How did an understanding of angiosperm reproduction allow floriculturists to develop a commercially successful poinsettia?

Poinsettias are short-day plants, using the long nights of winter as a signal for flowering. In fact, the poinsettia’s inductive dark period of 14 hours was among the first described in a flowering plant. To control photoperiod, the plants are grown in greenhouses, where the photoperiod is carefully regulated.

A second important factor in the poinsettia’s commercial success was the plant’s growth habit. In Mexico, the wild relatives of cultivated varieties grow up to 3 meters tall, with few branches (compare these plants with the 0.5-meter-tall plants in the photograph at the opening of this chapter). Paul Ecke found a variety that was much shorter and formed an attractive branching plant with axillary shoots. These compact poinsettias were initially propagated asexually by grafting to native plants.

Many new varieties of poinsettias have been generated by conventional sexual reproduction. You can look for the phenotypes of these varieties next holiday season. They include stiff stems, new colors (such as white), and longer lifetimes for the colorful bracts. How do you think these characteristics have been introduced into the cultivated varieties?

Future directions

Sequencing of the genomes of agriculturally important plants and genetic screening of mutant strains have led to an accumulation of knowledge of genes controlling the transition to flowering. Efforts are under way to use this understanding to breed crops that produce more seeds more often, by conventional means or biotechnology. This has enormous potential for human welfare. Consider legumes, which are in two general clades, most represented by peas and soybeans. The former is a long-day plant that responds to cold (vernalization), while the latter is more acclimated to warm climes and responds to short days to flower. Genes for the photoperiodic response and signal transduction for flowering have been identified for both species. For example, genetic changes in signaling could allow the plants to have more vegetative growth prior to flowering, which might lead to more floral meristems, more flowers, and more seeds.