Site of study

The study was carried out in the Papaloapan region, Oaxaca state, México; in commercial plots of Litchi Maurittius cultivar (average of 10 years of age), of the municipalities of Loma Bonita, San Juan Cotzocón, San Juan Yaveo and Santa María Jacatepec; during the 2011 production cycle.

Evaluation of fruit cracking incidence

The incidence of fruit cracking symptom was evaluated in four Mauritius litchi commercial plots in the municipalities of Loma Bonita, San Juan Cotzocón, San Juan Yaveo and Santa María Jacatepec, from Oaxaca state during the 2011 production cycle.

In each plot 15 lines of litchi trees were systematically selected and five litchi trees were marked on each line; of each tree four clusters of fruits were marked per cardinal point and of each cluster the total number of fruits and the number of cracked fruits was evaluated.

Nutritional analysis of fruits

Fifty litchi fruits of Mauritius cultivar at commercial maturity stage were collected for each type of symptom (cracked and asymptomatic) during the 2011 production cycle. At the laboratory, the fruits were longitudinally sectioned to separate pericarp, aryl and seed. Samples composed of fruits of the same symptom and same tissue were labeled and dried in an oven (60 °C) for 48 h, and then digested in a diacid mixture. The phosphorus content (P) was determined by photocolorimetry by reduction with molybdo-vanadate; the potassium content (K) was determined by flame emission spectrophotometry. The content of calcium (Ca), magnesium (Mg), iron (Fe), copper (Cu), zinc (Zn) and manganese (Mn) was determined by atomic absorption spectrophotometry. Finally, the boron content (B) was determined by atomic absorption spectrophotometry. Finally, the boron content (B) was determined by photocolorometry with azomethine-H.

Climatic factors assessment

The climatic variables: temperature (maximum, minimum and average), relative humidity and solar radiation were evaluated in one hour intervals, with a DataLogger WatchDog^® equipment during the period from January to May 2011, in the municipality of Santa María Jacatepec, Oaxaca.

The average values of each of the climatological variables evaluated were correlated with the incidence of fruit ripening in order to determine their effect on the disease development.

Effect of calcium and gibberellic acid on the incidence of fruit cracking

The study was developed in a commercial plot of Mauritius litchi in the municipality of Santa María Jacatepec, Oaxaca during the 2012 production cycle.

Eight treatments were evaluated: 1) absolute control, 2) gibberellic acid (AG) 25 ppm (BioGip^®), 3) AG 50 ppm, 4) AG 100 ppm, 5) AG 200 ppm; 6) Calcium (Ca) 50 mmol L^-1 (Poliquel Ca^®); 7) Ca 100 mmol L^-1 and 8) Ca 200 mmol L^-1. For each treatment, four trees were selected, and eight fruit clusters (two per cardinal point) were labeled per tree.

The treatments application was performed with a motorized sprinkler backpack type and with fan type nozzle; to each treatment adherent (Innex-^®) (1 ml L^-1 of water) was added. Three sprays were performed during the study: 1) when fruits were clustered; 2) one week after clustering; and 3) four weeks after fruit clustering.

The evaluation of fruit cracking incidence was carried out every week, from fruit clustering to commercial maturity. The incidence percentage of cracked fruits was determined by the ratio of total number of fruits per cluster, among the number of cracked fruits.

Incidence of fruit cracking

The incidence percentage of cracked litchi fruits in the four evaluated commercial plots was different (α= 0.05); the highest incidence percentages of cracked fruits were shown in the municipality of Cotzocón (8.2%) and Santa María Jacatepec (5.7%). In three of four evaluated plots, no statistical differences were observed in the percentage of incidence of cracked fruits per sampling orientation (Tukey, α= 0.05) (Table 1).

Table 1 Incidence of cracked fruits of litchi cv Mauritius according to the cardinal position of the cluster in four municipalities of Oaxaca state. 

