Abstracts

INTRODUCTION

Weeds are major biotic components of the agroecosystem that are capable of negatively interfering in crops. The negative effects are expressed on the quantity and quality of the agricultural production, a result of the competition for the environmental growth resources, allelopathy or agents that host pests and diseases, allowing their multiplication. In Brazil, it is believed that the weed interference on agricultural crops is responsible, on average, for yield reductions around 20 to 30% (Lorenzi, 2006LORENZI, H. Manual de identificação e controle de plantas daninhas: plantio direto e convencional. 6. ed. Nova Odessa: Instituto Plantarum, 2006. 339 p.).

There are several weed species present in agricultural areas, among which may be mentioned those classified in the Cyperaceae family. This botanical family consists of approximately 3,000 species, of which about 300 are classified as weeds, and around 42% of these belong to the genus Cyperus (Bendixen & Nandihalli, 1987BENDIXEN, L. E.; NANDIHALLI, U. B Worldwide distribution of purple and yellow nutsedge (Cyperus rotundus and C. esculentus) . Weed Technol., v. 1, n. 1, p. 61-65, 1987.), and purple nutsedge (Cyperus rotundus) significantly standing out.

The purple nutsedge is a herbaceous perennial plant, between 0.10 and 0.60 m high. It has a C4 type photosynthetic cycle, i.e., it is highly efficient in performing photosynthesis in high radiation and temperature scenarios. The development of shoots is discreet, with few leaves and inflorescences, whose seeds have minimum viability. On the other hand, it stands out due to its vegetative propagation, since it produces numerous rhizomes and tubers, which greatly promote its spread on agricultural fields.

It has been considered one of the most important weeds in the crops, being classified by Holm et al. (1977)HOLM, L. G. et al. The world's worst weeds: distribution and biology. Honolulu: University Press Hawaii, 1977. 609 p. as one of the ten worst weeds in the world. In Brazil, purple nutsedge can be found in all types of soils, climates and cultures, but is particularly undesirable in the areas of sugarcane (Saccharum spp.), where, besides competing for the resources of the environment, it exudes allelopathic compounds that inhibit budding of the crop (Durigan, 1991DURIGAN, J. C. Manejo da tiririca (Cyperus rotundus L.) antes e durante a implantação da cultura da cana-de-açúcar (Saccharum spp.). 1991. 336 f. Tese (Livre-Docência) - Universidade Estadual Paulista, Jaboticabal, 1991.; Kissmann, 1997KISSMANN, K. G Plantas infestantes e nocivas. 2. ed. São Paulo: BASF, 1997. v. 1. 825 p.). In this environment, the purple nutsedge stands out as an important weed, even in areas with harvest without previous burning (raw cane) (Foloni et al., 2008FOLONI, L. L. et al. Programa de manejo da tiririca (Cyperus rotundus) na cultura da cana-de-açúcar com aplicação isolada ou sequencial de MSMA. Planta Daninha, v. 26, n. 4, p. 883-892, 2008.). Kuva et al. (2007)KUVA, M. A. et al. Fitossociologia de comunidades infestantes de plantas daninhas em agroecossistema canacrua. Planta Daninha, v. 25, n. 3, p. 501-511, 2007. have assessed the weeds pestering cane raw agroecosystems and found that, of the 28 fields assessed, purple nutsedge had stood out in 20, being the main weed in 13 of them.

In this sense, it is considered that the analysis of the behavior of the weeds regarding the ecological factors, as well as their effect on the environment, mainly regarding their interference on other plants, contributes to the development of integrated management systems (Lucchesi, 1984LUCCHESI, A. A Utilização prática de análise de crescimento vegetal. Anais ESALQ, v. 41, n. 1, p. 181-202, 1984.; Bianco et al., 1995BIANCO, S. et al. Estimativa da área foliar de plantas daninhas XIII - Amaranthus retroflexus L. Ecossistema, v. 20, n. 1, p. 5-9, 1995.). Still, the growth characteristics of a certain species provide an important indicator of its competitive ability (Holt & Orkutt, 1991HOLT, J. S.; ORKUTT, D. R Functional relationships of growth and competitiveness in perennial weeds and cotton (Gossypium hirsutum) . Weed Sci., v. 39, n. 4, p. 575-584, 1991.).

