Journal of Experimental Botany, Vol. 53, No. 366, pp. 123-129,
January 1, 2002
© 2002 Oxford University Press
Original Papers |
Effects of treatments potentially influencing the supply of assimilate on its partitioning in sugarcane
1 School of Life and Environmental Sciences, George Campbell Building, University of Natal, Durban 4041 South Africa
2 SA Sugar Association Experiment Station, Mount Edgecombe, 4300 South Africa
Received 16 March 2001; Accepted 20 August 2001
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Abstract |
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Two pot experiments and one field experiment were conducted on sugarcane to assess the effects of treatments expected to change total carbon assimilation on the partitioning of assimilate. In the first experiment pots of cultivars CP and N14 were arranged to simulate normal field spacing. At 5 months, plants were partially defoliated or left intact. In the subsequent four months, defoliation resulted in a small (not significant) decrease in total dry mass increment; it increased the proportional partitioning of assimilates to leaves in N14, whilst in CP it increased the proportional partitioning to stems. In both cultivars defoliation increased proportional allocation to non-structural dry matter, and thus sucrose, in the stem. In the second experiment pots of cv. CP were grown at normal spacing for 4 months, then left untreated, shaded, or placed further apart. During the subsequent four months shading decreased total dry matter increment, but increased proportional partitioning to the stems, and within stems to non-structural dry matter, and so sucrose. Widened spacing increased total assimilation, but decreased proportional allocation to stems; partitioning within the stems was not affected. In the field experiment plants of both cultivars were partially defoliated at 6 months, or left intact. Defoliation resulted in only a very small decrease in stem dry mass increment during the subsequent four months (leaves were not measured). Within the stem partial defoliation caused proportionally increased partitioning to non-structural dry matter, hence to sucrose. The results suggest that sucrose storage receives priority in the allocation of assimilate, rather than representing the accumulation of assimilate not required for vegetative growth.
Key words: Assimilate production, homeostasis, partitioning, sink priority, sugarcane.
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Introduction |
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It was the sucrose accumulating ability of the stem of sugarcane (Saccharum sp. hybrid) that led to its domestication, and which has been subsequently improved by breeders. The control of sucrose accumulation by the plant is thus of genetical and physiological as well as agronomic interest. The components of yield in sugarcane are stem dry mass (stem numberxdry mass per stem), and the fraction of stem dry mass consisting of sucrose. Sucrose yield could simply amount to the accumulation in the stem of photosynthate in excess of that required for the growth of the plant body: leaves, stem, and root structure. Any curtailment of assimilation should then be reflected more in diminished sucrose accumulation than in diminished structural growth.
Leaf production and stem elongation take place throughout the life of the sugarcane crop, successive leaves and their subtending internodes expanding sequentially. Older leaves senesce, but remain attached to the stem (‘withered’ leaves). Some sucrose accumulation takes place concurrently with stem elongation, though most of the sucrose in an individual internode accumulates after the internode has stopped elongating. (Lingle, 1999
). This could be taken to suggest that the sugar accumulated by sugarcane consists of material not required for the production of plant structure and of respiration to support growth and maintenance (sugarcane does not reproduce sexually in normal agricultural practice). However, considering the intensive selection for sucrose production to which sugarcane has been subjected, the elongating stem and sucrose accumulating processes within the stem might have become potentially strong sinks for photoassimilate; potential sink strength is the intrinsic, genotypically determined, ability of the sink to receive or attract assimilates (Marcelis, 1996). Sucrose accumulation could then compete for photosynthate with structural parts of the plant.