Valores con la misma letra en cada columna (*) o hilera (**) son estadísticamente iguales, Tukey (p≤ 0.05). Letras mayúsculas corresponden a la comparación de medias entre tratamientos (columnas *); letras minúsculas representan a la comparación de medias entre sitios de evaluación (hileras **).

Nutritional analysis of fruits

The litchi fruits showed statistical differences (α= 0.05) with respect to the nutritional content of boron in the pulp, between fruits with cracking symptoms (42.36 mg kg^-1) and that of asymptomatic fruits (62.22 mg kg^-1); in addition, a direct relationship between the calcium content in the pericarp and the cracking symptom was not observed; however, a higher concentration of Fe, Cu, Zn, Mn and B was present in the pericarp of asymptomatic fruits (Table 2). These results contrast with studies that indicate that in litchi fruits, the pericarp of cracked fruits has a significantly lower concentration of calcium than asymptomatic fruits in the same cultivar (^{Sanyal et al., 1990}; ^{Li y Huang, 1995}; ^{Li et al., 1999}; ^{Lin, 2001}).

Table 2 Nutritional analysis of litchi cv Mauritius fruits, with cracking symptoms in pericarp and asymptomatic; Santa María Jacatepec municipality, Oaxaca. 

Evaluation of climatic factors on the incidence of cracked litchi fruits

The temperature had a positive correlation with the percentage of cracked litchi fruits, in this regard, a higher quantity of cracked fruits was shown by increasing the maximum temperature (r²= 0.547), minimum temperature (R= 0.629) and average temperature (R= 0.569) (Figure 1). These results had a development pattern similar to those reported by ^{Mitra et al. (2010)} for ‘Rose Scented’ ‘Early Muzaffarpur’ and ‘McLean’ cultivars, with a higher incidence of cracked fruits as temperatures increased.

Figure 1 Cracking incidence (% Inc.) in litchi cv Mauritius and its relation with temperature (T °C) and relative humidity (%). Study period 15 weeks after fruit clustering (sda), (January 29 to May 14, 2011). 

The relative humidity showed a negative correlation with the incidence of cracked fruits (r²= -0.56) (Figure 1). ^{Li et al. (2001)}; ^{Mitra et al. (2010)} mentioned that the incidence of ripened fruits is favored by periods of high relative humidity; however, our results were different. In this regard, experiments in India, Africa and Australia indicated that periods of drought can have different effects on yield, quality and cracking depending on the level and period of water deficit. In South África, a water deficit during fruit development increased the number of cracked or rippened fruits (41%) in relation to fruits harvested from trees with good irrigation (10%) (^{Menzel et al., 1995}). ^{Abbott et al. (1986)} showed reduction of cracking problems in tomato fruits, by increasing irrigation frequencies.

^{Mitra et al. (2010)} suggested that elevated temperatures and high relative humidity favor cracked litchi fruits in the ՙRose Scented՚ ՙEarly Muzaffarpur՚ and ՙMcLean՚ cultivars. The same authors reported that Kaolin sprays at 2% and 4% and AG₃ 40 mg^-1 (1.16%) reduced the ripening incidence by reducing the external temperature of the fruits.

In China, litchi fruits develop during the rainy season and are cracked during periods of high relative humidity and heavy rains, especially when aryl is under development (^{Li et al., 2001}).

Although it is considered that environmental factors such as drought, high temperatures and excessive rains induce shell cracking (^{Kanwar and Nijjar, 1984}; ^{Chande and Sharma, 1992}; ^{Li et al., 2001}), the incidence of cracking varies considerably between trees within the same orchard, which are under the same climatic conditions, indicating that internal plants factors also affect the incidence of cracked fruits.

The periods of light hours and its intensity had the greatest effect on the percentage of cracked fruits (correlation, R= 0.917) (Figure 2, Table 3). The effect of this factor external to the fruit may be related to the increase in the external temperature of the fruits (^{Mitra et al., 2010}), leading to stress conditions to the fruit and therefore a greater cracking susceptibility.

