Figure 1.
Schematic representation of Opuntia robusta.
Cladodes of O. robusta are almost circular. Roman numbers describe the cladode levels. We call “1st-level cladodes” the newest ones (I). The oldest are called “4th-level cladodes” (IV in the figure). No plant in our sample had 5th-level cladodes. A 1st-level cladode that produces either one or two daughter cladodes (it never produces more than two) converts to a 2nd-level cladode. Daughter cladodes may appear on the same parental cladode in different seasons. In the same season, on the same plant, new cladodes can be daughters to the cladodes from different levels.
Figure 2.
Sampling of the spines and of the total phenolic compounds.
We traced an imaginary cross on each cladode, dividing it into four equal sections, and measured the length of each spine (to the nearest 0.5 mm) as well as the number of spines on each areola overlapping the axes. We perforated the mid-section of the arc delimited by the border of the upper quarters of the cladodes, approximately 1 cm away from the border (or close, when an areola obstructed the perforation exactly in this place), using a stainless steel tube with sharpened borders (Ø = 2 cm).
Table 1.
Intersexual comparison of the traits analyzed in the study.
Figure 3.
State-dependent comparison of the average total phenolic compounds (TPCs) per cladode in terms of tannic acid (TA), in 1st-level cladodes.
a) Comparison of empty (non-parental) and parental, female, and hermaphrodite cladodes in the group of plants that bore empty and parental cladodes. Empty cladodes contained a higher TPC concentration than parental ones. Parental female cladodes contained a higher TPC concentration than parental hermaphrodites (significant Bonferroni test at 0.05). b) Comparison of the concentration of TPCs from empty, parental, and reproductive cladodes in the group of plants that bore each cladode from a different state. Parental hermaphrodite cladodes contained a lower TPC concentration than empty and reproductive cladodes.
Table 2.
Sex-dependent average tannic acid (TA) concentration [mg eqTA g−1 FB] per cladode.
Table 3.
Sex-dependent average tannic acid (TA) concentration [mg eqTA g−1 FB] per cladode.
Figure 4.
Relationship between defense traits.
a) Comparison of the regression lines between average areolae number on a cladode (X) and average spine number per areola, per plant (Y): Y = 19.43 – 1.38 X, and Y = 15.43 – 0.6 X, for females and hermaphrodites, respectively. The effect of average areola number was significant (F1, 3 = 8.33, P = 0.005). The slopes were not significantly different between sexes (F1, 3 = 1.1, P = 0.29), contrary to the intercepts (F1, 3 = 10.5, P = 0.002). The adjustment of the model was significant (F3, 71 = 6.66, P = 0.0005). b) Average spine number per plant, per areola (X), and average TPCs in terms of tannic acid concentration (Y) per plant. There was no significant intersexual difference between either slopes (F1, 3 = 3.11, P = 0.08) or intercepts (F1, 3 = 0.42, P = 0.52) for this relationship. The relationship for the pooled data was Y = 6.52+0.164 X (F1, 74 = 11.87, P = 0.0009; adj. r2 = 12.66%).
Figure 5.
Intersexual differences in reproductive output.
a) estimated as seed number per fruit for all the plants sampled (P = 0.04). b) With exclusion of plants bearing only one fruit on one cladode (P = 0.02).