Plant Physiology - Assimilate Partitioning

Plant Physiology - Assimilate Partitioning

• Photosynthesis and translocation occur simultaneously in green plants
• Early plant growth utilizes photosynthetic products for vegetative structures
• Later, products are used for storage organs (grains, fruits, tubers, seeds)
• If photosynthetic product supply exceeds demand, it's stored in stalks and leaves, then later transported to growing parts.
• Translocation occurs through phloem, the conducting tissue
• Plasmodesmata, tubelike strands, connect cells and move photosynthetic products.
• Leaves export a significant portion (70-80%) of photosynthate within 6 hours of photosynthesis
• C₄ plants have a faster translocation rate compared to C₃ plants.
• Efficiency of translocation in C₄ plants may be due to specialized Kranz anatomy and greater phloem amount.
• Factors like water stress, nutrient deficiencies, or extreme temperatures can reduce translocation in plants.
• Source organs are plant parts that produce carbohydrates (photosynthesis products) - example is leaves
• Mature leaves are the main source of assimilates produced during current photosynthesis, supplemented by stored reserves.
• Sink organs receive and utilize photosynthates. Examples include developing/growing organs like fruits, grains, seeds, tubers, and young leaves
• Young leaves act as sinks as they do not produce enough food for their growth
• The rate of photosynthesis is influenced by demand from sinks
• Removal of tubers or inhibition of tuber growth reduces net assimilation rate
• Removal of fruits or florets decreases net photosynthesis

Components of Yield

• Crop yield is the economically useful part for human or animal consumption, measured as grains or dry matter per unit land area.
• Yield is expressed in kg/ha or metric ton/ha
• Biological yield is total dry matter, whilst economic yield is the useful part.
• Yield components in cereals include panicles/ears per unit area, spikelets/grains per panicle/ear, and weight per spikelet.
• Yield = (no. of panicles/m²) * (no. of grains/panicle) * (weight/grain)
• Yield in legumes considers pods per unit area, seeds per pod, and weight per seed.
• Yield in tuber crops involves stems/m², tubers/stem, and average tuber weight
• Yield in forage crops involves plants/m², shoots/plant, and weight/shoot

Harvest Index

• Harvest index (HI) quantifies economic yield (grain/fruit/tuber) relative to the total biomass produced
• HI of rice is calculated as the ratio of grain weight to above-ground biomass.
• A higher HI signifies that most assimilates are partitioned into economically important parts.

Changes in Assimilate Partitioning

• Assimilate partitioning changes as the plant grows.
• Early plant growth directs assimilates to vegetative organs.
• Following reproductive development, assimilates are channeled to fruits, seeds, tubers.
• Soybean crop initially prioritizes leaf canopy, but later diverts more to flowers, stems and petioles (increasing proportion as flower population increases).
• In the final stages, assimilates are channeled to the grains.
• Crop also experiences dry matter loss from senescent leaves and stems which contribute to grain yield proportion in maturity, contributing high proportion of dry matter.

Limiting Factors of Crop Yield

• Several factors can limit crop yield, including water supply, nitrogen nutrition, temperature, solar radiation, plant population, and diseases/pests/weeds.

1. Water Supply

• Water status affects leaf size, longevity, carbohydrate assimilation, and grain-filling duration.
• Water deficits during canopy establishment can reduce leaf size.
• Water deficits reduce grain fill duration due to premature senescence.
• Specific developmental periods are sensitive to moderate water stress, especially root/leafy vegetables during respective growth periods.
• Yield depends on the quantity of transpired water.
• Crops with high water use efficiency (transpire less, absorb more) can better withstand water stress.

2. Nitrogen Nutrition

• Nitrogen status affects cereal crop grain yield by influencing canopy size and duration.
• This in turn influences the efficiency of solar radiation capture, and conversion into organic compounds.

3. Temperature

• Temperature affects photosynthesis and respiration rate, flowering, assimilate partitioning, leaf production and maturity.
• Extremes inhibit plant growth
• Plants grow faster within a favorable temperature range, provided other environmental factors like nutrients, water, and light aren't limiting.
• Different crops/varieties require specific temperatures for optimal performance.
• Temperature affects seed germination, water absorption, nutrient absorption and subsequent translocation and assimilation in the plant.
• Temperature influences the partitioning of growth between shoot and root, with a cold season favoring root weight and warm season favoring shoot yield

4. Solar Radiation

• Biomass yield under optimal agronomic practice depends on light interception by crop canopy
• Solar radiation affects plant growth type, food material synthesis and differentiation of tissues/organs, and maturity stages
• Light intensity and duration are critical aspects of solar radiation
• Photosynthesis rate increases linearly with light intensity until a saturation point is reached.
• The reproductive and ripening stages are critical in relation to solar radiation for active photosynthesis for assimilate transport to sink organs
• Responsiveness to irradiance is higher for plants with greater nitrogen nutrition and increased leaf area

5. Plant Population

• Yield components are sensitive to population density
• Overpopulation results in low weight/plant, reducing crop yield
• At very high densities, plants focus on above-ground growth to survive at the expense of yield components like seeds/roots

6. Diseases, Pests, and Weeds

• Diseases, pests, and weeds reduce nutrient/water use efficiency, thus increasing production cost
• These factors reduce crop quality lowering their price for growers

Conclusions and Practical Applications

• Crop yield predominantly depends on assimilate translocation to economic parts (i.e., grains, seeds, tubers).
• A certain portion of assimilates is lost during respiration
• Assimilation rate decreases when the crop canopy ages
• Mature leaves act as a primary source, supporting sink growth with current photosynthesis and stored reserves.
• Other storage organs such as stems mobilize assimilates to grains, seeds, fruits, and tubers.
• Regrowth in grasses after grazing depends on reserves in roots
• Weeds' persistence may be from rhizomes/stolons for sustaining rapid regeneration
• Improving harvest index is vital for boosting yield components in crops. Shorter stature cultivars reduce competition between stem and grains, thereby improving harvest index
• Maximizing yield hinges on maximizing light interception
• Crop water stress remains the primary yield limiting factor worldwide
• In areas with limited rainfall, annual crops over perennial ones are recommended, due to their short life cycle that ends before severe droughts
• Fertilizers should be applied strategically during critical developmental stages to improve nutrient uptake efficiency.