ELECTRICAL CONDUCTIVITY AND QUALITY OF DIFFERENT UMBEL ORDER CARROT SEEDS

Carrot seeds exhibit irregular vigor due to the wide blooming period of the crop. Results from germination tests are insuffi cient to evaluate the physiological potential. Thus, vigor tests are used in combination with germination tests. The objective of this work was to evaluate the quality of carrot seeds from diff erent umbel order and the electrical conductivity under diff erent temperatures and imbibing periods. Carrot seeds from cultivar Brasilia were used to perform the experiments. Content of water, the fi rst and the fi nal score of the germination test, index of germination rate, and length of roots and of the aerial portion were determined in the initial characterization. The electrical conductivity test was completed with replicates containing 50 seeds, in 25 mL deionized water at 20, 25 and 30C. Evaluations were performed with 2, 4, 6, 8, 12 and 24 hours of imbibing period. The experiment was accomplished under a completely random statistical design. The quality of carrot seeds varied according to the umbel order. Seeds of secondary umbels showed better physiological quality. The electrical conductivity test is effi cient to evaluate the vigor of carrot seeds with 8 hours of imbibing period at 25°C.


INTRODUCTION
Plants of carrot (Daucus carota L.) exhibit a wide blooming period, thus generating highly irregular vigor seeds, even when produced under uniform soil and climatic conditions and derived from the same population (Lima & Athanázio, 2009).
The employment of certifi ed quality seeds constitutes an important factor in the establishment of crops, allowing higher yields (Catão et al., 2013). According to Bittencourt et al. (2012), it is of vital importance that physiological potential of seeds has been obtained through tests which may supply reliable information, to take decisions during production and commercialization of those seeds.
Results provided by germination test are considered insuffi cient to evaluate the physiological potential of seeds in fi eld conditions (Ohlson et al., 2010), once this is performed under optimal water, aeration and temperature conditions (Brasil, 2009). Within this context, the use of fast, reliable and easy methods is fundamental to evaluate the physiological potential of seeds.
Vigor tests are used in conjunction to germination tests, to support decisions, increase agility and effi ciency of quality control (Bittencourt et al.;. Amongst the fastest tests studied, the electrical conductivity is related with the initial events of the deterioration sequence in seeds (Bewley & Black, 1994;Ortiz, et al., 2018). The results are obtained through indirect evaluation of the structural degree of the cell membranes by means of determining the quantity of leached ions in the imbibing solution (Vieira & Krzyzanowski, 1999;Marcos Filho, 2015).
Electrical conductivity has been widely used to evaluate seed's vigor with consistent results, especially for large crop seeds. However, some researchers are attempting to standardize this test to evaluate vegetable seeds, as in studies completed in pepper (Vidigal et al., 2008) and onions (Rosa et al., 2015).
Diverse factors may infl uence the results of the electrical conductivity test, among those the plant genotype, loss of structure in cell membranes, size and number of seeds in the sample, presence of pathogens, temperature and imbibing period as the most signifi cant (Wain-Tassi et al., 2012). These factors are the focus of research aiming more consistent results, especially when regarding the temperature and the imbibing period which are considered factors of extreme importance (Avelino et al., 2018). Besides, carrot seeds may show uneven vigor status and the availability of information concerning the techniques to perform such vigor evaluation are also scarce. The objective of this work was to evaluate the quality of carrot seeds from diff erent umbel order and electrical conductivity under diff erent temperatures and imbibing periods.

MATERIAL AND METHODS
The experiment was completed in the fi eld and in the Seed Laboratory from the Integrated Faculties from Ourinhos (FIO), Ourinhos-SP. The climatic conditions were typical from sub-tropical regions in Brazil, classifi ed as Cfa according Köppen. The mean annual temperature and rainfall were of 21.2°C and 1339 mm, respectively.
Carrot seeds from cultivar Brasília were produced in seedbeds with spacing of 20 cm between rows and 5 cm between plants. Cultural treatments were accomplished according to the crop needs, with thinning being executed 25 days after sowing. Seeds were harvested 120 days after blooming, collecting seeds from the primary, secondary and tertiary umbels separately.
The initial characterization of the physiological quality of seeds from diff erent umbel order was performed through the following tests: Water content: accomplished through the method of an oven at 105±3°C, using two sub-samples of 50 g of seeds each, according Brasil (2009).
Germination test and fi rst counting: seeds were sowed over two blotting paper foils, previously dampened with distilled water at a proportion of 2.5 times the weight of the dry substrate, in gerbox plastic boxes. Boxes with seeds remained in B.O.D. chambers at 20ºC under a 12 hours photoperiod. The evaluation constituted of two counts of normal plants, seven and fourteen days after the test started (Brasil, 2009). Four replicates with 50 seeds were analyzed and the results were expressed as percentage of normal plants.
Index of germination rate: this evaluation was performed simultaneously with the germination test, computing every day, the number of germinated seeds. Seeds were considered germinated when showing protrusion of the radicle. The index was calculated according to the formula proposed by Maguire (1962).
Length of the roots and the aerial portion: the analysis was achieved from seeds sowed on blotting paper as described previously. Four replicates with 10 seeds of each umbel order were used, arranged in alternated form. After 14 days the length of the roots and the aerial portion

