Using chlorophyll fluorescence as an indicator of heat stress on fruit crops.

1. Chlorophyll fluorescence as part of the photochemical process.

1.1. Photosynthesis and chlorophyll fluorescence.

The absorption of light to perform photosynthesis by photosynthetic autotrophic organisms is carried out by pigment-protein of light harvester complexes (LHCs) which are associated with reaction centers of photo-systems (Mülleret al., 2001). The LHCs. are embedded in the thylakoid membrane of green plants, algae and cyanobacteria, the charge separation and secondary electron transport for carrying out photochemical process (photosynthesis) is performed on the reaction center (RC) of photosystem II (PSII .) Reaction centers are surrounded by proteins from the center of antennas, capture and share the light absorbed by chlorophyll molecules in reaction centers efficiently (Andrizhiyevskaya et al., 2005). 

The energy received by the chlorophyll molecules from sunlight, can be delivered by 3 different ways: it can be used to carry the plant photosynthesis (photochemistry), other pathways may be heat dissipation, or can be re-emitted as light (chlorophyll fluorescence), the latter occurring when there is excess of light energy (Maxwell and Johnson, 2000), or when the photosynthetic apparatus has some sort of stress damage. Müller et al. (2001) notes that there is an extra option, indicated as a non-photochemical process by forming a triplet state of the molecule of chlorophyll (Chl*), which is responsible for producing a reactive oxygen species (O2 *), which constitutes a species which is highly damaging to the photosynthetic apparatus (Photo-oxidation).

Therefore one can conclude that the energy dissipation pathways mentioned before (photosynthesis, heat emission, fluorescence and triplet formation) occurs in competition, and an increase in one leads to a decrease in another. What already makes it clear that from a certain level, photosynthesis and chlorophyll fluorescenceare competing processes. 

According to Maxwell and Johnson (2000), presents the advantage that the measurement used to estimate chlorophyll fluorescence abouet photosynthesis in relation to the method that is based on CO2 fixation is that it indicates the functioning of photosystem II as acritical point of photosynthesis , while absorption of CO2, it is not representative, because this process competes with many others and their estimate is very biased.

1.2. The origin of chlorophyll fluorescence.

As mentioned before, during photosynthesis, photons (light particles) from solar energy are absorbed by complex systems called antenna pigments, which are responsible for capturing and channeling the energy to the photochemical function of plants. 

Photosystem II has been noted for long as the key link weaker, and the chief architect of chlorophyll fluorescence, but the damage in photosystem II occurs only at high temperatures, generally about 45 ° C (Sharkey, 2005). In fact Ducruet et al. (2007) indicates that Photosystem II is the only one with an ability to perform variable fluorescence, and fluorescence yield depends on 2 forms of energy dissipation processes, the photochemical dissipation process (photosynthesis) and non-photochemical dissipation processes (heat emission, fluorescence and triplet formation).Therefore, chlorophyll fluorescence occurs at the level of photosystem II. 

1.3. Fluorescence as a means of energy dissipation.

As noted above the chlorophyll fluorescence is one of the ways by which plants dissipate energy, the light absorption spectrum is therefore different from the spectrum of fluorescence. Sharkey(2005) notes that the fluorescence emission due to the decoupling of photosystem II, is a way to avoid damaging the entire system of post-cascade reactions, which occurs once the plastoquinone QA accepts electrons from the photolysis of water meditated by the solar energy absorbed by chlorophyll. This is due to the indications of Müller et al. (2001), because when the brake is not generated oxygen species produce a very high damage power, which can irreversibly damage the entire cell apparatus, thereby compromising the homeostasis, and plant life. So in a heat stress event, for any plant, is much cheaper, from a physiological point of view, decoupling the system to rebuild damaged structures and detoxify reactive oxygen species. To Allakhverdiev et al. (2008) this is very important because the damage produced by stress may be due to 2 reasons, first is the denaturation of proteins, and the second by the inhibition of de novo protein synthesis because of the reactive oxygen species. 