While the great importance of studies assessing the biology of weeds is recognized, few studies have discussed the growth and development of these species, mainly based on growing degree days - GDD (thermal units). The prediction of different phenological aspects of crops, weeds and other pests with simple thermal equations tends to be an excellent tool to provide practical solutions to crops problems (Ghersa & Holt, 1995GHERSA, C. M.; HOLT, J. S Using phenology prediction in weed management: a review. Weed Res., v. 35, n. 6, p. 461-470, 1995.). Thus this work was carried out with the objective of elaborating mathematical models to predict growth and development of purple nutsedge (Cyperus rotundus) based on days or thermal units (growing degree days).

MATERIALS AND METHODS

Two independent experiments were carried out in an experimental nursery at the Instituto Federal de Educação, Ciência e Tecnologia do Sul de Minas Gerais (Federal Institute of Education, Science and Technology of Southern Minas Gerais), Campus Machado, MG, Brazil (21^o 40' S; 45^o 55' W; 850 m of altitude). In each experiment, the growth and development of purple nutsedge (Cyperus rotundus) were assessed. The first experiment was developed between March and July 2013 (decreasing photoperiod), while the second one was developed between August and November of the same year (increasing photoperiod).

Cyperus rotundus tubers were collected in agricultural and non-agricultural areas of the Brazilian municipality of Machado, MG, washed and immediately planted in experimental plots, numbering 13 per plot. The experimental plots comprised plastic pots with a capacity of 4 L, filled with a proportion of a commercial substrate and vermiculite (3:1; v:v), appropriately fertilized with 25 g of fertilizer NPK 04:14:08 (N, P₂O₅ e K₂O) and 10 g of ammonium sulfate. Throughout the trial period, there was no manifestation of nutritional deficiency in the plants. The pots were watered whenever necessary, without water deficit. After the emergence of the seedlings, thinning of the pots was performed so as to obtain a constant final density of only one plant per pot.

In both experiments, the experimental design was of randomized blocks with ten treatments (growth evaluations) and three replications. In the first half of the experiments, due to the small variation of matter, growth assessments were spaced in 14 days. In the second half, the matter assessments were spaced in seven days, with total experimental cycles higher than 100 days. In each of the ten assessments, three plots (replicates) were randomly sampled by the destructive method. The plants were washed with tap water to remove the substrate remaining in the roots, and then all the material was dried at 70 °C for 72 hours. After drying, the total dry weight was measured (g per plant). On the same dates of the matter assessments, the population phenological estimate was also held, using the scale proposed by Hess et al. (1997)HESS, M. et al. Use of the extended BBCH scale - general for descriptions of the growth stages of mono-and dicotyledonous weed species. Weed Res., v. 37, n. 6, p. 433-441, 1997.. In this case, the growth stage was set when a certain development characteristic was observed for 50% + 1 of all remaining plants in the population.

The experiments were independently assessed by applying the F test for analysis of variance with 1% probability. The phenological data of purple nutsedge were adjusted to the time counting in days after planting (DAP) or to the accumulated thermal units (growing degree days - GDD) by means of the linear regression model y = ax, where y concerns the development of purple nutsedge according to the phenological scale (Hess et al., 1997HESS, M. et al. Use of the extended BBCH scale - general for descriptions of the growth stages of mono-and dicotyledonous weed species. Weed Res., v. 37, n. 6, p. 433-441, 1997.), x concerns the time scale used and a is the model parameter.

