Increase in the CO 2 concentration till it attains 5 c. At this concentration of CO 2 , energy is just sufficient to decompose it but not more.
If the concentration of CO 2 is raised still further, the rate of photosynthesis does not increase since light is now the limiting factor. When the light intensity is increased, then a higher concentration of CO 2 will be decomposed and the rate of photosynthesis increases till light again becomes a limiting factor.
Figure represents the whole concept graphically. This experiment evidently shows that the photosynthetic rate responds to one factor alone at a time and there would be sharp break in the curve and a plateau formed exactly at the point where another factor becomes limiting. For instance, with lower concentrations of the limiting factor e. Biology , Plant Physiology , Photosynthesis , Factors. Top Menu BiologyDiscussion. This is a question and answer forum for students, teachers and general visitors for exchanging articles, answers and notes.
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Others Others. Factors affecting photosynthesis Three factors can limit the rate of photosynthesis: light intensity , carbon dioxide concentration and temperature. Light intensity Without enough light, a plant cannot photosynthesise very quickly - even if there is plenty of water and carbon dioxide. Carbon dioxide concentration Even if there is plenty of light, a plant cannot photosynthesise if there is insufficient carbon dioxide.
Temperature If it gets too cold, the rate of photosynthesis will decrease. It has been observed that increased photosynthesis under e[CO 2 ] results in greater production of certain carbohydrates compared to others. The concentration of sucrose, the main product of photosynthesis, increases in all organs of pea plants exposed to e[CO 2 ] in growth chambers, however, glucose concentrations are largely unaltered Aranjuelo et al.
Glucose measurements may be inaccurate as glucose content can fluctuate throughout the day in some plants, increasing and then decreasing as the day progresses Seneweera et al. As such, hexose to sucrose ratio will differ depending on what time period the glucose levels are measured. Glucose measurements taken when glucose levels are naturally low, will give a lower hexose to sucrose ratio than if glucose was measured during a period of high glucose levels.
Sucrose levels also increased in castor oil plants grown in growth chambers under ppm CO 2 compared to ppm, increasing by an average of one third Grimmer et al. Levels of sucrose are higher than that of hexoses under e[CO 2 ] in both chamber and field studies Grimmer et al. Perhaps, such variation in hexose to sucrose ratio during plant development may affect plant source and sink activities. In addition, the preference of a plant to produce one type of carbohydrate over another could potentially be linked to the control of genes by a specific carbohydrate glucose, sucrose, etc.
For example, if a plant requires the presence of sucrose to initiate the repression of a specific gene, it would be ineffective to produce greater glucose quantities than sucrose. The effect that carbohydrates have on gene expression is a topic discussed further in this review, however, the impact that a change in sugar composition has on plant gene regulations is not well understood. Starch, a major storage carbohydrate in plants, is also increased in plants growing in e[CO 2 ] Aranjuelo et al.
The increase in starch likely contributes to the high levels of sucrose observed with e[CO 2 ], due to the conversion of starch to sucrose overnight. This conversion is important for normal plant growth under ambient conditions Smith et al. In plants grown under ambient [CO 2 ] the starch content builds up during the day and disappears overnight.
The increased production of starch under e[CO 2 ], however, means that not all of the plant's starch reserves are depleted during the night, leading to a gradual accumulation in leaves over time Grimmer et al. Different plant species accumulate different amounts of sucrose compared to starch, for example spinach accumulates more sucrose and cotton more starch Goldschmidt and Huber, These responses are likely to affect the sugar sensing pathways in either type of plant.
The degree of carbon partitioning between sucrose and starch is influenced by the length of daylight. In shorter periods of light, carbon partitioning shifts toward starch synthesis, while sucrose synthesis and consumption is decreased Pokhilko et al. Less starch is accumulated during days with long light periods, while sucrose synthesis is increased Pokhilko et al.
Sucrose content is greater during the day than night, but the amount of sucrose remaining at the end of the day, as well as the end of the night, decreases as day length decreases Sulpice et al. The degradation of starch at night is influenced by the amount of trehalosephosphate T6P. Increased T6P was found to inhibit starch degradation at night in Arabidopsis plants, resulting in much higher starch reserves at the end of the night Martins et al.
