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Weed control without chemicals is one the most important limitation to sugar beet cultivation in low input and/or organic agriculture. The aim of this work was to evaluate, in two sugar beet varieties (L1 and L2) characterized by high and low yield, root morpho-functional traits (total root length, root surface area and number of root tips), useful indexes of sugar beet competitiveness against weeds. The mentioned traits were evaluated on sixteen-day old seedlings grown on hydroponic solutions whereas root length density (RLD) and weed density were measured in field trials. The line L1 characterized, with respect L2, by the higher total root length, root surface area, number of root tips and root density, showed the lower weed density and, therefore, a superior competitive ability (less number of weeds per area unit) and productivity also in absence of weed control. These morpho-functional and molecular markers might allow the selection of sugar beet lines requiring minor technical inputs for weed control and, therefore, more suited to low input and/or organic agriculture.
Key words: sugar beet, root system, weeds, competition
The development of cultivated plants characterized by high competitiveness against weeds is needed to the diffusion of sustainable agriculture (Didon et al., 2003). The identification of cultivars less dependent on the use of herbicides especially occurred in cereals (Caton et al., 2003; Lemerle et al., 2006) and in rice it has been highlighted the fundamental role of root apparatus for the competiveness against weeds (Gibson et al. 1999; Fofana and Rauber, 2000). Also in sugar beet the development of more competitive varieties is now an urgent requirement. Because of its low stand (around 8-10 plants m-2), slow shoot development and limited height, sugar beet is a poor competitor against weeds (Dewar et al., 2003; Märländer, 2005; Deveikyte and Seibuits, 2006), especially in the first development phases (Ammen et al., 1986; Scott & Wilcockson, 1976; Scott et al., 1979; Heidari et al., 2007). Sugar beet productive decreases occurred also at low density of weeds (Wilson et al., 2001) until yield loss of 100% without weed control (Schweizer e May, 1993; Heidari et al., 2007). Studies on sugar beet sugar beet competitiveness against weeds have considered only the effect of leaf apparatus on the competition for light interception (Bremner et al., 1967; Märländer e Rover, 1994; Stibbe e Märländer, 2002). The superior sugar beet competitiveness against weeds has been related to the rapid leaf development at the early stage (Lotz et al., 1991). In these study, the competitive function of the roots has been not considered also if several studies highlighted the role of morphological traits (root length, root surface and number of root tips) and physiological (nutrient uptake rate after deprivation) that are strictly related to the soil nutrient acquisition (Roumet et al., 2006; Sorgonà et al., 2007). These traits have been considered determinant of the yield in maize (Saccomani et al., 1981; Vamerali et al., 2003) and sugar beet (Stevanato et al., 2004; Stevanato et al., 2007), might be also determinant of the superior competitiveness against weeds. The aim of this work was the identification, in two sugar beet lines characterized by high and low yield, the relationships between some characteristics of the root system and the competitiveness against weeds. The final aim was to identify useful indexes of sugar beet competitiveness against weeds.
Materials and methods
- Plant material
The plant material was composed of two sugar beet varieties (L1 e L2) selected at Lion Seeds (UK) and respectively characterized by high and low sugar yield.
- Analysis of root parameters
Root elongation rate, total root length, root surface area and number of root tips were determined by means of a scanner-based image analysis system (WINRHIZO Pro, Regent Instruments, Quebec, Canada) according to Saccomani et al. (2009). Nitrate and sulfate uptake rate was measured transferring the depleted seedlings into a complete nutrient solution containing 200 µM NO3- and MgSO4 following the procedures described by Stevanato et al. (2004). Root length density (RLD) were evaluated in 2005, 2006 and 2007 under field conditions. RLD was determined using eleven minirhizotrons per genotype installed at an angle of 45° to the vertical on the plant row at sowing. Roots were observed at 173 days after sowing in the 0-2.90 m deep layers using 4 m long plexiglass tubes (4 cm outer diameter). Roots intersecting the minirhizotron wall were measured at intervals of 35 cm with an optical fibre probe (Olympus IF 1303) connected to a video camera system (Sony video camera DXC-1079). Data were expressed as cm of root per cubic centimetres (cm3) of soil.
- Analysis of leaf parameters
The leaves traits and development of the sugar beet varieties were investigated in a trial grown in 2004 and settled to allow 20 weekly sampling beginning to early May. A randomized block design with four replications was used. In order to reduce the high variability of the leaves traits existing among plants and plots, 24 average developed beets were collected for each sub-plot. Six beets were selected, discharging the nine above and the nine below for root weight. The leaf area of the remaining beets was measured using an area meter Li-Cor 3000 (Li-Cor, Lincoln, NE, USA).
