Yield distribution of test tea varieties within 2 years
The average fruit yield per plant of a tea variety exhibited a positively skewed distribution (Fig. 2). The findings indicated that the higher the yield was, the lower was the proportion of varieties. The yields of 0–200, 201–400, 401–600, and 601–800 kg accounted for 83%, 9%, 5%, and only 3% of the total varieties, respectively, in 2013 and 52.2%, 21.8%, 15.7%, and 7.58% of the total varieties, respectively, in 2015. The yields of 801–1000, 1001–1200, and >1200 kg were observed only for one plant. Because of the annual change in tea fruit yield, the average fruit yield in 2 years was used as the benchmark for determining the corresponding flower characteristics in this study.
[Insert Figure 2 here]
Distribution of fruit yield between the anther and stigma superior groups
A total of 106 tea varieties with 20 individual plants for each variety were examined in this study. These individual plants had a median yield of 135.0 (IQR: 44.6–269.5) g. Among the 106 varieties, the average length of the pistil of 16 (15.1%) varieties was shorter than that of the stamen, with a median difference of −0.3 mm. These 16 varieties were included in the anther superior group. The remaining 90 (84.9%) varieties with a median difference of 1.0 mm between the length of the pistil and that of the stamen were included in the stigma superior group (Fig. 1). The yield of the anther superior group was significantly lower than that of the stigma superior group, with the median value of 73.6 (IQR: 27.9–145.3) versus 157.8 g (IQR: 57.3–292.3; P = 0.02; Fig. 3).
[Insert Figure 3 here]
The results showed that the fruit yield of the anther superior group was significantly lower than that of the stigma superior group. Due to the self-incompatibility of tea, it is speculated that in the varieties with a higher anther, the stigmas may have less opportunities to contact cross pollens, resulting in the decline of fruit yield.
Differences in phenotypic traits between groups
The stamen length of the anther superior group was comparable to that the stigma superior group (median value: 10.7 vs. 10.9 mm). Furthermore, the pistil length and stigma width of the anther superior group were significantly shorter and lower than those of the stigma superior group, respectively (median value: 10.1 vs. 12.2 mm and 3.0 vs. 4.0 mm, respectively; P < 0.001). In addition, the anther superior group had a significantly lower stigma width minus stamen bundle inner width than did the stigma superior group (median values: 0.6 vs. 1.4 mm; P = 0.023; Table 2).
[Insert Table 2 here]
Monotonic correlations between phenotypic traits and yield
As presented in Table 3, yield was significantly negatively correlated with the stamen length, stamen bundle outer width, and stamen bundle outer width minus stamen bundle inner width (Spearman’s ρ = −0.196, −0.243, and −0.254 and P = 0.044, 0.012, and 0.008, respectively). The strength of all the correlation coefficients was weak (absolute value of Spearman’s P < 0.4). In addition, yield was significantly positively correlated with the stigma width minus stamen bundle inner width, with a weak correlation coefficient (Spearman’s ρ = 0.202, P = 0.038).
[Insert Table 3 here]
Similar results were observed in the stigma superior group. Yield was weakly negatively correlated with the stamen length, stamen bundle outer width, and stamen bundle outer width minus stamen bundle inner width (Spearman’s ρ = −0.228, −0.303, and −0.301 and P = 0.031, 0.004, and 0.004, respectively). Furthermore, a weak positive correlation was observed between yield and stigma width minus stamen bundle inner width (Spearman’s ρ = 0.216, P = 0.041). In the anther superior group, yield was moderately negatively correlated with the stigma width minus stamen bundle inner width (Spearman’s ρ = −0.556, P = 0.025).
Effects of phenotypic traits on yield
General linear models were used to investigate the effects of phenotypic traits on yield, and the results are presented in Table 4. The five variables of phenotypic traits significantly affected the yield of the individual plants of the 106 tea varieties. First, yield increased with a 1-mm increase in pistil length minus stamen length, with a weight coefficient of 35.3 g (P = 0.037), and no such correlation was observed in both the anther and stigma superior groups. Second, yield decreased with a 1-mm increase in the stamen bundle outer width, with a weight coefficient of −15.3 g (P = 0.018). A similar result was observed in the stigma superior group, with a weight coefficient of −19.4 g (P = 0.006). Third, yield decreased with a 1-mm increase in stamen bundle outer width minus stamen bundle inner width, with a weight coefficient of −20.3 g (P = 0.008). A similar result was observed in the stigma superior group, with a weight coefficient of −25.0 g (P = 0.003). In addition, yield increased with a 1-mm increase in stigma width minus stamen bundle inner width, with a weight coefficient of 22.8 g (P = 0.039). A contrasting result was observed in the anther superior group, in which yield decreased with a 1-mm increase in stigma width minus stamen bundle inner width, with a weight coefficient of −67.1 g (P = 0.013). Finally, yield significantly increased in the stigma superior group compared with the anther superior group, with an estimated difference of 102.5 g (P = 0.023).
[Insert Table 4 here]
Figure 4 presents the relationship between the lengths of the pistil and stamen and the width of the style minus the inner width of the stamen bundle and the fruit yield of the tea varieties, as determined using regression analysis. When the pistil was longer than the stamen, the fruit yield of the tea varieties was higher (Fig. 4a). When the pistil was longer and the width of the style minus the inner width of the stamen bundle was greater, the fruit yield of the tea varieties was higher. These findings may be related to the fact that the stigma easily comes into contact with the pollen carried by visiting insects (Fig. 4b). When the anther is in the upper position and the width of the style minus the width of the stamen bundle is higher, the stigma and anther are more likely to contact each other, reducing the possibility of the stigma contacting cross pollens and thus decreasing the yield of fruits in tea plants (Fig. 4c).
[Insert Figure 4 here]