In addition, Martins et al. As such, increased T6P concentrations result in more starch at both the end of the day and night. Combined with limitations on starch degradation set by the plant's circadian clock, these findings suggested a model for overnight starch metabolism Martins et al. High sucrose demand causes lower T6P, alleviating the inhibition of starch degradation and increasing sucrose content. Under low sucrose demand, T6P increases and inhibits starch degradation.
The plant's circadian clock prevents the total depletion of starch at night by setting limits on starch degradation based on the length of the night period Martins et al.
The extra carbohydrates that accumulate in leaves are allocated to the rest of the plant in varying amounts, where some organs receive more of these carbohydrates than others. Little research has been done into the allocation of carbohydrates under e[CO 2 ], but the following studies have investigated this. Carbohydrate allocation under e[CO 2 ] varies with species. Some species allocate more carbon to the seeds and others to the shoots, leaves or roots Salsman et al.
For example, during the grain filling stage of rice e[CO 2 ] promotes the translocation of carbohydrates stored in vegetative tissues to the panicle, as well as allocating newly fixed carbohydrates to the panicle, where it is stored as starch Sasaki et al. A difference in carbon allocation between durum wheat and bread wheat occurs under e[CO 2 ]. Durum cultivars Blanqueta and Sula allocated more carbon into roots, rather than shoots Aljazairi et al.
Furthermore, Sula a modern cultivar allocated more carbon into spikes compared to Blanqueta a traditional cultivar , which allocated more carbon into non-reproductive shoot tissue. This indicates that variation exists within as well as between species and suggests that genetics contributes to these differences. In the case of the two durum cultivars, both differed in yield potential. Sula, which invested more carbon in spikes, is a higher yielding wheat than Blanqueta. Elevated [CO 2 ] also increased growth of roots and shoots of tepary bean, where the roots saw a ten-fold increase in starch Salsman et al.
Carbon dioxide concentration is not the sole regulator of carbohydrate partitioning, with many other environmental factors involved in shaping the outcome. Which carbohydrate the increased carbon is partitioned into can be affected by the method plants use to take up nitrogen.
An experiment by Aranjuelo et al. As e[CO 2 ] is known to affect the uptake and assimilation of N in plants Bloom et al. Plant growth method glasshouse, field, etc. Elevated [CO 2 ] causes increased carbon allocation to roots of perennial rye-grass resulting in increased root dry matter when grown in field conditions, however, no such results occur when grown in controlled environment chambers Suter et al.
This outcome in rye-grass was attributed to a difference in N availability, plant age and shoot sink strength. Results from Aranjuelo et al. This could mean that control of carbon allocation could be partially affected by the availability of carbon sinks.
Another factor that may affect the allocation of carbohydrates under e[CO 2 ] is the effect e[CO 2 ] has on leaf area, as appeared to be the case for N allocation in rice Makino et al. Plants which show less variable responses to leaf area under e[CO 2 ] e. For some plants, root growth is increased under e[CO 2 ] George et al. Carbon allocation under e[CO 2 ] can also be influenced by pH, as seen in plants grown in a low pH media under e[CO 2 ], where much of the carbon from photosynthesis accumulates in the shoots Hachiya et al.
There are many reviews already written on the role of sugars as signals in plants including Granot et al. However, to the best of our knowledge there are no reviews written specifically for sugar sensing in roots, which is a major focus of this review. As such, before moving on to our discussion of sugar sensing in roots, this section will serve to provide general information on sugar sensing not specific to roots.
There is much more information known on sugar sensing than written in this section, however, we direct you to other reviews, such as those mentioned above, for more detailed discussions on sugar sensing not specific to roots. Glucose has long been known to play a role in photosynthetic gene repression, with the enzyme hexokinase acting as a sensor Jang and Sheen, It has since been established that hexokinase is a central enzyme in glucose sugar signaling pathways Moore et al.
Through sugar sensing, hexokinase appears to be able to promote plant growth by causing greater cell expansion in roots, leaves, and inflorescences when exposed to high light conditions Moore et al. It has been observed that SnRK1 may be inhibited by the presence of sucrose.
KIN10, a part of the SnRK1 complex, is activated under sugar starvation, leading to up-regulation and down-regulation of various genes Baena-Gonzalez et al. SnRK1 also contributes to increasing sugar content in plants by phosphorylating both sucrose phosphate synthase SPS and trehalose-phosphate synthase TPS; Nukarinen et al. Sucrose concentrations are linked with T6P levels, as increased sucrose leads to stimulation of TPS which in turn increases T6P concentrations Yadav et al.
