Fertilization, Soils and Cultural Practices EFFECTS OF FLOODING ON SUGARCANE GROWTH. 2. BENEFITS DURING SUBSEQUENT DROUGHT

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1 Fertilization, Soils and Cultural Practices EFFECTS OF FLOODING ON SUGARCANE GROWTH. 2. BENEFITS DURING SUBSEQUENT DROUGHT B. W. Eavis Ministry of Agriculture, Science and Technology Barbados ABSTRACT Sugarcane varieties B 4744 and B were grown in lysimeters containing either disturbed montmorillonitic or kaolinitic clay, under waterlogged or drained conditions in a 23 factorial experiment. During a 7-month period the differences between growth and tiller production in the waterlogged treatments (water table 15 cm below surface) and the field capacity treatments (drained from 80 cm below surface) were small. During a drought which was imposed at 7 months the plants removed water until they reached permanent wilting point. The amount of available water removed from the disturbed montmorillonite was 66% greater than from the disturbed kaolin, and was 50% greater in both cases than from Hudson's undisturbed monoliths, thus indicating an important advantage of cultivation. Waterlogging increased the water available to the plants during the drought by 100 and 25% in the kaolinitic and montmorillonitic s, respectively, and there were 17 and 7 extra days for growth, respectively. Soil moisture content in all treatments was the same at permanent wilting point. Waterlogging increased total shoot elongation during the drought by 200 and 50% in the kaolinitic and montmorillonitic s, respectively. In the waterlogged treatments, 25 and 33% more shoot elongation occurred per cm of water transpired in the 2 s, respectively. There were no significant differences in variety performance during the drought. INTRODUCTION In many sugarcane growing areas the chief factor limiting yield is drought. If rainfall is irregularly distributed throughout the growing season, the ability of the to conserve water in rainy periods for subsequent use by the crop in a drought may have an important influence on yield. The water storage capacity of the is greatest when drainage is prevented and all the pore space is filled with water. Experiments reported here were carried out to determine whether the extra water stored as a result of waterlogging and drainage restriction could be used by the plants subsequently during a drought. Previous work by Erickson et al. (I) showed that: subsurface asphalt barriers increased the water holding capacity of sandy s and sugarcane yields. This work shows that overdrainage can a1.o occur in some clay s and that this may influence yields in areas subjected to drought. ''?.,

2 716 FERTILIZATION, ETC. MATERIALS AND METHODS Weighable lysimeters were constructed from 45-gal oil drums. Eight of these were filled with kaolinitic top (Barbados type 60) and 8 with montmorillonitic top (Barbados type 20). Drainage was allowed to occur rhrough perforations in the drum made either 15 cm below the surface,(waterlogged treatment) or 80 cm below the surface (field capacity treatment). Hudson's (2) system for weighing and transporting the lysimeters was used. Each drum could be lifted and carried on a spider cart to a platform scale for weighing. One man could comfortably weigh 16 of the 600-lb lysimeters in 2 hours each morning. Eight 1-bud cuttings of either B 4744 or B were planted in each lysimeter and were allowed to establish for 6 weeks before the waterlogging was imposed. Tiller elongation and tiller production were measured weekly over the next 6 months, during which half the treatments were waterlogged to 15 cm below the surface and the remainder drained to field capacity. These treatments were maintained by daily additions of water. When the plants were 7 months of age, no more water was added and the plants were shielded from rainfall by placing them under a shelter with a transparent plastic roof. Subsequently, during the drought the daily loss of water and the total loss of water by transpiration were measured until the plants were at permanent wilting point. Tiller elongation rate was measured daily throughout the drought on 8 tillers in each lysimeter, using Hudson's method described in the previous paper. Permanent wilting point was arbitrarily chosen as the stage when all but 3 of the central spindle leaves had turned brown. At this stage the fresh weights of the shoots were measured, and the root systems were separated by washing the through a wire grid. The volume was determined both before and after the drought in order to measure shrinkage. The treatments- type, drainage regime, and varieties-were combined in a 2 X 2 X 2 factorial experiment with 2 replications, giving 16 units in total: Soil type: kaolin clay and montmorillonite clay; Drainage: waterlogged to within 15 cm of surface and drained to field capacity; Variety: B 4744 and B Initial 6 Months Preceding Drought RESULTS AND DISCUSSION Maintaining a water table 15 cm below the surface for 6 months (but not during the establishment period) did not significantly (P = 0.05) reduce tiller elongation rate in either type or variety (Table 1). Tiller production was also similar in waterlogged and drained s. This result can be compared with that reported in the previous paper where the flooded to the surface adversely affected tiller elongation rate and tiller production. No deleterious effect was observed in the waterlogged treatments during 1-6 months of age, because the watertable was below the level of the underground stem tissue, and the buds were healthy and in an aerobic environment. Root pro-

