Effects of different cropping systems and weed management methods on free energy and content of pigments in maize

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INTRODUCTION
Maize monoculture is still present on large areas owing to the domination of maize in a general sowing structure.On the other hand, rotation is a cropping system that has many advantages, reflected in better crop growth and yielding.The most widespread rotational practice is a two-crop rotation (winter wheatmaize), while three-crop rotation (winter wheat-maizesoybean) is the next most frequent.It is well-known that maize rotation with other crops can decrease the number of pathogens, pests and weeds, maintain or increase soil fertility, enabling better conditions for maize growth and development, so that higher yields can be expected (Dolijanović et al., 2006;Stranger andLauer, 2008; Riedell et al., 2009).
Parallel with rotation, combined application of other cropping measures (tillage, fertilization, sowing, cultivation, etc.) can provide optimal crop density for maximal growth and photosynthesis efficiency since continual application of the same measures in agricultural practice leads to disturbances in the agro-ecosystem.It would be unreal to expect that a single measure could have a satisfactory effect in weed control and lower potential weediness in a maize crop because a great number of weed species all have different life cycles and surviving types (Simić et al., 2013).Saudy (2013), for example, reported an increased maize grain yield, grain number ear -1 and leaf area index along with a reduction in weed dry weight of 90.5 by combining herbicide (metribuzin) treatment and hoeing.Moreover, the interdependence between yielding potential and growth parameters, such as leaf area, is emphasized.However, the relation between leaf area and an achieved grain yield depends on many factors, complicating to understand their interdependence (Milošev et al., 2008).
The complexity of interrelationships between measured plant parameters and yielding potential can be understood better by measuring some biochemical or thermodynamic parameters.Nemeny (2009) emphasized that stress in an agro-ecological system, the maximum energy input, can be calculated and quantified by thermodynamics.Plant systems transform solar energy mainly into chemical energy and then use it for a range of different reactions which ensure system stability, its growth and transition through different developmental phases.Sun (2002) recognized free energy input by water as a presumable plant growth factor.According to the Hess Law, free energy is cumulative, irrespective of its origin; hence, all the potential energy present in a plant system is given by the sum of individual energy states, which could be successfully used by the system in coupled reactions (Kadem and Caplan, 1965;Dragičević and Sredojević, 2011).It is also well-known that plants react to stress by alterating their chlorophyll and carotenoid contents.Mayfield and Taylor (1984) underlined a connection between a decrease in light harvest of maize plants and lower carotenoid content.Some other researchers, e.g.Bónis et al. (2006), have detected positive effects of some herbicides to chlorophyll synthesis, which may lead to an increase in maize yielding potential.
The objective of this study was to investigate the influence of different cropping systems: monoculture and two rotations, including maize, wheat and soybean, and different weed management methods (hoeing and two herbicide doses) on weed biomass and maize growth parameters -the leaf area index, free energy, contents of chlorophyll and carotenoids, and grain yield, as well as their possible relationships.

