Effect of drying method on the longevity and quality of seeds from Bauhinia purpurea. II. Storage in cold-storage room
Marlen Navarro y J. C. Lezcano
Estación Experimental de Pastos y Forrajes "Indio Hatuey". Central España Republicana, CP 44280, Matanzas, Cuba
The combined influence of different drying methods, the storage conditions and the physiological age of seeds from Bauhinia purpurea, on their germinative capacity and quality was determined. The treatments for drying were: A) in the pods; B) artificial; and C) under sunlight. The moisture content (MC), germination and viability of the seeds were determined during the 12 months of storage in the cold-storage room. For the information processing multivariate analyses were used. In A the highest value of the germinative capacity (98,67%) was observed in the first evaluation, as in B (100%), where the differences between months were minimal in the whole study. In C the germinative capacity was 79,33% at 12 months, which represented a quantitative increase of longevity of the seeds with regards to A. The germinative process in A, between 0 and 9 months of storage, was completed 9 days after the test started, while in the other evaluations it occurred after 6 days. Between the seeds of B and C a similar performance was observed, only with the difference that the germinative potential could be expressed after 6 days in B at 0 and 2 months; in C at 0, 1, 2 and 11 months; while in B germination began after 6 days for 9 and 10 months. It is concluded that the treatments essayed in this work can be applied to bauhinia seeds, although the best results were obtained when using silica gel (B).
Key words: Seed storage, Bauhinia purpurea, quality
The seeds from Bauhinia purpurea show an accelerated postharvest deterioration rate, which favors the loss of their physiological activity (physiological death) and, hence, their germinative capacity when they are stored under the ambient conditions prevailing in Cuba. On the other hand, in the country there is scarcity of seeds from this species, for which within the management strategies it is beneficial, from the economic point of view, to establish the conditions that propitiate that they reach a larger life span with the minimum available resources, which will allow having reserves of the species for sowing in livestock production systems.
Navarro and Lezcano (2007) determined that the factors that influence the most the rate of quality loss of B. purpurea seeds stored under ambient conditions were moisture content and physiological age.
The storage period is often limited by the physiological potential, i.e. the time in which one seed in particular survives under the available conditions. To maintain viability for long periods it is important that the ambient of the storehouse has the optimum conditions for each species (ISTA, 1999). For that, in this work the combined influence of different drying methods, the storage conditions in cold-storage room and the physiological age of seeds from B. purpurea, on their germinative capacity and quality, was determined.
MATERIALS AND METHODS
The pods from B. purpurea were collected in (approximately) 20 trees of spontaneous generation, located at 22º46'N and 81º08'W, in March, 2000, and were carried to the Laboratory of Seed Analysis of the EEPF "Indio Hatuey".
The newly harvested pods were divided into three portions, which constituted the experimental treatments used for the study (table 1).
In all the treatments the seeds were manually separated from the pods and the inert matters composed by sick or broken seeds, as well as remains of pods, petioles and leaves, were eliminated, always avoiding damage to the external and internal structure of the seed.
The seeds of each treatment were put in amber glass flasks, with semi-airtight lids, which were stored in a cold-storage room (temperature: 10ºC and relative humidity 23 ± 2%). In each flask a seed quantity was put higher than that used in the determinations of moisture content, germination and viability, corresponding to each of the twelve evaluations carried out during the storage period for each drying method, i.e., the flasks were opened only at the moment of performing the monthly determinations and were discarded afterwards.
The moisture content (MC), germination (standard test) and viability of the seeds (topographic tetrazolium essay) were determined monthly, during the 12 months of storage, and in each evaluation daily counts were performed for 21 days (ISTA, 1999). Those seeds in which the radicle reached a length higher than 1 mm were considered germinated.
As B. purpurea is a species with a mean daily germination rate higher than 10, it was not possible to determine the germination rate according to the formula proposed by Djavanshir and Pourbeik (1976); in this case the highest value of such indicator was determined, which represents only the measure of the germination rate and it is obtained by successively dividing the cumulative germination by the relevant incubation time (Czabator, 1962).
Multivariate analyses were carried out to determine the influence of the drying method (DM) and physiological age of the bauhinia seeds (PA), on their germination capacity; the GLM (General Linear Models) procedure for the variance analysis and the matrix of multiple mean comparison of Student Newman Keuls (SNK) of SAS ® (1996) were used, and the variable germination was adjusted through polynomial regressions. The differences were tested using the option PDIFF of SAS®, declared significant at values of P<0,05 and the trends discussed at P<0,15.
