Potato ( Solanum tuberosum L . ) and Sulla ( Hedysarum coronarium L . ) Intercropping in Tunisia : Effects in Water Consumption and Water Use Efficiency

This experiment was carried out at the Technical Center of Potato (C.T.P) situated in the low valley of Medjerda river at Tunisia, during two growing seasons (2006-2007) and (2008-2009). It aims at evaluating the effect of intercropping potato (Solanum tuberosum cv. Spunta) and sulla (Hedysarum coronarium L. cv. Bikra 21) grown under 1:1 intercropping row arrangement and sole cropping on water consumption and water use efficiency of one potato variety and one sulla variety. In addition, land equivalent ratio (LER) was determined to assess the efficiency of the intercropping system. Intercropping had no significant effect on the above parameters and tuber dry biomass production of potato and sulla. However, it increased the total dry biomass of the intercropping system. This increase occurred during the two cultivating seasons of 2007 and 2009 respectively from 12.5 to 14.8% compared to the potato in sole cropping. Intercropping has led to a similar reduction of water consumption (WC) of both crops. This reduction was 36 and 28% for 2007 and 2009 seasons respectively and for both crops. However, intercropping has led to an increase in the overall (WC) of the mixed system by 9.1 and 13.8% in 2007 and 2009 respectively. Water Use Efficiency (WUE) has been improved from 35.4 to 45.6% for potato in sole intercropping and from 25 to 37.5% for sulla intercropped. The values of total LER were higher than one, which indicates the superiority of intercropping system over the sole cropping system.


Introduction
Increasing crops productivity and saving irrigation water are two interrelated issues raising a lot of concern these days in Tunisia.For the arid and semi-arid areas of the world, water use is of great importance in determining resource utilization in intercropping systems.Ogindo and Walker (2005) confirmed that under intercropping, crops conserve water largely because of early high leaf area index and higher leaf area.In intercropping, agricultural income, nutrient, water and radiation resources may be used more effectively than in sole cropping (Willey, 1990;Rodrigo et al., 2001).Intercropping has been reported to reduce water evaporation, and improve conservation of the soil moisture compared with sole cropping (Ghanbari et al., 2010).Usually, complementary use of resources occurs when the component species of an intercrop use qualitatively different resources or they use the same resources at different places or at different times (Tofinga et al., 1993).Therefore, crops selection that differs in competitive ability in time or space is essential for an efficient intercropping system.Due to these issues, numerous research based in indirect measurements of water consumption by intercrop reported an increased water use efficiency in this system (Reddy & Willey, 1981;Mandal et al., 1996;Walker & Ogindo, 2003).Nevertheless, it is very difficult to separate the water use by two crops during the co-growth period (Adiku et al., 2001;Morris & Garrity, 1993).Morris and Garrity (1993) report no significant differences in total water uptake between intercrops and sole cr basis for in Garrity (19 efficiency 2001) The observations were made on leaf area index, above-ground dry matter, tuber dry matter and total dry matter (g m -2 ).Plants were harvested for growth analysis at 52, 56, 62, 69, 75, 83, 90, 96, 103, 111 DAP (potato) and110, 114, 120, 127, 133, 141 DAS (sulla) in (2006-2007) and at 48, 55, 62, 71, 76, 83, 90, 104, 118 DAP (potato) and107, 115, 122, 129, 138 and143 DAS (sulla) in (2008-2009).At each sampling, three plants by plot (potato and/or sulla) were collected.After separation of the plant organs, leaf area and fresh weight were measured.The weightings were made using a precision balance (Sartorius, Model PB3001).Leaf area was measured using a LICOR LI 3100 leaf-area meter then all material was dried at 65°C to constant weight.

Growing Degree Days (GDD)
Growing degree days (GDD), based on air temperatures were used as the explanatory variable in the regression analysis (AGDM, TUBDM and WUE) and accumulated from the date of planting.For each day, GDD was calculated according to the following formula: GDD Tb (1) Where Tmax and Tmin are daily maximum and minimum air temperature, respectively, and Tb is the base temperature.A base temperature (Tb) of 5°C was used as the minimum temperature for sulla growth (Ben Jeddi, 2005) and the Tb for potato was equal to 7°C (Sands et al., 1979).

