The onion (Allium cepa) is the second most important vegetable worldwide after tomato, with approximately 85 795 190 t produced in the world, where Mexico contributes 1.48% (^{FAOSTAT, 2014}). In Mexico 2014 were harvested 47 430.57 ha with a production of 1 368 183.69 t (^{SIAP, 2016}). While in Sinaloa for the same year 1 480.7 ha were harvested; however, the average yield was only 23.04 t ha^-1 in 2014 (^{SIAP, 2016}).

The factors influencing the onion bulb formation are day length, temperature and variety (^{Lancaster et al., 1996}; ^{Brewster, 2008}). Bulbification occurs when certain heat hours have accumulated (^{Lancaster et al., 1996}). According to ^{Tapia (1999)}, the onion is cold season and is moderately frost resistant. High temperatures can stress the onion plant, causing physiological disturbances, decreasing leaf development speed and number (^{Tesfay et al., 2011}).

On the other hand, the influence of photoperiod is affected by the quality and intensity of the light, because the light infra red and high light intensities favor the development of the bulb (^{Bertaud, 1986}). As the duration of the day decreases, the light intensity decreases, the leaves perceive it and send signals to other parts of the plant, initiating dormancy (^{Chope et al., 2012}). According to the above, the combined effect of temperature and photoperiod induces the formation of onion bulbs, although it may be that in tropical areas temperature is a more determinant factor (^{Tesfay et al., 2011}).

The nutrients of the onion initially pass from the soil to the plant, leading to the area where photosynthesis takes place. From there they are transported to the growing areas and to the storage regions. ^{Palacios et al. (2005)} observed that the onion began to absorb K at 52 days after transplantation.

They also found that during the 90 days after the transplant they extracted 94% of the total K; the leaves began to extract K at 38 days after transplantation and in a period of 70 days extracted 96% of their total K. From day 122 translocation to the reserve organs began; also observed that accumulation of K was initiated in the bulbs at 94 days after transplantation; In addition, the percentage of K was always higher in leaves than in bulbs (^{Palacios et al., 2005}). Based on the above, the objective of the present work was to determine the increase in size and weight of the bulb that occurs after folding of the leaf, due to the translocation of nutrients.

The study was conducted in the autumn-winter cycle 2015-2016 in a San Lorenzo Valley in Culiacán, Sinaloa, located at 24° 27’ 27.53” north latitude, 107° 16’ 02.32” west longitude of the Greenwich meridian, and an altitude of 38 m, the climate of the region is Bs (h’), very warm semiarid with a precipitation / temperature quotient greater than 22.9 (^{García, 1988}).

The Nunhems company’s “Carta Blanca” seed was used, this is short days with a white bulb, round shape, tolerant to flowering and maturing 170-175 days after planting. The seedling was produced in seedbeds using a polypropylene microtunnel.

The ground was prepared by subsoiling, tracking, and leveling with a plank, finally proceeding to form beds of 1.6 m wide center. In each bed six plant lines were formed with a separation of 12 cm between them. The distance between plants was also 12 cm arranged in staggered. The experiment was performed under drip irrigation conditions, using tape (T-Tape, John Deere, USA) with droppers at 20 cm separation and a cost of one liter per hour per dropper. In each bed three tapes were used.

Transplantation was started 60 days after sowing, when the seedling reached approximately 4 mm in diameter. To do this the plant was removed from the soil and pruned leaving the leaves about ten centimeters high to avoid dehydration during the transplant. Before the transplant, the econil fungicide (chlorothalonil) was applied to the seedling to prevent the attack of pathogenic microorganisms found in the soil. In addition, 295 kg h^-1 of nitrogen, 100 kg h^-1 of phosphorus and 110 kg h^-1 of potassium were applied during the crop cycle, where the main source of nitrogen was urea fertilizer 46-00-00), phosphorus was monoammonium phosphate (12-61-00), potassium was potassium nitrate (12-00-45).

A simple random sampling was performed that was representative of a frame constituted by the number of onions planted in an area of three hectares (approximately 775 000 plants). The sampling unit was each onion. The statistical parameter to estimate the sample size was the mean and variance of the equatorial diameter of the bulb. A random sample of the population was obtained without distinguishing between its elements, in such a way that all the units had the same probability of being selected, for that the rows of onions were enumerated. At the beginning of folding of the onion leaf, the measurements of the equatorial diameter of the bulb were carried out, with an interval of one week between each one.

The normality of the data obtained for each of the tests was checked, taking into account the data of the first measurement: mean= 7.48 cm and standard deviation= 0.926, a normality check of data was made from a test of goodness of adjustment of normality using chi square (^{Zar, 1999}). A Levene homogeneity test was also performed (^{Correa et al., 2006}) and the analysis of variance of the differences between the means of the studied populations was performed, demonstrating that there is homogeneity between the variances. Subsequently, the data were submitted to a variance analysis and means comparison test, applying the student’s t test (^{Zar, 1999}) with the SAS statistical package 9.0 (^{SAS Institute, 2002}).

It was observed that at the beginning of the folding of the leaf, the average of the equatorial diameter of the bulb was of 7.48 cm, whereas a week later they had bulbs of average 8.02 cm; that is, a difference of 0.53 cm, which was statistically significant. This difference represents a percentage rate of change of 7.12. The growth of the bulb continued after the first week of measurement, so that in the second week the difference with respect to the beginning of the folding of leaves was 0.78 cm. This difference means a percentage rate of change of 10.48. Also, when comparing week 1 with respect to week 2 a difference of 0.25 cm was observed, however, it would only have a significant difference if α= 10. For the third week practically the onion bulb stopped having growth, time at which the leaf lost the green color. This makes it clear that after 15 days the onion bulb did not grow. Likewise, a correlation of 0.93 between the diameter of the onion and the weight of the onion was found (Figure 1).

Figure 1 Relationship between diameter and weight of onion bulb.  

The analysis of variance showed that there is a high significance, with a critical value of F of 7.98E-28. El análisis de varianza mostró que hay una alta significancia, con un valor crítico de F de 7.98E-28. Asimismo, el valor de la recta de la regresión fue de 84.01, es decir, que por cada centímetro que aumenta el diámetro ecuatorial de la cebolla, incrementa 84 g. Por lo anterior, las ventajas de posponer la cosecha después del doblado de las hojas son evidentes. Por lo anterior, se debe procurar que la hoja se encuentre lo más sana posible para que la mayor parte de los nutrientes que se encuentran en ella pasen al bulbo. Es decir, hay que evitar lo más posible la salida de energía de la planta hacia los consumidores, como el trips (Thrips tabaci), el gusano minador (Liriomysa trifolii) y enfermedades como el mildiú (Peronospora destructor).

In the Table 1 shows a sensitivity analysis, based on a price of four pesos (MXN) per kilogram, which shows that it may be profitable to wait a certain time to raise the onion of the land. However, it is also important to consider the probable meteorological conditions for the period when the onion would be exposed in the field, because of precipitation in that time, the product could have lower quality at the time of sale and damage the shelf life.

Table 1 Sensitivity analysis, considering estimated selling price. 

With an estimated sale price: $4.00 kg; TPV of diameter week 1: 7.12 (0.78 cm); TPV of diameter week 2: 10.48 (0.53 cm); TPV difference between week 1 and week 2: 3.14 (0.25 cm); value of the regression slope: 84 g cm^-1 diameter; correlation coefficient= 0.93.