Image 1.- Light absorption and chlorophyll fluorescene emission. Plant Physiology. Taiz-Zeiger.

1.4. Processes associated with chlorophyll fluorescence.

As a result of physiological adjustments that occur in the plant, chlorophyll fluorescence can be measured as a derivative of heat stress, including changes that occur in the plant associated with it, we can mention a stimulation of dark reduction of plastoquinone, and increased cyclic electron flow to light, also increases the leakage of electrons from the thylakoid, there may be adeactivation of Rubisco (ribulose 1.5 bi-phosphatecarboxylase- oxygenase), and the generation of reactive oxygen species such as the O2 * and H2O2. In this line Sharkey (2005) indicates that isoprene is the function of providing thermo-tolerance through its action on reactive oxygen species, indicating that this action is very fast, but has serious implications on carbon stocks in woody plants. Changes also occur in the lipid composition of thylakoid membranes and the supply of isoprene, which has a role in responding to heat stress (Sharkey, 2005), in this sense Ducruet et al. (2007) notes that the membrane has the ability to pass such a fluid membrane liquid-crystalline to solid gel type, it generates radical changes in membrane permeability, which decreases the ion exchange and has its effects on photosynthesis. One line of research has developed in the study of the activity of heat shock proteins (HSPs), which were earlier identified as responsible for the responses to different types of stress, especially in the heat stress (Kotak et al. , 2007). Regarding the importance of HSPs and chaperones, Sun et al. (2002) indicate that the great diversity and unusual amount within the plant makes them seem very important, this is reflected in the need of plants to adapt to different types of stress, HSPs are indeed able to assemble, translocate and degrade many proteins, stabilize membranes and proteins under stress conditions (Wang et al., 2004). In this case Wang et al. (2004) notes that for the plants is very necessary to maintain the native conformations of proteins, and that they are not distorted, as this allows cells to survive under stress conditions. Schrader et al. (2007) note that the pool of ribulose 1.5bi-phosphate drops sharply when plants are rapidly subjected to a heat stress, but can recover if they are kept at such temperatures, which may indicate a denaturation of the RUBISCO in response to stress, but says it is a mechanism to avoid the damage that this can generate, through the production of free radicals by its oxygenase activity. 

1.5. Measurement of chlorophyll fluorescence.

"The basis on which underpin the analysis is quite simple chlorophyll fluorescence" (Maxwell and Johnson, 2000) and indicate that chlorophyll fluorescence measure allows us to understand the behavior of photosystem II.

The 3 processes of dissipation of light energy that reaches the leaf (Photochemical, heat, fluorescence) occurs in competition with each other, therefore an increase in one or another to the detrimentof the others. Thus the measurement of the efficiency of any of them delivered immediately an idea of the behavior of other processes (Maxwell and Johnson, 2000). 

To measure chlorophyll fluorescence, Maxwell and Johnson (2000) note that one should "turn off" one of the 2 mechanisms by which energy is dissipated in the photosynthetic systems, mainly the photochemical mechanism goes through the darkening of photosynthetic material so it breaks up the entire electron transport mechanism that allows the realization of photosynthesis, this serves to determine the maximum chlorophyll fluorescence (Fm) or in other words the minimum photosynthetic quantum efficiency (photon energy can be stored as energy chemical bond). The maximum chlorophyll fluorescence yield and maximum chlorophyll fluorescence (Fmax) is obtained by illuminating the dark material, which will have no way to dissipate energy faster than the fluorescence. After a few minutes, the system restores electron transport and chlorophyll fluorescence yield reaches a stable minimum (Fmin). The difference between Fmax and Fmin (Fv = Fmax-Fmin) determines the variable fluorescence yield or Fv. The grace of Fv, as its name suggests, is a value that can vary over time between 2 minimum and maximum values​​, depending on the conditions in which photosystem II is. This variation is determined by the Fmax, since depending on the level of stress that has the photosystem, it can increase your rate by increased levels of chlorophyll fluorescence (energy dissipation that competes with the photosynthetic efficiency). Photochemical yield, or quantum yield of photosynthesis (quantum yield) is determined by the ratio Fv /Fmax = (Fmax-Fmin) / Fmax, so if Fmax increases, the value of quantum yield would tend to zero, reflecting a decrease in performance efficiency of photosynthesis. According to Maxwell and Johnson (2000), for most species the optimal value of quantum yield is 0.83 (a value far from zero), and point out that lower values ​​indicate that plants have undergone or suffered naturally some kind of stress, especially by photo-inhibition or damage in photosystem II (photo-oxidation). 