Several studies on this topic have been carried out on sugar beet (Beta vulgaris L.), a species which, like sugarcane, is cultivated for its sucrose production. Ulrich concluded that only assimilate in excess of that required for the growth of leaves, and the structure of storage and functional roots is accumulated as sucrose (Ulrich, 1952, 1955). Later studies (Watson et al., 1972; Milford and Thorne, 1973; Wyse, 1980) showed that decreased assimilation, brought about by the shading of plants, caused a proportionally greater reduction in the growth in dry mass of the storage root than in that of the shoot (leaf laminae and petioles); shading did not, however, affect the proportional distribution of assimilate between sucrose and structural dry matter in the storage root. Watson et al. concluded that there must be some mechanism in the storage root that maintains a nearly constant ratio of sucrose to other dry matter, structural or non-structural, and thus that sucrose accumulation did not simply represent the accumulation of carbohydrate not required for vegetative growth (Watson et al., 1972).
This paper describes two pot experiments and one field experiment on sugarcane, performed to assess the relative priority of various sinks, particularly sucrose accumulation. Treatments comprised partial defoliation, shading, and increased spacing of plants, intended to vary total assimilation per plant. Two cultivars were used; CP 66/1043 (CP) from Florida, USA, and a South African cultivar, N14. Cultivar CP produces a comparatively small number of stems per unit land area, and matures, or ‘ripens’ early, i.e. it has a high stem sucrose concentration from an early stage of growth; it was bred in a region where the active growing season is less than nine months. N14 produces a comparatively large number of stems per unit land area, and ‘ripens’ late, i.e. there is a low stem sucrose concentration until near the normal time of harvest at 12 months. The field experiment was undertaken to compare responses in the field with some of those obtained with potted plants; use was made of a few rows of the two cultivars which happened to be available; they had been established side-by-side to fill an unused part of an experimental field.
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Materials and methods |
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Planting material
Sugarcane is propagated vegetatively in agriculture. A crop is established by planting sections of stem, about half a metre in length, in parallel furrows, previously drawn at row spacing, and covered with soil. Nodal buds sprout and give rise to shoots, which tiller to produce a dense within-row stalk population; individual plants cannot be distinguished. For the present experiment, buds, each on a short (c. 2.5 cm) section of stem, were sprouted in potting compost in small plastic pots, to produce ‘transplants’. This was done to facilitate establishment of individual plants singly in pots, and so as to be able to distinguish individual plants in the field. Transplants were used when they were 10–15 cm tall.
Pot experiments
These were undertaken at an outdoor, near level paved area on the South African Sugar Association Experiment Station, Mount Edgecombe near Durban, South Africa; no trees or buildings were close enough to cast shade on the area at any time of the day.
Cultural methods
Transplants, selected by eye for uniformity, were planted singly in pots each holding 20 l of washed river sand. Pots were stood in trays 10 cm deep to which water and nutrients were added. Nutrients were supplied at concentrations commonly used in nutrient culture work, with concentrations of nitrogen and phosphorus being 7.5 and 1.1 mmol l-1, respectively (Hewitt, 1952); this concentration of N in the nutrient solution appears to correspond well with soil N fertility levels in well-managed field crops (JCS Allison and NW Pammenter, unpublished observations). Nitrogen was supplied in the nitrate form. Trays were refilled daily with tap water (the amount required per tray measured with a flow meter), and the amount of stock solution needed to maintain nutrient concentrations added. Trays were drained and pots flushed with tap water at fortnightly intervals.
Experiment 1: pots, cultivars CP and N14, partial defoliation
Pots were placed in rows 1.2 m apart, with pots placed 40 cm apart in rows; this arrangement gave a density of 2.1 plants m-2, which approximated plant spacing in the field. Pots were arranged in a block consisting of four replicates, each of two single-row plots, one per cultivar. There were six pots per plot. The whole block was guarded on both sides with a row of pots and with three pots at the end of every treatment row.
At 155 d after planting samples, each of two plants, cut at soil level, were harvested from every plot; the fresh weight of stems and green and withered leaves was determined (in these cultivars dead leaves remain attached to the stem throughout the growth period). Subsamples of green and withered leaves were oven-dried to determine dry matter concentrations. The composition of stems was determined by standard methods (Anonymous, 1985). After being weighed fresh, stems were shredded and two samples of the shredded material taken. One was weighed before and after oven drying. The total non-structural dry matter (TND) concentration of liquid expressed from the other sample was determined by hydrometry and the sucrose fraction of the TND by polarimetry. The structural fraction of the stem dry matter was obtained as the difference between stem dry mass and TND mass. (Apart from sucrose, the TND consists of small quantities of reducing sugars, inorganic acids, amino and other organic acids, and proteins. The structural dry mass consists mostly of cellulose, hemicellulose and lignin.)
