Floral characters and breeding systems in the dimorphic annual plant Diptychocarpus strictus (Cruciferae)

Abstract

Diptychocarpus strictus is an early spring ephemeral plant mainly distributed in Junggar Basin of China. It is one of the few species of Cruciferae that has heterocarpy on the same raceme. The upper and lower flowers in a raceme develop into two kinds of siliques that differ in morphology and in manner of dehiscence. This paper deals primarily with the floral syndromes and breeding system of D. strictus. The results can be summarized as follows. (1) This species has two floral morphs, purple-flowered and white-flowered, and the color of flowers is stably inherited. The ratio of the two morphs within a population is close to 1:1. (2) There are significant differences between the two morphs in anther length of the longer stamens in the upper part flowers of a raceme and in length of petal and style of the lower flowers in a raceme (P0.01). Additionally, there are significant differences between the two morphs in length of outer sepals, petals, filaments of longer stamens, ovary, and style between upper and lower flowers in a raceme (P0.01). The data of morphological characters of the upper flowers in a raceme are larger than those of the lower flowers. (3) Number of pollen grains per flower in upper and lower flowers of a raceme in purple-flowered and white-flowered morphs are 7589.07±976.64 and 5428.93±900.45 and 7044.73±557.37 and 5138.60±813.48, respectively; number of ovules per ovary, 31.83±3.36 and 25.03±3.72 and 32.03±3.43 and 25.70±2.61, respectively; and P/O ratios, 241.31±41.85 and 222.33± 52.51 and 222.19±27.80 and 201.84±37.52, respectively. Number of pollen grains per anther is greater in the shorter stamens than they are in the longer stamens of the same flower (P0.01). Abortion rate of pollen is very low, and it differs between longer and shorter stamens (P0.05). (4) In the same raceme of the two morphs, the upper flowers have more pollen grains, higher P/O ratios, and more ovules than the lower flowers (P0.01). Also, there are significant differences between the two morphs in number of pollen grains per anther in the longer stamens of the upper flowers in a raceme (P0.01). (5) Pollen longevity and stigma receptivity barely overlap; the period of highest pollen vigor is just the same as the peak of stigma receptivity. Dynamic curves of pollen viability of the longer and the shorter stamens in the two morphs are similar. At the beginning of anther dehiscence, the pollen vigor of both longer and shorter stamens is 90%. (6) Few pollinators were present during flowering, and self-pollen could germinate normally on the stigma. In the two morphs, fruit-set was 88.2%and 85.7%, respectively, in no emasculation and bagging treatments, and 95.0% and 90.0%, respectively, under conditions of natural pollination, indicating that D. strictus is a facultative selfer.