SOURCE-SINK relationship.pptx

SOURCE - SINK RELATIONSHIPS AND LIMITATIONS ON PLANT GROWTH RATE AND YIELD

1. PLANT NUTRITION AND CROP PRODUCTIVITY SAID H. MARZOUK    SOURCE - SINK RELATIONSHIPS AND LIMITATIONS ON PLANT GROWTH RATE AND YIELD [email protected]

2. Source – sink Source : regions of photoassimilate productions Sink: regions of photoassimilate storage Source: Any transporting organ capable of mobilizing organic compounds or producing photosynthate in excess of its own needs , e.g., mature leaf . Sink: Non photosynthetic organs and organs that do not produce enough photoassimilate to meet their own requirements , e.g., roots , tubers, developing fruits , immature leaves . [email protected]

3. The flow of water in plants is almost always from roots to leaves. Translocation of sucrose can be in any direction – depending on source and sink location and strength. Examples: Beta maritima (wild beet) root is a sink during the first growing season. In the second season the root becomes a source, sugars are mobilized and used to produce a new shoot. In contrast, in cultivated sugar beets roots are sinks during all phases of development [email protected]

4. The continued growth of any plant depends primarily on photosynthetic activity in the leaves and the transport of organic compounds from the leaves to heterotrophic cells . The source supplies assimilates (C-compounds) to the sink. The sink accepts and consumes the assimilates for its own growth or accumulates them for a certain period. [email protected]

5. Translocation Translocation is the movement of organic solutes e.g. SUCROSE, from a source to a sink through the phloem by means of mass flow. The sucrose transporter gene in Rice is OsSUT1 [email protected]

6. Phloem loading  is the process of loading carbon into the phloem for transport to different 'sinks' in a plant. Sinks include  metabolism , growth , storage , and other processes or organs that need carbon solutes to persist [email protected]

7. Plants take up a range of resources from the environment in order to sustain growth, including water, carbon dioxide and mineral nutrients . Plant organs responsible for taking up a particular resource from the environment are known as the ‘source’ for that resource. Whilst organs utilizing the resource for growth, metabolism or storage are the ‘sink’ for that resource. Source and sink organs must be in balance in order for plants to function effectively in their environment , and this balance depends upon a coordinated relationship between sources and sinks. [email protected]

8. Source and Sink Organs for Carbon and Nitrogen In the simplest case, consider a plant composed of two organs only: leaf and root. Leaves carry out photosynthesis and are net sources of carbon , whilst roots are net sinks for carbon. In contrast, roots are net sources for nitrogen which they take up from the soil , whilst leaves are net sinks for this element [email protected]

9. Since both leaves and roots require both carbon and nitrogen , the net role played by each organ represents the overall balance of resource transfer. All plant cells use carbon for respiration, growth and the building of essential metabolites , and nitrogen for building proteins, enzymes and genetic material. Thus, mature photosynthetic leaves have a relatively small carbon sink activity and a relatively large carbon source activity, meaning that they are net sources for carbon . Roots have a small nitrogen sink activity and a greater nitrogen source activity making them net sources for nitrogen . [email protected]

10. CO2 from the atmosphere is fixed by plants in the process of photosynthesis , to generate triose phosphate which is converted to sucrose , which is a readily available store of carbon, or starch, for longer-term storage. The amount of starch stored in leaves varies between different species ; many temperate grass species also use the polymer fructan for long-term carbon storage (Paul and Foyer, 2009). Structural components – such as cellulose, which is a major component of plant cell walls – are also important repositories for fixed carbon. Besides sucrose, some plant species transport oligosaccharides and sugar alcohols. [email protected]

11. Nitrogen is reduced and then used to create amino acids, the building blocks of proteins, a process which occurs in both root and shoot. Once assimilated into amino acids , nitrogen can be stored as protein for long-term storage , or amino acids for readily available nitrogen to be combined with carbon fixed in photosynthesis . In tropical legumes and some other species, nitrogen is stored as ureides (Rao et al., 2017). [email protected]