In practice, parameter a of this equation can be understood as the percentage of GDD effectively converted into plant phenology units, allowing an estimation of the speed of development of plants in a given season or sowing date (Machado et al., 2014MACHADO, E. C. R. et al. Initial growth and development of southern sandbur based on thermal units. Planta Daninha, v. 32, n. 2, p. 335-343, 2014.). When necessary, the comparison of two straight lines was made by overlapping the confidence interval of the analysis for parameter a of the equation. Thus, two straight lines were considered equal in the existence of an overlapping of the confidence intervals (Carvalho & Christoffoleti, 2007CARVALHO, S. J. P.; CHRISTOFFOLETI, P. J Estimativa da área foliar de cinco espécies do gênero Amaranthus usando dimensões lineares do limbo foliar.Planta Daninha, v. 25, n. 2, p. 317-324, 2007.).

For calculating the GDD, the equation by Gilmore Jr. & Rogers (1958)GILMORE JR., E. C.; ROGERS, J. S Heat units as a method of measuring maturity in corn. Agron. J., v. 50, n. 10, p. 611-615, 1958. was used:

where: Tmax is the maximum daily temperature; Tmin is the minimum daily temperature; and Tb concerns the basal temperature of the purple nutsedge, assessed at 10, 12 or 15 ^oC. The daily maximum and minimum temperatures were obtained by the meteorological station installed on campus Machado and made available by Instituto Nacional de Pesquisas Espaciais - INPE (National Institute for Space Research, a research unit of the Brazilian Ministry of Science and Technology) (Figure 1).

The total dry matter was adjusted to logistic type nonlinear regressions, also based on DAP or GDD. The model proposed by Streibig (1988)STREIBIG, J. C Herbicide bioassay. Weed Res., v. 28, n. 6, p. 479-484, 1988. was adopted:

where: y is the total dry matter (g per plant), x is the temporal scale (DAP or GDD) and a, b and c are estimated parameters of the equation (a is the existing amplitude between the maximum and minimum point of the variable, b corresponds to the value of the temporal scale necessary for the occurrence of 50% of the response of the variable and c is the slope of the curve around b).

RESULTS AND DISCUSSION

Depending on the events that may occur during plant development, there is a need for the adoption of numerical scales to establish levels for this period. Traditionally, days have been used as a cycle timing, but it is a variable that is very much subject to environmental interferences which indirectly are also expressed on the phenology (Silva et al., 2014SILVA, A. P. P. et al. Growth and development of honey weed based on days or thermal units. Planta Daninha, v. 32, n. 1, p. 81-89, 2014.). Therefore, temperature has been considered the most important climatic element to predict physiological events, if there is no water deficit (Russelle et al., 1984RUSSELLE, M. P. et al. Growth analysis based on degree days. Crop Sci., v. 24, n. 1, p. 28-32, 1984.; Gadioli et al., 2000GADIOLI, J. L. et al. Temperatura do ar, rendimento de grãos de milho e caracterização fenológica associada à soma calórica. Sci. Agric., v. 57, n. 3, p. 377-383, 2000.).

The method of growing degree days - GDD is based on the premise that a plant needs a certain amount of energy, represented by the sum of the thermal degrees required to complete a certain phenological phase or even the total cycle. It allows, in addition, a linear relationship between the increase of temperature and the plant development (Gadioli et al., 2000GADIOLI, J. L. et al. Temperatura do ar, rendimento de grãos de milho e caracterização fenológica associada à soma calórica. Sci. Agric., v. 57, n. 3, p. 377-383, 2000.). Thus, it becomes possible to use mathematical models and simulation routines that use the concept of GDD (Medeiros et al., 2000MEDEIROS, G. A. et al. Crescimento vegetativo e coeficiente de cultura do feijoeiro relacionados a graus-dia acumulados. Pesq. Agropec. Bras., v. 35, n. 9, p. 1733-1742, 2000.).