Figure 2 Incidence of cracked (% Inc.) fruits of litchi and its relationship with relative humidity (%) and light intensity (lum/ft-2). (January 29 to May 14, 2011). 

Table 3 Correlation analysis between incidence values of cracked fruits and climatological variables of Santa María Jacatepec site, Oaxaca. 

Effect of calcium and gibberellic acid on the incidence of fruit cracking

The highest incidence percentage of cracked fruits was observed after four weeks of fruit clustering; which corresponds to the phenological stage of commercial maturity.

The highest incidence percentage of cracked fruits was observed in the fourth evaluation in the fruits of the control plants (5 582%); while in the plants treated with doses of 50 and 200 mmol L^-1, no cracked fruits were observed, which were statistically different (Table 4).

Table 4 Effect of treatments on the incidence of cracked fruits of litchi cv Mauritius, Santa María Jacatepec municipality, Oaxaca. 2012 production cycle. 

Valores con la misma letra en cada columna (*) o en cada hilera (**), son estadísticamente iguales, Tukey (p≤ 0.05). Letras mayúsculas corresponden a la comparación de medias entre tratamientos (columnas *), letras minúsculas representan a la comparación de medias entre fechas de evaluación (hileras **).

Previous results suggest that calcium applications reduce the cracking percentage in other fruits such as berries (^{Tukey, 1984}; ^{Ruper et al., 1997}; ^{Lang et al., 1998}; ^{Richardson y Ystaas, 1998}; ^{Fernández et al., 1998}; ^{Howard et al., 1998}; ^{Marshall y Weaver, 1999}), apples (^{Moon et al., 1999}), peaches (^{Sun and Liu, 1997}), oranges (^{Xu et al., 1994}, ^{Zou and Xu, 1995}), figs (^{Aksoy et al., 1994}). It is also reported that combined calcium-boron sprays in melon (^{Combrink et al., 1995}) or with copper in berries (^{Brown et al., 1995}) show to be more effective than calcium-only sprays.

For the litchi case, ^{Peng et al. (2001)} reported that the spraying of 100 mmol L^-1 Ca (NO₃)₂ in full flowering, 19 and 39 days after this stage, reduced the incidence percentage of cracked fruits in the Noumici cultivar (14.4 to 10.4%). Similarly, applications of calcium chelates (180 mmol.L^-1 Ca) (^{Li et al., 1999}) and 132 mg L^-1 Ca (^{Qiu et al., 2000}), reduced the incidence of ripened fruits of Noumici cultivar litchi (27.7 to 17.5% and 47.2 to 15.4%, respectively).

^{Huang et al. (2005)} reported that calcium applications generally reduce the incidence of cracking; however, the effect depends on the application period and the formulation of applied calcium. In the same way, it is pointed out that because the cracking incidence can be variable between each of the evaluated trees in the same cultivar, the effects of the applications might not be significant based on a statistical analysis.

It is important to note that when calcium is applied via soil, it is absorbed by the roots and then transported through the vessels; however, fruits generally have a lower transpiration rate than leaves, so they will have a lower calcium concentration (^{Huang et al., 2005}); in addition, during drought periods, calcium in litchi cultivation is absorbed by the leaves, and at a mínimum percentage by fruits, which is reflected by higher rates of fruit cracking (^{Li and Huang, 1995}).

The percentage incidence of cracked fruits in the treatments based on gibberellic acid (AG) varied from 1.25 to 4.367 in the fourth evaluation and did not show statistical differences with respect to fruits of the control plants (Table 2). Although ^{Peng et al. (2001)} indicated that the AG reduces fruit cracking by reducing cellulose activity, it is considered necessary to make new evaluations in the Mauritius cultivar in order to determine the doses that allow reducing the incidence percentage of cracked litchi fruits.