MATERIAL AND METHODS
An experiment was set up in the field of the Maize Research Institute at Zemun Polje in 2009 on a slightly calcareous chernozem type of soil under rain-fed conditions.After three years of experiment, which included fields with monoculture and two types of rotation: maize-soybean-wheat (MSW) and maize-wheat-soybean (MWS), the first comprehensive results of these cropping practices were obtained in 2012.The fields were ploughed to the depth of 30 cm every autumn before the sowing of any of the three crops.An amount of 30 t ha -1 of manure was applied to each field before the experiment, and to the rotation fields in the autumn of 2011.Two maize hybrids, ZP 677 (H1) and ZP 606 (H2), were sown in the fields with monoculture and rotation in the third decade of April at the density of 59.500 plants ha -1 .Mineral fertilization included monoammonium phosphate (150 kg ha -1 ) incorporated every autumn, as well as side dressing at the stage of 5-6 leaves, based on soil analysis (available nutrients content).
All plots, monoculture and rotations, included subtreatments with different weed management: hoeing (weeds were removed from entire subplots); weedy check (weeds stayed throughout vegetation); application of the herbicides isoxaflutole + acetochlor (750 + 768 g a.i.), which were used at the recommended dose (RD) and half of recommended dose (0.5RD).Each sub-treatment included four replications.
The level of weed infestation was evaluated 50 days after herbicide application by measuring fresh weed biomass m -2 .Maize leaf area, chlorophyll content and free energy were measured at the end of the anthesis phase by sampling three plants.Leaf area was determined using a LI -3100 area meter (LI Cor, Lincoln, NE) and its relation with the soil area under plants represented the leaf area index (LAI).From the same leaves, chlorophyll content (chlorophyll a + b) and total carotenoids were determined spectrophotometrically after extraction with 70% acetone.Chlorophyll a was determined at λ=662 nm, chlorophyll b at λ=644 nm and carotenoids at λ=440 nm.The difference between dry (after drying at 130 °C) and fresh biomass referred to the contents of free, bulk and chemically bound water, which was used for calculation of thermodynamic parameters by using the sorption isotherm as suggested by Davies (1961) and Sun (2002): where, a w is the relative water content achieved after drying at T (130 °C); R is the gas constant (8.3145J mol -1 K -1 ); ∆G is differential free energy, which represents the amount of work that the system can perform.Its decrease signifies a domination of exergonic (spontaneous) processes which release free energy, and its increase signifies endergonic (nonspontaneous) processes which actually consume energy.
The maize grain yield was measured from two inner rows of each subplot and calculated to 14 % moisture.The experimental data of maize grain yield, LAI and weed biomass were statistically processed by the analysis of variance (ANOVA) and analysed by the LSD-test (5 %), while free energy, carotenoids and chlorophyll contents were presented with standard deviation (SD).Interdependences among maize grain yield, free energy, chlorophyll and carotenoid contents were processed by regression analysis.

Meteorological conditions
The highest average temperature in 2012 was reached in July and August (27.1 °C and 26.6 °C, respectively, Table 1).Moreover, the lowest precipitation amount of only 4 mm was recorded in August.Such conditions may have affected grain filling, and possibly also reflected on the relatively low grain yield.

