Response of Sugar Beet Yield to Deficit Irrigation under Drip Irrigation System

Table (2). Some soil physical and chemical properties of experimental site

                     used in the present study

Table (3). Chemical analysis of water used for irrigation

            According to Pollach et al. (1996), polarimetric assay of sugar measures the sucrose content of molasses sometimes more exactly than value corrected for raffinose.

Thick purity juice (TPJ % = meq 100g^-1) was expressed as:

Sucrose % (Pol,%): Juice sugar content of each treatment was estimated in fresh samples of sugar beet root by using Saccharometer according to the method described by A.O.A.C. (Ahadi and Sobhani, 2005).

Total soluble solids (TSS), %

Recoverable sugar yield (kg/ha) was deduced as described by Mohamed (2002), applying the following formulae:

Recoverable sugar yield (kg ha^-1) = roots yield (kg ha^-1) ×recoverable sugar percent (%)

Recoverable sugar percent (%) was deduced according to Harvey and Dutton (1993) as:

Where: Pol. %( Sucrose %), and K, Na, and α- amino-N were determined as me 100 g^-1 beet.

Gross sugar yield, GSY (kg ha^-1) = root yield (kg ha^-1) x gross sugar percentage

White sugar yield, WSY (kg ha^-1) = root yield (kg ha^-1) x white sugar percentage

Losses sugar yield, LSY (kg ha^-1) = root yield (kg ha^-1) x loss sugar percentage

Loss sugar content, % = Gross sugar content (%) - white sugar content (%)

Juice purity percentage calculated as:

Statistical Analysis

            All collected data for sugar beet yield and quality were subjected to analysis of variance (ANOVA) according to Snedecor and Cochran (1991). The mean values were compared according to least significant difference Test (LSD test), Williams and Abdi (2010). All statistical analyses were performed using analysis of variance technique of "Statistix 8" computer software package (Statistix, 2003).

RESULTS AND DISCUSSION

Sugar beet growth characters

            The results presented in Table (4) show the response of sugar beet growth characters to different irrigation regimes. The results clearly indicated a significant effect of different irrigation regimes on all sugar beet growth characters except leaf area per plant and foliage water content as compared with control (100% of ET₀). Irrigation regime at 100% ET₀ gave the highest values of no. of leaves per plant (42.3), leaf area per plant (6205.7 cm²/plant) and total chlorophyll content(49.73 mg/100g leaf fresh weight), but the highest values of foliage fresh weight (1517.7 g/plant) and top yield (101.1 ton/ha) were attained at 60% of ET₀. The percent increase in top yield of sugar beet at 60% of ET₀ was account as 16.50% over the common treatment (100% of ET₀).

Table (4).  Growth characters of sugar beet as affected by irrigation regimes

Yield and yield components

            Table (5) shows sugar beet yield and yield component parameters of sugar beet as influenced by irrigation regime treatments. The resultsclearly indicate a significant effect of irrigation regimes on sugar yield and yield components. Irrigation at 40% of ET₀ gave a highest value of root length (32.7cm) and root diameter(13.0 cm), but 60 % of ET₀ gave the highest values of average root fresh weight (1500 g/plant) and root gross yield (100.0 ton/ha). The highest value of root/top ratio (1.31) was attained at irrigation with 120% of ET₀. The percent increase of root gross yield of sugar beet at 60 of ET₀ was account as 49.99% over the common treatment (100% of ET₀).

            According to the present results, to get maximum root yield of sugar beet under the present condition of Nubaria region, it might be recommended irrigation at 60% of ET₀. This case of suitable soil water resulted in healthy plants, also highest foliage yield consequently higher yield could be obtained and vise versa regards the extra or less soil water availability. These results are in agreement with those of Bailey (1990) and Emara (1996).

Table (5). Sugar beet yield and yield components as affected by irrigation              regimes

Juice quality and impurities contents

            The results of juice quality and impurities content are illustrated in Table (6-1, 6-2). The different irrigation regimes significantly affected the juice quality and impurities contents of sugar beet. Irrigation at 40 and 60% of ET₀ gave the highest value of TSS (total soluble solids), 23.0%, but the highest values of K (6.62 meq/100g root), a-amino N(4.39 meq/100g root), white sugar yield (13.71 ton/ha), gross sugar yield(16.97 ton/ha) and loss sugar yield (3.26 ton/ha) were attained at 60% of ET₀. The highest values of polarity or sucrose content (17.76 %), effective polarimetric assay of sugar (15.46%),thick purity juice, TPJ (90.66%), white sugar content (14.94%) and juice purity (84.13%) were attained at 80% of ET₀. Irrigation at 120% of ET₀ gave a highest value of Na content (1.69 meq/100g root).

            The increase in sugar yield was due to both increase in sugar content and root yield in which sugar yield was adversely affected by water deficit. Increasing the impurities in the root of stressed plants decreased extraction of white sugar. So, deficit irrigation improved sugar beet quality by reducing these impurities.

