It used to be rare to come across plants that bind CO2 mainly at night: CAM plants. But it’s no longer an exception in the horticultural sector. Nowadays the best-selling pot plant in the Netherlands – phalaenopsis – belongs to this group. This then raises questions such as: When do CAM-plants behave according to the book and when don’t they? And when does it make sense to dose with CO2 and provide lighting?
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Plants that use the most common photosynthetic tactic open their stomata during the day. They take in CO2, which together with water is converted in a series of processes (the Calvin cycle) into sugars, with the aid of the enzyme Rubisco. This requires energy and this comes from the sun.
Own death sentence
If CAM-plants did this in the wild they would be signing their own death sentence. They mostly grow in hot, dry conditions where it is too dangerous to keep their stomata open during the day. They would dry out in no time. Therefore they have another tactic. They open the stomata at night when it is cooler and moister and then they take in CO2. With the help of another enzyme – PEP-carboxylase – they fix CO2 and store it in the form of malate in the cell vacuoles.
However, its processing into sugars via the photosynthetic process cannot take place at night. This requires light (energy). So there is a limit to how much CO2 can enter: At a certain moment the storage capacity is full. When it becomes light the CAM-plant closes its stomata again to prevent it drying out.
The malate that was stored in the vacuole during the night goes to the chloroplasts. There photosynthesis and sugar formation occur in the usual way. So, as soon as the stored malate is used up, photosynthesis stops. CAM plants therefore are not very fast growers.
Slow growth
Of course in practice it’s a lot more complicated than the theory. Some plants are strictly CAM; others can also switch to the ‘normal’ method of photosynthesis (which is called C3-photosynthesis). Research conducted at Wageningen UR under the supervision of Tom Dueck, shows that young phalaenopsis leaves behave in the C3-way while the older ones behave in the CAM-way.
Probably the consistent selection for good growth has ensured that few horticultural crops are strictly CAM-plants. CAM is accompanied by slow growth and the best growing specimens will therefore often have C3-traits. It’s clear that for growers the C3-way is more attractive: The plant grows faster and decisions about cultivation measures (such as CO2 and lighting) are easier.
From CAM to C3 or vice versa
There are two issues of note: The CAM-phases and the factors that cause CAM to switch to C3 or vice versa. CAM-plants normally have four phases:
1. Night: The stomata are open, incoming CO2 is tied up in the form of malate.
2. Early morning: The activity of the enzyme PEP-carboxylase reduces and that of Rubisco (under the influence of light) increases. Both enzymes are temporarily active. In other words: Photosynthesis takes place while CO2 is fixed as malate.
3. During the day: The stomata are tightly closed. The stored CO2 (in the form of malate) is used in photosynthesis until it has all been used up.
4. End of the dag: The malate is all gone. The stomata open and, temporarily, the incoming CO2 is immediately used in photosynthesis in the C3-way.
Opportunity for selection
The relationship between these phases is different per species and per cultivar which opens an opportunity for breeding and selection. In addition, these phases can be influenced by cultivation measures. Certainly a good water supply is important. Then the plants take up more CO2 in the light. Lack of water leads firstly to a reduction or an elimination of phase 4, then phase 2 disappears and then, during a more serious water shortage, phase 1 is delayed by a few hours so that eventually only a little CO2 is taken up. During a very severe drought plants go into survival mode and close their stomata both day and night.
Switch or shift of phases
Just as interesting is that some species can switch between C3 and CAM. Champion in this area is Clusia, a genus of trees, of which one species – the Clusia rosea – is grown as a pot plant. Within a few hours they can switch from one type of photosynthesis to the other.
The main factors that affect this are temperature, water and light. The greater the difference between the night and day temperature, the more the plant shows its CAM-side. Drought also leads to CAM-tendencies. High radiation has the same effect, but only during water stress. So a good water supply (and perhaps a higher RH) prevents the undesirable transition.
German research shows that blue and UV-A light are responsible for the switch. If the plants only receive light with a wavelength above 530 nm they remain in the C3 position. There is also the question whether it is really a switch or a shifting of the phases. If phase 2 and 4 are very pronounced and phase 3 isn’t, it looks exactly like C3, apart from the CO2-uptake at night.
One clear switcher is known among Bromeliaceae, namely Guzmania monostachia. When the water supply is good it chooses for C3. But also other species such as the Vriesea seem to be able to switch.
Production regime for each species
The young plants of Kalanchoe blossfeldiana behave in the C3-way. The older plants show the CAM-characteristics. It is therefore important to take this into account during the cultivation measures. In addition, a long day encourages the plants to remain C3 and a good nitrogen supply also helps.
When should you dose CAM-plants with CO2 and provide additional lighting? It’s clear from the account above that there’s no simple answer. The principles are clear: It only makes sense to dose with CO2 if the stomata are open and provide lighting if assimilation is taking place, in other words while malate is available. But the splitting into phases and the transition to C3 make the job complicated. In fact, each species (and sometimes even varieties) need their own production regime.
Summary
CO2 enters CAM-plants at night and is processed during the day via photosynthesis. This process has four distinct phases. Due to constant selection we see very few strictly CAM-plants in the horticultural sector. Usually they exhibit a mixture of CAM and ordinary photosynthesis. It only makes sense to dose CAM-plants with CO2 if the stomata are open and provide lighting when assimilation is taking place.
Text: Ep Heuvelink (Wageningen University) and Tijs Kierkels. Photos: Wilma Slegers
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