3 B. W. EAVIS 717 Table 1. Average weekly tiller elongation rates during 5-month period preceding drought (cmlday). Kaolinitic Montmorillonitic B B 4744 B B 4744 Waterlogged to within 15 cm of surface Drained to field capacity duction was also normal in the top 15 cm of, but only fine roots grew into the saturated. Fine roots were found throughout the volume to the depth of 80 cm. Results During the Period of Drought At the start of the drought the amount of water present in each lysimeter varied according to type and drainage regime, and its utilisation by the plants in growth and transpiration was measured (Fig. 1). The amount of,/.-i311 SHRINKAGE 5% A a, l - 60] Water logged Solid I p t. - p q Water Available tor. \ Growth -+-auamj&l Solid Mineral* Matter Fig. 1. Volumetric proportions of water, air and solid mineral matter as influenced by type and drainage. water for growth is that removed between the commencement of the drought and the cessation of elongation of the central spindle leaves. The water available for survival is that removed subsequently up to the permanent wilting point. There was no difference between the ability of the 2 varieties to remove this water, and in Table 2 averages for both varieties are, therefore, combined. Also included in Table 2 are results obtained by Hudson for the same s taken from the field in the undisturbed state. Hudson worked with cylindrical monoliths removed from the field in 45-gal oil drum cases, and he established cane under conditions similar to those used in these experiments. Four months after planting he allowed the cane plants to dry the to permanent wilting point, and

4 718 FERTILIZATION, ETC. Table 2. The effect of type and drainage on the water available to sugarcane plants in a drought. Undisturbed monolith (81 cm deep) K 7 3 dried from field capacity (Hudson's results) M 12 5 Water available to sugarcane plants for: Total shoot Shoot elongation elongation per cm of water made during transpired in Growth Survival drought growing period (cm) ( 4 (cm) (cm) Disturbed top (81 cm deep) K cm in 2.07 dried from field capacity 12 days M cm in days Disturbed top (81 cm deep) K cm in 3.14 dried after being waterlogged from 29 days 15 cm of surface M cm in days Note: K = kaolinitic M - montmorillonitic called the relationship between spindle growth rate and water deficit a profile moisture release curve. His methods have been followed in these experiments. Hudson's undisturbed monoliths were at field capacity at the start of the drought. Table 2 shows that between field capacity and permanent wilting point the plants were able to remove 70% more water from the montmorillonitic clay than from the kaolin clay. The smaller amount of available water retained by the kaolinitic clay was partly a consequence of a greater volume of large pore spaces which drained at field capacity; the kaolin clay also had a smaller pore volume at high negative water potentials, which contained water available for survival but not for growth. The porosity (total pore volume) of the 2 clays did not differ significantly. Hudson's results can be compared with those 'obtained in these experiments using the same volume of disturbed tops from the same sites. Both montmorillonitic and kaolinitic s when dried from field capacity to permanent wilting point, released 50% more water in the disturbed condition as compared with the undisturbed conditions described above. This indicates that the smaller bulk density (larger total pore volume) affected the pore volume holding water between field capacity and permanent wilting point, and did not merely increase the air-filled pore volume at field capacity. Soil cultivation is therefore able to increase the available water capacity in the case of these 2 s by 50%. In the disturbed state the montmorillonitic clay held 66% more available water than the kaolinitic clay. The available water removed from the disturbed s held initially at field capacity can be compared with that removed from the same s dried from the waterlogged condition (Table 2). The waterlogged plants were able