Also at 155 d pots were moved up to close the gaps left by sampling; two of the remaining four plants in a plot were left intact (I), and two were ‘half’ defoliated (D): alternate fully emerged laminae were removed by cutting them with scissors at the junction with the sheath; this procedure decreased lamina area by c. 40%. Laminae reaching full emergence (ligule exposed) after day 155 were removed at about 10 d intervals. The mean decrease in lamina area during the period between day 155 and final harvest at 273 d was probably at least 35%. All laminae removed were dried and weighed, the masses added to those measured at final harvest.
A final harvest was made 273 d after planting. The same measurements as at the first harvest were made on samples of two plants from each of the four replicates of every treatment. In addition, the nitrogen concentration of leaves and stem was determined by the Kjeldahl method, on milled dry material from each replicate of every treatment.
Experiment 2: pots, cv. CP, shading and increased spacing of plants
This experiment was carried out after Experiment 1 had been completed. Pots of cv. CP were grown in a block, arranged as in Experiment 1 (120x40 cm, 2.1 plants m-2).
At 121 d after planting 12 randomly chosen plants were harvested from inside rows, avoiding the three plants at the end of a row. The same measurements as in Experiment 1 were made on the plants.
On the next day three treatments were imposed, each on one of three adjacent blocks of pots: untreated (U), shaded (S), and widened spacing (W). Spacing of U and S was unchanged from the initial spacing (2.1 plants m-2). That for treatment W was rows spaced at 150 cm with pots spaced at 80 cm within rows, to give a density of 0.83 plants m-2. For treatment S a 40% black shade cloth ‘tent’ was placed over the plants; the tent, which was raised as plants grew, covered both top and sides of the green leaf canopy. Measurements, made at intervals throughout a clear day, showed that the shade cloth transmitted c. 50% of incident photosynthetically active radiation (PAR), and on a later day, in overcast conditions, between 25% and 40% of incident PAR.
Treatment rows were surrounded by guard rows and there were three guard plants at each end of every row. This ‘side-by-side’ arrangement meant that treatments were not replicated; replication would have been impractical because of the limited experimental area. A final harvest was made 240 d after planting, when the same measurements as before were made on six samples, each of two plants, from each of the three treatment blocks.
Experiment 3: field, cultivars CP and N14, partial defoliation
The soil was a deep, uniform coastal sand. Transplants had been planted 50 cm apart in a plot consisting of 12 rows, 15 m in length, spaced at 1.2 m, giving a plant density of 1.7 m-2; cv. CP occupied rows one to six, and cv. N14 rows seven to twelve; thus cultivars were not replicated. Fertilizer was applied at rates recommended for commercial crops on this soil. A light irrigation was applied to the plot on five occasions during periods of scanty rainfall, and the plants never showed any visible symptoms of water stress.
Six months after planting 10 triplets of CP plants (three consecutive plants in a row) were tagged, five in the third and five in the fifth row of the plot. One plant was left untagged between successive triplets. No plants were tagged within 3 m of the ends of rows. Ten triplets of N14 plants were similarly tagged in the eighth and tenth row of the plot.
On day 189 after planting the middle plant of each triplet was harvested by cutting at ground level. Leaves were stripped from each harvested plant, and the number of stems counted. The mass and composition of stems was determined by the same methods as in Experiment 1. Leaf mass was not determined.
At the same time one of the other two plants of each triplet, chosen randomly, was ‘half-defoliated’, in the same way as in Experiment 1, to decrease lamina area by c. 40%. Laminae reaching full emergence after day 189 were removed at about 10 d intervals, and the mean decrease in lamina area after day 189 was probably at least 35%. The two remaining plants of each triplet were harvested on day 311, 122 d after the first harvest. The same measurements as at the first harvest were made on the stems of these plants; and their nitrogen concentration determined by the Kjeldahl method.