Chlorophyll fluorescence is quite small in normal conditions, and comprises between 1 and 2% of total absorbed light (Maxwell andJohnson, 2000).

By having a different spectrum measurement can be performed by exposing a sheet or tissue at a certain wavelength and re-measurethe light emitted at a longer wavelength, but this measurement is only relative because there is always a loss of light in the measurement process. Measurement of chlorophyll fluorescence fluorometers can be done by both lab and field with the new portable models (Maxwell and Johnson, 2000). 

1.6. Changes in chlorophyll fluorescence.

Kautsky and colleagues found that exposing photosynthetic material from darkness to light, this very significantly increased the chlorophyll fluorescence yield in short periods of up to 1s (Maxwelland Johnson, 2000). Maxwell and Johnson (2000) note that due to a decrease in the acceptor of electrons in the supply chain, especially the plastoquinone, and very important plastoquinone QA. This has the ability to receive electrons from the photolysis of water and delivered one by one to the plastoquinone QB, to continue the chain of electrons. So while the QA have associated an electron (reduced) will be in a "closed"state and will not receive another electron until transferred the first, becoming so again in an "open" state. This means that an increase in the closed state of the plastoquinone A, allows a reduction in photochemical efficiency (photosynthesis) and an increase in non-photochemical efficiency (chlorophyll fluorescence). When the leaves are a dark state, its electron acceptor states are closed quickly when exposed to light, however, this state can last a few minutes because of the production of more acceptor and a concomitant increase in the ability to carry out the photochemical process. By increasing the pool of plastoquinone and electron acceptors, and the stimulation of stomatal opening by light, leading to restore the metabolism of photochemical reduction of CO2. Another reason is referred to an increase of dissipation through heat loss. 

2. Stress in higher plants.

2.1. Stress.

Stress is any factor that produce negative changes at the biochemical, physiological and molecular level (Wang et al., 2004). Stress can have 2 major sources, can be caused by living organisms, or can be caused by non-biological (environmental) factors, refered in the first case as biotic stress and abiotic stress in the second. This paper speaks only the second by being more closely related to chlorophyll fluorescence

Plants to be immobile organisms are always exposed to sudden changes in temperature and other abiotic stress constituents.

Wang et al. (2004) notes that both abiotic stress (such as drought stress, salinity, high temperatures, chemical toxicity) and oxidative stress are very serious for agriculture and also create major damage to the environment. Vinocur and Altman (2005) notes that are the most important crop losses worldwide. 

Abiotic stress, mainly produces a loss of functionality of proteins (Wang et al., 2004), this may be due to the loss of native conformation of proteins, albeit momentarily, let there be changes in pH or by temperature changes. 

2.2. The heat stress.

All living organisms are under stress from high temperatures when the temperature at which they are exposed is above the normal optimum development (Kotak et al., 2007). Wahid and Close (2007) defines heat stress as an increase in temperature for a fixed term, which is capable of causing irreversible damage to the development of plants.