Electrical conductivity and quality of diff erent umbel order carrot seeds
were measured with the aid of a graduated ruler, and the results expressed in centimeters.
The electrical conductivity test was performed by the bulk method, using four replicates with 50 seeds for each umbel order. Seeds were weighed in analytical balance (precision of 0.001g) and conditioned in plastic cups with 200 mL capacity, containing 25 mL of deionized water. Then, cups remained in B.O.D. chambers at temperatures of 20, 25 and 30°C, with evaluations were acomplished in six imbibing periods with 2, 4, 6, 8, 12 and 24 hours.
Electrical conductivity measurements were accessed with a Tecnal TEC-4MP digital conductivity meter, registering the results at the end of the evaluation. The equipment was calibrated with readings of up to 2000 µS.cm -1 , variation of 2. 02% and standard solution of 146.9 µS.cm -1 at 25 o C. The experimental design used was completely random with four replicates in a 3x6 factorial scheme, with 3 umbel orders and 6 imbibing periods (2, 4, 6, 8, 12 and 24 hours), isolated at temperatures of 20, 25 and 30ºC. Data was analyzed statistically using the F test and variance analysis at 5% probability, the Scott-Knott test at 5% probability was used when signifi cant eff ects were detected, with the aid of the software SISVAR 5.0 (Ferreira, 2011).

