INTRODUCTION

Modern agriculture has increasingly sought to maximize available resources, aimed not only at increasing productivity, as in the past, but also at seeking higher quality food. Nowadays, due to the greater degree of knowledge about the quality of products, consumers are giving preference to fresh produce that is smaller in size and has greater nutritional value (^{Hoppu et al., 2020}).

The chemical composition of carrot roots is variable and influenced by genetic factors and cultivation conditions, such as crop system, soil type and physical properties, planting time, rainfall, and temperature during the growing season (^{Seljåsen et al., 2013}), as well as phytosanitary aspects, fertilization, and planting density.

Among the climatic and management factors that affect carrot quality, precipitation and irrigation are very important (^{Soltoft, 2010}). Stress from high or low water availability in the soil can induce the production of undesirable compounds (^{Reid & Gillespie, 2017}). Other important factors are temperature and light conditions. Warm and humid climates seem to affect quality by inducing a turpentine-flavored, less sweet-tasting carrot compared to carrots grown in cooler and drier climates. It is known that low cultivation temperatures (9 - 21 ºC) affect the sensory aspect by increasing the sweet taste (+35%) and content of fructose (+49%) and glucose (+28%) and reducing the bitter taste (−30%) and content of sucrose (−33%) and β-carotene (−40%) (^{Seljåsen et al., 2013}).

Therefore, understanding the environmental conditions in different regions and climates where a crop is grown is crucial when choosing a cultivar.

In order to overcome the limitations imposed by high temperatures, Brazilian researchers have developed cultivars that perform well in the temperature range of 18 to 25 ºC. The new varieties, in addition to good climate adaptability, have resistance to diseases caused by fungi and nematodes. This makes it possible to cultivate carrots in regions and states where the temperatures are higher, such as Bahia and Goiás (^{Silva et al., 2011}).

The use of summer cultivars and seeds from primary umbels has been the main strategy to enable carrot cultivation in regions where this vegetable was not cultivated in the past (^{Resende et al., 2016}).

Although the summer cultivar Brasilia is still one of the most widely used in warm regions during summer, in recent years the planted area using imported hybrid cultivars has also increased and research using these new materials, mainly in high temperature conditions, is still incipient. Among the advantages of hybrid cultivars over open-pollinated cultivars are the higher degree of hybrid heterosis and vigor, as well as greater uniformity of internal root color and low presence of white halo.

However, in some states, even with the advent of summer and hybrid cultivars, carrot production is insufficient to meet the domestic demand. In the state of Rio Grande do Norte, where virtually all commercialized carrots come from Bahia, the price of this product in local markets is usually well above the average of the Brazilian market (^{Bezerra Neto et al., 2014}).

Additional research is needed due to the difficulties of carrot production in warm and lower elevation regions, starting with the selection of cultivars with genetic potential for quality root production under high temperatures.

Thus, it is assumed that the planting date influences the postharvest quality of carrot roots due to climatic and environmental variations, such as precipitation, temperature, and cloudiness.

In order to provide more information about the behavior of new carrot cultivars under limiting environmental conditions, the objective of this study was to evaluate the effects of planting dateson the root quality of ten carrot cultivars under semiarid conditions, in Mossoró, Rio Grande do Norte.

MATERIAL AND METHODS

RESULTS AND DISCUSSION

According to the joint variance analysis, there was an interaction between the cultivar and planting time factors for all of the post-harvest quality related characteristics, except for beta-carotene content (Table 3).

Regarding the white halo percentage, there was a significant difference between cultivars for all planting times. In general, the percentage of carrots with white halo increased with each planting date, from 25% in Season 1 to 76.2% in Season 4. The cultivars that showed the highest percentages of the disorder were Brasília, Kuronan and Suprema. The hybrid cultivar Nativa had the lowest percentage (Table 4).

Although the open-pollinated cultivars were more susceptible to this disorder, hybrids were also susceptible. The Amanda cultivar did not exhibit the disorder in Season 1. In seasons 2 and 3, almost half of the roots were affected by white halo. In Season 4, nearly all of the roots of the Amanda cultivar showed the physiological disorder. A similar pattern was observed for the Francine cultivar.

This evolution of the physiological disturbance throughout the planting dates is certainly related to changes in climatic conditions that the plants were subjected to throughout the year, especially temperature, relative humidity and solar radiation. Although high temperatures are determinant for the occurrence of this physiological disorder, some cultivars appeared to be more tolerant (i.e., Melinda and Nativa). It was also observed that the Meloidogyne sp. infection tends to aggravate the problem. Therefore, the increased incidence of white halo in seasons 3 and 4 may also be related to a higher incidence of this disease in the study area.

White halo is a physiological disorder that affects the xylem vessels in carrot roots. It begins as a whitish halo around the vascular bundle and progresses to complete discoloration of the vascular bundle (^{Grangeiro et al. 2012}). Carrot roots with this disorder have low market acceptance. ^{Pereira et al. (2015}) point out that in the Brazilian market, for consumption in natura, there is a preference for roots with a pronounced orange color and little differentiation between the colors of the xylem and phloem.

Carrot hybrids showed higher levels of soluble solids compared to open-pollinated cultivars, except for the hybrid Nativa, which was not statistically different from open-pollinated cultivars, and the Suprema cultivar, which was the only open-pollinated cultivar with a soluble solids content statistically similar to the one observed for hybrid cultivars during seasons 1, 3 and 4 (Table 5). The means of total soluble solids obtained in the present study are above the range of 6.5 to 7.5º Brix, which is recommended for harvest and consumption (^{Paulus et al., 2012}).