The high temperature stress occurs when a plant interrupt its most important mechanism for heat dissipation, this is the way of transpiracional cooling, this can be developed by an event of water stress (Sharkey, 2005), stomatal closure not allow sweat, or other event that disrupts this pathway, such as the use of herbicides. Salvucci et al. (2004a, b, c) states that facilities with adequate access to water, keep their stomata open, transpiration cooling using to reduce the temperature of their leaves. In fact, this mechanism can generate a differential of up to 10 ° C between air temperature and leaf temperature in warm and semi-arid areas, with air temperature at the top. However, claims that this ability to respond to heat stress may be diminished by a high relative humidity and a low evaporative process (transpiracional) due to water stress. 

The leaves of the plants are mostly very thin and flat to increase photon capture area, however this quality entails having a minimal ability to regulate its temperature, and thus highly susceptible to sudden environmental changes, so thus, when exposed to all the solar energy can heat up substantially, over the air temperature.The use of thermocouples of fine wire has shown that low rates of transpiration the plants suffer frequent episodes of high temperature in the leaves, which can reach 15 º C above air temperature (Sharkey, 2005), and fluctuations may be the order up to 10 ° C in 1s (Schrader et al., 2007). However Allakhverdiev et al.(2008) notes that the changes that generate heat stress, will depend on the degree of development of photosynthetic organisms, and summarizes that the young leaves and older respond differently to heat stress. 

Photosynthesis in one of the physiological processes of plants more sensitive to heat stress (Sage and Kubie, 2007, Zhang andSharkey, 2009), the photosynthetic apparatus and electron transport chain are the main targets of temperature stress, is this cold or heat (Ducruet et al., 2007). According Allkhverdiev et al.(2008) there are three processes that are affected by stress caused by high temperatures, and these are, the photosystems and most important is the photosystem II, ATP generation and carbon assimilation process. But Sharkey et al. (2005) indicates that moderate heat stress (35 to 40 ° C) promotes little or no damage to PSII, while stating that the photosynthetic rate can be reduced to zero. This may explain why according Pshybytko et al. (2008) oxidative water complex is disrupted, leading to the electron transport stops and the energy can not be stored as chemical bonds, this has an effect on the reaction center and the LHCs of photosystem II.

The rate of photosynthesis increases gradually with temperature until it reaches a maximum, which represents the optimum temperature for photo-assimilation, then drops sharply by a high temperature stress, and before the optimum temperature may cause low temperature stress, even damage or become a sub-optimal temperature for photosynthesis, lowering the efficiency of this process (Ducruet et al., 2007).
Taub et al. (2000) notes that plants growing under higher CO2 concentration to better tolerate heat stress, both in laboratory and field.

2.3. The complexity of the response to heat stress.

Plants in their evolution have developed a number of responses to heat stress to prevent or minimize damage and ensure protection of the cellular balance (Kotak et al., 2007). Recent studies of stress responses indicate that the response to them is complex due to a stress condition never found itself in field conditions, therefore, the response may be not associated with only one factor , Since many ofthe responses are consistent for more than one condition and are not exclusive to just one. Thus physiological and metabolic responses to heat stress and water stress is the same (Mittler, 2006). 

The RUBISCO catalyzes the first steps of 2-way competitive, CO2 reduction and photo- respiration, which are determined by the carboxylase and oxygenase activity, respectively (Salvucci et al., 2004a, b, c).
Sharkey (2005) points out that even without damage to the photosystem be expected to decrease the efficiency of photosynthesis due to increased photo-respiration due to a rise in temperature above the optimum for this process in each species. Yet not all of this loss can be explained by the effect of photo-respiration. 

Damage to photosystem II occurs only at temperatures exceeding 45 º C, however this damage is reversible but they have a detrimental effect on the rate and thus on the photosynthetic efficiency (Sharkey, 2005), and the damage on photosynthesis is recoverable (Zhang and Sharkey, 2009). This irreversible damage may be due to increased RUBISCO oxygenase activity at high temperatures, which is detrimental to the carboxylase activity, which allows the reduction of atmospheric CO2 (Schrader et al., 2007). In this sense Salvucci et al. (2004a, b, c) state that the RUBISCO increases its catalytic ability, but his affinity for CO2 decreases, also decreases the solubility of CO2 compared to the solubility of O2 and increases its nature oxygenase, thereby limiting the potential increase net photosynthesis with increasing temperature.
Moderate heat stress reduces carbohydrate metabolism and electron transport in photosynthesis. (Zhang and Sharkey, 2009). 

