3.1. Experiment 1
Neither the type of seed nor the year of production significantly affected (
p ≤ 0.05) the moisture content of the seed at the beginning of the viability and germination tests, ranging, on average, from 9.35% and 9.51% (
Table 1).
The type of seed was the only factor that significantly (
p ≤ 0.05) influenced the seed viability, considering both S and S + WV (
Table 2), corresponding to the highest value to OS, which is followed by type IV seeds, and the lowest viability corresponded to CS. The low viability of type IV seeds (31.3% for S) and particularly that of CS (6.3% for S) is notable, since they should not present viability nor germination restrictions given their age (6 months) [
13].
Although the viability obtained for CS was extremely low, it doubles that obtained by [
18] (on average 3.15% for S) in a commercial seed lot produced in 2018 and that was provided by the same company. Using a stereoscopic microscope, based on the integrity of the seed coat, these authors classified the seeds of that lot into four groups: intact, scraped, cracked, and broken seeds. In the 2018 seed lot, the deterioration of the seed coat considerably contributed to the low viability of the seeds, since cracked and broken seeds supposed 31% and 16%, respectively, of the total seeds, their respective viability being low (7.5% and 0%, respectively, for S criterion). This suggested that in the 2019 and 2020 lots, the deterioration of the seed coat could have contributed considerably to the low viability of the seeds; nevertheless, cracked and broken seeds were considerably reduced in these lots, the cracked seeds accounting for 1.3% and the 1.8% and the broken seeds accounting for 0.5% and 1.0% of the total seeds in 2019 and 2020, respectively. It seems that, although slowly, this company has been improving the quality of their caper seeds.
The low viability of both type IV and CS seeds agrees with [
26], who stated that some studies confirmed that the seed quality (i.e., viability, germination rate) in a number of species, such as Arabidopsis, beans, melon, rape, pepper, and tomato, continues to increase after physiological maturity, but once the seeds have dried below about 20% moisture content, their metabolism has ceased, and deterioration may begin. Probably for this reason, according to [
26], the seeds of some crops are harvested with relatively high moisture contents and then carefully dried to avoid deterioration in the field and to preserve the highest quality. Seeds of type IV and probably also part of CS were quickly dried in the field, which could lead to a reduced viability.
As expected, given the low viability, the accumulated germination (
G) values were low for OS and very low for IV and CS. The coefficient of determination (R
2) for 48 curves (four replicates from three combinations of variation sources: three types of seeds, GA
3 or water addition to the germination substrate and both years of production) ranged from 0.891 to 0.995, with F ratio values of the model statistically significant (
p ≤ 0.01; data not shown). This indicates that the use of the logistic function is suitable for analysing caper seed germination as it was in similar studies carried out both in caper [
12,
18] as in other crops [
23,
27], weeds [
28], and fungi [
29].
Figure 2 presents the cumulative germination curves fitted to the logistic model obtained for the average values of each type of seed and saturation solution combination.
The cumulative germination (
G) and the final germination percentage (
A) values are similar (both in this experiment,
Table 2, and in experiments 2 and 3), being both significantly affected by the same factors; thus, only
A values will be discussed.
Both the type of seed and the substrate saturation solution as well as their interaction influenced (
p ≤ 0.05) in
A, the highest value corresponding to OS and the lowest value corresponding to CS. The GA
3 addition to the substrate increased the value of
A. When analysing their interaction (
Figure 2), it is observed that GA
3 considerably increased germination in OS and, to a lesser extent, in the type IV seeds, but it did not increase germination in the CS. The germination obtained with the GA
3 addition can be considered acceptable (69.3%) in OS, very low in the type IV seeds (16%), and practically negligible in CS (1.9%). These values agree with the viability obtained (S + WV, 73.8% for OS). It seems that as previously stated and according to [
26], deterioration begins with the seed drying after reaching its physiological maturity.
As can be seen in
Figure 2 and
Table 2, the caper seeds germination is very slow, so that on average, the time required to reach 50% of
G has exceeded 50 d, as has been obtained in previous studies [
15,
18]. The GA
3 application reduced the
Gt50 (on average from 74.4 to 32.4 d;
p ≤ 0.01), and the significant interaction of this factor with the year of production (
p ≤ 0.01) indicates that this decrease was greater in seeds produced in 2019 than in those produced in 2020.
