Evaluated material

The sowing of Gandul varieties (Negro and EGV22), was carried out in black plastic bags with a capacity of approximately 4 kg, two seeds were placed per bag to later leave a plant as an experimental unit. The soil used was of volcanic origin (Andisol-USDA) that was collected at a depth of 0 to 20 cm (Table 1).

Description of treatments

Two types of fertilization with nitrogen (+N) were considered, which in the early stages of the growth of legumes requires nitrogen fertilization, because the symbiosis between legumes and rhizobium has not been developed. While the other treatment was without nitrogen (-N), soil fertilization was performed at the time of preparing the planting. For the -N treatment, 54.0 kg ha^-1 of phosphorus (P, Super simple phosphate, FERTISA, Guayaquil, Ecuador) and 194.0, 86.0 and 44.0 kg ha^-1 of potassium (K, potassium chloride), sulfur were incorporated into the soil. (S) and magnesium (Mg) (K-mag, FERTISA, Guayaquil, Ecuador).

While for the treatment +N, 137.0 kg ha^-1 of N (Urea, FERTISA, Guayaquil, Ecuador), 81.0 kg ha^-1 of P (Diammonium Phosphate, FERTISA, Guayaquil, Ecuador), 137.0, 53.0 and 44.0 kg ha^-1 were applied of K, S and Mg (K-mag), respectively, according to the recommendations of the manual of nutrition and pasture fertilization (^{Bernal and Espinosa, 2003}).

In addition, during the period of plant growth, fertilizations were carried out at the foot of the plants, fifteen days after germination and then every 30 days; for -N they were applied 1.0 g of K-mag fertilizer, which corresponded to doses of 297.0, 243.0 and 297.0 kg ha^-1 of K, S and Mg, respectively. While for the +N treatment, 1.0 g of ammonium sulfate was added, which corresponded to a dose of 284.0 kg ha^-1 of N.

Study variables

In the present work we measured: the length of the root (LR), from the neck to the cap; number of nodules (NN), healthy (pink color) and vain (gray); the nodules were extracted in water to avoid damage; nodule weight (WN); plant height (HP), from the base of the stem to the apex; number of branches (NR); number of leaves (NL); length of branches (Lr) and total dry matter weight (TDMW). The samples were dried in an oven at 65 °C until constant weight was obtained; leaf area (LA); using the methodology suggested by ^{Rincon et al. (2012)}.

The physiological efficiency of the plant (EFP) was determined through growth as a function of time (weeks). To make this type of analysis, two basic measurements were used, the dry matter production of the aerial fraction of the plant material and the foliar area (^{Carranza et al., 2009}). The parameters used to estimate the physiological efficiency of the plant were: leaf area index (LAI), specific leaf area (SLA) and rate of the crop growth (RCG); determined by the formulas established by ^{Hunt (1990)}; rate of net assimilation (RNA) and rate of relative growth (RRG) determined by the formulas described by ^{Tayeb (2012)}.

Regression analysis and experimental design

To estimate the growth curves or trend lines over time of each of the two nitrogenous fertilization treatments, a regression analysis was used with 12 destructive samplings (one sampling per week after germination) and five repetitions within each sampling (^{SAS, 2012}).

For the analysis of continuous variables, a randomized complete block design with factorial arrangement (2 x 2) was used and a comparison test of means was carried out with the Tukey test at 5% (^{SAS, 2012}).

Effect of nitrogen in the aerial part of the pigeon pea

Statistical significance was found (P <0.05) for HP between the varieties and effect of fertilization, surpassing the variety EGV22 in 7.3 cm p^-1 to the Negro variety; and for the effect of fertilization the -N treatment, allowed to obtain taller plants with an approximate average of 10.0 cm p^-1 in comparison with the treatment + N; conversely, for the interaction of treatments, no statistical significance was found (P> 0.05).

Regarding the TDM, statistical significance was observed (P <0.05) in the variety interaction by fertilization, where the Negro -N variety with 6181.6 kg ha^-1 showed to have greater production of biomass; although statistically similar to the variety EGV22 with + N (6129.0 kg ha^-1) and -N (5502.2 kg ha^-1), being able to overcome with 1223.0 kg ha^-1 the variety Negro +N (Table 4). The same behavior and result of TDM was obtained in LAI, since both the variety EGV22 with +N and Negro with -N, showed the highest LAI with 6.29 and 5.76, respectively. In Figure 2, the initial growth of the aerial part of the legumes is observed, which showed a positive growth behavior over time (weeks).

Figure 2 Height of plant (HP), total dry matter (TDM) and leaf area index (LAI), for two varieties of pigeon pea (Cajanus cajan, Negro and EGV22) with (+N) and without (-N) nitrogenated fertilization in Santo Domingo, Ecuador. 

