of 50 cm from after planting until treatments were applied. Four water table treatments, each replicated three times, were first imposed on 4 July 2000. One treatment that served as a control was a water table depth continuously maintained at 50 cm. The three other water table treatments included flooding for the first 7 d of five 21-d cycles and drainage to depths of 16, 33, and 50 cm for the remaining 14 d of each cycle in the experiment conducted in 2000. During flood, water height ranged from at the soil surface to about 2.5 cm above the soil surface. Flooding durations of 7 d were chosen because this duration approximates the longest duration to which commercial sugarcane in the EAA is sometimes exposed. The 50-cm drainage depth was chosen because it is a desired commercial depth, and two incrementally higher depths were chosen because such depths are becoming increasingly common for extended durations in commercial fields.
In the second experiment, planted on 1 Feb. 2001, the water treatments began on 17 Apr. 2001 and continued for nine flood–drain cycles. A third experiment was planted on 23 Jan. 2002, and a total of nine flood–drain cycles began on 6 May 2002. Flood–drain cycles in all experiments began when the interrow space was covered by the plant leaves and were discontinued in the final half of October to coincide with the dry season in Florida. After the final flood–drain cycle of each experiment, water tables were maintained at their prescribed drainage depths until harvest.
Each year, all sugarcane stalks were cut at their base from each row of each lysimeter. Immature stalks (suckers) were discarded. After removal of their top four internodes, all remaining stalks were weighed to determine cane yield measured as kilograms per square meter. Except for five stalks, all stalks were then milled to extract juice and determine theoretical recoverable sugar (TRS, measured as g sugar kg–1 cane), calculated using a previously described procedure (Legendre, 1992). Sugar yield (kg sugar m–2) was calculated as follows:
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Harvest dates were 19 Jan. 2001, 14 Nov. 2001, and 10 Dec. 2002 for the first, second, and third experiments, respectively.
Soon after the harvest of the second experiment, while digging out the sugarcane stools, white grub (Ligyrus subtropicus Blatchley) infestations were detected in the three lysimeters that were maintained at a continuous 50-cm water table depth (not flooded). The nine lysimeters that were cyclically flooded for 7-d did not have white grubs, as expected (Cherry, 1984). Based on their life cycle, the white grubs probably began causing damage in August or September in the lysimeters that were not flooded (Cherry, 1991). In the final experiment, all lysimeters were flooded from 25 Sept. 2002 to 3 Oct. 2002 as a control measure. No white grubs were found in any lysimeters when digging up sugarcane stools after the first and third experiments.
Morphological characters were measured on the five stalks not used to measure TRS. Nodes from the bottom of the cut stalk were counted, and diameter was measured on the second internode from the bottom of each stalk. Also on the internal portion of the second internode from the bottom of the cut stalk, a subjective rating was assigned for relative size of pith and pipe (air cavity) combined. (Any pipe was always within the pithy portion of the stalk.) We are not aware that the pipes within this area of pith have been described as aerenchyma in sugarcane, but these pipes are probably similar to what has been characterized in stems of other species as aerenchyma. Aerenchyma formation in stems and roots is due to cell separation during development (schizogeny) or cell death and dissolution (lysigeny) (Drew, 1997). Aerenchyma formation has been reported to be routine in sugarcane roots (Ray et al., 1996; Van Der Heyden et al., 1998). The ratings of pithy area with aerenchyma formation ranged from 0 for none to 5 for an area that was equal to about 70% of the stalk diameter. Leaves were collected and separated into brown and green leaves. Brown leaves, green leaves, and stalks were oven-dried, and dry weights were recorded for each.