The
k/2 was affected (
p ≤ 0.01) by the type of seeds, the highest value corresponding to CS, but it is necessary to emphasise that only less than 2% of these seeds germinated. As in viability, it can be stated that overall, the values of the germination parameters obtained with the CS agree with those obtained in previous studies [
18].
3.2. Experiment 2
There were significant differences (
p ≤ 0.01) in the moisture content depending on the type of seeds, decreasing with increasing fruit maturity (
Table 3). The moisture content of type IV seeds (14.7%) was clearly lower than that of other types of seeds (on average 27.8%). Logically, this seed moisture content was much higher than that presented in Experiment 1 (on average 9.4%), since results were obtained after drying and storing for six months.
In the case of fruits that are kept in the field after dehiscence, the seeds dry out before being collected, reducing their weight and, consequently, their density. These seeds can be both mature and immature. When the seeds have a density below a threshold, they float in the water; thus, it is not feasible to separate the mature seeds from the immature seeds by flotation method [
19]. On average, the percentage (on weight basis) of mature seeds in seed types I, II, and III (not floating in tap water) represented 61.5% and 67.0% of the total seeds of 2019 and 2020, respectively. In the type IV seeds, the desiccation led to the flotation of all the seeds. This fact agrees with the lower seed moisture content of the type IV seeds (14.7%) in relation to the other seed types (on average 27.6%). According to the results obtained for the flotation method, the mature seeds (those that did not float) of types I, II, and III, and all seeds of the type IV, were used in the viability and germination tests.
The viability of type IV seeds, considering both the seeds of category S and those of category S + WV, was lower (
p ≤ 0.05) than that of type III, which in turn was lower (
p ≤ 0.05) than of types I and II, with no differences (
p ≤ 0.05) between them (
Table 3). Although the difference between the viability of the seeds of types I and II was not significant
(p ≤ 0.05), the proportion of WV was lower in seeds of type II (5%) than in those of type I (13.8%). As previously cited, ref. [
26] reported that the quality of the seed is maximum during and shortly after its physiological maturity.
The viability of the type IV seeds was very low (<38%, considering S + WV), although it must be considered that these lots include both mature and immature seeds (because all of them floated in tap water); thus, these figures could underestimate (although in a small percentage) the viability of the mature seeds, as mentioned afterwards. In addition, it is common for ants, wasps, and birds to take seeds, especially the best quality seeds, when the fruits dry in the field.
The coefficients of determination (R
2) for 64 curves (four replicates from three combinations of variation sources: four types of seeds, GA
3 or water addition to the germination substrate, and two years of production) ranged from 0.931 to 0.996, with F ratio values of the model statistically significant (
p ≤ 0.01; data not shown), meaning that the use of the logistic function is suitable to analyse the germination of caper seeds in this experiment, as shown in Experiment 1 (
Figure 3).
The type of seed influenced (p ≤ 0.05) its germination, so that the type II seeds presented the highest A values (p ≤ 0.05), while the lowest value was obtained for the type IV seeds. The year of seed production did not affect A (p ≤ 0.05). As mentioned, the GA3 application increased (p ≤ 0.05) the A values, as well as the k/2, decreasing Gt50 (p ≤ 0.05).
The interaction between the type of seed and the GA
3 application significantly affected
A (
p ≤ 0.01;
Table 4). It can be observed in
Figure 3 that while with the GA
3 application, there were no differences (
p ≤ 0.05) between the germination obtained in seeds of type I (78.3%) and type II (74.3%), being high in both cases, when only water was applied to the substrate, the germination obtained by the type II seeds (63.5%) was higher (
p ≤ 0.05) than that obtained by those of type I (41.4%). On the other hand, the
A value was higher (
p ≤ 0.05) with the GA
3 application in type I seeds (78.3%) than in type III seeds (61.0%), while there were no differences when only water was applied (41.4% and 42.8% for I and III, respectively;
p ≤ 0.05). It is worth noting the high germination obtained with type II seeds, both with water and GA
3 application, which agrees with that stated by [
26], in the sense that the ideal state for the seed collection is just in the fruit dehiscence, since the seed quality is maximum during and shortly after the physiological maturity of the seeds.
Germination of the type IV seeds may be considered as very low (30.9% with GA3 and 9.0% with water; on average A ≈ 20%), although it must be considered that these lots included both mature and immature seeds (because all of them floated in tap water, as mentioned above). Thus, these figures could underestimate, by a small percentage, the germination of the mature seeds. Specifically, considering for the seed types I, II, and III that on average, 35.75% (on weight basis) of the seeds floated, and that 13.15% of these seeds germinated (parallel studies; data not shown), the cited underestimation could be around 5% (specifically 4.7%). Therefore, the germination of the mature seeds in type IV could reach percentages up to 36% with the AG3 application.
