Pod shattering resistance
The average shattering for sixteen soybean genotypes are presented in Table 1. The shattered pods of all genotypes were mostly started at 50°C with an average of 9% and then the average of shattering was increased up to 28% at 60°C. The shattering variation which started at 50°C of the oven-dry method was also reported in the shattering evaluation of the F
6 soybean population
(Krisnawati et al., 2019b).
Perusal of Table 1 showed that there were two resistant genotypes, seven moderately resistant genotypes, six susceptible genotypes and one highly susceptible genotype. The degree of resistance ranged from resistant to highly susceptible and no any genotype found as highly resistant. Similarly, the previous studies did not find a highly resistant genotypes or with zero shattering
(Adeyeye et al., 2014; Antwi-Boasiako, 2017). However, other studies have successfully obtained very resistant genotypes (Romkaew and Umezaki, 2006; Adie and Krisnawati, 2016). The varying results on the shattering resistance in those studies could be caused by the difference in the genetic background of the genotypes used and the quantifying method for shattering resistance
(Bhor et al., 2014; Gan et al., 2008). The shatter-resistant genotypes identified from the present study could be used for the improvement of the plant to minimize the yield losses in soybean.
Shattering at different pod positions
The pod shattering on three pod positions on the stem shows different patterns (Fig 1). The highest percentage of the shattered pods was in the lower part of the stem (52%) followed by the middle part (32%) and the upper part (15%), respectively. Thus, pods at the lower part of the stem contributed more than 50% of shattering in a soybean plant. This result is in line with the results obtained by Krisnawati and Adie (2017a) using an ambient temperature method. According to
Romkaew et al., (2007), the differences in the shattering resistance was due to the moisture content of the pods.
Summarizing the pod shattering pattern of three pod positions (Fig 1), it appears that the pod shattering pattern of pod position in the upper part shows a rather flat curve. Meanwhile, the pod shattering pattern of pod position in the middle part shows a curve with a slightly raised edge. The pod shattering pattern of pod position in the lower part shows a curve with a raising edge.
The patterns of pod shattering for each part position of the pods for each genotype are presented in Fig 2. Each soybean genotype shows different pod shattering patterns, either due to differences in shattering among pod positions or due to the shattering resistance. The pod shattering of resistant genotype was started at 60°C and the shattered pods were mostly derived from the lower part of the stem. The group of moderate genotypes showed the shattered pods were mostly derived from the lower and middle parts of the stem. However, the percentage of shattering in the middle part was lower than those of the susceptible genotypes. The groups of susceptible genotypes showed the shattered pods derived from the lower, middle and upper parts of the stem. The susceptible genotypes showed a high percentage of shattering at the lower as well as middle parts of the stem and few shattered pods at the upper part of the stem. The highly susceptible genotype showed shattered pods in all parts of the stem.
The pods at the lower part of the stem aged older than the pods at the middle- and upper-part positions. According to the previous study, the flowering of cultivated and wild-type plants proceeded from the basal order racemes to the upper order racemes
(Saitoh, et al., 1998). In mungbean, it was reported that the indeterminate flowering habit leads to a spread of flowering and pod maturity on a single plant over the entire reproductive phase. Consequently, pods that develop at the earliest flower may shatter prior to 100% pod maturity
(Vairam et al., 2017).
Determinant factor for pod shattering resistance
The determinant factors for pod shattering resistance were investigated through the characters of pod morphology, seed morphology, plant height, pod weight and seed weight. The mean data of pod shattering and plant characters is presented in Table 2. All the plant characters, except the plant height and weight of individual seed, show no significant correlation with pod shattering at all pod positions (Table 3). The plant height showed a significant negative correlation with the pod shattering in all three parts position of the pods in the stem,
i.e. r = -0.589** (upper part), r = -641** (middle part) and r = -0.505** (lower part). This shows that a shorter plant tended to have a higher percentage of pod shattering compared to the higher plant, especially if associated with the position of the pods.
The pod shattering from the middle part of the stem showed a significant positive correlation with the weight of the individual seed (r = 0.467*). This indicates that the heavier individual seeds will increase the percentage of pod shattering. Related to the seed characters,
Kataliko et al., (2019) reported an increase in resistance to pod shattering towards a reduced number of seeds within a pod. In the present study, we use the detached sample pods from each part of pod positions in the stem. However, in the field, soybean pods will remain attached to the stem, hence a further study is needed to get closer to the simulation of the screening for shattering resistance by using the undetached pods.