Water table treatments (lysimeters) were arranged as main plots in a randomized complete block design. All water table treatments were replicated three times. Genotypes were arranged as split plots in lysimeters. All statistical analyses were performed using PROC MIXED of SAS (SAS Inst., 1999). Data were analyzed for each year separately. Analyses of morphological characters included five samples of each experimental unit. Analyses were also conducted with the combined data of all three experiments (years) for all characters. In analyses of separate years, or analyses combined across years, replication and sample or year (when present), and any interaction including these terms, were classified as random effects. The water table and genotype treatments were treated as fixed effects.
Significant effects identified by analysis of variance were further analyzed by separating least square means with t tests. Also, the contrast statement in SAS (SAS Inst., 1999) was used to calculate single degree-of-freedom comparisons that tested significance, for each genotype, of linear regression on water table depth during drainage for treatments that were periodically flooded and drained to 16, 33, and 50 cm. Regressions were calculated using recorded water table depths that differed moderately each year (Table 1). To simplify presentation, graphs and other results are reported as responding to depths of 16, 33, and 50 cm. Differences were identified as significant at P = 0.05 and as highly significant at P = 0.01.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The linear responses combined across genotypes of cane and sugar yields to water table depth were significant (Table 2). However, analysis of each genotype separately revealed that only CP 95-1376 responded linearly to water table depth. For CP 95-1376 flooding treatments, cane and sugar yields increased by 0.25 and 0.03 kg m–2, respectively, for each additional centimeter of water table depth from 16 to 50 cm during drainage (Fig. 1). Cane and sugar yields of each genotype exposed to the continuously drained treatment were not significantly higher than those of any of the periodically flooded treatments (Tables 4 and 5).
Soon after the 2001 harvest, unlike in the previous harvest, white grub infestations of 12.5 grubs m–1 row of both genotypes were detected in the continuously drained lysimeters. No grubs were present in any lysimeter treated with a periodic flood. Sosa (1984) reported that white grub infestations of 12.1 m–1 row reduced yields of cane and sugar by 28 and 39%, respectively. This grub infestation explains the significantly reduced cane yields of each genotype in the continuously drained treatment compared with its treatment that was exposed to nine cycles of 1-wk flooding followed by water table depths of 50 cm for 2 wk (Table 4). Also, the periodically flooded treatment of CP 95-1429 that was maintained at a water table depth of 33 cm during drainage had significantly higher cane yield than CP 95-1429 maintained at a continuously drained water table. Water table did not affect TRS in years when grubs were and were not present. This suggests that the damage from grubs did not reduce yields of TRS in 2001 (Table 3). Sosa (1984) found significant reductions in sucrose and juice purity due to grub infestations of 12.1 m–1 row.
Yields, Year 2002
Similar to results of the previous 2 yr, water table treatment did not affect TRS, and as in 2001, CP 95-1376 had higher yields of TRS, cane, and sugar than CP 95-1429 (Tables 2 and 3). No significant regressions were identified for cane and sugar yield, but water table depth during drainage and water table depth x genotype significantly affected both characters (Table 2). The significant interactions were caused by distinct genotype reactions under periodic flooding compared with continuous drain. Cane and sugar yields of CP 95-1376 were significantly higher in the continuously drained treatment than in all three of the treatments where it was exposed to nine cycles of periodic flooding (Tables 4 and 5). Conversely, CP 95-1429 had similar yields under all four water table treatments. These genotype responses to periodic flooding were similar to their responses in 2000. These responses were not detected in 2001, probably due to the infestation of white grubs.
Unlike in 2000 and 2001, CP 95-1376 yields did not respond linearly to water table depth during drainage. A possible explanation is the amount of time that elapsed between planting and initiation of flood–drain cycles each year. Flood–drain cycles were initiated each year when the sugarcane leaves covered the interrow space in the lysimeters. In 2000 and 2001, flood–drain cycles began 50 and 75 d after planting, respectively. However, in 2002, the experiment was planted earlier, the plants grew more slowly, and the leaves did not cover the rows until 103 d after planting. Perhaps the increased time before exposure to flooding and water table treatments enabled CP 95-1376 to respond more favorably to the 16- and 33-cm water table depths in 2002.
