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African Crop Science Journal, Vol. 10. No. 3, 2002, pp. 231-237 EFFECTS OF SEASON OF SOWING ON WATER USE AND YIELD OF TOMATO IN THE HUMID SOUTH OF NIGERIA S.O. Agele, A.Olufayo1 and G.O. Iremiren2 Department of Crop Production, Federal
University of Technology, PMB 704, Akure,
Nigeria (Received 9 May, 2000; accepted 5 May, 2002) Code Number: cs02023 ABSTRACT Soil water availability is a major constraint to crop production in the post-rainy season period in the humid tropics. The seasonal pattern of water use was, therefore, studied in field-grown tomato (Lycopersicum esculentum) during two consecutive rainy and post-rainy seasons in a humid zone of Nigeria. Soil moisture content was monitored by the gravimetric method. Drainage micro-lysimeters were installed to monitor maximum evapotranspiration (ETM). For the post-rainy season sowing, the relative water use (ETa/Eo) values (the ratio of actual evapotranspiration, ETa to open water evaporation, Eo) varied from 1.14 at the beginning of the growing season to 0.04 at crop maturity. The values of ETa/Eo and evapotranspiration deficit (ETd) indicated that during the crop cycle, there were increasing intensities of soil moisture deficits, decreased transpiring leaf area and canopy developments and shortened vegetative and reproductive phases. The vertical profile of root distribution revealed significantly larger depths of roots and higher root mass and values of root length to maximum leaf area in the post-rainy season crop of tomato. Key Words: Consumptive use, evapotranspiration, growth, Lycopersicum esculentum, lysimeter, water balance, yield RÉSUMÉ La diosponibilité de l’eau dans le sol est une contrainte majeure à la production agricole dans la période post saison pluvieuse dans la région tropicale humide. La tendance saisonnière de l’utilisation de l’eau était alors étudiée dans un champs de tomates (Lycopersicum esculenta) pendant deux saisons pluvieuses et post-pluvieuses consécutives dans une région humide du Nigeria. L’humidité du sol était examinée par la méthode gravimétrique. Des micro-lesimètres étaient installés pour déterminer l’evapotranspiration maximale (ETM). Pour la période post semis, la valeur relative de l’utilisation de l’eau (Eta/Eo) (Taux de l’evapotranspiration naturelle, Eta ; l’évaporation à la surface de l’eau, Eo) a varié de 1,14 au debut du semis à 0,04 à la maturité. La valeur de Eta/Eo et le déficit à l’evapotranspiration (ETd) indiqués que pendant le cycle de la plante, il y avait une intensité croissante du déficit dans l’humidité du sol, une chute de la surface de feuilles transpirantes et le dévelopement de la couverture végétale et l’écourtement de la phase végétative et reproductive. Le profile vertical de la distribution des racines a révélé de racines profondes, de masses et de longueurs à la surface de feuilles maximale élévées pendant la période post saison pluvieuse pour la tomate. Mots Clés: Usage à la consommation, evapotranspiration, croissance, Lycopersicum esculenta, Lesimètre, bilan en eau INTRODUCTION The importance of tomato (Lycopersicum esculentum, Mill) as a dietary staple vegetable in Nigeria, its high income potentials and the need for increased production to meet year-round food demand compels farmers to cultivate the crop in and out of season. However, in the humid tropics, tomato cultivation is concentrated mainly in the wet rainy season, which is characterised by high incidence of pests and diseases, low fruit set and poor fruit quality (IAR&T, 1991). Conceivably, the post-rainy season tomato could give higher yield and more remuneration, but this period is occasioned by high soil temperatures and limiting soil moisture status, which have profound influence on growth and yield. Although weather factors greatly influence crop yield, there is scanty information for tomato production on weather-yield relationships, especially in the humid tropics (Fakorede and Opeke, 1985). Relative water use, defined as the ratio of actual evapotranspiration (ETa) to open water evaporation (Eo), is dependent on soil moisture status and is an index of crop production. Seasonal ratios of ETa/Eo in the range of 0.77 and 0.80 were reported for cowpea (Vigna unguiculata) and maize (Zea mays) intercrop towards the end of rainy season in south western Nigeria (Hulugalle and Lal, 1986). The amount of water transpired by a crop depends on the evaporating surface intercepting radiant energy. Approaches to maximise water use at periods of limited soil water availability should aim at realising maximum yield for the same amount of available soil water. Yield increases may be due to reduction in the amount of evaporation and by selecting crops which attain high harvest index. Earliness to anthesis has been reported as a method of ensuring water availability for the completion of reproductive growth (Sinclair, 1988). These attributes are achievable by a crop’s ability to increase the percentage of water use after anthesis by altering rooting properties. Passioura (1982) estimated a root density of 0.5 cm3 as a threshold value above which extraction of water from soil by roots should proceed at a considerable rate and a value of at least 1 cm3.cm3 for droughted crop. The study of growth responses to soil moisture reserve is basic to