Statistical methods
For each of the three experiments, means and standard errors of the means were calculated for attributes measured at the first harvest. For Experiments 1 and 2 changes in the values of attributes during the interval between the harvests were calculated as the difference between the mean value at the first harvest and each of the replicate values at the second harvest. For Experiment 3 changes in attribute values were calculated as the difference between that of the early-harvested plant of each triplet of plants, and that of each of the late-harvested plants of the triplet. Data recorded as percentages were ‘logit’-transformed for statistical analysis (Dyke, 1997).
For Experiment 1 two factor analysis of variance was carried out on attributes measured at the second harvest, and on changes occurring during the period between the harvests. At the second harvest in Experiment 2 treatments were equivalent to blocks; treatment means were compared by means of t-tests. For Experiment 3 comparisons were made by means of t-tests.
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Results |
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Dry matter accumulation
In Experiment 1 the increment in total dry mass during the period between harvests was c. 60% greater in N14 than in CP (Table 1
). In Experiment 2 (CP only) the increment in total dry mass (Table 2) was substantially greater than that of CP in Experiment 1 (the interval between the harvests was the same, c. 120 d). This was probably mainly due to warmer weather in Experiment 2; mean daily growing degrees (average temperature, °C, minus 12; 12 is commonly used as a base temperature for sugarcane) during the period between the harvests was 8.0 in Experiment 2 and 5.5 in Experiment 1; the latter is a low value for a tropical crop. Photosynthetically active radiation (PAR) differed by only 6%.
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In Experiment 1 the increment in total dry mass was not much larger in intact than in half-defoliated plants (Table 1
; the difference was c. 10% averaged over the cultivars; P>0.30). In Experiment 2, shading, which decreased PAR by c. 50%, resulted in a decrease of 29% in total dry mass increment during the period between the harvests (Table 2; P<0.01). With increased spacing, which decreased plant density from 2.1 to 0.83 m-2, and thus leaf area per unit land area (leaf area index) by about 60%, the increment in total dry mass per plant increased by 18% (Table 2; P<0.05).
In the field (Experiment 3) the increment in stem dry mass during the period between the harvests was substantially greater in N14 than in CP (Table 3; P<0.05). The difference was proportionally similar to that in dry mass accumulation between the cultivars in pots (Experiment 1). The effects of half-defoliation were similarly small to those in pots; it caused a decrease of only 5% (not statistically significant) in the increment in stem dry mass in both cultivars during the period between the harvests (Table 3; cf. Table 1).
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Distribution of dry matter
In Experiment 1 the main effect of partial defoliation on the distribution of dry matter accumulated in the interval between harvests was not significant (assessed by a two-way analysis of variance; Table 1). This was because the cultivarxdefoliation interaction for stem increment as a percentage of total increment was significant, (P<0.05). The two cultivars showed opposite effects: partial defoliation resulted in proportionally increased partitioning of the total dry mass increment to leaves in N14, but partitioning to stems was proportionally greater with partial defoliation in CP (Table 1). In Experiment 2, on CP only, shading resulted in stems receiving an increased fraction (P<0.05) of the total increment in dry mass (Table 2). With widened spacing there was a small, but significant (P<0.05), decrease in the proportional distribution of dry matter to stems (Table 2).
Partial defoliation in Experiment 1 and shading in Experiment 2 both resulted in proportionally increased partitioning to TND in the stem (Tables 1, 2). Wider spacing had no effect on the proportional partitioning of dry matter between non-structural and structural material in the stem (Table 2). Purity (the fraction of TND consisting of sucrose) was not much affected by either partial defoliation or shading; consequently sucrose yield was approximately proportional to that of TND (Tables 1, 2).