RESULTS AND DISCUSSION
The obtained results showed significantly higher grain yield and LAI of H2 (17% and 12%, respectively) than H1 (Tables 2 and 3).H2 also had a significantly higher grain yield in MSW rotation and LAI in MWS rotation.Moreover, the interactions between hybrid and treatments exposed the lowest values of grain yield and LAI of both hybrids in weedy check.A general impact of the cropping system reflected only on LAI, which had two times higher values in MWS rotation.On the other hand, the interaction between cropping system and treatments emphasized a negative impact of monoculture with significantly lowest yield and LAI, which was particularly evident in weedy check.The influence of cropping systems on LAI had been also reported by Wozniak (2008).Such influence was reduced by the application of herbicides, increasing LAI in 0.5 RD and RD variants in monoculture up to the level observed in MSW rotation (Table 3).This is supported by the results of Malik et al. (2006) and Saudy ( 2013), who had also achieved better maize growth, higher LAI and grain yield under the positive influence of herbicides.The highest impact of rotation on LAI was observed in MWS rotation, and the highest values were recorded in the variants with 0.5 RD and hoeing, while hoeing mainly affected grain yield, increasing it significantly in monoculture and MWS rotation (Table 2).It is well-known that hoeing and application of herbicides can significantly increase maize grain yield (Khan et al., 2002;Saudy, 2013).Dolijanović et al. (2006) had reported the highest effect of a three-crop rotation (winter wheat-maize-soybean) on maize yield increase.
The mentioned lowest grain yield and LAI of both hybrids (Tables 2 and 3) were associated with the highest weed biomass in weedy check, where its values were about 2.5 times higher than the average values in both rotations (Table 4).From that aspect, weedy check was the only variant in which weed biomass (Table 4) significantly affected maize development and yielding potential, as reflected by grain yield and LAI decrease.Nakova et al. (2004) had similarly reported a significant suppression of maize LAI and grain yield under the negative effect of high weed infestation.Significant variations in weed biomass were also present under the interaction of cropping system and treatments, but the highest values were in monoculture (particularly weedy check).The lowest values of weed biomass were obtained in MSW rotation, mainly in the RD treatment, approaching the values in treatment with hoeing, where weeds were manually removed.Stoimenova et al. (2004) and Simić and Stefanović (2008) had also reported negative correlations between maize yield and weed biomass, depending on the level of competition.
According to the results shown in Figure 1, the lowest ∆G values of H1 were found in the monoculture the cropping systems, with a domination of exergonic processes and energy releasing.Such energy can be released into the external environment or spent on some other processes, such as biosynthesis in coupled reactions (Kadem and Caplan, 1965;Dragičević and Sredojević, 2011).From that aspect, the lowest ∆G values obtained in the variants with hoeing and RD in rotational fields (both hybrids) correspond with the highest values of grain yield (Table 2), which, coupled with the lower average ∆G obtained in MSW rotation, is an indication of system efficiency provided by treatments.This could also signify a domination of exergonic reactions (Sun, 2002;Dragičević and Sredojević, 2011), which are spontaneous.This energy release could contribute to an increase in growth/yielding potential of the system and is reflected in the negative interdependence between ∆G and grain yield in MSW and MWS rotations (R 2 =0.860 and R 2 =0.467, respectively, Figure 4).The variations in weed biomass that were influenced by cropping system have no significant impact on ∆G, except in MSW rotation (R 2 =0.578, Figure 5).Lower average contents of carotenoids and chlorophyll, together with their higher variations among hybrids and treatments, were detected in both rotation fields (Figures 2 and 3).In monoculture, the highest carotenoids (about 14%, compared to other treatments) and the lowest chlorophyll (about 11%, compared to other treatments) contents were recorded in weedy check, which could, together with the lowest values of the other parameters (LAI and grain yield) and relatively high ∆G, indicate a presence of stress caused by high weed infestation (Table 4).Such situation is supported by a negative and significant interdependence of grain yield and carotenoids (R 2 =0.475; Figure 4), and by positive and significant interdependence between weed biomass and carotenoids (R 2 =0.516; Figure 5) present in monoculture.This is important because lower content of carotenoids is associated with lower light harvest by maize plants (Mayfield and Taylor, 1984).Maize plants in the treatment with hoeing and both rotational fields had higher average carotenoids and chlorophyll contents (27% and 15% respectively) than in weedy check (Figures 2 and 3).The same trend was also observed in the variant 0.5 RD in MSW with the average values of up to 37% and 18% for carotenoids and chlorophyll.Bónis et al. (2006) had also reported an increase in chlorophyll content in maize leaves under the influence of herbicides.The impact of the tested cropping systems on maize growth and their interactions with weeds can be evaluated based on positive correlations of grain yield and chlorophyll and carotenoid contents in MWS rotation (R 2 =0.360 and R 2 =0.349, respectively, Figure 4) and significant negative correlations of weed biomass and chlorophyll and carotenoid contents in MSW rotation (R 2 =0.438 and R 2 =0.527, respectively, Figure 5).This situation supports a conclusion that only a combination of different measures can have an adequate weed control effect and bear the crop yielding potential in full (Simić et al., 2013).Grain yield (t ha -1 ) ∆G Carot. Clf.

MWS rotation
It can be inferred from the obtained results that the decrease in maize yielding potential in monoculture can be associated with relatively high weed infestation, and with stress reflecting on reduced LAI and chlorophyll content, and increased ∆G and carotenoids content.On the other hand, rotation, particularly if it is combined with herbicide treatment or hoeing, had a positive effect on the yielding potential by increasing LAI, ∆G, chlorophyll and carotenoids contents.Hoeing had an advantage as a treatment that had the highest impact on the measured parameters and grain yields but, considering also the labour involved in weed removal, the combination of RD and MSW rotation can also give satisfactory results, particularly if the increased energy potential (lower ∆G values) is also regarded.

Figure 1 .Figure 2 .
Figure 1.Free energy of two maize hybrids (H1 and H2) grown under different rotation systems and weed control treatments ± SD

Figure 3 .Figure 4 .
Figure3.Chlorophyll content (mg g -1 ) in leaves of two maize hybrids (H1 and H2) grown under different rotation systems and weed control treatments ± SD

Figure 5 .
Figure 5. Interdependence between weed biomass and free energy (∆G), carotenoids and chlorophyll content in maize leaves under different rotation systems

Table 1 .
Average daily temperatures and precipitation sum during vegetative period of 2012

Table 2 .
Grain yield (t ha -1 ) of two maize hybrids (H1 and H2) grown under different rotation systems and weed control treatments

Table 3 .
Leaf area index (m 2 m -2 ) of two maize hybrids (H1 and H2) grown under different rotation systems and weed control treatments