Plant Physiology - LU 8 Assimilate Partitioning - 2018-19 Past Paper PDF

STT 1043 PLANT PHYSIOLOGY LU 8 Assimilate Partitioning  Translocation of Photosynthate Photosynthesis and subsequent translocation of photosynthetic product take place simultaneously in the green plants. In the early stages of plant growth, the photosyn...

STT 1043 PLANT PHYSIOLOGY LU 8 Assimilate Partitioning  Translocation of Photosynthate Photosynthesis and subsequent translocation of photosynthetic product take place simultaneously in the green plants. In the early stages of plant growth, the photosynthetic product is utilized for the formation of vegetative parts of the plant. In later stages, it is used in the formation of storage organs such as grains, fruits, tubers, seeds.  Translocation of Photosynthate If the supply of photosynthate is higher than the demand, it will be stored in the stalks and leaves, and later on, it is translocated to the growing parts of the plant.  Rate of translocation Photosynthate is transported to the various plant parts through the conducting tissues, called phloem.  Rate of translocation It is moved to phloem through cell interconnecting tubelike strands known as plasmodesmata.  Rate of translocation Leaves may export 70-80% of the photosynthate within 6 hours, from the time of photosynthesis. There is a difference between C4 and C3 plant species in translocation of photosynthetic product. In the C4 species, translocation rate is faster compared to C3.  Rate of translocation The efficiency of translocation in C4 plants may be related to the specialized kranz anatomy and a greater amount of phloem in their leaves. Other factors such as water stress, nutrient deficiencies or toxicities and extreme temperatures, may reduce translocation of photosynthate in plants. Kranz anatomy  Source-Sink Relationship Plant organs that manufacture carbohydrates (photosynthesis products) are known as source. Example of source organs are plant leaves which carry out photosynthesis activity. Mature leaves are the primary sources of assimilate, from current photosynthesis, but this can be supplemented by the mobilization of stored reserves.  Source-Sink Relationship Sink refers to plant organs where photosynthate are deposited and utilized. Example of sink organs: developing and growing organs such as fruit, grains, seeds, tubers and young leaves. Young leaves are sinks because they cannot fix enough carbon to support their growth.  Source-Sink Relationship The rate of photosynthesis by a leaf is influenced by the demand for assimilate imposed upon it. Early work led to the hypothesis that photosynthesis was controlled by the concentration of assimilate in the leaf. If the sinks’ demand for assimilate were low, the resultant accumulation of sugars or starch in the leaf suppress photosynthesis.  Source-Sink Relationship Eg. removal of tubers from a potato plant, or inhibition of their growth by low soil temperatures, led to a reduction in net assimilation rate. Similarly, removal of fruits from tomato plants, or florets from cereal ears, has led to reduction in net photosynthesis. Source-Sink Relationship  Source-Sink Relationship Conversely, increasing the demand for assimilate made upon the leaves, has been shown to increase the rate of photosynthesis. As the plant canopy undergoes senescence, the photosynthesis activity declines, leading to a reduction in the rate of nutrient assimilation. At this stage, assimilates will be translocated from storage organs (mature leaves and stems) to the sinks (grains, fruits, seeds, tubers).  Components of yield Crop yield is the economic part of the plant used for human or animal consumption and measured as grains or dry matter per unit land area. It is normally expressed in kilograms per hectare (kg/ha) or metric ton per hectare (t/ha). The biological yield refers to total dry matter (everything), and the economic yield refers to the economically useful part of biological yield.  Components of yield The economic part of the plant may be grains and seeds for cereals and legumes, fresh or dry matter for forages, and tubers and roots for tuber and root crops. Field crops yield is determined by various ratios of yield components. It is important to have a knowledge of formation and contribution of each component in the yield.  Components of yield The important yield components in cereals are panicles or ears per unit area, number of spikelets (grains) per panicle or ear, and weight of spikelets. Cereal yield = no. of panicles/m2 x no. of grains/panicle x weight/grain  Components of yield Each individual grain is called a spikelet Panicle of a rice plant Components of yield Ear of a corn  Components of yield In legumes, yield is determined by number of pods per unit area, seeds per pod and weight of grain. Legume yield = Pods/m2 x seeds/pod x weight/seed Components of yield Pods of a garden pea crop  Components of yield In tuber crops such as potatoes, yield components are: – No. of stems/m2 x no. of tubers/stem x average tuber weight In forage plants, such as alfalfa, yield components are: – No. of plants/m2 x no. of shoots/plant x weight/shoot  Harvest index The term “harvest index” is used in agriculture to quantify the economic yield (grain/tuber/fruit) of a crop species versus the total amount of biomass that has been produced. Eg. Harvest index of rice = Grain weight Above ground biomass A crop with high harvest index simply means that most of the assimilates are partitioned to the yield components (economically important parts), thus resulting on high yield.  Changes in assimilate partitioning Assimilate partitioning changes with TIME as the plant progresses in chronological events. Early stages of plant growth = assimilates are distributed to the vegetative organs. Following reproductive development = assimilates are distributes to fruits, seeds, tubers.  Changes in assimilate partitioning Eg. Soybean crop. At vegetative stage, most of the plant dry matter is partitioned to the leaf canopy, but after the initiation of the 1st flower, an increasing proportion is devoted to stem and petiole tissues, supporting an expanding population of flowers. Once grain filling started, an increase in dry matter is partitioned successively to pods and grains. In the last days of the green crop, all new assimilates and stores located in the stem and pods are channeled to the grains.  Changes in assimilate partitioning At the same time, the crop also experience a decline in dry matter of the vegetative organs (stems, branches and leaves), due to losses of senescent leaves and petioles. This results in, by maturity, the grains constituted a high proportion of dry matter.  