3. The fruit plants.

3.1. Fruit plants and its origin.

Vegetables, so also fruit trees, have an optimum domain of temperature depending on their area of ​​origin, long-term adaptation is creating the appearance of ecotypes that somehow tolerate or withstand temperatures colder or warmer compared to their parents (Ducruet et al., 2007). 

To describe the species according to their origin in terms of temperature and behavior of the same to this variable, Ducruet et al. (2007) defines 5 domains: Domain of cold (1 to 18 º C), Domain of optimum temperature (18 to 30 ° C), Domain warm (30 to 40 º C), Domain of heat stress (40 to 60 º C) and Domain of very high temperature (above 60 º C). Most fruit species of tropical or subtropical origin, are damaged or even die in the cold domain, precisely because it is not resistant to low temperatures. In the domain of optimal temperature, the plants may be faced with a particular stress, which in this case the author draws the photo-inhibition and that critical point, also in this domain indicates that the plants reach their maximum photosynthetic efficiency (quantum yield). In the case of the Domain warm, one of the determinants of stress is the reduction of carbon assimilation, derived from the decrease in the affinity of RUBISCO with this substrate (CO2), and increased oxygenase activity, which generates reactive oxygen species, as well as significant changes occur in the thylakoid membrane with its effects on permeability, this is where it begins to increase chlorophyll fluorescence, and where are the signs of early damage indices of stress generated by high temperatures. In the domain of heat stress, as its name suggests, is rightfully where the plants are damaged by this factor and in which the plants are constantly subjected to damage mentioned before are related to chlorophyll fluorescence. And in the domain of very high temperatures, few of plant species that can cope with any acceptable degree, but at these temperatures, most plants show severe damage to the level of the thylakoid membrane, which is impossible carry out photosynthesis, and thus live and produce. 

3.2. Importance of temperature on fruit species.

Physiology, distribution and productivity of plants, including fruit trees, is controlled mainly by temperature, with this important effects on physiological activity at all spatial and temporal scales (Sage andKubie, 2007). Taub et al. (2000) add that it is responsible for growth restriction, and that with the climate change process, the plants will be exposed to heat stress events with greater frequency, either in natural environments and in agro-ecosystems.
C3 plants (including fruit trees) show an optimum temperature of photosynthesis ranging from 20º C to 35 º C, with a maximum carbon assimilation with a temperature below 30 º C (Schrader etal., 2007).

In the world heavy losses in the agricultural sector can be attributed to heat stress on crops, especially if accompanied by drought or other stress factors (Vinocur and Altman, 2005; Mittler, 2006, Kotak et al. , 2007). The heat stress is considered one of the major stress factors affecting biomass production and productivity, so much tropical countries, such as those subtropical and warm (Allakhverdied et al., 2008). Sharkey (2005) states that for every degree Celsius increase in average temperature of the growing season results in a decrease in yields an approximate value of 17%.
The inhibition of photosynthesis due to heat stress is common for crops in tropical and subtropical areas, and is experienced periodically in warmer areas (Salvucci et al., 2004a, b, c). Abiotic stress is the main cause of losses in agriculture worldwide, accounting losses that may reach up to 50% of production, or even more (Wang et al., 2004).

The leaf temperature in natural conditions of environment and therefore also in agro-environmental conditions, varies during the day, from one day to another even varies within the location that has the blade on the ground and the degree of its exposure to light (Huve et al., 2006).
Saakov (2001) notes that there is a depression in CO2 assimilation rate at midday, resulting from high temperatures, further states that this not only occurs in plants under conditions of desert environments, it is very important in species is developed in areas where summers are very hot. Furthermore, this author points out that if the plants are subjected to temperatures exceeding 35 ºC (optimum for photosynthesis) the photosynthetic capacity is affected up to 80%, and goes beyond saying that once restored the system, the plant is at capacity 70% of which had prior to the stressful situation. 

