Abstract: In field trials conducted in central Greece during two growing seasons, effects of irrigation system and fertilization on the host-parasite association tobacco-Orobanche ramosa were quantified. The experiment was designed as split plot design with four replicates, two main plots (drip and sprinkler irrigation) and three sub-plots (vetch as green manure, red clover as green manure and control (without fertilization)). Drip irrigation was the only treatment that significantly reduced broomrape infestation throughout the growing season. Also, drip irrigation reduced Orobanche dry weight by 70-76%. Finally, the use of drip irrigation system appears to be a promising cultural practice in the management of Orobanche ramosa in tobacco.
INTRODUCTION
Orobanche ramosa is a root holoparasite in the family Orobanchaceae. Orobanche infestations can be devastating to crops and remove otherwise productive land from effective use for very long periods of time (Rubiales et al., 2003a). Orobanche sp. parasitize crops such as sunflower (Helianthus annuus; Orobanche cumara), tomato (Lycopersicon esculentum), potato (Solanum tuberosum), tobacco (Nicotiana tabaccum), rapeseed (Brassica napus), (Orobanche ramosa) and legumes (Orobanche crenata and Orobanche aegyptiaca) (Matusova et al., 2005). The broomrapes (Orobanche sp.) are widespread in Mediterranean areas, in Asia and Southern Europe, attacking dicotyledonous crops and depend entirely on their hosts for all their nutritional requirements (Fernandez-Aparicio et al., 2006).
Broomrape is very common in Mediterranean region and maintained by the fact that it has a broad host range; second, broomrape seeds can be dispersed short distances by wind; third, soil seed-bank can increase rapidly, as a single broomrape plant can produce hundred thousands of seeds and fourh, the seeds remain viable in the soil for many years and germinate only after stimulation by root exudates of the host (Rubiales et al., 2003a). In the last decade, broomrape has become the most important weed in tobacco production in East Mediterranean countries.
Many efforts have been development to fighting this pest, but to date there are no completely satisfactory control measures. Some success has obtained using chemical control (Qasem, 1998; Goldwasser et al., 2003), resistant varietes (Perez-de-Luque et al., 2004; Rubiales et al., 2003a) and agronomical practices (Acharya et al., 2002; Grenz et al., 2005; Haidar and Sidahmed, 2000). In recent years, the attention of researchers has shifted more towards cultural ways to alleviate the parasitic problems. The concept of integrated management is gaining importance to overcome broomrape problem and use of different irrigation system and organic fertilizers are some of its important components.
Organic fertilization are commonly used in cropping systems to increase soil organic matter, structural stability, water holding and cation exchange capacities and as a good sources of nutrients (Mandal et al., 2003). Many researchers investigated the effect of organic fertilizers on the seed germination of many plants (Haidar et al., 1999; Haidar and Sidahmand, 2000). For example, Haidar et al. (2003) reported that goat manure significantly reduced broomrape infestation.
Another control method is the use of drip irrigation system. In regions where summers are hot and dry and rainfall is infrequent, irrigation practices in row crops that reduce soil wetting would reduce the germination of weed seeds. Drip irrigation significantly reduced the weed density and dry weed compared to furrow or sprinkler irrigation systems (Sabra, 2000; Veeraputhiran and Kandasmy, 2001). The aim of present study was to study the effects of irrigation systems and fertilization on the establishment and development of Orobanche ramosa in tobacco (Nicotiana tabaccum).
MATERIALS AND METHODS
Experimental design: A field experiment was conducted during 2004-2005 and 2005-2006. A Virginia (flue-cured) tobacco crop (Nicotiana tabaccum cv. NC71) was established in field in the Domokos (22.3381E, 39.0329N) district 200 km to the north of Athens. A naturally heavily infested area with O. ramosa was selected. The soil in the experimental area was a clay (50.7% clay, 23.3% silt and 26% sand) with a pH of 7.6, 2.2% organic matter and EC of 0.32 mS cm-1. The crops before tobacco were vetch (Vicia sativa cv. Alexandros) and red clover (Trifollium pratense cv. Nemaro) which was incorporated into the soil. Prior to this study the field was under wheat cultivation. The experiment was set up on an area of 1200 m2 according to the split plot design with four replicates, two main plots (irrigation systems) and the three sub-plots (vetch as green manure, red clover as green manure, control (without fertilization)). The plot size for each irrigation systems was 15x10 m and the sub-plot size was 5x10 m.
