More than 20 years ago it was reported a set of pioneering studies that emphasized the importance of the relationship between elevated temperature, light and photosynthesis in plants, showing that adaptation of the daily metabolism to periods of a higher heat is advantageous because it allows the acclimatization of the plants to a superior luminous intensity, that occurs in the summer period.
The balance between light capture and its use in the photosynthesis is influenced by oscillations of environmental factors, and this balance was suggested as key for the regulation of the biotic and abiotic interactions in plants. This means that there must be something that integrates these relationships and can distribute its effects. The discovery of a group of genes that results in the regulation of the activity of a molecule (plastoquinone) that links the reactions of energy synthesis to the reactions of carbon fixation synthesis, both during the photosynthesis, allowed to consider that there is a “conductor” to maintain oxidative stress homeostasis in plants in the contexts of an active photosynthesis.
If high temperature affects firstly the plastoquinone, then the light reactions would be the first mechanisms to be affected and the acclimatization to the luminosity would be obligatory in the short term, because if there is more energy and it is obtained through the light, it is necessary to reduce the ability to capture light in order to reduce the amount of energy obtained by it. It is no coincidence that plants with a metabolism adapted to higher temperatures seems to have in common high evapotranspiration rates and a photosynthesis rate correlated with the luminous intensity except in the periods of greater luminosity, in which some can drastically lower the photosynthesis. Even when exposed to combinations of water and heat stress, the first response of the plant is to cool the leaves, although it is at the expense of water loss, that only after is mitigated, often with osmotic adjustment.
Water loss mitigation needs to be done at the expenditure of a reduction of the evapotranspiration, that is, through stomatal closure, which obviously implies a fine regulation between the control of this process and the control of the leaf cooling, since one occurs at the expense of the other. This regulation seems to be achieved by the balance between the levels of free absicisic acid (ABA) and salicylic acid (SA), and when the ABA dominates we have a bigger closure of the stomas and the consequent decrease of the cooling rate of the leaves; when the SA dominates we have a higher rate of evapotranspiration at the expense of a higher water loss. It turns out that a higher xylemic sap flow, as a consequence of the SA domain over ABA, also seems to be a motor of the hormonal balance beeing completely different from that established when the ABA dominates over SA and in this context the mechanisms of dissipation of excessive energy that accumulates in the energy generation phase of photosynthesis and occurs in thylakoid membranes.
Several studies have been reveling that the submission of plant to a moderate heat, within the range that we can be considered for each species, can largely benefit its ability to adapt to both high temperatures and high light intensities, something that occurs annually in the spring period and that occurs daily in the adaptation to the daily light and thermal amplitudes. More quickly and efficient the plants respond to these variations, better will be their ability to adapt their metabolism in a timely manner and better will be their survival ability. And this could be achieved through plant “practices”, which we call “acclimatization”, and that are always carried out, for example, when moving the plants from the greenhouse to an external environment. However, challenging plants with controlled factors results in a bigger factor-driven adaptation. The main result is, not only the acclimatization to two environmental factors (light and temperature) but also to a third factor, the attack of pathogenic agents, particularly biotrophics. Maintaining SA domain over ABA, promoted by the maintenance of high xylemic flow and mechanism of excessive energy dissipation in thylakoids, seems to stimulate the immunity of plants and contribute to generate a hypersensitive response, or amplify that they already had, an infection in the initial phase or control the number and size of infection foci in plant structures. In conclusion, agricultural production may benefit a lot, in the short and medium terms, from a technological development that allows to capture and retain more water under various physical states and to promote acclimatization-driven plants to face global warming.