Abstract
Chlorophyll determination with a portable chlorophyll meter can indicate the period of highest N demand of plants and whether sidedressing is required or not. In this sense, defining the optimal timing of N application to common bean is fundamental to increase N use efficiency, increase yields and reduce the cost of fertilization. The objectives of this study were to evaluate the efficiency of N sufficiency index (NSI) calculated based on the relative chlorophyll index (RCI) in leaves, measured with a portable chlorophyll meter, as an indicator of time of N sidedressing fertilization and to verify which NSI (90 and 95 %) value is the most appropriate to indicate the moment of N fertilization of common bean cultivar Perola. The experiment was carried out in the rainy and dry growing seasons of the agricultural year 2009/10 on a dystroferric Red Nitosol, in Botucatu, São Paulo State, Brazil. The experiment was arranged in a randomized complete block design with five treatments, consisting of N managements (M1: 200 kg ha-1 N (40 kg at sowing + 80 kg 15 days after emergence (DAE) + 80 kg 30 DAE); M2: 100 kg ha-1 N (20 kg at sowing + 40 kg 15 DAE + 40 kg 30 DAE); M3: 20 kg ha-1 N at sowing + 30 kg ha-1 when chlorophyll meter readings indicated NSI < 95 %; M4: 20 kg ha-1 N at sowing + 30 kg ha-1 N when chlorophyll meter readings indicated NSI < 90 % and, M5: control (without N application)) and four replications. The variables RCI, aboveground dry matter, total leaf N concentration, production components, grain yield, relative yield, and N use efficiency were evaluated. The RCI correlated with leaf N concentrations. By monitoring the RCI with the chlorophyll meter, the period of N sidedressing of common bean could be defined, improving N use efficiency and avoiding unnecessary N supply to common bean. The NSI 90 % of the reference area was more efficient to define the moment of N sidedressing of common bean, to increase N use efficiency.
Highlights
This could mean that chlorophyll concentration in the leaves stabilized after flowering, and that after this growth phase, the crop response to N is low, probably due to the higher contribution of biological N2 fixation and redistribution of N in the plant to other tissues, in line with the results obtained by Silveira et al (2003) and Barbosa Filho et al (2008, 2009)
In the rainy growing season, in the early growth phase of common bean, the relative chlorophyll index (RCI) values of the control treatment (M5) were below the readings obtained on the fertilized plots, but after 50 days after emergence (DAE), the RCI values of the plants in M5 were very close to those of the fertilized plots (Figure 1a)
Barbosa Filho et al (2008) observed that, after flowering, the green of the bean plants in the control treatment was more intense, resulting from RCI values very close to those read on the fertilized plots, and reported that this is a result of N mineralization of the plant residues left on the soil surface
Summary
Nitrogen is the nutrient taken up in highest amounts by common bean (Phaseolus vulgaris L.), since it is a constituent of amino acids, proteins, enzymes and the chlorophyll molecule, with a marked effect on photosynthesis and on crop growth and yield (Malavolta et al, 1997; Soratto et al, 2004). The common bean satisfies part of its requirements for N by symbiosis with bacteria of the genus Rhizobium (Pelegrin et al, 2009), N fertilizer must be supplied to achieve high yields (Santos et al, 2003; Silveira et al, 2003; Soratto et al, 2004, 2005, 2006; Farinelli et al, 2006; Barbosa Filho 2008, 2009; Pelegrin et al, 2009). If simple recommendations are followed, the moment of N fertilization may fail to coincide with the period of highest crop demand for this nutrient (Barbosa Filho et al, 2009)
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