In the field (Experiment 3) the TND fraction of the increment in stem dry mass was increased by half defoliation in both cultivars (Table 3; P<0.05 for CP, and <0.01 for N14), while purity was little affected. Sucrose yield per plant was slightly, though not significantly, greater in half-defoliated than in intact plants (Table 3).
Nitrogen concentration
In Experiment 1 at the second harvest N concentration in the leaf laminae was similar in intact and half defoliated plants of both cultivars (Table 4); N concentration of the withered leaf and of the stem was somewhat smaller in half-defoliated than in intact plants; however, the only significant difference was that for withered leaf in CP (P<0.05). In Experiment 3 N concentration of the stem was slightly smaller in half-defoliated than in intact plants, but the effect was not significant; N concentration was, however, appreciably greater in the stems of CP than in those of N14 (P<0.001).
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Discussion |
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Partial defoliation in Experiments 1 and 3 resulted in proportional decreases in above-ground growth in dry mass smaller than the proportional decreases in leaf area (any changes in below-ground dry mass were not measured). In Experiment 1 a near 40% decrease in leaf area resulted in a (non-significant) 10% decrease in total dry mass increment (Table 1
); in Experiment 3 there was a small (non-significant) decrease of c. 5% in stem dry mass increment (leaves were not measured) when leaf area was decreased by, probably, about 35% (Table 3). Evidently, defoliation resulted in an increased rate of assimilation by the remaining leaf surface. This could have been as a consequence of either increased illumination incident on, and/or a change in the photosynthetic characteristics of, the remaining leaves. A regression of crop growth rate, C (rate of increase of total dry mass per unit land area) on leaf area index, L (leaf area:land area ratio) determined for maize (Allison, 1969), which has a similar leaf arrangement to that of sugarcane in respect of light penetration, suggests that a decrease in L of 35–40% should have resulted in a c. 25% decrease in dry mass increment. Thus partial defoliation appears to have resulted not only in increased radiation interception, but also in an increase in rate of assimilation per unit radiation flux density. The stimulation of the rate of photosynthesis by partial defoliation has been obtained in sugarcane (Cock et al., 1997), with wheat (Wareing et al., 1968), and an understorey palm (Anten and Ackerly, 2001).
In Experiment 2 a 50% reduction in PAR flux density resulted in a decrease of c. 30% in total dry mass increment per plant (Table 2); however, at an L of c. 4.0, as in this experiment (data not shown) the decrease in dry mass increment would be expected to be nearly proportional to the decrease in radiation. An increase in spacing, which reduced L by about 60%, resulted in an increase of c. 20% in total dry mass increment per plant (Table 2); however, the relationship between C and L derived for maize (Allison, 1969) predicts that such a large decrease in L should have resulted in an increase in dry mass per plant of about 45%.
These results from defoliation and shading are equivalent to response coefficients (proportional change in growth/proportional change in photosynthetic capacity) of less than unity, implying that growth was controlled partially by sinks (Jones and Lynn, 1994; Farrar, 1999). The results thus suggest that the rate of assimilation was being influenced by demand for assimilate by expanding parts of the plant, particularly the elongating stem (cf. Green and Vaidyanathan, 1986); the stem accounted for the bulk of the dry mass increment during the period between the harvests (Tables 1, 2).
Treatments that potentially affected assimilate availability also influenced dry matter distribution. In CP, partial defoliation in Experiment 1 and shading in Experiment 2, caused assimilate to be partitioned preferentially to stems at the expense of leaves (Tables 1, 2). Conversely, widened spacing (Experiment 2), which would have increased radiation penetration through the canopy, caused a decrease in proportional allocation to stems (Table 2). In N14, however, leaves received precedence in the distribution of assimilate following partial defoliation of plants in pots (Experiment 1; Table 1). A possible explanation for this difference in allocation between the cultivars is that it was a consequence of a difference in stage of development at the time that defoliation was carried out. Plants of CP had by then completed about two-thirds of their normal period of active growth, of about 8 months, while N14 plants were less than half-way through their usual growth period (12–14 months). Changes in the priority of different sinks with stage of development are probably a common feature of plant growth; for example, Ho quotes results from tomato showing that precedence in the distribution of assimilate changes from leaves in young plants to expanding fruits (the principal storage organ) in older plants (Ho, 1988).