Mittler (2006) conducted a matrix (stress matrix) which linked the different factors that can offer some degree of stress to plants, and noted that from the standpoint of agriculture, heat stress has a high importance when combined with stress salinity, water stress, atmospheric ozone, pathogens, and ultraviolet radiation, while the damage by frost or cold (low temperatures) is unimportant, since they do not occur simultaneously, and it is not clear how important its combination with nutrient deficiencies that may exist.   

3.3. The domestication and its problems

Cultures are born or are forged through the emergence of agriculture. The man for thousands of years has been selecting plants and animals according to their own needs (food, shelter, wood, etc.). Therefore the problem of domestication has always existed and will continue to operate. Able to domesticate aspecies means selecting the best material to suit the new conditions of growth and development, and provide better conditions for their development. In the case of fruit plants life cycles are longer and varietal improvement processes last long, therefore, it is necessary to measure the damage that is generated in one species to change their place of origin, the ultimate purpose of determining management practices that enable the species to respond in the best possible way to obtain the expected benefits. 

4. Understanding the use of chlorophyll fluorescence as an indicator of heat stress on fruit crops.

4.1. Fluorescence as an indicator of stress

It is important to recognize that the chlorophyll fluorescence provides information about the state of photosystem II (Maxwelland Johnson, 2000). Changes in fluorescence indicate not only changes in the state of the photosystem, but also relates to extrapolating to other events occurring simultaneously that clearly indicate the condition of some kind of stress (see section 1.4).

4.2. Adaptation to heat stress.

In the short term, plants have the ability to adapt or acclimate to temperature (Ducruet et al., 2007). Wang et al. (2004) notes that adaptation to stress involves a series of stress-response mechanisms that act in a coordinated and even synergistic, to avoid the damage or to restore cell function. HSPs play a fundamental role in both normal growth and development in conditions of stress.  

Pshybytko et al. (2008) indicated that the more complex is the stress response system, the individual is more likely to adapt to stress conditions in the photosynthetic apparatus and the specific protection of photosystem II. This makes clear the importance of diversity in plants of proteins HSPs.Kotak et al. (2007) notes that the primary means of adaptation in the evolution of plants, has been forged through conservation and diversification of HSPs proteins, of which there are groups of location or action at the cytosolic, nuclear, mitochondrial, chloroplast , at the rough endoplasmic reticulum, etc. each of which helps protect individually or synergistically to different organelles and cell compartments. The conservation of these proteins have been carried out through the conservation of transcription factors (HSFs), which are activated once the plant is some kind of stress, particularly an event of heat stress. The role of these proteins is to prevent damage to the native conformation of proteins and enzymes so the plant does not lose its physiological activity, on the other hand are able to renature the proteins that have lost their quaternary structure, so that we can recover some of the damage caused by stress. Huve et al. (2006), for its part that the power of plant adaptation is focused on to respond optimally to the average temperature to which they are exposed during a given period, making it clear that under no circumstances, plants will to adapt to the maximum temperature at which they faced look. This, as far analyzed, it would be very easy to understand, because, if they were tailored to withstand temperatures above average, would be demanding his entire apparatus protein enzyme and respond to higher temperatures, which is not an advantage from a viewpoint of enzyme kinetics. The same author notes that a more efficient way for the plant is to use the xanthophyll cycle to change the stiffness of the membranes and allow cells to be more or less permeable depending on their need versus temperature. 