Soluble solids content is directly related to the taste and sweetness of plant products. Therefore, the high soluble solids content value is a positive result, since sweetness is a desirable attribute that increases the quality of carrots (^{Schifferstein et al., 2018}).

Planting in July favored higher levels of soluble solids in the roots in all cultivars (Table 5). The high solar irradiance observed during the cycle of the plants sown in July, especially between September and December 2017, may have caused the greatest accumulation of soluble solids. This is because high luminosity favors an increase in soluble solids content in the plants (by reducing the root weight) and a decrease in acidity, resulting in better-tasting carrots (^{Mattiuz, 2007}).

For the Shin Kuroda cultivar, which belongs to the Kuronan group, the average soluble solids content observed by ^{Paulus et al. (2012}) was 9.5 ºBrix, with the highest averages observed for carrots grown in the winter. In Mossoró, ^{Grangeiro et al. (2012}) report an average of 7.62 - 8.90 ºBrix for the Brasília cultivar.

Regarding titratable acidity, the cultivars did not differ statistically from each other for the planting dates of May and August. In June, the cultivars Brasília, Kuronan and Mariana did not differ from each other and were the ones with the greatest acidity (Table 6). In July, in addition to the cultivars mentioned above, the cultivars Amanda and BRS Planalto also formed the group of cultivars with the highest acidity.

The synthesis of organic acids in a plant is directly related to the photosynthetic capacity of the plant (^{Taiz & Zeiger, 2017}). Thus, environmental factors such as temperature and solar radiation, associated with planting density, may have influenced the decrease in the acid content in the roots during the planting dates.

In general, the titratable acidity means found here were similar or approximate to those in the literature. ^{Alves at al. (2010}) obtained 0.167% malic acid for the Brasília cultivar. ^{Pereira (2014}) reports that the titratable acidity of conventional and organic carrot samples was 0.19 and 0.2% malic acid, respectively, with no significant differences between the two cultivation systems.

For the ratio of soluble solids/titratable acidity, there was no significant difference between cultivars in Season 1. Seasons 3 and 4 had the highest soluble solids/total titratable acid (SS/TA) ratios. The increase in the SS/TA ratio during the planting dates was observed for practically all cultivars, except Érica, Melinda and Nativa that showed an increase in Season 3 and a decrease in Season 4 (Table 7).

The Francine, Mariana and Nativa cultivars had the highest SS/TA averages and were statistically equal. The other cultivars did not statistically differ from each other and had averages ranging from 62.61 to 70.78 (Table 7).

The increase in the SS/TA ratio in seasons 3 and 4 is related to the increase in the content of soluble solids in those seasons and to the decrease in the titratable acidity observed in the same seasons.

The results obtained in the present study were superior to those obtained by ^{Figueiredo Neto et al. (2010}) (30.8 °Brix/% for Brasília cultivar) and ^{Alves et al. (2010}) (°Brix/50.15% for Brasília carrot). This difference can be attributed to the difference in soil, spacing, management and local average temperature.

The total soluble sugar levels ranged from 6.46 to 8.28%. A significant difference was observed between cultivars in seasons 1, 2 and 3. In seasons 1 and 2, the cultivars that stood out were the hybrids Amanda, Érica, Francine, and Mariana (Table 8).

In general, it was observed that the concentration of total soluble sugars tended to decrease throughout the year. This shows that planting carrots at the hottest times of the year will result in root quality loss. The lowest concentration of sugars in the hottest times of the year is related to the metabolism of the carrot plant. The carrot is a C3 metabolism plant and, therefore, it tends to undergo photorespiration with increased temperature and solar irradiance, and can also suffer photoinhibition due to high solar radiation, which can damage the photosynthetic apparatus and, consequently, have a direct impact on the ability to synthesize carbohydrates (^{Taiz & Zeiger, 2017}).

The mean sugar contents found here are in accordance with ^{Umuhoza et al. (2014}), who reported that in Brasília type cultivars the sugar content varies in the range of 9.9 to 10.27%, values ​​similar to those found in American cultivars.

The means found by ^{Alves et al. (2010}), for the Mossoró region, ranged between 5.12 and 6.32% and are, therefore, lower than the averages identified in this study, even for the most unfavorable planting times for carrot cultivation in the region.

For the levels of beta-carotene, there was a significant effect of the cultivar factors and planting times alone (Table 9). The cultivars Brasília and Kuronan were similar to each other. The Brasília cultivar, which has been the most recommended cultivar for summer cultivation, once again exhibited the worst performance among the ten cultivars studied.

Regarding the planting dates, carrots planted in July showed, on average, the highest content of beta-carotene in the roots, while carrots planted in August (Season 4) had the lowest content.

One of the main climatic factors that influences the content of carotenoids in carrots is air temperature. Air temperatures ranging from 16 to 25 ºC are considered ideal for the synthesis of beta-carotene (^{Vieira & Pessoa, 1997}). The average temperatures in the study area, during the entire period of the experiment, were above the ideal range for the synthesis of beta-carotene, which would justify the low levels of this pigment found here.

CONCLUSIONS

The interaction between the carrot cultivars and the planting dates had a significant influence on postharvest quality of carrot roots.

For the study area, the cultivation of carrots in May and June is most recommended, since the July and August plantings resulted in a decrease in marketability, mainly due to a higher occurrence of white halo in the roots.

Finally, further studies should be conducted to find ways to attenuate the occurrence of white halo and cultivate carrots throughout the year in regions with hot climates.