High T6P then causes a decline in sucrose content which prevents further increases in T6P Yadav et al. The regulation of T6P content is primarily linked with sucrose content, as studies have shown that only sucrose and hexoses able to be converted to sucrose have a significant effect on T6P levels Lunn et al. Sucrose and T6P may also be involved together with nitrogen assimilation, where increases in T6P signal the plant to synthesize organic and amino acids rather than sucrose Figueroa et al. In conjunction with T6P other similar sugar phosphates, glucose 1-phosphate G1P and glucose 6-phosphate, are able to inhibit SnRK1, with G1P working together with T6P to significantly increase this inhibition Nunes et al.
Sugar signaling in plants begins as early as seed development and germination. At low levels, sugars are able to delay germination of Arabidopsis thaliana seeds.
Other sugars have displayed this function as well, with sucrose, glucose, and the non-metabolically active glucose analog 3-O-methyl glucose exhibiting a greater delay on germination than others Dekkers et al. The ability of the glucose analog to delay germination indicates a pathway independent of hexokinase. Sucrose functions as a signaling molecule in a variety of ways.
During the G1 phase of the cell cycle, sucrose induces the expression of the two CycD cyclins Cyc2 and Cyc3 , which influence cell cycle progression and cell division Riou-Khamlichi et al. The role of sucrose in regulating the cell cycle likely correlates with its role in plant growth. As a plant produces more sugars, sucrose stimulates the cell cycle and allows utilization of the produced sugars for growth.
As such, e[CO 2 ] is likely to facilitate this process. The greater sugar production caused by e[CO 2 ] could stimulate the cell cycle and allow the excess sugars to be used to produce greater plant biomass Seneweera and Conroy, Sugar signaling pathways also interact with hormones.
For example, glucose increases the biosynthesis of auxin, therefore affecting processes regulated by this hormone Sairanen et al. Evidence also suggests that sugars interact with pathways of both abscisic acid Cheng et al. Among other functions, abscisic acid has an enhancing effect on some genes regulated by sugar Rook et al. These findings show the various roles of sugars in gene regulation and thus their contribution to plant growth and development by way of sugar sensing.
Currently there is a lack of understanding about the effect of e[CO 2 ] on sugar sensing, however, many studies have conducted experiments applying exogenous carbohydrates to plant roots, thus creating conditions of increased root sugar content which may mirror the conditions of greater root sugar content resulting from increased photosynthesis under e[CO 2 ].
Most of the research into the role carbohydrates play in plant roots has focussed on sucrose exclusively. While some research has brought to light several effects of other carbohydrates, such as, glucose and fructose, there may yet be many more roles that non-sucrose carbohydrates play.
Much of this work is limited to A. The following section discusses the potential outcomes for roots of plants grown under e[CO 2 ], whereby excess carbohydrates in leaves are transported to roots and lead to altered gene expression Figure 1. The effects of sugar sensing in roots has had less attention then in shoots, as is especially the case for sugar sensing under e[CO 2 ]. As such, there is insufficient data to draw conclusions at this time, however, we provide an insight into how e[CO 2 ] may affect sugar sensing in roots, as well as sugar crosstalk with hormones.
NO 3 - uptake is diurnally regulated in a variety of plants Lejay et al. This could mean that if sugars accumulate in roots of e[CO 2 ] grown plants during the night, the diurnal cycle of NO 3 - transport will be affected. In plants that store starch in their roots, this could lead to an accumulation of sucrose in roots throughout the night, leading to altered gene transcription overnight.
Sucrose concentration is also responsible for transcriptional regulation of other diurnally-regulated root ion transporters. Though sucrose has the ability to regulate root ion transporters, they are not all regulated by the same mechanism. Lejay et al. Most genes ten appeared to be regulated by a pathway dependent on the catabolic activity of hexokinase, rather than its sensing function, whereby the downstream metabolites of glycolysis act as signals for gene regulation. Hexokinase sensing was proposed as the third pathway, which affected a single gene.
All three pathways are briefly reviewed in Rolland et al. Among these genes, the majority appeared to also respond to [CO 2 ] Lejay et al. If no sucrose was applied exogenously to the plants, 11 of the 16 genes responded to light exposure, provided there was also CO 2 in the atmosphere. This may suggest that these genes display a varied response depending on the amount of photosynthate produced.