In HP throughout the experiment the variety EGV22 was positioned above the Negro variety; while the TDM of the legume varieties was affected by the incorporation or restriction of N, from the sixth week. One of the moments in which the fertilization treatment was applied, it was observed that in the +N treatment, the EGV22 variety had a greater increase in TDM; conversely, the Negro variety proved to have more biomass with the -N treatment, a similar tendency observed in TDM was presented in the LAI, where the variety EGV22 required N to express a greater leaf area, since in the absence of N as shown in figure 2, from the sixth week the LAI began to decline; while in the Negro variety the application or restriction of N did not infer in the manifestation of leaf area (P> 0.05).

Effect of Nitrogen for the physiological efficiency of the pigeon pea

In SLA, no statistical significance was found (P> 0.05) for varieties, fertilization treatments and their interaction; that is, they developed the same leaf diameter and area; which were not affected by the application or restriction of N, although their trend over time was negative. In the treatment with +N the Negro variety after the first two weeks of the study until the seventh week, presented a greater decrease of the SLA, to later claim and surpass the EGV22 variety; while in the -N treatment, the two varieties presented a similar trend.

In the case of RNA, only statistical significance (P <0.05) between varieties was observed; being the Negro variety (0.00043 g cm² d^-1), which showed the highest photosynthetic efficiency since the beginning of the biological cycle, independently of the application or N restriction (Figure 3).

Figure 3 Specific leaf area (SLA), net accumulation rate (RNA), relative growth rate (RRG) and crop growth rate (RCG), for two varieties of pigeon pea (Cajanus cajan; Negro and EGV22) with (+N) and without (-N) nitrogenous fertilization in Santo Domingo, Ecuador 

While for the RRG no statistical significance was found (P> 0.05) in any of the sources of variation, the Negro variety without -N and with + N had RRG of 0.058 and 0.052 g g^-1 d^-1, respectively.

Similarly, in RCG the Negro variety obtained in the -N treatment the highest growth (0.0036 g g^-1 d^-1), statistically different (P <0.05) to the variety EGV22 -N (0.0020 g g^-1 d^-1). The highest RNA and RRG was observed in the first week of the crop cycle, and later declined gradually during the rest of the investigation; this corroborates the inversely proportional relationship that they have with RCG, as a result of the possible shading present in the leaf blades between them (Figure 3).

In all the efficiency variables, the Negro variety exceeded (P <0.05) the variety EGV22, especially in the -N treatment, where approximately from the sixth week the variety EGV22 decreased NRA, RRG and RCG, observing a differential behavior between the varieties (Figure 3). In the variety EGV22 the application of nitrogen fertilizer improved (P <0.05) the variables of biomass production and crop efficiency, allowing an adequate growth and development of the plants.

Effect of nitrogen in the aerial part of the pigeon pea

^{Martínez et al. (2003)} who evaluated 25 lines of Cajanus cajan, found that the varieties most recommended for the production of forage are those of high size. In addition, ^{Mayz (2007)} observed that legumes treated with nitrogen tend to have higher biomass in initial stages, since according to ^{Pliego et al. (2003)} the application of N can cover the needs of the plants, increasing their total biomass. The results obtained in this work are contrary to those of ^{Pliego et al. (2003)}, since the EGV22 variety presented higher plants, without obtaining higher TDM.

However, the application of N in the EVG22 variety was essential to improve the IAF, but the development will depend on each species (^{Gómez-Carabalí et al., 2011}). For ^{Higuera et al. (2001)} the differences in the morphological components of the plants are due to the duration of physiological stages of the legumes.

Effect of Nitrogen for the physiological efficiency of the pigeon pea

For the SLA, the trend was negative over the weeks, which could be due to the fact that with the passage of time the thickness of the leaf increases (^{Villar et al., 2008}). According to ^{Grazia et al. (2001)} fertilization with nitrogen affects the development of foliar structure (leaf area, leaf area duration and leaf expansion rate), but does not modify the leaf area per unit of mass.

A negative trend was observed in the weeks of study for NRA and RRG these same results were mentioned by ^{Hoyos et al. (2009)}, which could be due to the greater accumulation of dry matter, in relation to the production of photoassimilates during the growth of the crop. While the increase in RCG was the result of constant cell division in meristematic tissues and plant architecture (^{Hernández et al., 1995}).

In the variety interaction by fertilization for RCG, according to ^{Villar et al. (2008)} commented that RCG is influenced by the environmental conditions and genetic components of each species. ^{Hoyos et al. (2009)} mentioned that RCG can be altered with the application of nitrogenous fertilizer.