Irrigation systems: Overhead sprinkler and drip irrigation systems (Table 1) were set up on adjacent blocks within the same field, but far enough apart to avoid cross irrigation. A water meter was used to measure the amount of water applied.
The drip system consisted of PE laterals of 20 mm in diameter, laterals have in line drippers and at 0.40 cm distance. The drippers had a discharge rate of 4 L h-1 under an operation pressure of 1 atm. The sprinkler irrigation systems consisted mainly pipes of 40 mm diameter, laterals pipes of 40 mm diameter and rotor type sprinklers (radius: 5 m, discharge rate: 16.35 L min-1) with 2 m risers. Laterals pipes were 15 m long with sprinklers spaced at 5 m in each lateral.
Planting and weed control: Vetch and red clover were sown by hand in rows 20 cm apart at a depth of 1.5 cm. The field was sown on 5 October 2004 and 1 October 2005 at a rate 100 and 20 kg ha-1 for vetch and red clover, respectively. Vetch and red clover was incorporated into the soil at the beginning of May.
Seeds of tobacco were sown on the 5th of March in a seedbed under plastic house conditions (to protect seedling from cold weather). After hardening, seeding transplanting on 22 May 2005 and 16 May 2006. Seedling was transplanted by hand in rows 90 cm apart.
| Table 1: | Irrigation details of the experiment |
Transplants were set at 30 cm between each other. The weed were controlled by hand.
Sampling, measurements
Legumes: Biomass samples were taken at the end of April. To define biomass it was cut 1 m in 2 different places of each plot. The total N from biomass were investigated with the Kjeldahl method (Bremer, 1960).
Tobacco: The sampling was made 110 days after transplanting date of tobacco seedlings. To define biomass it was cut 1 m in 2 different places of each plot. Root samples were taken 110 days after transplanting date of tobacco. Samples were collected for each plot at soil depth 0-30 cm with a sampler 30x30 cm. In each sample roots were separated from soil after standing for 24 h in water.
Weed: Broomrape was measured using 1x1 m quadrates, 3 per subplot, on two dates (80 and 110 Days After Transplanting, DAT). All weeds were collected from a measured area and weighed to determine the weed dry matter.
Statistical analysis: For calculating analysis of variance and means comparisons the software Statshoft was used (Anonymous, 1996).
RESULTS AND DISCUSSION
Tobacco growth: Green manures increased tobacco roots dry weight (Table 2) and the most significant impact was when vetch was applied to soil. This species which has a high N content (Table 3) stimulates root growth to a greater extent than red clover. Also, the yields of tobacco crops (Table 4) were higher in 2005 than those in 2006. The main reasons for higher yields in Domokos may be attributed to precipitation. The precipitation during growing season in 2005 (97 mm) was higher than those in 2006 (42 mm). Moreover, there was found irrigation x green manure interaction (Table 4, Analysis of Variance) in yield of tobacco. Data obtained by others researchers (Mandal et al., 2003; Sangakkara et al., 2004; Stopes et al., 1996) clearly demonstrated the beneficial effects of legumes on the root growth of the following crops (wheat, maize, rice). The reduced biomass of the root system caused a reduction in the supply of nutritional elements to the plant, which resulted in the development of smaller plants.
Drip irrigation was characterised by a smaller amount of water (Table 1) applied to the soil. The tobacco yield (dry weight of leaves) was not affected by the reduced water application.
| Table 2: | Influence of irrigation system (Drip irrigation: D; Sprinkler irrigation: S) and fertilization (control, vetch, red clover) on dry weight of roots (g plant-1) |
| **Significant at p = 0.01; ns: Not significant | |
| Table 3: | Amount of N (kg ha-1) that accumulated in the roots and in above-ground parts of the vetch and red clover plants (multiplying dry weight kg ha-1 with N%) |
| Table 4: | Influence of irrigation system (Drip irrigation: D; Sprinkler irrigation: S) and fertilization (control, vetch, red clover) on dry weight of leaves (g plant-1) |
| **Significant at p = 0.01; ns: Not significant | |
The yields of tobacco crops under drip irrigation were higher than those with sprinkler irrigation (Table 4). The yield difference is statistically significant. In a similar study with cotton Cetin and Bilgel (2002) also observed that drip irrigation method produced 30% more seed-cotton than the sprinkler method. Other investigators have also reported high yields for crops under drip irrigation (Malash et al., 2005; Tiwari et al., 2003; Yohannes and Tadesse, 1998). Also, the higher tobacco yield in drip irrigation may be attributed to higher infection by broomrape in sprinkler irrigation (Table 7 and 8). For the first year, there were no significant differences between the drip and sprinkler irrigation for root biomass.