Mathematical model used:
Yijk = µ + DMi + PAj + DM*PAij + eijk
Yijk = ijk-eth value of germination.
µ = mean of all the observations
DMi = i-eth effect of drying method
PAj = j-eth effect of storage period
DM*PAij = ij-eth effect of the interaction between drying method and storage period
eijk = ijk-eth effect of random error
RESULTS AND DISCUSSION
For the drying treatment of the seeds within the pods (A) the highest values of the germinative capacity were recorded in the first three evaluations (98,67; 94,00 and 90,67%), among which there were no significant differences according to the SNK test, as well as between 2, 3, 4 and 5 months, although 2 did not differ from 0 and 1 month. The decrease of this indicator in A was gradual (fig. 1); even at the end of the study germination and viability reached 71,80%, which represents a reduction of only 26,87% in 12 months of permanence in the cold-storage room; while for this same drying treatment, but with seeds stored under ambient conditions, Navarro and Lezcano (2007) detected a 96% decrease of the germinative capacity in only 7 months.
The rate of loss of germinative capacity in B was remarkably slower than in A; the better adjustment to a cubic polynomial adjustment was found (table 2), in which the highest value (100%) was represented by the evaluation carried out at the moment of beginning the storage and the lowest value (87,33%) was recorded at the end of the study, i.e. at 12 months of physiological age of the bauhinia seeds. It must be emphasized that the differences among the months for the variables germination and viability were minimum in the whole study and the dispersion field of such values varied between 0,34 and 2%.
In the case of seed drying under sunlight similarities were found with the treatment in A, with the difference that at the beginning of the storage they showed 100% viability and germination; nevertheless, this value did not differ from the one recorded in A (98,67%), and there were no differences either between these two treatments at 1, 2, 3 and 4 months after the beginning of the permanence in the storehouse. Since 5 months they differed statistically, but the numeric values were not very distant. For these seeds that were dried under sunlight the germinative capacity was 79,33% at 12 months, which represents a quantitative increase of their longevity in C with regards to A.
By the end of the study (12 months) the highest viability and germination values were recorded in treatment B, with 87,33%, higher than the value reported by Navarro and Lezcano (2007) for that same age of the bauhinia seeds stored under ambient conditions (70,0%). The difference between both percentages is obviously due to the effect of relative humidity and temperature of the facilities where the seeds were stored, because in this study these indicators did not suffer variations given the characteristics of the cold-storage room. For such reason, it can be stated that the extrinsic factors modified the intrinsic factors, the latter represented by the MC of the seeds and their germinative capacity, which coincides with the reports by Sanhewe and Ellis (1996), who stated that postharvest longevity depends on the species and external factors, such as humidity content, temperature and composition of the gaseous atmosphere during storage.
Table 3 shows the cumulative germination, regarding the 21 days established for the performance of the standard germination test (ISTA, 1999), in each one of the evaluations during the permanence of the seeds in the cold-storage room. When analyzing the germination kinetics of the bauhinia seeds corresponding to treatment A, the germinative process was deduced to begin at 3 days. In addition, between 0 and 9 months of storage the germinative process was completed 9 days after starting the test; while in the other evaluations (10, 11 and 12 months) this occurred 6 days after the beginning of the test. For the artificially dried seed, and the ones dried under sunlight a similar performance was observed, only with the difference that the germinative potential could be expressed 6 days after starting the test in B at 0 and 2 months; in C at 0, 1, 2 and 11 months; while in B germination started at 6 days for 9 and 10 months. These results indicate that the highest contribution to the final percentage was made by the seeds that germinated in the interval between 3 and 9 days.
Figure 2 shows, through a histogram, the results of the MC tests in each drying treatment; for that the values expressed in percentage in each month were grouped, based on the number of evaluations that coincided with the MC value. The homogeneity of such values in A, B and C was observed.
Although the flasks in which the seeds were put had semi-airtight lids, the fluctuations of MC were discreet and there were no significant differences among the values. This could be related to the ambient of the storehouse, where temperature and relative humidity remained constant, contrary to the storage under ambient conditions, in which there were remarkable differences between maximum and minimum temperature and RH.
It is concluded that one of the strategies to preserve the bauhinia seeds over periods equal to or higher than 12 months could be the use of cold-storage rooms and any of the desiccation treatments applied in this work could be applied to them, because they were efficient with regards to conservation under ambient conditions.