There are possible causes of the effects on dry matter distribution, other than direct consequences of defoliation or shading. Defoliation removes the nutrients in the excised leaves, and any resulting shortage of nutrients, particularly of N, in the plant might limit leaf expansion; Bazzaz refers to work showing a strong relationship between plant N concentration and the fraction of dry mass allocated to leaves (Bazzaz, 1997). Any reduction in N concentration could result in proportionally decreased partitioning of assimilate to leaves. However, in Experiment 1 partitioning to leaves only decreased in CP; in N14 it increased (Table 1). The plants were well supplied with nutrients, and the N concentration of leaves and stem was not much affected by defoliation (Table 4). It appears that where mineral nutrition is adequate and plants are at a stage of development at which assimilate is devoted primarily to the production of the photosynthetic system, the loss of N with defoliation may not necessarily limit leaf growth. When maize plants were partially defoliated during the vegetative phase, subsequent leaf expansion was unaffected, although the excision of leaves resulted in the loss of about one-third of the N in the shoot (Allison et al., 1975). Similarly, Schäufele and Schnyder concluded that N supply did not limit leaf expansion of defoliated young ryegrass plants (Schäufele and Schnyder, 2001).
A possible cause of a bias in partitioning towards the stem with shading, as in Experiment 2, would be any increase in the far-red to red ratio of the incident illumination caused by the shade cloth; this could induce ‘shade-avoidance’ behaviour (Smith, 1986), and thus proportionally greater partitioning to stem, at the expense of leaf growth. However, the black shade cloth used did not alter the far-red:red ratio of the transmitted radiation, and it is unlikely that such a photomorphogenic effect was involved in the partitioning of assimilate in the shaded plants.
The treatments affected not only proportional assimilate partitioning to stems, they also influenced allocation within the stem. Partial defoliation in Experiments 1 and 3 (Tables 1, 3), and shading in Experiment 2 (Table 2), resulted in the dry matter within the stem being biased towards non-structural material, hence sucrose production, at the expense of structural growth of the stem. With widened spacing in Experiment 2 (Table 2), proportional allocation to non-structural material was not affected.
The net result of changes in assimilation, partitioning of assimilate between stems and leaves, and partitioning between structural and non-structural material within stems, is that, within a variety, none of the treatments had a significant effect on the sucrose yield per plant. This is the case for both varieties, despite their different growth characteristics. The most likely explanation of these effects is that sucrose accumulation constitutes a high priority sink, at least in superior (with respect to sucrose yield) sugarcane varieties.
In conclusion, in sugarcane the elongating stem, and sucrose accumulation in it, evidently constitute a high priority sink (‘dominant’ according to the terminology of Wardlaw, 1990), i.e. one receiving priority in the allocation of assimilate when the supply of assimilate is potentially decreased. Additionally, increased assimilate supply from widened plant spacing resulted in a decrease in proportional allocation to sucrose. It is tempting to regard these results as providing an example of developmental homeostasis, i.e. constancy of attributes of adaptive value as a consequence of physiological flexibility (Shank and Adams, 1960; Geiger et al., 1996). The property under consideration in this instance is sugar accumulation by sugarcane stems. This property has ‘adaptive’ value in that sugarcane has been selected over a long period to maximize the average yield of sugar over a run of more or less favourable years.
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Acknowledgments |
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The authors would like to thank MJ Savage, School of Applied Environmental Sciences, University of Natal, Pietermaritzburg, for measuring the effect of the shade cloth on the spectral characteristics of transmitted radiation.
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Notes |
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3 To whom correspondence should be addressed. Fax: +27 31 260 2029. E-mail: pammente{at}biology.und.ac.za
4 Present address: School of Life and Environmental Sciences, George Campbell Building, University of Natal, Durban, 4041 South Africa.
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