In my last post (06-10-21 Sticky Geranium) I noted that the sticky geranium is a protocarnivore. That was a new term for me. So what is a protocarnivore?
A true carnivorous plant has visual or scent cues to attract and capture prey, actively digests the insect with its own enzymes or resident microbes and absorbs the products of digestion using specialized structures. On the opposite end of the spectrum a photoautotroph is totally capable of synthesizing its own food from inorganic substances and light via photosynthesis. In between these two extremes are plants that incorporate mechanisms found in both fully autotrophic and fully carnivorous species.
There are many terms, in addition to protocarnivore, for these plants: paracarnivore, subcarnivore, near carnivore and borderline carnivore to name a few. Generally protocarnivores grow where there is a significant nutrient deficiency, but not the severe lack of N and P seen in environments where full carnivores grow.
In 1999 George Spomer found that sticky geraniums exhibited protease activity, dissolving proteins that became trapped on their sticky surfaces. If C14-labeled proteins were placed on the plant surfaces, the C14 was absorbed into the plants. Whether this process is carried out by enzymes manufactured by the plant itself or by surface microbes was not determined. But it does suggest that sticky geranium can to some extent digest proteins and absorb nutrients, yet these plants are not directly capturing, digesting and absorbing nutrients like totally carnivorous plants – a protocarnivore.
It is suggested that many plants with glands, surface hairs and sticky surfaces may be protocarnivorous.