12. The balance between carbon- and nitrogen-containing metabolites is an important indicator of source–sink status. For example, the ratio of free amino acids to sucrose expresses the relative availability of nitrogen and carbon With a high ratio indicating an excess of available nitrogen and a low ratio indicating an excess of available carbon. This balance is attuned to enable plants to optimize their growth and development. For example, the rate of photosynthesis is correlated with the rate of nitrogen assimilation, whilst increased availability of nitrogen leads to a reduction of starch synthesis, making carbon available for assimilation into amino acids. [email protected]

13. HOW TO DIFFERENTIATE SOURCE AND SINK There are 3 criteria to differentiate source from sink Morphology : Seeds, fruits, roots, tubers, are generally considered as sinks . Mature leaf is considered as source . 2.Direction of transport: Source is a plant part which exports the photosynthates sink imports the assimilates . Contrast: Leaf at young stage is a sink till it gets 75% of its total expansion it will become source. A fully expanded mature leaf is a source. A senescing leaf is also treated as a source because the stored food material from it will be exported back to growing points . [email protected]

14. Measures of sink Sinks are measured in terms of carbohydrates, oils, and proteins etc. largely based on the biological energy units they produce. In general sinks are measured in terms of number (coconut), (in this case weight is not important but number is important) Volume (timber), the stem biomass Weight (agricultural crops). Eg rice [email protected]

15. Strength of source and sink The strength of a sink or source depends upon the size of the organ and the rate at which it is taking up or utilizing a particular resource: Source strength = source size × source activity (1) Sink strength = sink size × sink activity (2) Where strength is net transport, size refers to the biomass of the organ (g) and activity is the specific resource uptake rate. based on Geiger and Shieh (1993). [email protected]

16. Measures of source : Source Strength = source Size * Source Activity = Leaf area/plant * Average photosynthetic rate per unit area. • Eg: Leaf Area Index*Net Assimilation Rate The size means how much assimilates are available for export. [email protected]

17. The source size and activity is highly dynamic or sensitive to , nutritional and climatic factors . Eg. N deficiency significantly decrease the photosynthetic CO2 assimilation capacity of leaves. (Havlin et al., 2005). Potassium (K) is essential nutrients for all plants and is involved in many vital plant functions such as plant enzyme activation , energy capture from photosynthesis , N uptake and protein synthesis [email protected]

18. Starch is a primary insoluble carbohydrate produced by higher plants and consists of amylose and amylopectin as a major fraction. The enzymes responsible for synthesis of starch, starch synthetase , is activated by potassium. Under inadequate potassium levels, the level of starch declines, while soluble carbohydrates and nitrogen-based compounds accumulate (Mae ,1997). [email protected]

19. Other nutrients like Mg, Mn and Co also play major role in photosynthesis, their by acting upon oxidation reduction in PSII and PSI Other factors like light intensity , CO2 concentration , temperature and moisture also play a major role in photosynthesis and so affects source strength. [email protected]

20. Types of assimilates are: 1.Current assimilates : They are translocated and linked to photosynthesis Eg. Sugars produced through carbon fixation - these assimilate contribute for maximum phytomass. These are carried out by expanded leaves. [email protected]

21. 2. Accumulated assimilate: Mainly as stored food material to serve as some for seedling growth. These accumulated assimilates are from plant parts . Eg. Endosperm of germinating seeds , cotyledons , Sugarcane sets. [email protected]

22. Sink strength Sink Strength = Sink size * sink Activity • Sink size: It is the maximum space available for the accumulation of photosynthetic products. • In grain crops it is expressed as number and size of grains . • Eg: no. of panicles/plant *avg. no. of spikelets per panicle*specific grain weight, sink activity refer to photoassimilate use and storage. Eg tubers, seeds If rate of conversion of sucrose to starch is more ↓ Translocation gradient will be more. ↓ Higher will be the rate of sink [email protected]

23. Eg. The major part of the starch in rice grains at harvest is the photosynthetic product of the leaves , which is translocated from the leaves directly to the growing grains after flowering. The ripening of rice grains can be considered a process of accumulation of contents in a container. The grain yield may be limited by either the size of the container or the amount of its contents . If the container is small the yield cannot be high . On the other hand, even if the container large , the yield cannot be high if the contents are limited . [email protected]