As such, these results may support our argument that greater photosynthesis caused by e[CO 2 ] will change the level of expression of some genes in roots. Sucrose can also stimulate nitrogen assimilation via the oxidative pentose phosphate pathway OPPP.
An increase in sucrose concentration in roots of A. This induction requires plants to have a functional plastidial OPPP, which suggests that sucrose influences the OPPP to produce a signal that leads to transcription of N assimilation genes Bussell et al. Not only is the OPPP important for sucrose mediated nitrogen assimilation, but it is also required for glucose mediated Nrt2. Glucose also appears to post-transcriptionally regulate Nrt2.
Utilization of the glucose-insensitive gin mutant, which lacks the hexokinase sugar sensing mechanism, showed that glucose regulates Nrt2. However, it is not known how these genes function under dynamic changes to sugar composition at e[CO 2 ].
Given that sucrose and glucose can affect the OPPP in roots, it is reasonable that a similar system may exist in leaf tissue. The down-regulation seen in Vicente et al. As increased sucrose in roots cause induction of OPPP genes, an increase in sucrose due to increased photosynthesis under e[CO 2 ] may cause a similar interaction in leaves, but down-regulating the genes instead.
Sugars may also contribute to nutrient uptake by control of genes involved in root formation. The cyclin CYCD4;1 is expressed in pericycle cells of the root apical meristem and is involved in lateral root primordia formation Nieuwland et al. This may be, in part, how sugars are able to regulate root growth, as discussed in the next section.
In addition, this may explain one way that e[CO 2 ] is able to increase root growth Lee-Ho et al. Sugars are important regulators in phosphate deficient plants. During phosphate starvation, carbohydrates are used to regulate various phosphate starvation induced PSI genes Karthikeyan et al. Glucose and fructose can stimulate PSI genes to an extent, however, optimal responses occur with sucrose.
During phosphorus deficiency, sucrose is able to increase the expression of a phosphate transporter gene LaPT1 and a phosphoenolpyruvate carboxylase gene LaPEPC3 ; Zhou et al. Sucrose also promotes growth of root hairs in phosphate deficient A. The increased sugar production under e[CO 2 ] likely leads to lower inorganic phosphorus in plants due to the use of phosphorus in sugars such as triose phosphate, the synthesis of which will likely increase under e[CO 2 ]. The lower phosphorus concentration then becomes limiting in ATP synthesis and regeneration of ribulose bisphosphate Farquhar and Sharkey, Whether the increased sugar production under e[CO 2 ] provokes the same expression of PSI genes mentioned above, is not currently known.
There may be many genes in the root that are unrelated to nutrient acquisition which are activated by a sugar signal. For example, almost every aspect of auxin metabolism appears to be affected or regulated by glucose. Out of auxin regulated genes in A. Amino acid synthesis may also be impacted by sugar sensing. Silvente et al. This brings to light more ways that e[CO 2 ] could affect root processes through sugar sensing.
Research needs to be conducted in this area before any conclusions can be drawn, however, given that e[CO 2 ] has been shown to affect sugar regulation of genes in roots Lejay et al. Under e[CO 2 ] conditions, Jauregui et al. The main finding of this study, however, showed that supplying e[CO 2 ] treated A. This suggests that altering the nitrogen availability of plants may affect the plant's sugar sensing capabilities, as altering the plant's photosynthetic capacity will ultimately alter the carbohydrate content of plants.
The mechanism by which e[CO 2 ] affected the 48 genes was not explored in the paper and as such, we don't know whether they were affected via sugar sensing pathways. The sugar content of the roots under e[CO 2 ] did not differ significantly from roots of plants grown under ambient [CO 2 ], however, there was a slight increase in sucrose content.
Whether this small increase is enough to alter gene expression in roots is uncertain. Another possibility is that faster sugar catabolism may promote gene expression, however, the process is totally unknown and more research into the effect of e[CO 2 ] on gene expression in roots is required. Lower nutrient concentration in grains has been widely reported under e[CO 2 ] Taub et al. Storage of the accumulated carbon under e[CO 2 ] is not consistent across all plants.
How this extra carbon affects roots is not well understood, however, understanding the extent that sugars affect roots will provide a starting point for research into the effect of e[CO 2 ] on roots. Elevated [CO 2 ] has a similar effect on root growth as increased sucrose concentrations.