Crop Coefficient
According to the FAO-56 method (Allen et al., 1998), Ea in sole cropping is modeled as: Where Kc is the crop coefficient under adequate soil moisture (unitless), Ks is the soil moisture coefficient (0-1, unitless), and Ep is the potential evapotranspiration for a reference grass in mm d -1 , KcKs is the combined crop coefficient and includes the effect of both crop type and soil moisture stress.Soil moisture stress is indicated when Ks is less than 1.0 (Allen et al., 1998).
In intercropping when the fractions of ground covered by each crop are different, the K C for an intercropped field can be estimated by weighting the K C values for the individual crops according to the fraction of area covered by each crop and by the height of the crop: In the FAO-56 method (Allen et al., 1998), crop coefficient in intercropping (Kc intercrop ) is modeled as: Where F1 and F2 are respectively the fractions of ground covered by the potato and sulla, H1 and H2 are respectively the heights of potato and sulla, and KC1 and KC2 are the KC values for potato and sulla.
Thus, the crop coefficient of potato and sulla in the sole intercropping is Kc sulla intercropping * * * * (5)

Water Consumption (WC)
Water consumption was estimated with soil water balance equation as follows (Hillel, 1998): Where P is the effective precipitation (mm), I the irrigation (mm), U the upward capillary flow into the root zone (mm), R the runoff (mm), DW the downward drainage out the root zone (mm) and ΔS the change of soil water stored in soil layer of 0-80 cm (mm).The upward and downward flow was estimated using Darcy's law (Kar et al., 2007;De Medeiros et al., 2005).Results indicated that these two parameters were insignificant at the experimental site.Runoff was also insignificant during the two growing seasons.Soil water content was measured once a week with neutron probe (Brand Nardeux, Model Solo 25).Soil water content data were collected for every 10 cm intervals in a soil profile of 10-80 cm.Soil water content of soil layer of 0-20 cm was measured gravimetrically.Some measurements were added before and after irrigation and heavy rain events.Neutron probe access tubes were placed in sole cropping (potato and sulla) at the center of two rows, and in sole intercropping at the center of potato and sulla rows.
2.5.4Water Use Efficiency (WUE) and Land Equivalent Ratio (LER) WUE in sole cropping (potato and sulla) was calculated using the following equation: Where WUE is the water use efficiency (kg m -3 ), TDM the total dry matter production (kg), and WC is the total water consumption over the whole growing season (mm).
WUE in sulla sole intercropping was calculated using the following equation: WUE in potato sole intercropping was calculated using the following equation: Fwater potato intercrop (11) The intercropping advantage was assessed by calculating the land equivalent ratio (LER), an index of intercropping advantage and a reflection of the degree of interspecific competition or facilitation in an intercropping system.The formula is defined as follows (Vandermeer, 1992;Li et al., 1999;Fetene, 2003): Where Y PM and Y PI are TDM of potato in sole cropping and intercropping (kg ha -1 ), respectively; Y SM and Y SI is TDM of sulla in sole cropping and intercropping, respectively (kg ha -1 ).An LER of 1.0 indicates that the two intercropped species have similar demands on the same limiting resources.An LER greater than 1.0 reveals an intercropping advantage or a demonstration that interspecific facilitation (or complementarity) is higher than interspecific competition so that intercropping results in greater land-use efficiency.An LER less than 1.0 reveals mutual antagonism in the intercropping system.Thus, an LER less than 1 has no intercropping advantage and indicates that interspecific competition is stronger than interspecific facilitation in the intercropping system (Wahla et al., 2009).

Statistical Analysis
The results were subjected to variance analysis of one factor by General Linear Model (GLM).This analysis was performed using SPSS 10.0 software.The ensemble was completed by multiple comparisons of means with Student Newman Keuls test (S-N-K).

Effect of Intercropping Potato-Sulla on Production
In order to identify the effect of intercropping potato-sulla on production, we calculated the potato tuber fresh matter production (TUBFM) and sulla above-ground fresh matter production (AGFM).
Similarly, the economic advantage of the intercropping system has been calculated on the basis of LER.The results obtained during the two measurement campaigns (2006-2007) and (2008-2009) were represented in Table 1.
The results (Table 1) showed that for the potato in sole cropping (M2), the TUBFM has respectively varied for the two measurement campaigns (2006-2007) and (2008-2009) from 59.5 to 60.6 t ha -1 .In sole intercropping (M3), the TUBFM was respectively equal to 59.1 and 57.8 t ha -1 for the both experiments.However, statistical analysis showed no significant differences (P ˃ 0.05) in TUBFM between M2 and M3.Besides, the intercropped sulla provided an additional forage production, with AGFM production estimated respectively for the two campaigns equivalent to 19.3 and 22.9 t ha -1 (Table 1).Similarly, during the two growing seasons the total LER of intercropping system potato-sulla has varied from 1.5 in (2006-2007) to 1.4 in (2008-2009).Moreover, the values of total (LER) were higher than one, which indicates the superiority of intercropping system over the sole cropping system.TUBFM is the tuber fresh matter of potato; AFM is the above-ground fresh matter of sulla; LER is the Land Equivalent Ratio and LSD is the least significant difference at 5%.