In the case of the purple nutsedge, independently in each experiment, a proper adjustment of the phenological development to the time scales used was obtained in DAP or GDD by means of the first-degree linear equation, with coefficients of determination always above 90% (Table 1). For all scales, there was an overlap of confidence intervals of parameter a of the equation between the experiments developed in increasing and decreasing photoperiods, which indicates a similar behavior for the development of purple nutsedge in different seasons (Carvalho & Christoffoleti, 2007CARVALHO, S. J. P.; CHRISTOFFOLETI, P. J Estimativa da área foliar de cinco espécies do gênero Amaranthus usando dimensões lineares do limbo foliar.Planta Daninha, v. 25, n. 2, p. 317-324, 2007.). In this case, a decision was made for performing a joint analysis of the experiments (Table 1) by means of the general cumulative points (Figure 2).

Also in the case of the joint analysis an appropriate mathematical adjustment for all scales used was obtained, with coefficients of determination higher than 85% (Table 1; Figure 2). Adoption of the base temperature (Tb) of 12 ^oC must be highlighted, which resulted in lower mean square of the residue (MS_residue = 67.652), greater significance of the model to F test (F = 1184.270**) and a higher coefficient of determination (R² = 0.918) (Table 1).

Base temperature (Tb) is the minimal temperature for growth of a given species, below which growth ceases or is greatly reduced. In literature are considered values of Tb = 0 ^oC for weeds and temperate climate crops such as barley (Hordeum vulgare) and wheat (Triticum aestivum) (Cao & Moss, 1989CAO, W.; MOSS, D. N Temperature and daylength interaction on phyllochron in wheat and barley. Crop Sci., v. 29, n. 4, p. 1046-1048, 1989.; Kirkby, 1995KIRKBY, E. J M. Factors affecting rate of leaf emergence in barley and wheat. Crop Sci., v. 35, n. 1, p. 11-19, 1995.). For sunflower (Helianthus annuus), Granier & Tardieu (1998)GRANIER, C.; TARDIEU,F Is thermal time adequate for expressing the effects of temperature on sunflower leaf development? . Plant Cell Environ., v. 21, n. 7, p. 695-703, 1998. suggested base temperature around 4.8 ^oC.

For redroot pigweed (Amaranthus retroflexus), species with photosynthetic cycle type C₄, Gramig & Stoltenberg (2007)GRAMIG, G. G.; STOLTENBERG, D. E Leaf appearance, base temperature and phyllochron for common grass and broadleaf weed species. Weed Technol., v. 21, n. 1, p. 249-254, 2007. have recorded Tb = 8.5. Base temperatures around 10 ^oC were reported for Leonurus sibiricus (Silva et al., 2014SILVA, A. P. P. et al. Growth and development of honey weed based on days or thermal units. Planta Daninha, v. 32, n. 1, p. 81-89, 2014.), for common beans (Phaseolus vulgaris) (Medeiros et al., 2000MEDEIROS, G. A. et al. Crescimento vegetativo e coeficiente de cultura do feijoeiro relacionados a graus-dia acumulados. Pesq. Agropec. Bras., v. 35, n. 9, p. 1733-1742, 2000.), for maize crop (Gadioli et al., 2000GADIOLI, J. L. et al. Temperatura do ar, rendimento de grãos de milho e caracterização fenológica associada à soma calórica. Sci. Agric., v. 57, n. 3, p. 377-383, 2000.) and for bunch grass Panicum virgatum (Sanderson & Wolf, 1995SANDERSON, M. A.; WOLF, D. D Morphological development of switchgrass in diverse environments. Agron. J., v. 87, n. 5, p. 908-914, 1995.). Machado et al. (2014)MACHADO, E. C. R. et al. Initial growth and development of southern sandbur based on thermal units. Planta Daninha, v. 32, n. 2, p. 335-343, 2014. have obtained Tb = 12 ^oC for southern sandbur (Cenchrus echinatus). Also, Villa Nova et al. (1999)VILLA NOVA, N. A. et al. Modelo para previsão da produtividade do capim elefante em função da temperatura do ar, fotoperíodo e frequência de desfolha. R. Bras. Agrometriol., v. 7, n. 1, p. 75-79, 1999. have used Tb = 15 ^oC for elephant grass cv. Napier (Pennisetum purpureum), especially a Poaceae family plant of tropical climate.