This may suggest that the way in which e[CO 2 ] affects root growth is through the increased sugars allocated to roots. Elevated [CO 2 ] increases both total root number and length in A. Increasing sucrose concentration in plants grown under ambient [CO 2 ] also gives results similar to e[CO 2 ] Lee-Ho et al. Elevated [CO 2 ] may increase root growth in order to balance nutrient uptake with the rate of sugar production from increased photosynthesis or perhaps a larger root system acts as a sink to store excess sugars.
Gaining a better understanding of how e[CO 2 ] affects the growth of roots could help explain the changes in nutrient status that occur under e[CO 2 ], such as the deficiencies of iron and zinc in wheat Myers et al. With both e[CO 2 ] and sugars increasing plant root growth, you would expect greater uptake rates of nutrients, thus relieving nutrient deficiencies. While there are other mechanisms that are affected by e[CO 2 ] that lead to nutrient deficiencies, their discussion is outside the scope of this review.
The role that roots play in causing or alleviating nutrient deficiencies needs to be further elucidated. The carbohydrate status of plants can strongly influence root architecture. For example, increasing concentrations of the hexoses glucose and fructose in the growing regions of A. Not all hexoses work to promote root elongation, however, as mannose inhibits root elongation by a signaling pathway initiated by hexokinase Baskin et al. Galactose, another hexose, also inhibited root elongation in the study by Baskin et al.
Psicose, an analog of fructose, is a third hexose capable of inhibiting root growth. It was found to inhibit root growth of lettuce seedlings, however, in contrast with mannose, it does not appear to cause the inhibition through a hexokinase-mediated pathway Kato-Noguchi et al. In Sedum alfredii , e[CO 2 ] is found to increase both root elongation and branching Li et al. The meta-analysis also found that increased fine root biomass was the main component of the total biomass increase.
This may suggest that if e[CO 2 ] plays a role in the sugar stimulated increase in root growth, more carbon is partitioned into sugars such as glucose, which is capable of increasing root growth, rather than psicose or mannose.
Therefore, understanding how diurnal changes in sugar composition is affected under e[CO 2 ] will provide a greater insight into the role that sugars have on root growth and gene expression in response to e[CO 2 ]. The role of glucose in A. It has also demonstrated the ability to control root growth direction in A.
The directional change induced by glucose occurs via both hexokinase dependent and independent methods Singh et al. The hexokinase glucose sensing pathway also leads to increased lateral root production Gupta et al. Furthermore, root meristem activation is stimulated by glucose via a target-of-rapamycin TOR signaling network Xiong et al.
The control of root meristem activation by the glucose-TOR interaction relies on glycolysis—mitochondrial energy relays. This signal network in turn promotes root growth. Sucrose has been identified as a necessary signal to stimulate primary root growth in A.
In addition, secondary root growth is also promoted by sucrose Freixes et al. Sucrose also has the ability to rescue plants from certain factors which inhibit root growth. The inhibition of root growth caused by both psicose and mannose, as previously mentioned, is overcome by the addition of sucrose Kato-Noguchi et al.
This means that in plants that produce more sucrose under e[CO 2 ] than hexoses, the inhibition by psicose and mannose is unlikely to occur. As previously mentioned, sucrose is also involved in promoting lateral root primordia formation, however, Macgregor et al. They concluded this on the basis that sucrose and its downstream metabolites glucose, fructose, and glucosephosphate, all promoted lateral root primordia formation, but the non-metabolized glucose analog 3- O -methyl glucose did not, combined with the observation that exogenous sucrose promoted lateral root primordia formation in the hexokinase mutant gin2.
It could instead be argued that these sugars operate as signals independently of hexokinase, particularly sucrose which is not sensed by hexokinase. Despite evidence that sugars promote lateral root development, a recent study concluded that sucrose and glucose promote the expression of the A. Adding to the complexity surrounding the influence of sugars on regulatory pathways, auxin, a hormone that promotes lateral root development and is upregulated by sugars, represses WOX7 expression Kong et al.
Further aspects of the ability for sugars to control root architecture are discussed in the next section, where crosstalk with various plant hormones is required to bring about changes in root architecture. Along with the ability for sugars to control gene expression and root growth, they also are known to interact with hormones, extending their potential effect as a signaling molecule. For instance, sucrose-mediated induction of the Nrt gene may be due to its capability to crosstalk with auxin, a hormone which, among other functions, regulates the A.
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