The interaction between the type of seed and the GA
3 application also significantly affected (
p ≤ 0.01) the
Gt50 (
Table 4), in the sense that
Gt50 values obtained in the four types of seeds with the GA
3 application did not differ between them, requiring, on average, 23.3 d to reach the 50% of the corresponding final germination. However, when only water was applied to the germination substrate, the highest time (
p ≤ 0.05) was required by type III seeds (113.2 d) and the lowest time (
p ≤ 0.01) was required by type IV (80.6 d), but this shorter time of the latter is related with its low germination percentage (
Figure 3).
3.3. Experiment 3
As expected, the state of the seed at the start of the viability and germination test (consequence of the period between the extraction of the seeds from the fruits, which was carried out immediately after the collection, and the start of the tests) significantly affected (
p ≤ 0.01) the moisture content of the seed (
Table 5). The moisture content of the FS was higher (
p ≤ 0.05) than that of DS, which in turn was higher (
p ≤ 0.05) than that of SS, although the difference between the last two are small in absolute value (about 1%), which indicates that the desiccation of the seed occurs mainly in the first days of drying.
The state of the seed significantly affected (
p ≤ 0.01) its viability, considering both the S and S + WV criteria (
Table 5). The lowest values corresponded to the SS without differences between the FS and DS (
p ≤ 0.05), which means that seed viability decreases with storage, decreasing the proportion of S and increasing those of WV (0%, 10%, and 30% in FS, DS, and SS, respectively).
Both the state of the seeds and substrate saturation solution, as well as their interaction, influenced the final germination (
Figure 4 and
Table 6), the highest
A value corresponding to FS and the lowest corresponding to SS (
p ≤ 0.05;
Table 6). As occurred in experiment 2, the GA
3 application to the substrate increased the value of
A. When analysing the interaction of both factors (
Figure 4), the final germination percentage obtained with the GA
3 addition can be considered as very high in FS (89.2%) and high in DS and SS (72.1% and 72.6%, respectively). These
A values were expected considering the viability obtained with the S criterion for FS and DS (97.5% and 82.5%, respectively) and with that obtained with the S + WV criterion for SS (77.5%). It can be observed in
Figure 4 that GA
3 considerably increased germination percentages compared to those germinated with water, in SS (64.2%), to a lesser extent in DS (35.6%), and even less in FS (25.8%). This seems to be related to a weakening of a part of the seed coat or to the low embryo growth potential, so that the increase in germination obtained with the GA
3 addition (compared to water) increases with storage, that is, the GA
3 effect was more important when the seeds had been stored and their viability had decreased. The high germination percentage obtained with seeds stored for one month (with the GA
3 application), which allows the distribution and sow of the seeds during this period, not being necessary to apply techniques such as priming. Priming is a practice used by the seed industry to increase the performance of commercial seed lots, improving the germination rate and uniformity [
19,
30]. Nevertheless, priming tends to shorten seed life in storage, and the benefits of priming can be lost during storage [
19]. Recently harvested seeds (FS and DS) presented higher viability and germination values than those harvested six months before (SS), thereby agreeing with [
13], who recommended a storage period for caper seeds no longer than two years because during this period, its viability does not significantly decrease, and high germination percentages can be obtained, although
Gt50 increased for the seeds stored for one year compared to those stored for just one month. This highest viability and germination obtained in FS differs from seeds of other common families such as Asteraceae and Poaceae, among others [
31], which present nondeep physiological dormancy and undergo after-ripening, that is, dormancy break during dry storage. The herein presented results, as well as those reported in [
13,
18], indicate that caper seed viability and germination not only do not increase with dry storage, but they decrease.
The GA3 addition to the substrate reduced Gt50 (p ≤ 0.01), obtaining similar values for the three periods (23.6 d on average, with no differences (p ≤ 0.05) between them), while without the GA3 application, the highest value (p ≤ 0.05) corresponded to FS (103.7 d; with the highest final germination) and the lowest value (p ≤ 0.05) corresponded to SS (56 d; related to its low germination value). The GA3 addition to the germination substrate increased k/2 (p ≤ 0.01).