Thus, considering the phenological development on the proposed scales, data consistency (Table 1), size of the purple nutsedge and the fact that it is a Cyperaceae family species with a C₄ type photosynthetic cycle and rating as a perennial weed, assigning Tb = 12 ^oC is suggested as appropriate for future studies.

In a condition of decreasing photoperiod, the beginning of the vegetative propagation (stage 40 - Hess et al. (1997)HESS, M. et al. Use of the extended BBCH scale - general for descriptions of the growth stages of mono-and dicotyledonous weed species. Weed Res., v. 37, n. 6, p. 433-441, 1997.) it was recorded at 20 DAP, by means of the formation of rhizomes with an accumulation of 200 GDD for Tb = 12 ^oC. At the end of the cycle, an average of five tubers formed per plant was observed. For the increasing photoperiod, beginning of vegetative propagation at 27 DAP was recorded, with an accumulation of 150 GDD for Tb = 12 ^oC. At the end of the cycle, an average of 3.5 tubers formed per plant was recorded.

The use of growth analysis is one of the easiest and more accurate ways to infer the contribution of different physiological processes for plant growth, enabling the knowledge of plant biomass production kinetics, its distribution and efficiency during ontogeny (Benincasa, 2004BENINCASA, M. M P. Análise de crescimento de plantas noções básicas. Jaboticabal: FUNEP, 2004 42 p. .). Accordingly, the total dry matter production is recognized as a basic process of plant growth (Radosevich et al., 1997RADOSEVICH, S.; HOLT, J. S.; GHERSA, C. Weed ecology: implications for vegetation management. New York: John Willey, 1997. 589 p.).

In the experiments, in an independent analysis, the total dry matter data were properly fitting to the model, with coefficients of determination above 95% (Table 2). Slow initial growth was identified for the species, with subsequent exponential accumulation of dry matter after 300 GDD (Tb = 12 ^oC) and maximum values exceeding 40 g per plant (Figure 3). All equations based on GDD have set the dry matter accumulation of purple nutsedge with proximity between the slopes of the two experimental conditions, and Tb = 12 ^oC again standing out. The wider gap between the slopes was observed for the use of days as a temporal scale (Figure 3).

Due to the close proximity of the adjustments between different photoperiod conditions, implementing a joint analysis of the experiments based on all the different temporal scales was chosen (Table 2; Figure 4). Also in this case, the best overall adjustment was obtained for Tb = 12 ^oC, highlighting the lowest mean square of the residue (MS_residue = 11.498), the greatest significance of the model to the F test (F = 210.485**) and the highest coefficient of determination (R² = 0.959) (Table 2).

Thus, the overall analysis of the results allows one to assume that the growth and development of this weed can be adequately predicted with the use of mathematical models based on accumulated thermal units, especially for the adoption of Tb = 12 ^oC. Taking into account the overlap of the straight lines (Table 1), there was no phenological variation among different photoperiod conditions, which allows the joint analysis of the data (Figure 2). Similarly, the variation in matter accumulation was negligible, and overall adjustment of the data can be observed with a high coefficient of determination that is higher than the adjustment obtained for the scale in days. Considering growing degree days calculated with Tb = 12 ^oC, the phenology of the purple nutsedge can be predicted by y = 0.113x, while the species growth can be predicted by y = 37.678/(1+(x/509.353)^-7.047).

ACKNOWLEDGMENT

The authors would like to thank the Instituto Federal de Educação, Ciência e Tecnologia do Sul de Minas Gerais - IFSULDEMINAS, specially campus Machado, for fostering the development of this work.

LITERATURE CITED

Publication Dates

History