How do plants sleep?
By Eduardo Fonseca Jr
It is quite common for aquarists to prolong the photoperiod to accelerate plant growth beyond 8 hours. In a way, plants grow longer with longer light periods, but that’s not entirely true. It is one thing to accelerate growth and another to keep them growing longer. They may look the same, but the effects of these two things are completely different.
Let’s look at some fundamentals to clarify a few things:
As studied quite extensively in Book 2, light does not directly imply growth but rather accumulation of energy. It is the “photo” phase of photosynthesis, what was formerly called the “light phase” of photosynthesis. Light basically produces energy for growth, but not growth itself. The “synthesis” phase of photosynthesis, or the “dark phase”, assimilates the carbon that will be destined for two different functions: the production of fuel by the production of sugars and the production of carbon skeleton structures, from which all the organic molecules are built up (remember that almost 50% of the dry weight of organic molecules is made of these carbon structures). Don’t confuse the “dark phase” as if carbon synthesis only takes place in the dark. It was called that because before it was believed that carbon synthesis (the Calvin Cycle operated by RuBisCO, so to speak) did not depend on light to occur, but today it is known that the enzyme RuBisCO has a light dependent component for work.
At that point, photosynthesis completes its role and then the plant is ready to grow. The metabolism that effectively involves the growth of plants is cellular respiration, which does operate much more strongly in the dark period. In fact, growth is still the result of respiration, but we don’t need to go into that dimension of detail. What we need to keep in mind is that plants grow in the dark and not in the light. Try leaving your tank in the dark for two whole days and see how the plants stretch. It is an elongated growth (estiolation) that is not wanted by aquascapers, but it can be very easily observed.
Okay, now we can get back to the subject.
A very long photoperiod can increase the development of plants, but after a certain point it can cause them a stress called photoinhibition and hinder the development of the aquarium in a very similar way to a person who does not sleep properly. The effects become even more harmful when the light intensity is stronger in order to stimulate a faster metabolic rate. Therefore, we can quickly conclude that the DURATION and QUALITY of the dark period is as or more important as the light period and needs to be respected.
Unfortunately, I dismissed the description of the plant’s mechanism for realizing “bedtime” as I found it too precious, but after realizing that many aquarists abuse the photoperiod in an attempt to energize the plants (and get into trouble), I felt I needed to write about this to clear up once and for all the nature of this misunderstanding.
Plants have a photoreceptor pigment called phytochrome, which has two modules, Fv and Fve. The Fv module absorbs the wavelength at 660 nm, while the Fve absorbs up to a maximum of 730 nm. If you look in the spectral band, you will notice that 730 nm is already at the boundary or already a little bit within the infrared spectra. When there is more absorbance by the Fv, ie long red light, in relation to what the Fve is absorbing, then the plant has active photosynthesis. For it, the day is clear. When this ratio inverts, that is, when the long red is no longer absorbed by the Fv, but there is still a lot of infrared being absorbed by the Fve, the plant realizes that the sun has set and it’s time to sleep, stopping photosynthesis. It is important to point out that phytochromes do not participate in photosynthesis like chlorophylls, they only guide plants in relation to the photoperiod.
Figure 1 – Absorption graph of the phytochrome modules.
Now we have two conditions, the natural and the aquarium.
In the natural condition, this ratio between 660 nm and 730 nm reverses slowly, taking about 2 hours to effectively “put the plants to sleep”. After these two hours, it still takes about 10 uninterrupted hours of darkness for the nocturnal metabolic cycles to complete. In other words, 12 hours of darkness are required for the plants to rest in a natural condition. Note that long days and short days due to the seasons influence this metabolism, causing certain plants to flower on short days, such as Cymbidium orchids and ipe trees that flower in autumn and winter, and others on long days, like most tropical angiosperms.
In the aquarium condition, in turn, there is no such inversion in the composition of the 660 nm and 730 nm spectra. There may not even be the 730 nm infrared emission in any significant amount. When the lights go out, plants simply take hours to be induced to sleep without Fve stimulation and may take even longer to complete the nocturnal metabolic cycle. We don’t know if the regularity of the aquarium’s photoperiod conditions plants to go into night mode more quickly, but in any case, we know that it takes at least 10 undisturbed hours of darkness for them to breathe and grow in peace. Based on this, it is not worth it to extend the light period beyond 8 hours to stimulate the plants and risk sacrificing their physiological cycle of maintenance and growth. It is better to increase the light intensity (increasing the system rhythm) and preserve the dark longer and healthier. It is known by the most experienced that 7 or 8 hours of light period is enough to get a strong and well-rested tank.
Figure 2 – Rotala raising the leaves as a sign of too much light or too long a photoperiod.
There are already luminaires that simulate this inversion of 660-730 nm, but not for aquariums. They are used in indoor horticulture to speed up consumption production or encourage flowering. These luminaires make the inversion ramp much more accentuated, which reduces the “sleeping” time of the plants from 2 hours to just 15 minutes. Particularly for aquariums, I don’t know where the advantage of this would be, as in high-rate systems there is already the stress of over-pruning and accelerating growth even more can age the aquariums faster. However, when it comes to improving the quality of the dark period of plants, we do not know what effects this could have on us. Either way, I believe the benefits would be irrelevant.
Already anticipating the doubts, some would ask if there is a problem for those aquariums that light up at night and spend the day in semi-darkness taking indirect light from windows and balconies. Yes, totally. These plants are active photosynthesis without CO2 and are not sleeping well. They may not grow very well, respond poorly to pruning, and may not redden easily because they may be suffering at some level of photoinhibition, not to mention the photorespiratory problem we won’t address here (this one is well described in Book 2 and its consequences in Book 3). The ideal is to adjust the photoperiod to minimize the time of disturbed darkness that can be easily noticed with plants leaning towards the unwanted light source, which is clear evidence that they have active photosynthesis in a period that does not should be. In cases where there is a lot of unwanted light exposure it may be a good idea to cover the tank with dark fabric or blocker. I know it’s not pleasant to do this, but for some this may be the only solution to synchronizing the time they are at home with active light in fertilizing conditions (always fertilize in light) and the period of darkness without disturbance.