- Analysis of weed density
Growth and yield performances of varieties in absence and presence of weeds was evaluated in three years (2004-2006) field trials grown at Rovigo (Italy). A split-plot design replicated four times was used with the varieties as main plots. Subplots consisted in two treatments (presence and absence of weeds). The absence was achieved by hand hoeing every two-three weeks. Each subplot included six rows 5 m long, planted 0.45 m apart. A regular stand of ten beets pro m2 was obtained by hand singling. Check sub-plots without beets were also included in each main plot for assessment of the potential weed density and development. The weed population was evaluated beginning to 40 days after beet emergence, in order to determine the species present in the field and the respective density. Other five observations every 2-3 weeks were performed. The harvest of beets took place around the middle September, together with the separate collection of weeds grown on the plots for the determination of the weight of the single species.
- Data analysis
Field data were processed for the analysis of variance (ANOVA) using the Plabstat software package (Utz, 1995). A post hoc comparison was calculated by Duncan test.
Significative differences have been observed for the root morpho-functional traits of the root apparatus between the two sugar beet varieties.
The sugar beet varieties L1 showed higher root length, surface area and number of root tips with respect the variety L2.
In the field trials, the sugar beet L1 showed higher root length density along the soil profile and a lower density and fresh weight of weeds.
Conversely, not significant differences were observed for emergence, growth, and leaves traits (development, area, shape of the shoot).
The 26 identified weed species and their density in the plots are reported in table 1. The 85% of weeds was represented by Sorghum halepense (L.) Pers., Solanum nigrum L., and Abutilon theophrasti Med.
A better understanding of weed biology and populations dynamics can improve the results in point out sustainable methods of control for maintaining the weed populations at manageable levels (Bond and Groundy, 2001). The study of growth dynamics in six sugar beet varieties provided to support some predictive indices of cultivar suppressive ability against weeds. The more competitive variety L06 was characterized by a superior total root length, total root surface, number of root tips, and root density. Not significant differences were observed for field emergence, leaves development, growth habit, and other aboveground traits. The dynamics of weed emergence confirm the importance of early development in sugar beet. In field experiments, the three more competitive weeds were also most important during the first growth phase of the establishment of sugar beet seedlings. Moreover, sugar yield depends strongly on seasonal climatic factors (Loomis et al., 1971). Their occurrences has been evaluated in about 90%, while an incidence of 10% is due to ??????. An early growth is essential for the suitable growing of sugar beets (Boiffin et al., 1992).
Following studies are required to understand a potential implication of allelopathic activity in weed-crop root interaction. The higher number of root tips of the more competitive variety L06, could be an index of its allelopatic activity (Weston et al., 2002). Root proliferation in deep layers can minimize the overlapping and interference of adjacent roots and alleviate stress of root competition (Xie et al., 2006). Tilman & Wedin (1991b) found that the competitively dominant species that most depleted soil nitrate also had high root biomass, as would be expected if high root biomass leads to high N uptake. Seedling is an important plant trait in determining plant strategy and relative competitiveness of weeds and crops. Rapid seedling growth is a characteristic of weeds adapted to the frequently disturbed, highly fertile environment of the arable fields (Grime et al., 1988). For example, in some herbaceous plants the root:shoot ratio is initially very high because of early root growth and establishment in the soil, but it drops rapidly few weeks after emergence (e.g. Ledig et al. 1970: Bazzaz et 01. 1989). Theory and laboratory experiments (Hodge et al. 1999; Robinson et al. 1999) suggest that species take up N in proportion to their RLD. Thus traits associated with the production of high root biomass and high RLD may be well correlated with R* and competitive ability. Cousens et al. (1991) found that wheat and barley decrease their ability to control wild oat (Avena fatua L.) due to the continuous growth of weed, whereas the development of the crops stops in the stage of ripening. In a study of competition dynamics between barley (Hordeum vulgare L.) and oat cultivars (Avena sativa L), Jönsson et al. (1994) showed that the differences in root growth among barely cultivars explained about 50% of the variance in weed biomass, while the part due to shoot growth was not significant. In oat, on the other hand, differences in early shoot and root development explained equally parts. Competition for nutrients and water could justify some of the results, but also differences in allelopathic activity may have played a significant roleIt is unclear if these traits / weed-suppressive ability are correlated with a higher yield crop, despite this trade off is considered a basic / a primary condition for breeding program and to
Weeds represent the more yield-limiting factor for annual crops especially in organic farming. The seed produced by weeds and remaining vital for long time in the soil is another problem, considering that at least 1-5% of the weed potential population survive and develop seeds even applying the best possible treatments with herbicides (May and Wilson, 2006). Seed production is much higher in organic cultivations, thus increasing rapidly the weed infestation in the subsequent crop rotation (Bond and Grundy, 2001).
