Location and characteristics of the study area

The research was conducted during the summer-fall 2011 cycle in the locality "Ampuero" located in Torreon, Coahuila, Mexico (25° 27' north latitude and 103° 22' west longitude), at an altitude of 1 137 m. The physical and chemical soil analysis made in the substrate 0 to 0.3 m deep, clay-sandy texture moderately alkaline (pH= 8.5), no salt problems (EC of saturation extract 1.48 dS m^-1), percentage of organic matter 1.59% and bulk density of 1.31 g cm^-3.

Agronomical management

The yellow hybrid high-yield (HT-7887), which corresponds to a corn whose adaptation area is the northern and central Mexico, plant height is 2.5 to 2.7 m, intermediate cycle, with good yield and foliage quality. The trial was established on June 30, 2011. The seeds were placed in separate grooves 0.75 and 0.13 m between seeds for planting density of 102 564 plants ha^-1. Two melgas of 10 m wide by 250 m long (2 500 m²) were established. Irrigation was applied pre planting on May 15. Subsequently three irrigations of an average sheet of 17 cm with an interval between irrigations of approximately 30 days using siphons to derive water from the channel into the plots; fall armyworm (Spodoptera frugiperda) was controlled with two applications of Palgus (Spinoteram) (125 mL ha^-1). For pest control, Latigo (Chlorpyrifos-cypermethrin) (1.25 L ha^-1) was applied. Weed control was performed by applying Primagram (Atrazine-Smetolachlor) (1.25 L ha^-1) at 15 and 28 days after planting.

Treatments evaluated and experimental design

In one of the melgas described before the Algaenzims^MR product was applied, which is a biological product based on macro and micro seaweed with alkaline pH (8.2). The application was made in the pre sowing irrigation at a dose of 1 L ha^-1. Subsequently, to plants of the same melga was applied (foliar) 0.5 L ha^-1 of AlZincB^MR containing growth regulators such as auxins, gibberellins and cytokinins as an inherent part of seaweed extracts (Sargasuum spp.) plus trace elements like zinc (10%, w/w) and boron (0.5%, w/w). Applications were made 41 days after planting. The other melga was kept as control (without application of biological fertilization). Both melgas received mineral fertilization that the producer regularly applies, which consisted of 25 kg of ammonium sulfate and 100 kg of monoammonium phosphate, applied to the soil one day before planting.

A completely randomized design with two treatments (with and without application of biological fertilizers) and 10 replications was used; one plant represented the experimental unit. 10 plants in each melga were randomly selected in the central part of the plot. Means comparison was performed using Duncan test (p≤ 0.05).

Relative chlorophyll content

The effect of biofertilizer in relative chlorophyll content of leaves was performed spectrophotometrically using SPAD 502 (Minolta Inc.) sensor, which measures the relative chlorophyll concentration by light transmitted through the leaf at a wavelength of 650 to 940 nm (^{Sainz-Rozas and Echeverria, 1998}). This sensor, estimates instantaneously relative chlorophyll content (ranging from 0 to 199) in leaves nondestructively with the following relationship:

Where: SPAD is the value of chlorophyll level; K is the extinction coefficient of the medium (leaf chlorophyll) to applied radiation (650 nm); IR_t is the intensity of radiation that leaf transmits at 940 nm; IR_o is the intensity of light emitted by the sensor at 940 nm; Rr is the intensity of radiation that leaf transmits at 650 nm; R_o is the intensity of light emitted by the sensor at 650 nm.

Measurements were made in 10 plants from the center of each melga. Eight measurements distributed along the third leaf of each plant (in four places on each side) were made. The data was obtained from the middle between the base and the apex. The records were made at 36 and 71 days after planting.

Electron transport rate (ETR)

The electron transport rate was determined through eight measurements per leaf on each of the 10 plants of each melga (with and without application of biofertilizers), 71 days after sowing (das). For this a pulse-modulated fluorometer FMS2 model (Hansatech Instruments) was used, which obtains ETR with the following equation:

Where: ETR is the electron transport rate (µmol m^-2 s^-1), ΦPSII is the quantum efficiency of photosystem II, α is the ratio of light absorption by the photosynthetic tissue (0.84), f is the proportion of light transferred to each of the photosystems (PSII and PSI), which corresponds to 0.54, and PAR is incident photosynthetically active radiation (µmol m^-2 s^-1).

The quantum efficiency of photosystem II is obtained by the following equation:

Where: FM 'is the real or effective maximum fluorescence and F is the initial fluorescence.

Plant growth components evaluated

Plant height was measured from ground surface to the last knot from the stem. The first evaluation was performed at 71 and the second at 99 das. 10 plants were selected randomly from each treatment. Stem diameter was measured at 5 cm from ground surface with digital vernier surtek (Electronic digital caliper 122200). Measurements were made in 10 plants per treatment, randomly selected at 99 das. Plants from each melga were harvested at ground level at 100 das these were placed in a drying oven at 70 ^oC for 24 h to obtain plant dry weight.