Huve et al. (2006) notes that a change in sugar concentration also creates a greater resistance to stress, which may be related to reports of Zhang and Sharkey (2009), who observed a pattern of change in carbon metabolism. And part of this discussion may shed light on the possibility that the chloroplast of storing large amounts of starch. We can say that there would be readily available way against thermal shock, which would encourage the cells to lose water to enable evaporative cooling, but can not lose as much water as it could have if accompanied plasmolysis water stress (and it is usual that) (Mittler, 2006). Therefore this carbon could be used to generate compatible solutes that allow much more water bind to the cell under stress. Otherwise Mittler (2006) notes that the occurrence of an event of water stress and heat stress involves stomatal closure, increased leaf temperature, a loss of photosynthetic capacity and high photo-respiration. 

The ability to recover from the damage of photosystem II is to Allakhverdiev et al. (2008) the basis for acclimation that may occur in plants under stress conditions. The same author notes that there are six important factors in tolerance to heat stress, listing the following: HSPs, antioxidants, unsaturated lipid membranes, protein stability to heat and solute accumulation. However for Vinocur and Altman (2005) adaptation to stress is given by the cascades of reaction networks in perception, signal transduction, and expression of specific genes and metabolites related to stress.

4.3. The fluorescence and fruticulture.

This section will discuss the application in fruit can have the use of the measurement of chlorophyll fluorescence in relation to the information supplied on the condition of heat stress. It was already clear the relationship between chlorophyll fluorescence and photosynthesis, also noted the complex processes that occur with increasing temperature leads to higher fluorescence. So, are related fluorescence measurements with different types of damage that can be inferred, are occurring in the plant through the increase in fluorescence with temperature. Knowing that there are increases in fluorescence indicates that stress and ultimately production is being disadvantaged. In fruit, as in all areas of agriculture, is not desired stressed plants, since plants that means they are not expressing their maximum productivity. Plants that do not express their maximum productivity, lead to crops that low yields are obtained, which determines lower revenues per unit area. So every time you increase the fluorescence one can think of a decline in economic and productive efficiency of the system under analysis.This is very important in the search for new orchard management technologies, which allow to decrease the damage created by excess heat.

Conclusions.

The use of chlorophyll fluorescence as an indicator of stress can have a view of the state in which photosystem II is therefore derived from this, the capacity at which the plant as a whole is responding in terms of photosynthetic assimilation.A plant with good indicators of chlorophyll fluorescence (near 0.83) means that it is a plant that is free of stress in photosystem. A plant that works to the highest level with their photosystems, is a plant that can have an optimal quantum yield. With an optimal quantum yield, the plants can easily meet your needs for photo-assimilated and less appropriate to partition, which ensures that no fruit or no structure shall be sub-optimal conditions. Measurements of chlorophyll fluorescence in its latest order, measure the degree of photoinhibition in which plants are, therefore, the degree of stress you have. The usefulness of this indicator can be, use it to find maneuvers by which minimized the damage to the days when both radiation, water supply and temperature are limiting, this is of vital importance for the fruit crops that develops in Mediterranean-type climates, semi-desert and even in tropical climates. This technique will determine the optimum temperature for photosynthesis of all species, the light level (PAR) needed to have a photosystem not stressed, but at its maximum quantum efficiency. In short term, can be used for research purposes, but the contribution that provides for better performance of plants is very high value because it is often easy to determine when species occur or vegetate bad stress conditions most commonly known (water stress, nutrient deficiency, pH, chemical toxicity, etc..), however, there are many situations where these factors are handled well and even then not achieved optimum yields. In the future, no one can doubt that the use of remote sensing technology and global positioning, given the possibility of generating two-dimensional images of gardens and natural ecosystems, and the use of these technologies, in most cases is related to the fluorescence emission from the elements investigated. Therefore, today it is necessary to understand how you can use the reference that delivers state of photosystem chlorophyll fluorescence in order to develop better design and management of orchards. Tomorrow, it will be possible to get this technology in real time by computer and every day to estimate the effect of photoinhibition stress as it happens, either through excess of light, temperature or water deficit.

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