Tobacco infection by broomrape: Green manure had no effect on broomrape number and dry weight (Table 5-8). In contrast, Haidar et al. (2003) reported that organic soil amendments reduced broomrape infestation. Also, Westwood and Foy (1999) observed that nitrogen inhibit the germination of broomrape seeds.
Moreover, the drip irrigation reduced broomrape number and dry weight (at harvest, 110 DAT). Also, at 80 DAT, the combined analysis of variance (Table 5 and 6) of the two years data indicated that the tobacco infection by broomrape was higher in 2005 than those in 2006. The main reasons for higher infection in 2005 may be attributed to precipitation. The precipitation during growing season in 2005 (97 mm) was higher than those in 2006 (42 mm). The infection of tobacco by broomrape was not dependent on tobacco root biomass. In addition, Manschadi et al. (2001) indicated that the positive effect of irrigation on the numbers and dry weights of parasites can be attributed to a better development of the faba bean root system.
Moreover, Sabra (2000) and Veeraputhiran and Kandasmy (2001) reported that drip irrigation significantly reduced the weed density and dry weed compared to furrow or sprinkler irrigation system. High correlation (R2 = 0.97; p = 0.001, Fig. 1) between the number of broomrape plants and total quantity of water was registered. R2 was calculated using the linear equation as follow:
It is well known that water-stressed roots synthesize ABA (Zhang and Outlaw, 2001). Moreover, Matusova et al. (2005) reported that for Striga and broomrape the strigolactone germination stimulants are derived from the carotenoid pathway and the abscisic acid strongly reduced root exudates induced parasite germination.
| Table 5: | Influence of irrigation system (Drip irrigation: D; Sprinkler irrigation: S) and fertilization(control, vetch, red clover) on number of broomrape plants m-2, 80 DAT |
| *Significant at p = 0.05; ns: Not significant | |
| Table 6: | Influence of irrigation system (Drip irrigation: D; Sprinkler irrigation: S) and fertilization (control, vetch, red clover) on dry weight of broomrape (kg ha-1), 80 DAT |
| *, **Significant at p = 0.05, p = 0.01, respectively; ns: Not significant | |
However, the significant effect of drip irrigation on broomrape growth was not possible to attribute to ABA accumulation. Additional studies on the relationship between ABA accumulation in tobacco roots and Orobanche infestation was recommended.
Moreover, Manschadi et al. (2001) reported that O. crenata infestation was higher under sufficient than under limiting water conditions. Also, the level of infection depends on soil moisture (El-Far, 2001; Perez-de-Luque et al., 2004; Grenz et al., 2005).
| Table 7: | Influence of irrigation system (Drip irrigation: D; Sprinkler irrigation: S) and fertilization (control, vetch, red clover) on number of plants m-2, 110 DAT |
| **Significant at p = 0.01; ns: Not significant | |
| Table 8: | Influence of irrigation system (Drip irrigation: D; Sprinkler irrigation: S) and fertilization (control, vetch, red clover) on dry weight of broomrape (kg ha-1), 110 DAT |
| **Significant at p = 0.01; ns: Not significant | |
For example, Rubiales et al. (2003b) reported that the infection of chickpea by broomrape was favoured by rainy autumns and springs.
In contrast, in Striga, an evolutionary related species, Bukar et al. (1996) observed that the number of Striga hermothica haustoria attached to millet roots increased with water deficit stress. Also, the number of haustoria per unit root length increased with water stress.
| Fig. 1: | Correlation between the number of broomrape plants and total quantity of water (n = 12) |
Also Dugje et al. (2006) and Gbehounou et al. (2004) reported that the infection of crops by Striga was suppressed by heavy showers.
CONCLUSIONS
The irrigation system greatly affected the infection of tobacco by broomrape. The drip irrigation system reduced broomrape number and dry weight. The highest yield of tobacco was measured under drip irrigation. Parasitism had influence on yield of the host. Also, all fertilization treatments had no effect on broomrape number and dry weight.
Present results indicate that broomrape infestation in tobacco can be suppressed by drip irrigation.
ACKNOWLEDGMENT
We thank the Greek State Scholarships Foundation (I.K.Y) for funding.