From the Digest

Cactus Coloration

By Joe Merkelbach (July 2010)

One of the valued uses of grafting cacti – and perhaps other succulents, although I do not know of specific examples – is to keep alive specimens that lack the ability to make their own food via photosynthesis.

Since I’m presently teaching a biology course chapter on photosynthesis, I thought I would use that as a springboard for this month’s notes.

Cacti mostly have unusual means of conducting photosynthesis. With the exception of primitive types like Pereskia, which have typical leaves; and Pereskiopsis, Maihuenia, Quiabentia and Austrocylindropuntia, which have succulent leaves; most cacti photosynthesize via chloroplasts in their stems. The opuntias, including all the genera in that subfamily, have small temporary leaves on new growth that soon fall off.

The chloroplasts are organelles of all green plant cells that are the sites of photosynthesis. They take in sunlight energy, the ultimate source of almost all energy for life on earth, and use it to convert carbon dioxide and water to six-carbon sugars and oxygen. These sugars and the many subsequently formed complex biological molecules are food for almost all forms of life.

Chloroplasts are essential to harvesting light energy, but they are not the only colored organelles within plant cells. Plastids, and chloroplasts are green plastids, contain pigments in plants for various purposes such as flower color.

In photosynthetic tissue, the chloroplasts are so predominant that they govern appearance, but other plastids colored red, yellow, purple, brown, etc. also are present. When chloroplasts are few to absent, the other colors come to the fore. The best-known example of this shift is fall coloration, when the plastids found in leaves throughout the growing season become apparent as chloroplasts diminish in the autumn.

The white, yellow and red stems of some cacti lack chloroplasts but have other plastids. They have just enough energy stored in their sprouting seeds to begin to grow, but will die unless they are grafted to stock that can photosynthesize enough food to supply energy.

The “redtop,” Gymnocalicium mihanovichii cv. Hibotan, is the most commercially successful grafted cactus. The natural species has a purple coloration that probably manifests whenever chloroplasts are absent. Some G. mihanovichii exhibit partial green, partial red coloration, probably the naturally occurring interim step that allowed human selection of oddities to graft and preserve as a line.

Many other choice species give rise to variegated color stems, which are the frequent targets of successful grafts to achieve added interest for cacti and succulents.

Bioportal –