How Does the Role of Senescence Affect Crop Productivity?

• Senescence can determine and increase crop productivity.
• Selecting for cultivars with delayed senescence improves yield only in some species.
• Senescence increases plant phenotypic plasticity, helping it to adapt to abiotic stress.

Senescence is a significant agricultural trait that affects crop growth, stress adaptation, yield, and postharvest storage. Plants are unusual as they can time senescence at the organ and whole plant levels to improve survival chances and viability. Find out the intriguing roles of this phenomenon in crops.

Senescence In Plants

Senescence is no longer considered synonymous with aging. Though it does occur as the last stage of aging, plants’ ability to time senescence makes it a part of crop growth and development.

Over 10,000 years, agricultural practices have resulted in a coordinated timing of crop senescence to improve yields.

Senescence occurs due to internal factors like age, reproduction, and phytohormone production; external environmental cues like photoperiod; abiotic stresses such as drought, nutrient deficiency, wounding, shading, and ozone; and biotic stresses like pests and diseases.

Senescence Mechanism

Senescence occurs through programmed cell death. It is a degenerative process but is strictly controlled by genetic, biochemical, and physiological processes.

During senescence, structures and functions are orderedly lost, aiming to move the cells’ nutrients and building blocks (macromolecules and lipids) out of the cells to other parts of the plant. The other plant parts that act as sinks are growing organs (leaves and fruits) or storage organs (stem and bark).

Typically, senescence has three stages, even though overlap and differences may exist.

1. Storage mobilization: In the first phase, specific molecules are selectively degraded to ensure the senescence plant part can still function. So, the mobilized materials are considered nutrient storage materials being mobilized for transport out of the cell. Senescence can be reversed at this stage.
2. Generalized breakdown: In the second stage, the breakdown of components becomes generalized and extensive, and the plant part loses its physiological functions. All useable materials are transported out of the senescent structure, and the senescent process is now irreversible.
3. Abscission: The final stage occurs when all useful material is removed from the senescent structure. In abscission, the senescent part is shed from the rest of the plant and dies. Abscission involves complex biochemical and physiological processes.

Senescence occurs at various levels: cell organelles (chloroplasts and mitochondria), cells, tissues (xylem and phloem), organs (leaves, flowers, and roots), and whole plants. Types of senescence and their timings have varying roles and importance for crops.

Senescence Type in Crops

Based on which part of the plant is undergoing senescence, there are two types important for crops- whole plant and organ senescence.

Whole plant senescence

Whole plant senescence occurs in monocarpic crops, which die after producing seeds only once. The whole plant undergoes senescence, and the resources mobilized are transported to the seeds. These crops can be annuals like grains and pulses, biennials like cabbage, or perennials like bamboo.

Organ senescence

The senescence of two organs is critical for crops- leaves and flowers.

Leaf senescence is the most vital trait in determining yield and nutritional value for all species of monocarpic and polycarpic plants. Polycarpic types include orchard trees that bear fruits several times before whole plant senescence.

Leaf senescence is necessary to recycle nutrients like proteins, nitrogen, phosphorus, and lipids.

In monocarpic species, leaf senescence is coordinated with plant development, including whole-plant senescence. Under optimal conditions, leaf senescence occurs based on age. Scientists have devised many strategies to control the timing of leaf senescence to improve crop growth, survival, and productivity.

In monocarpic species, flowering senescence or floral arrest is vital in determining the length of the reproductive stage and the time available for optimum fruit and seed development. The reproductive stage occurs by remobilizing nutrients from vegetative parts to developing fruits and seeds.

Root senescence also occurs and is vital for nutrient mobilization from legumes.

In monocarpic plants, the timing of these senescence is also essential and determines fitness.

Senescence Timing

The timing of senescence is vital and depends on the environmental conditions of crops (see Figure 1). Under optimal conditions, stress-free plants undergo developmental senescence, which can be sequential or reproductive. Stress results in precocious senescence.

Sequential senescence: This type refers to leaves death and is age-based. Older leaves senesce, and their nutrients are allocated to young growing plant parts. Sequential leaf senescence occurs before the start of reproduction and is independent of this crop stage.

Reproductive senescence: This senescence affects the whole plant and occurs in monocarpic plants during reproduction. It supports seed development, viability, yield, and quality. During reproductive senescence, leaves are shed nearly simultaneously to promote grain filling.