understanding crop adaptation and yield stability. It is, therefore, necessary to closely match crop phenology and yield to periods and amount of soil moisture extraction. Very little work has been done in this respect in tomato production in the humid tropics, particularly during the post-rainy season period. The objective of this study was, therefore, to relate soil water extraction pattern to growth and yield of tomato in the wet and post-rainy season period (terminal drought situation) in a humid zone of Nigeria. MATERIALS AND METHODS The study was conducted at the Teaching and Research Farm of the Federal University of Technology, Akure (7o 5‘ N, 15o 10‘ E), Nigeria. The soil at the experimental site is an Alfisol (USDA) with a pH level of 6.3; N 0.9%; extractable P 11.2 mg kg-1 and 0.2, 1.3 and 0.3 cmol. kg-1 soil for exchangeable K+, Ca2+ and Mg2+, respectively. The soil was enhanced by applying NPK fertiliser at a rate of 60 kg N, 30 kg P2O5 and 30 kg K2O ha-1 as recommended (IAR&T, 1991). Three week old seedlings of a local variety of tomato (Akure) raised in the nursery, were transplanted into the field and planted in plots measuring 20 m x 12 m, at a spacing of 90 cm x 60 cm on May 6th, 1994 (wet season crop) and September 21st 1994 (late season crop). Chemical sprays were applied to protect the crop against pests, diseases and weed infestation. Reference crop evapotranspiration was estimated from the Blaney-Morin equation
PET = rf (0.45Ta = 8)(520 – R1.31) (1) 100 where PET is the potential evapotranspiration (mm/day), Ta is the mean monthly temperature oC, R is the mean monthly relative humidity (%) and rf is the ratio of monthly temperature oC to annual radiation (g-Cal/cm2/day). Evapo-transpiration deficit defined as reference crop evapotranspiration (PET) minus actual evapotranspiration (ETa) observed during a growth phase, was normalised using the mean saturation deficit (kPa) of air. The normalised evapotranspiration deficit (ETd) was used as an index to reflect the magnitude of water deficit experienced by a crop (Singh, 1991). To measure maximum evapotranspiration (ETM) of the crop, 60 cm diameter and 60 cm depth drainage micro-lysimeters were installed at the centre of the plot. The seasonal soil water balance was estimated for the late season crop of tomato for the experimental site using water balance equation: P + I = DS + R + D + ETa (2) Where P is precipitation (mm), I is irrigation (mm), DS is change in soil water storage (mm), R is runoff (mm) which was assumed zero since erosion was negligible on the plots < 3% slope. drainage component D of the soil water balance was also assumed negligible during the post-rainy season period since there was little rainfall during this period. Root, shoot biomass and leaf area were sampled at 50% flowering date (between 7 and 8 weeks after transplanting (WAT). Leaf area was measured with leaf area meter LiCor 2000 (Mayashi Denko, Japan) fortnightly starting from 3 to 13 WAT. Days to first flowering and fruit harvest were recorded and percent fruit set and fruit harvest were calculated. Repetitive samples from at least five portions of each plot were considered when computing average values of each parameter measured. The vertical distribution of roots was estimated for the rainy and post-rainy season crops of tomato by the trench profile method. A trench (0.7 m wide x 0.7 m deep x 1 m long) was dug perpendicular to the rows of crops. A cubic coring tool (10 cm x 10 cm x 10 cm) was inserted horizontally into the face of the trench and the roots washed from the samples using a 2 mm sieve. Samples were taken at 10 cm interval to a depth of 60 cm. The trench face was advanced at least 50 cm along the row before sampling to avoid edge effects. The lengths of about 100 exposed roots in each core sample was measured in consecutive 10 cm layers by the line intercept technique (Connor and Jones, 1985) and the lengths of roots in the remaining samples were estimated using the relations obtained between length and weight/unit soil volume. Root and shoot weight/plant were determined after oven drying fresh biomass for 24 hours at 80o C. Sampling was repeated during the second year (1995) and similar procedures as for the first season were followed for data collection. Data collected on growth and yield parameters of tomato were subjected to a two way analysis of variance (ANOVA) test (Steel et al., 1997) to determine the influence of year, seasons of sowing and their interactions on tomato performance. RESULTS AND DISCUSSION Table 1 presents the climatic data during the period of study. The average monthly rainfall was less than 25 mm during the post-rainy season period whereas a higher rainfall amounts per month were received during the wet season. The humid south of Nigeria is characterised by a bimodal rainfall pattern with the first cycle occurring between April and July and the second between August through October (Akintola, 1986). At the beginning of the post-rainy season period, the soil water content was especially high in the top (0 - 20 cm) layer (Fig. 1). This is due to stored water from the second peak of the bimodal rainfall pattern which terminates at the end of October. Usually, the soil water content is brought to field capacity in the month of September by the late season rains. Thereafter, the