Yield Response to Flooding
Sugarcane in commercial fields in Florida is intermittently exposed to floods, sometimes for durations of approximately 1 wk. The effect of repeated 7-d flooding on yields can be measured by comparing yields (in years 2000 and 2002 when there were no grub infestations) of the treatment continuously drained to 50 cm with those of the treatment flooded and drained to 50 cm for five cycles in 2000 and nine cycles in 2002. Responses to these treatments differed for each genotype. In 2000, cane and sugar yields of CP 95-1376 were 21 and 18% higher, respectively, in the continuously drained treatment compared with the periodically flooded treatment (Tables 4 and 5). In 2002, cane and sugar yields of CP 95-1376 were each 28% higher in the drained treatment compared with the periodically flooded treatment. Yields between flooded and continuously drained CP 95-1429 did not differ significantly in either year.
On the basis of a total of 35 flooded days in 2000, each day of flooding reduced CP 95-1376 cane and sugar yields by 0.17 and 0.02 kg m–2, respectively. In 2002, when total days of flooding numbered 63, each day of flooding reduced cane and sugar yields of CP 95-1376 by 0.21 and 0.03 kg m–2, respectively. Each of these predictions is made from two data points (0 and 35 d in 2000 and 0 and 63 d in 2002). Further research with more than one flood duration may discover that sugarcane yield losses due to flooding are not best explained by linear responses to duration of flood exposure. Periodic flooding did not affect cane and sugar yields of CP 95-1429. Due to restrictions on total daily drainage to public canals, EAA farmers sometimes must choose which of their flooded fields to drain. The losses in yield because of repeated 7-d flooding for one and not the other genotype in this study emphasize the importance of learning the reaction to short-duration floods of existing and future EAA sugarcane cultivars.
Morphological Responses and Implications
Effects of treatments on several morphological characters were also measured. For stalk diameter, number of nodes per stalk, and stalk weight, there were no significant water table effects (data not shown). For green-leaf and brown-leaf weights, no consistent effects of water table were identified (Table 6).
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Water table depth during drainage of periodically flooded treatments did not result in significant linear responses of aerenchyma ratings (Table 6). However, averaged across genotypes, the treatment not periodically flooded had a significantly lower aerenchyma rating than all three periodically flooded treatments in all 3 yr (Table 7). Thus, sugarcane responded to periodic flooding by forming more aerenchyma, at least on the bottom of its stalk, the only portion of the stalk we examined. In 2000 and 2001, no aerenchyma formed in the CP 95-1376 that was not exposed to periodic flooding. In 2002, the continuously drained treatments were flooded for 7 d to control grubs. This 7-d flooding explains the aerenchyma formation in CP 95-1376 in 2002. These responses suggest that CP 95-1376 needed exposure to flood to develop aerenchyma whereas CP 95-1429 developed constitutive aerenchyma.
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It is probable that the greater yield reductions due to periodic flooding in CP 95-1376 compared with CP 95-1429 were due to differences in aerenchyma formation between the two genotypes. In CP 95-1376, lack of constitutive aerenchyma formation may have resulted in ineffective O2 transport for an unknown time until sufficient aerenchyma formation occurred in response to periodic flooding. Although in 2002, one 7-d flood was sufficient to cause stalk aerenchyma formation in CP 95-1376, it is not known how long it took for this aerenchyma formation to occur after exposure to the flood. Also, the CP 95-1376 exposed to the one flood in 2002 was about 2 mo older than other treatments of CP 95-1376 routinely exposed to flooding. These results suggest that selection for the ability to form constitutive stalk aerenchyma (such as occurred with CP 95-1429) and high yield would be a useful approach for identifying flood tolerance among sugarcane genotypes. It has already been shown that constitutive aerenchyma formation in sugarcane roots is a routine process (Ray et al., 1996; Van Der Heyden et al., 1998).
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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