5 B. W. EAVIS 719 to remove all the extra water present. At permanent wilting point the moisture contents of the s were the same regardless of variety, type or drainage regime. Waterlogging increased the amount of water available in the disturbed kaolin by 100% compared with the field capacity treatment, and this extra water was contained in large pores which drained at a negative water potential of less than 80 cm of water. In the montmorillonitic clay the volume of pores in this size range was smaller, and the extra water available to the plants as a result of waterlogging was increased only 25% over the field capacity treatment (Table 2). As a consequence of the extra water present in the waterlogged s, there were 17 and 7 extra days for growth in the kaolinitic and montmorillonitic clay, respectively. There were 25 and 4 extra days for both growth and survival. During the drought, total tiller elongation (stem and spindle elongation) was 200 and 50% greater for the plants previously waterlogged in the kaolin and montmorillonite, respectively (Fig. 2). This greater growth was partly a result of e" : -2 9 PWP LT -4 - W Day 1 Drought MONTMORILLONITIC SO1 L PWP MONTMORlLLONlTIC SOIL PWP I I cms of water (taking PWP as datum) ---- Survival Period ( But no growth) pwp ( Permanent Wilt~ng Rxnt) was Z8'l.f 1.5'1. In all Fig. 2. Tiller eloogation rates of sugarcane during a drought imposed at 7 months of age. the extra number of growing days; however, the tiller elongation,/,cm of water transpired during the drought was also greater in the formerly w,$&1ogged. Efficiency of utilisation of available water (tiller elongation/unit*~ofswater lost) was 25 and 33% greater in the waterlogged treatments than in the field capacity treatments. The number of days in which the plants were suffering from water i I,,

6 720 FERTILIZATION, ETC. stress was similar in all cases, but as a proportion of the total number of drought days it was smaller in the case of the waterlogged treatments. This probably accounts for the better utilisation of water in terms of growth obtainedlunit of water transpired during the drought. The daily transpiration rates were similar in both treatments;' waterlogging did not restrict transpiration, or increase wilting. The daily loss of water in the early stages of drought when the plants were not suffering from water stress was chiefly dependent on evaporative conditions, and remained at between 0.80 and 1.40 cm/day until within 4 days of the cessation of growth. During the latter 4 days transpiration rates were rapidly curtailed and remained less than 0.30 cm/day during the survival period up to permanent wilting point. Differences in type, variety and rooting characteristics were apparently not great enough to cause differences in the transpiration patterns-under these climatic conditions. The type of root system found at the end of the experiment in the waterlogged treatments differed greatly from that found in the field capacity treatments. The latter consisted of a network of thick primary roots with a relatively small proportion by weight of small diameter laterals. The root system developed in the waterlogged treatment consisted of a very dense mat of fine laterals with a small proportion of thicker roots. Table 3 shows that the fibrous Table 3. Root weights and ratio of fibrous:thick roots by weight, determined at the end of the drought. Kaolinitic - -. Montrnorillonitic B B 4744 B B 4744 Waterlogged to 15 an Total root weight (gm) of surface fibrous: thick root ratio Drained to field Total root weight (gm) capacity fibrous: thick root ratio root:thick root ratio was 150% greater in the waterlogged treatments. The ratio was also 200% greater in the montmorillonitic, as compared with the kaolinitic. The proliferation of fine roots seems to be associated with the wetness of the ; in other experiments the development of a surface mat of roots under a mulch was also observed to occur only under very wet conditions. Sugarcane is thus able to adapt its root system to accommodate an anaerobic environment. A thin root has a smaller oxygen requirement and the diffusion pathlength for entry of oxygen into the respiring tissue is less than in the case of a thicker root. The total harvested root weights with the variety B were 90 (kaolin) and 50% (montmorillonite) greater when the had been dried from the waterlogged condition (Table 3). With B 4744 the differences were not as consistent. The mean root weights were 36% greater in the montmorillonitic s as compared with the kaolinitic s and this was associated with the proliferation of thin roots under wetter conditions.

7 REFERENCES c G,< 1. Erickson, A. E., et al Subsurface asphalt barriers for the improvement of sugarcane production and the conservation of water on sand. Proc. ISSCT, 13: Hudson, J. C The availability of water. Ph.D. thesis, University of the West Indies. 1