Relative chlorophyll content

Application of biological fertilizers seaweed based had a significant effect (p≤ 0.05) in the relative chlorophyll content of leaves (Figure 1). In the first assessment (36 das), chlorophyll content was 8.1% higher in plants treated with biological fertilizers compared to treatment without biological fertilizer. In the second assessment (71 das) the difference between treatments increased to 32.4% (Figure 1).

Figure 1 The relative content of chlorophyll in leaves of corn plants with biological fertilization (FB) (Algaenzims^MR and ALZincB^MR) and biological plants without fertilization (no FB), observed on August 05 (36 days after planting) and 9 September (71 days after sowing), 2011, Torreon, Coahuila, Mexico. 

Previous studies have shown that seaweed extracts contain significant amounts of cytokinins, auxins and betaines, when applied to soil or crop increases chlorophyll concentration in the leaves (^{Whapham et al., 1993}; ^{Blunden et al., 1996}; ^{Schwab and Raab, 2004}).

Electron transport rate (ETR)

The application of biofertilizers had a significant effect (p≤ 0.05) in the electron transfer rate (ETR) in leaves (Figure 2). At 71 days after sowing, ETR on leaves from treated plants was 17.8% higher than plants without application of biofertilizer (Figure 2). This result also indicated that the degree of stress in treated plants was lower, since the electron transport rate is an indicator of plant stress (^{Krause and Weis, 1991}; ^{Wajahatullah et al., 2009}).

Figure 2 Electron transport rate (ETR) in leaves of corn plants with biological fertilization (FB) (Algaenzims^MR and ALZincB^MR) and biological plants without fertilization (no FB) observed on 9 September (71 days after seeding) 2011, Torreon, Coahuila, Mexico. 

In this regard, ^{Nabati et al. (1994)} reported an increased tolerance to salinity in Kentucky blue grass (Poa pratensis L.) in response to the application of seaweed extract. Similarly, ^{Nahed et al. (2011)} reported that the application of seaweed extract (Ascophyllum nodosum) in amaranth (Amaranthus tricolor L.) reduces salinity stress and increases length, stem diameter, root length and leaf number. While ^{Spann and Little (2011)} found that the application of seaweed extract (Ascophyllum nodosum) to the soil or foliar to orange (Citrus sinensis L.) improves tolerance to water stress.

Plant height

Figure 3 shows that the application of biological fertilizer increased plant height (p≤ 0.05). In the first assessment (71 das), plants with biological fertilizer were 17.3% higher than plants without applications. In the second evaluation (99 das) height difference was 7.7% in plants with biological fertilization. These results can be attributed to marine algae are a natural source of auxins and cytokinins, which are a group of bio-regulators (^{Crouch and van Staden, 1993}). Similar results were reported by ^{Zodape et al. (2011)} finding that foliar application seaweed sap (Kappaphycus alvarezii) (5%), increased 34% plant height and 45% root length in tomato plants. Similarly ^{Sivasankari et al. (2006)} reported that the application of seaweed (Sargassum wightii and Caulerpa chemnitzia) increased plant growth on beans (Phaseolus vulgaris L.).

Figure 3 Height of corn plants with biological fertilization (FB) (Algaenzims^MR and ALZincB^MR) and biological plants without fertilization (no FB), observed on 09 September (71 days after planting) and October 7 (99 dds) 2011, Torreon, Coahuila, Mexico. 

Stem diameter

Stem diameter also showed increases (p≤ 0.05) by the application of biological fertilizers, since at 99 das, stem diameter was 10.5% higher than that observed in plants without biological fertilization (Figure 4). This can be attributed to seaweed extracts provide to plants major and minor elements content, plus enzymes and growth regulators (^{Canales-López, 1997}). In this regard, some studies report that seaweed extracts are a source of macro and micro nutrients, amino acids, vitamins, cytokinins, auxins and abscisic acid (ABA), which affect cell metabolism in plants leading crops to higher growth and yields (^{Durand et al., 2003}; ^{Stirk et al., 2003}; ^{Ordog et al., 2004}).

Figure 4 Diameter of plant corn stalk (mm) with biological fertilization (FB) (Algaenzims^MR and ALZincB^MR) and biological plants without fertilization, observed on October 07 (99 days after sowing), 2011, Torreon, Coahuila, Mexico. 

Plant dry weight

Application of biofertilizers derived from seaweed increased plant dry weight (p≤ 0.05) (Figure 5). At 99 days after transplantation, the average plant dry weight was 405.6 g plant^-1, and without applications to plants was 353.2 g plant^-1, this represented an increase of 14.8%. ^{Rathore et al. (2009)} reported higher growth and grain yield on soybean (Glycine max L.) with foliar application of seaweed extracts. Similarly, ^{Jeannin et al. (1991)} found an increased fresh weight on corn seedlings treated with seaweed extracts.

Figure 5 Dry weight of corn plants with biological fertilization (FB) (Algaenzims^MR and ALZincB^MR) and biological plants without fertilization (no FB), observed on October 07 (99 days after sowing), 2011, Torreon, Coahuila, Mexico.