24. Limitations of source and sink Yield constraints or limitations may be due to source limitation or sink limitation or both. Source size limitation: Source size is limiting due to defoliation (decrease in LAI) caused by certain environmental factors or due to diseases or sucking insects i.e. any factor that limits the leaf area development affects the source size . • Similarly source activity refers to NAR which is limited due to 1. Certain environmental factors like light intensity . If light intensity is not sufficient then source activity in decreased. [email protected]

25. 2. Mutual shading of leaves within the crop. Sink Limitation: It is due to A. Floret sterility ( cereals) B. Insect damage C. Flower drop [email protected]

26. Possible reasons for flower drops: 1. deficiency of P synthesis. 2. deficiency of N availability. 3. Reduced light intensity in plant canopies. 4. Canopy temperature and Water logging 5. Shading and salinity 6. Hormonal factors [email protected]

27. Relationship between source-sink and growth Growth is controlled by contiguous physiological and developmental mechanisms , but initially depends upon ecological adaptations and evolutionary history . Plants with different growth strategies succeed in different ecosystems , and in different niches within those ecosystems. Proximate causes of growth rate variation include both external and internal factors (Körner, 1991). [email protected]

28. Externally, plants are affected by a number of abiotic and biotic factors including nutrient and light levels , temperature , competition , and all of which influence the supply and demand for essential resources , and plants must ensure that growth rates are adjusted accordingly (Bloom et al. , 1985). Internally, plant growth is constrained by molecular , physiological , and developmental processes . These internal processes can all be understood within the framework of source–sink interactions. [email protected]

29. Since plants are sessile can influence external factors to a limited degree, the internal factors are well controlled. As a consequence, it is these internal interactions of source and sink activity that must be responsible for the large fundamental variation in relative growth rate among species under common environmental conditions. The primary focus for many of these is boosting photosynthetic carbon acquisition (source activity), yet sink activity is also believed to limit grain development in many major crops (Acreche and Slafer, 2009). [email protected]

30. Sink activity is maintained through breeding and genetic techniques to provide crops which is more productive (Reynolds et al. , 2012). [email protected]

31. Improve Harvest index (HI) Increase biomass production Synchronized development of reproductive organ Reproductively determinate Reduced growth of non harvestable organ Reduced leaf growth at reproductive stage Decline in duration of Vegetative growth and increased duration of Reproductive growth . [email protected]

32. Summary Sources and sinks affects both growth and yield. The growth may be controlled by both source and sink strengths. Such control involve physically manipulating the plant or its environment : for example, source activity may be altered by elevated CO2, or shading, while sink activity may be altered by sink removal or sink chilling . However, modern genetic approaches may now be used to alter source and sink activity and enhance growth and production. Also application of nutrients at right time, right amounts and right place is necessary [email protected]

33. REFERENCES Gray, S. B., Dermody , O., Klein, S. P., Locke, A. M., Mcgrath , J. M., Paul, R. E., et al. (2016). Intensifying drought eliminates the expected benefits of elevated carbon dioxide for soybean. Nat. Plants 2:16132. doi: 10.1038/nplants.2016.132.  Paul, M. J., and Foyer, C. H. (2001). Sink regulation of photosynthesis. J. Exp. Bot. 52, 1383–1400. doi: 10.1093/ jexbot /52.360.1383.  Rao, I. M., Beebe, S. E., Polania , J., Grajales , M., Cajiao , C., Ricaurte , J., et al. (2017). Evidence for genotypic differences among elite lines of common bean in the ability to remobilize photosynthate to increase yield under drought. J. Agric. Sci. 155, 857–875. doi: 10.1017/S0021859616000915  Hav_e M, Marmagne A, Chardon F, Masclaux-Daubresse C. 2016. Nitrogen remobilisation during leaf senescence: lessons from Arabidopsis to crops. Journal of Experimental Botany 68: 2513–2529. Forde BG, Cutler SR, Zaman N, Krysan PJ. 2013. Glutamate signalling via a MEKK1 kinase-dependent pathway induces changes in Arabidopsis root architecture. Plant Journal 75: 1–10. [email protected]

34. THANK YOU [email protected]