            Alkaline coefficient (AK) is considered as an indicator to determine the juice impurity. The Ac is affected by both the sodium+ potassium (Na+K) as nominator and a-amino-nitrogen (a-amino N) as dominator. So, increasing the dominator, the Ac will be decreased and vise versa. The threshold value of Ac is 1.8%, the values higher than 1.8% indicated that high purity sugar beet. The chemical characteristic of sugar beet juice was mainly affected by the sugar crystallization process. There are high sucrose content associated with low contents of K, Na and a-amino-N contents. It is also important for stability of juice in the factory that the content of a-amino N would be maintained low in relation to that of K and Na.

            The effect of irrigation water levels on Na content of roots was not consistent throughout the treatments. Sodium had significant variation among treatment. Na value ranged from 0.83 me/100g root for 60% deficit irrigation to 1.69 meq/100g root for 120% deficit irrigation. Several researchers (Fabeiro et al., 2003; Ober et al., 2005) reported that the effect of water deficit on Na content is less clear and varies between treatments. However, some studies showed that as deficit water increased, Na content decreased (Tognetti et al., 2003; Maralian et al., 2008).

            Bosemark (1993) reported that the chemical characteristics of sugar beet juice were mainly affected by the sugar crystallization process. There are high sucrose content associated with low contents of K, Na and alpha-amino-N and betaine contents. It is also important for stability of juice in the factory that the content of alpha-amino-N would be maintained low in relation to that of K and Na ions.

            Sugar content was affected by irrigation regimes. Therefore, root sugar content was generally increased in response to deficit irrigation treatment. Sugar beet roots accumulated more sugar (16.97% under 60% of ET₀ deficit irrigation than under any of the full and other deficit irrigation levels. Sucrose production from sugar beet depends on maximizing storage root growth over along growing season. It is necessary to apply a suitable irrigation program together with appropriate agricultural measures for taking a high sugar rate accumulation in the sugar beet production (Ucan and Gencoglan, 2004). The increase in the sucrose rate of fresh root is due to a slower accumulation of water. Excess irrigation increased sugar beet yield, but sugar rates decreased (Bilgin, 1992).

            The increase in sucrose content was 1.72% at 80% of ET₀ over common treatment (100% of ET₀). The economical yield of sugar beet is white sugar content, white sugar yield and gross sugar yield. The corresponding increases were 4.66, 44.16 and 45.66% at 60% of ET₀ over the common treatment (100% of ET₀), respectively.

Table (6-1). Juice quality and impurities contents of sugar beet as affected by   irrigation regimes

Table (6-2). Juice quality and impurities contents of sugar beet as affected by irrigation regimes

            Multi-regression results showed that there are a regression among sucrose content, root yield and root water content with white and gross sugar yields (as below), sugar content having highest effect on sugar yield.

Deficit irrigation also entails a number of constraints such as:

• crop response to drought stress should be studied carefully (Hsiao, 1973). Determining optimal timing of irrigation applications is particularly difficult for crops with crop water production functions in which maximal water productivity is found within a small optimum range of ET;
• irrigators should have unrestricted accress to irrigation water during sensitive growth stages. This is not always the case during periods of water storage (Zhang, 2003);
• a minimum quantity of irrigation water should always be available for application ( Zhang and Oweis, 1999; Kang et al., 2002; Geerts et al., 2008b). This is not always possible in extremely dry regions where irrigation water is scarce (Enfors and Gordon, 2008)

The increases in water productivity under deficit irrigation can be attributed to the following reasons:

• water loss through evaporation is reduced;
• the negative effect of drought stress during specific phenological stages on biomass partitioning between reproductive and vegetative biomass(harvest index) is reduced (Fereres and Soriano, 2007; Reynolds and Tuberosa, 2008) due to increases the reproductive organs(Karam et al., 2009);
• water production for the net assimilations of biomass is increased as drought stress is mitigated or crops become more hardened. This effect due to conservative behavior of biomass growth in response to transpiration (Steduto et al., 2007);
• water productivity for the net assimilations of biomass is increased due to the synergy between irrigation and fertilization (Steduto and Albrizio, 2005);
• negative agronomic conditions are avoided during crop growth, such as pests, diseases, anaeobic conditions in the root zone due to water logging (Pereira et al., 2002 ; Geerts et al., 2008a).

            In conclusion, the present study revealed that if water is limited and deficit irrigation is spread over growth season. The present findings were that deficit irrigation of sugar beet led to decrease in root and sugar yields and seasonal evapotranspiration. Water use efficiency values increased slightly with increase in water deficit. Irrigation at 60% of reference evapotranspiration (corresponding to 50% of soil moisture depletion) could be used for sugar beet grown in semi-arid regions such as Nubaria without decrease of root yield and white sugar yield. The findings in this study strongly recommend that up to 60% deficit irrigation of sugar beet would be advantage if the farmers target is to maximize root and white sugar yield. But if the target is to put more area into production under limited water supply, irrigation at 40% of reference evapotranspiration in sugar beet may be feasible. Also, this present results are good idea for scheduling irrigation with weather data instead of soil moisture data as confirmed with this study. Scheduling irrigation of sugar beet using weather data collected from nearby weather station is simple and accurate, in which requires the weather data and calculation of crop evapotranspiration using Penman-Monteith equation (Allen et al., 1998) or using any available computer software such as CROPWAT 8.0 (FAO, 2011) or AquaCrop 4.0          (Raes et al., 2012).