Discussion
Results of both experiments showed that the AGDM and the TUBDM production by the intercrop potato and sulla were not affected by this association.Thus, it had increased the AGDM and the TUBDM of the intercropping system.These results are consistent with several researchers who have reported that the above ground dry matter production by intercropping system was higher than in sole cropping (Hauggaard-Nielsen et al, 2001;Thorsted et al, 2006;Gao et al., 2009).Similarly, Bouwe et al. (2000) found that in intercropping system potato-beans, the yield of potato was not significantly different from that in sole cropping.This was due to the fact that the photosynthetic assimilation of potato plants was not hampered by the bean plants.Consequently, there was an increase of 9% in tuber yield per plant of potato in sole intercropping compared to potato sole cropping.The intercropping system potato-sulla has reduced the water consumption (WC) by potato and sulla in sole intercropping compared to that in sole cropping.This reduction has varied during the two experiments (2007 and 2009) from 36 to 27.8%.However, it has increased the total WC by the intercropping system.This increase has also varied from 9.1% in 2007 to13.8% in 2009.
According to Morris and Garrity (1993), the total water consumption of the intercropping system is often slightly different from sole cropping.They have conducted several experiments in which the seasonal rainfall varies from 84 to 575 mm.They have finally concluded that the overall water consumption in the intercropping system is relatively higher than 7% compared to that in monoculture.Thus, the morphological and physiological differences between the two cultures in association (potato and sulla), allow them to occupy different soil depth.Similarly, the mechanisms by which the two intercrops are adapted to semi-arid environments did make interspecific facilitation (or complementarities) between these two species in water consumption (same reduction in water consumption for both intercrops).This explains that the AGDM production by the intercrop potato and sulla and the TUBDM production by potato were not affected by the intercropping system compared to sole cropping.This finding is similar to that of Cohen (1970) for both crops intercropped cowpea and sorghum.Intercropping potato with sulla improved WUE of potato and sulla in sole intercropping compared to sole cropping.These results are consistent with several researchers who have reported that the WUE in intercropping system was higher than in sole cropping (Reddy & Willey, 1981;Morris & Garrity, 1993;Mandal et al., 1996;Walker & Ogindo, 2003).Similarly, it has been shown that the intercropping system potato-sulla had no impact on the total dry matter production by potato and by sulla in sole intercropping.This explains the improvement of the WUE in M3 and in M5.In fact, the increase in the WUE of the intercropping system can be explained by the elevated increase in total dry matter production with lowed increase in the total water consumption of the system.These results are in agreement with those of Black and Ong (2000).These authors have shown that the intercropping system can increase the proportion of water used for transpiration by a significant development of the canopy and reduced soil evaporation.This allows reducing the water consumption by crop in sole intercropping compared to sole cropping without affecting the total biomass production.Similarly, Karray (2006) reported that the study of certain types of intercropping showed an increase in the WUE of these crops compared to pure cultures without significant increase in consumption water.
Indeed, the proportion of transpiration flow in evapotranspiration is higher in intercropping than in monoculture.This led to the expansion of cover vegetation increases the intercropping system evapotranspiration and reduces the evaporation from the soil.Guvenc and Yildirim (1999) showed the benefits of intercropping systems in horticulture.These systems improved the efficiency of water use and land occupation.Other studies in intercropping system based on the dephasing of natural resources (water, fertilizers ...) peak demand for each culture showed the superiority of intercropping system productivity compared to monoculture (Hulugalle and Lal, 1986).Oluwasemire et al. (2002) found in intercropping system millet -cowpea that the WUE of millet in sole cropping was lower (2.49kg m -3 ) than in the intercropping system (2.89 kg m -3 ).Similarly, Caviglia et al. (2004) found that the WUE of the intercropping system wheat-soybean was greater than (3.12 kg m -3 ) the soybean in sole cropping (1.64 kg m -3 ).The total LER for intercropping system potato-sulla was higher than one which confirmed the superiority of intercropping system over sole cropping.

Conclusion
Four main results were reached in this study.First, no differences in total dry matter production were found between potato and sulla in sole cropping and intercropping which means that the potato sulla intercropped increased the TDM of the intercropping system.Second, the intercrop potato and sulla consumed less water than in sole cropping but it had increased the total water consumption by the intercropping system.Third, potato and sulla in sole intercropping used water more efficiently and had greater WUE than in the sole cropping.Fourth, the total (LER) was higher than one, which indicates the superiority of intercropping system over the sole cropping system.
In conclusion, Intercropping potato with sulla had no effect in the yield of potato in sole intercropping on one hand and on the other hand the intercropping system was generated a value of 50% of the cultivated area and also was allowed a additional forage production which ranged from 19.3 to 22.9 t/ha.Therefore, this intercropping system was beneficial for the potato small grower.

Table 1 .
Yield of fresh matter (Tuber PDT and above-ground sulla) and LER