The current need for more ecological and less expansive approaches in weed control necessitates the improvement of alternative and non-chemical tools to reduce weed development. This objective is quite difficult and can be partially satisfied through adequate crop rotations, mulching, use of cover crops with or without mowing (Ross et al., 2001), and a range of carefully applied soil management (Bond and Grundy, 2001). Machines for inter-row or intra-row weeding were also employed (Heisel et al., 2001). Among the alternative strategies, the enhancement of competition ability of the crops received attention in order to improve the efficacy of the weed management without chemicals.
Despite early studies indicated the existence of some degree of variability useful for possible selections, breeding for weed competitiveness remained unemployed (Callaway and Forcella, 1992). The cultivation of more antagonistic varieties could results in economical and environmental benefits not only in organic, but also in low input and in conventional farming (Christensen 1994; Lemerle et al. 1996; Wall, 1996; Kim et al. 1997).
Selection for yield, disease resistances etc. in commercial varieties was paid, among other things, with a reduced competitive ability against weeds (Rhoades, 1979; Evans, 1993). Opposite behavior on wheat/wimmera ryegrass (Triticum aestivum L./ Lolium rigidum Gaud.) competition was found by Lemerle et al. (2000), thus suggesting that local adaptation of the crops is an additional factor for increasing the antagonism against weeds. Currently, in the main crops only small differences exist regarding the expression of this character (Callaway, 1990; Challaih et al. 1986; Hucl, 1997; Malik 1990; Townley-Smith and Wright 1994).
The ability of the crops in weed limiting was correlated with a number of morphological and physiological traits of the shoot (rapid development, height, leaves size, leaves position), and other qualities useful for light interception (Kropff, 1993; Lotz et al., 1991; Bond and Grundy, 2001). Correlation between aboveground weed competition and yield resulted quite erratic in the major crops (Jennings and Aquino, 1968; Moody and De Datta, 1982; Christensen, 1995)
Recently, the investigations of genetic variability for root system received attention in order to identify adaptive traits to ensure non-conflicting relationships between crop production and environmental and economical sustainability ( ). Other studies evidenced also the importance of belowground competition for limiting the development of weeds (Sinoquet and Caldwell, 1995). The improvement of root traits associated with plant ability to grow under hostile or low fertility soil conditions may represent an advantage, since the belowground competition in several situations reduces the plants performances much more than the aboveground one, especially in the first phases of development (Casper and Jackson, 1997). If belowground competition occurs in nutrient limited conditions, the indisturbance of fertile soil has been largely revised, because fluctuations in nutrients and water take place frequently as well in fertilized and irrigated soil (Bruckler et al., 1997; Farley and Fitter, 1999; Doussan et al., 2003). A positive correlation between competitive ability and abiotic stress tolerance has been supposed, because weed-crop antagonism occurs especially for the uptake of limiting resources ( ). Therefore, the intensity of competition was associated to the ability in rapidly develop extensive root systems with reduced diameter of fibrous roots and with high absorbent surface (Cralle et al., 2003). The early root development also increases the release of compounds enhancing the allelopathic activity, if present ( ). In barley (Hordeum vulgare L.), as other traits useful for weed suppression, the selection for yield and quality reduced the allelopathic activity in the varieties used to day (Bertholdsson, 2003).
During the crop establishment, the belowground antagonism may be more important than the competition for light, especially in organic farming, where the low availability of nutrients is frequent (Bertholdsson and Jönsson, 1994). Later on, competition for light becomes more important and the growth rate and habit of the shoots starts to play a prevalent action. Faster ground cover by the crop is strongly correlated with the limited development of weeds up to full canopy cover until harvest (Dock Gustavsson, 1989).
Therefore, an early and rapid establishment ensuring a regular and uniform distribution of plants on the soil confers, with a reduced intraspecific competition, a superior interspecific weed-suppressive ability (Stibbe and Märländer, 2002). This trait has been associated with the decrease in weed seedling development and in a more efficient carbon fixation of sugar beets (Paolini et al. 1999).