soil water content diminishes in the top soil layer. The crop root zone, which is confined to the top layers of the soil (0 - 20 cm) at the beginning of the growing season, extends to subsoil (20 - 50 cm) at the end of December thus, enabling the crop to extract water from the subsoil. This contrasts with the wet season crop when tomato root zone did not proliferate to this depth (Fig. 2). Although the effects of seasons of sowing was significant on tomato performance, non-significant differences were found for yearly response and interactions of season with year. There was a restricted production and expansion of the leaf following anthesis and rapid leaf senescence (Table 2). This may be due to high evaporative demand and continuously declining soil water status from the expanding root system (Fig. 2). From the vertical profiles of root density presented in Figure 2, the distribution of roots (mean weight of roots/unit volume of soil) declined with depth. Roots were well developed in the surface (ploughed) layers (0 - 20 cm) and slightly lower in deeper soil layers (20 - 60 cm), a greater proportion of total root weight was observed at depth of greater than 20 cm, while less than 50% root weight was observed within 20 cm depth. The early exploration of the top-soil is marked by change in root density between the upper and lower layers, a pattern of root distribution which must have shifted soil water extraction to deeper layers of the profile. The pattern of root development at depth ensured root ramification of large soil volume. Nevertheless, a huge investment of dry matter is required to attain this capacity, this is evident from the shoot biomass development. Total root lengths of tomato in this study were within the range reported for unirrigated crops (Robertson et al., 1980). The maximum leaf area (Table 3) that was supported by the root biomass is an indicator of potential capacity for water supply relative to demand for water in the canopy (Connor and Jones,1985). Higher ratio of root density to maximum leaf area (Table 3) was attained by post-rainy season tomato, thus, balancing demand for water to the available supply in order to maintain functional activity of the restricted canopy. The higher ratio might be in response to declining soil water status and increased evaporative demand in the post- rainy season period. Limitations in soil moisture status might have led to lower plant performance presumably from limitations in light interception and assimilation. The soil water balance for post-rainy season tomato crop from 1994 to 1995 is presented in Table 3. The evaporative demand as indicated by the reference crop evapotranspiration (PET) was high (for example, 157 and 189 mm in November and December, respectively) as opposed to rainfall amounts of less than 5 mm during the same period, which led to the crop dependence on soil water reserve. It was noted, however, that the PET values in the wet (rainy) season were not higher than 100 mm (Agele, unpublished report). The actual evapotranspiration (ETa) values were low in the months of November corresponding to flowering and fruit maturity stages in the tomato crop. Low values of ETa of 61.3 and 5.8 mm in November and January (Table 3), are indicative of increasing intensities of soil moisture deficits. This was confirmed by the values of relative water use (ETa/Eo) which fell below 0.5, the point considered as the threshold value for well watered crops (Baldy et al., 1993). A sudden drop in the values of relative water use was also observed in the months of November/December. Hulugalle and Lal (1986) also noted that soil water reserve was not sufficient to meet crop evapotranspiration demand for maize in the post-rainy season in south western Nigeria. They noted the existence of large negative gradients in the hydraulic potential with little upward flux of water below the root zone. The tomato crop coefficient (k) was obtained from drainge lysimeters using water balance equation. The values of k (Fig. 3) are comparable to those reported by Romisio and Manuel (1992). The values of k were high in November corresponding to flowering and fruit setting stage when tomato water requirements peaked (Romisio and Manuel, 1992). The k values in post-rainy season tomato far exceeded unity as observed for most crops indicating that the crop has high water consumption capacity. Growth stages were shorter in the late season as tomato attained flowering and fruit harvest 10 days earlier compared to wet season crop (Table 2). This is in agreement with earlier observation that water constraint hastens the attainment of growth stages (Sinclair, 1988). There was no appreciable decrease in fruit yield of post-rainy season tomato compared to wet season crop in spite of the low relative water use in the former. Tomato fruit yield was lower in the rainy season period despite the significantly higher shoot biomass yield. This trend may be associated with overcast skies (Fakorede and Opeke, 1985) and its consequencies on fruit set and fruit drop (IAR&T, 1991). REFERENCES
©2002, African Crop Science Society The following images related to this document are available:Photo images[cs02023f3.jpg] [cs02023t2.jpg] [cs02023f2.jpg] [cs02023t1.jpg] [cs02023t3.jpg] [cs02023f1.jpg] |
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