RESULTS AND DISCUSSION
According to the results showed in Table 1, it was possible to observe signifi cant diff erences in the physiological quality of carrot seeds. Data regarding the water content were similar among seeds from diff erent umbel order. Water content in seeds during the test was of 7.3%, 6.7% and 7.4% in seeds of the primary, secondary and tertiary umbel, respectively. Guedes et al. (2011) stresses that diff erences from 1 to 2% in water content between samples are not signifi cant, allowing the tests to proceed, that is to say, this percentage is considered within the range which does not infl uence the results of the electrical conductivity test. Concerning the fi rst germination count, a lower percentage of normal plants was observed in seeds from the tertiary umbel. According Marcos Filho (2015), the fi rst count is extremely important to evaluate seed vigor, the higher the germination percentage of a determined seed lot, the higher its vigor. The lowest germination percentage was also observed in seeds from the tertiary umbel. It must be emphasized that germination test was performed under ideal environmental conditions of luminosity, temperature and relative humidity. Therefore, such test not always will reveal diff erences in performance between seed lots (Mattioni et al., 2015).
The highest viability was observed in seeds from fi rst and secondary umbel (Table 1). According to the index of germination rate results, seeds from tertiary umbels were less vigorous than seeds form primary and secondary umbels. Similar results were observed by Rodo (2001), demonstrating there was no decrease in seed germination as umbel order increased. However, Santos et al. (2010) observed that seeds from the primary umbel showed a reduction in germination.
Regarding the length of the aerial portion, seeds from the tertiary umbel diff ered statistically form the rest. Such phenomenon was not observed concerning root length, which did not diff erentiate statistically. According Santos et al. (2010), diff erences in the aerial portion and root length constitute a signal of the existence of physiological quality diff erences among seeds. Thus, in the present study, tests performed to initially characterize the physiological quality were effi cient to diff erentiate seeds from each umbel order. Consequently, seeds from the tertiary umbel showed lesser vigor, although no diff erences were verifi ed for seeds from the primary and secondary umbels.
When analyzing Table 2 it is possible to observe signifi cant diff erences between the umbel orders and the imbibing periods of the electrical conductivity test, as well as for the interaction of the evaluated factors (P<0.05). The deterioration process causes degenerative changes of the membrane system, reducing its integrity and/or selectivity, producing loos of control in water and solute exchanges between the cells and the exterior environment, determining the reduction of seed viability (Binotti et al., 2008). Thus, the electrical conductivity test used to evaluate seed vigor in function of the leachate quantities, varied according to the methods used in the present study (Table 2). When the electrical conductivity test was performed at a temperature of 20°C, it was possible to stratify seeds from three diff erent umbel orders in only two vigor categories. Seeds from primary umbel showed a higher deterioration level and higher leachate quantities. Seeds from superior umbels showed a bigger size, thus may have greater mechanical damages and higher deterioration than lower umbel seeds (Rodo et al., 2001). Rodo et al. (2001) also observed that seeds from primary umbels have and early maturation in the fi eld, losing quality over time, until the other umbels are mature, and the harvest is done. This fact explains the higher deterioration level of such seeds.
A slight diff erence in the quality of carrot seeds was observed at a temperature of 20°C in the present work. However, Domiciano et al. (2015) observed that only at a temperature of 20°C a diff erence was observed in carrot seed lots. Seeds from secondary and tertiary umbels had no statistical diff erence in any of the imbibing periods tested. Thus, these seeds hold a better quality and vigor according to the tests applied in the present work, corroborating results obtained by Torres et al. (2015).
The electrical conductivity test is generally applied at 25°C (Marcos Filho, 2015). Such temperature is considered standard when applying this test, allowing to diff erentiate seed lots in vigor levels. As observed in Table  Electrical conductivity and quality of diff erent umbel order carrot seeds 2, at this temperature was also possible to stratify the seeds. The constitution of three vigor classes was verifi ed in the imbibing period of 8 hours, classes which remained the same in the following periods. Seeds from primary umbel showed a higher level of deterioration, followed by seeds from tertiary and secondary umbels. Therefore, seeds from secondary umbels are more vigorous. Domiciano et al. (2015) observed that at a temperature of 25°C lots of carrot seeds did not diff er statistically. Although Rosa et al. (2015) concluded that at a temperature of 25°C lots of onion seeds diff ered, once they showed higher quantities of leachates in water, this may allow inferring that such standard temperature of 25°C may be effi cient or not, depending on the crop to be evaluated.
At a temperature of 30°C, seeds from primary and secondary umbels were statistically equal until the imbibing period of 24 hours. Such imbibition temperature propitiated an equality of means, reducing the diff erence between seed vigor levels. Carvalho et al. (2009) emphasized that high temperatures and long exposition periods promote the concentration and stabilization of mean values for leachates of the electrical conductivity test.
At a temperature of 30°C a high quantity of electrolytes in the imbibing water was also evidenced. According to Bewley & Black (1994), high temperatures may promote an increase in leachates and this may be due to the loss of structure of the cell membrane system, which is correlated with membrane integrity, thus inhibiting the re-organization of membranes and therefore increasing the leachates in the seeds' imbibing solution.
Imbibing periods of 12 and 24 hours, at a temperature of 30°C, were ineffi cient to evaluate seed vigor. In these periods a high quantity of leachates in the water was observed, independently form umbel order, turning the use of conductivity tests unfeasible. According to Marcos Filho (2015), temperature has a great importance because it is related with the exposition period of seeds, therefore, the higher the temperature, the higher the electrolyte leachates will be. At 30°C the electrolyte leaching was lower in seeds from tertiary umbels, when compared with other seeds. This may have occurred due to the seeds being smaller. Corroborating with the results observed in the present work, Domiciano et al. (2015) verifi ed that a temperature of 30°C it was not possible to identify diff erences in vigor of carrot seeds. On the other hand, Dutra & Vieira (2006) verifi ed that at 30°C temperature it was possible to diff erentiate the quality of summer squash seed lots. Before that information, we observed temperatures of 30°C allows to diff erentiate seed lots, however depending on the crop seed evaluated.
As the imbibing period increased there was an escalation in the quantities of released electrolytes by the seeds with a tendency to a stabilization and, therefore, with the possibility of an anticipated evaluation of seed vigor (Table 2). Electrical conductivity was also sustained from 8 until 24 hours of imbibing period while evaluating seeds of Vigna unguiculata by Moura et al. (2017).
Due to the great number of samples demanding evaluations in the laboratories it is primordial to obtain promptly results, without jeopardizing consistence and reliability. Thus, the increment of leachate electrolytes during the imbibing period of 8 hours at 25°C allowed a safe evaluation of carrot seed quality. The reduction of imbibing period is of extreme importance for the industry of seeds, to rapidly determine the quality of seeds.
The imbibing period of 24 hours is established for readings of electrical conductivity (Vieira & Krzyzanowski, 1999). However, Dutra and Vieira (2006) determined also a period of 8 hours as being suffi cient to evaluate the vigor of summer squash seeds, once the contents of leachates conserved a tendency. Vidigal et al. (2008) while evaluating the quality of pepper seeds verifi ed effi cient results for vigor analysis after an hour of imbibing period. The reduction of the imbibing period for the electrical conductivity test was also observed during the evaluation of seed vigor in beans (Silva et al., 2014). Xavier et al. (2017) determined a period of 4 hours as suffi cient to separate lots with 50 seeds of Blackeyed pea. Using 50 seeds of coriander, Torres et al. (2015) verifi ed the evaluation of vigor in the seed lots was effi cient as soon as 2 hours of imbibing.
Consequently, the present work confi rmed that electrical conductivity test was effi cient to evaluate the physiological potential of seeds with diff erent umbel order, allowing to diff erentiate them with extreme effi ciency. This reinforces the fact that methodologies used in the imbibing periods were coherent to evaluate the vigor of carrot seeds.

CONCLUSIONS
Quality of carrot seeds varied according to the umbel order. Seeds from secondary umbels showed better physiological quality. The electrical conductivity test is effi cient to evaluate the vigor of carrot seeds with 8 hours of imbibing period at 25°C.