Limiting factors of a crop yield The yield of a crop is influenced by several factors such as water, nutrients, temperature, solar radiation, plant population and disease, pests and weeds infestation.  Limiting factors of a crop yield 1. Water supply The water status of a crop can influence leaf size and longevity, the rate of carbohydrate assimilation and the duration of grain filling. Eg. In cereal crops, the size of leaves can be reduced by water deficit during canopy establishment, and the duration of grain filling can be shortened due to premature senescence.  Limiting factors of a crop yield 1. Water supply Specifically, there is a critical development period where crop is sensitive to a mild water stress. For leafy vegetable: during active vegetative growth For root crops: during the formation and development of the storage organ or tuber. For fruit and seed crops: during reproductive stage.  Limiting factors of a crop yield 1. Water supply Under limitation of water supply, the yield is depends on the quantity of water transpired by the crop. Thus, crop species with high water use efficiency (transpire less, absorbs more water) has the ability to cope with water stress.  Limiting factors of a crop yield 2. Nitrogen nutrition The nitrogen status of a cereal crop can influence grain yield by affecting the size and duration of canopy expansion. This in turn, will result in the efficiency of solar radiation capture to convert into organic compound.  Limiting factors of a crop yield 3. Temperature Temperature controls the rate of photosynthesis and respiration, flowering, assimilate partitioning, leaf production and maturity. Extremes of temperature adversely affect physiological processes and inhibit plant growth.  Limiting factors of a crop yield 3. Temperature Within a favourable temperature range, plants grow faster as temperature increases, provided other environmental factors such as nutrients, water and light are not limiting. Different crops, and even varieties of the same species require specific temperature ranges for best performance.  Limiting factors of a crop yield 3. Temperature Germination is sensitive to soil temperature because imbibition of water by the seed requires enzymatic reaction which is controlled by temperature. Temperature affects nutrients absorption by plant roots and subsequently, their translocation and assimilation in the plant.  Limiting factors of a crop yield 3. Temperature Temperature, among environmental factors, is the main driver of plant development. Temperature influences portioning of growth between shoot and root. – During cold season (autumn –winter), most of the dry matter is distributed to the below ground, hence more root weight. – During warm season (spring-summer), the dry matter is partitioned to the above ground, resulting in higher shoot yield.  Limiting factors of a crop yield 4. Solar radiation Under optimum agronomic practice, the biomass yield depends on the amount of light intercepted by the crop canopy. Solar radiation affects the type of growth, synthesis of food materials, differentiation of tissues and organs, and maturity of various crops.  Limiting factors of a crop yield 4. Solar radiation The critical aspects of solar radiation as are light intensity and light duration. The rate of photosynthesis increases almost linearly with light intensity, until the light saturation point, at which photosynthesis becomes independent of light intensity.  Limiting factors of a crop yield 4. Solar radiation The critical periods in relation to solar radiation are the reproductive and ripening stages. Active photosynthesis is essential to transport assimilates to the sinking organs. Responsiveness to irradiance is greater with nitrogen nutrition – chlorophyll formation, increases leaf area.  Limiting factors of a crop yield 5. Plant population In general, yield components are very sensitive to changes in population density. Over-populated plants will result in low weight per plant, which can reduce the economic yield of a crop.  Limiting factors of a crop yield 5. Plant population At very high population densities, plants may partition assimilate primarily into above-ground parts to attempt to ensure plant survival. This can leave little carbohydrate available for seeds or roots.  Limiting factors of a crop yield 6. Disease, pests and weeds infestation Diseases, insects and weeds reduce the nutrient and water use efficiency, thereby increasing the cost of production. They also reduce crop quality and thereby, lower the price of agricultural produce for the growers.  Conclusions & practical applications The yield of the economically important parts of most crops depends on the translocation of assimilates, in most species largely sucrose, from the leaves or other photosynthetic tissues. However, it is important to remember that a certain proportion of assimilates (carbohydrates) are lost via respiration. The rate of assimilation declines with the senescence of the crop canopy.  Conclusions & practical applications The mature leaves, as the primary sources, support the growth of sinks by maintaining a supply of assimilate from current photosynthesis and stored reserves. Besides mature leaves, other storage organ such as stems can mobilized assimilates to grains, seeds, fruits and tubers.  Conclusions & practical applications In grasses, regrowth after cutting of grazing may be dependent on reserves stored in the roots. Thus, the success of many perennial weeds can be attributed to rapid regeneration sustained by reserves in their rhizomes or stolons. Improving harvest index has been practical to increase the yield components of some important cereal crops.  Conclusions & practical applications Selection of a shorter stature cultivars of cereal crops will result in less competition for dry matter partitioning between stem and grains (thus, increasing harvest index) The key to maximizing yield is to maximize radiation interception. The single most important factor limiting productivity of crop yield on a worldwide basis is drought.  Conclusions & practical applications In places that receive low amount of rainfall, it is recommended to choose annual instead of perennial crops. Annual crops have a shorter life span, therefore, they can set seeds and die before the onset of drought. To increase the efficiency of nutrient uptake, fertilizers should be applied during critical developmental stages.