Precocious senescence: When plants are under stress (salinity, drought, shading, nutrient deficiency), leaf senescence is started as an acclimation response regardless of the age of organs or plants. Also called premature senescence, this can reduce biomass accumulation time and crop yield and quality.

Thus, under normal conditions, senescence occurs as part of aging, but plants can start organ and whole plant senescence independent of age in certain situations.

Figure 1.: “Overview of nutrient remobilization and transport during developmental and precocious senescence,” Schippers et al. 2015. ( Image credits: https://doi.org/10.1104/pp.15.00498)

Significance of Senescence for Crops

Leaf senescence is the most important type, specifically for crops. Next is whole-plant senescence. Both these traits are used in crop breeding programs. The effects of senescence on crops can be positive and negative. The positive effects are the remobilization of nutrients, increased yield, improved survival and productivity under stress, and manipulation of crop cycles. The negative impacts are reduced postharvest quality and increased risk of diseases. Each effect is discussed below.

1. Remobilization of nutrients

Remobilization of nutrients in plants usually involves leaf senescence. Leaves switch from carbon capture through photosynthesis to remobilize nutrients, especially nitrogen. However, there are other sources of nutrients, such as roots and stems. Root senescence that controls nodules in legumes can also release nitrogen for remobilization in crop plants. Stems are temporary organs for the storage of photosynthates and nitrogen-containing compounds. During whole plant senescence, stems release nitrogen, carbon, and micronutrients to support fruit and seed formation. The timing of senescence that determines remobilization efficiency can influence the yield and quality of seeds.

Since nutrient uptake is energy-intensive, recovering nutrients from older leaves and roots and directing them for newer growth or seed filling increases plant nutrient-use efficiency and reduces fertilizer use.

2. Determines yield

Leaf senescence determines the leaf area and time available for photosynthesis to accumulate biomass. Senescence overlaps with reproduction. While remobilization of nutrients is helpful for seed formation, premature leaf death can reduce yield. Late senescence increases yield, as observations over 50 years in hybrid maize show. Delayed senescence is a sought-after trait in green fodder crops as it allows ratooning and multi-cuts.

3. Adaptation to stress

One of the significant benefits that senescence provides crops is a strategy to adapt to environmental change and respond to stress. The means through which senescence improves phenotypic plasticity are many, as listed below:

• Leaf senescence remobilizes precious resources to help in the survival of plants during nutrient deficiency or other stress.
• Altering whole plant senescence timing, which usually starts early to avoid stress, can improve crop productivity during drought. Catching for stay-green cultivars prevents precocious senescence and can confer drought resistance or better performance during nitrogen deficiency. For example, stay-green traits are associated with drought resistance in transgenic plants.

4. Manipulating crop cycles

Senescence provides scientists with a means to manipulate crop cycles. Cultivars with shorter lifespans can be developed for faster growth or have longer vegetative periods in response to changing seasons. Since senescence is a plant adaptation strategy, it can be leveraged to produce cultivars that grow on marginal land or in sub-optimal environmental conditions to increase food production.

5. Postharvest storage and quality

Leaves are the organs that synthesize carbohydrates and several bio compounds needed for plants that influence seed quality, which will be affected by senescence. Moreover, during the early stages of leaf senescence, some substances that are broken down are chloroplasts containing chlorophyll, amino acids, proteins, and vitamins. So early senescence results in poor leaf quality, reducing postharvest quality and nutritional value of leafy vegetables, animal feed, and potted plants.

6. Increased vulnerability to diseases

Senescing leaves are more vulnerable to pathogens, especially fungus. This is often seen in postharvest stages. Fodder or leafy vegetables infected with fungus can produce toxic substances, making food unsafe.

A proper understanding of the senescence process at various levels is essential for scientists to leverage plants’ natural autonomous attributes to address the current challenges of food security and climate change. Postharvest management and crop breeding can also be changed to delay senescence and its associated adverse effects. Hence, fine-tuning organ and plant senescence can make food production more sustainable.

Measuring Crop Senescence

Crop senescence can be tracked at the organ and whole plant levels in research and food supply chains.

Spectroscopy can record chlorophyll degradation and leaf color changes due to stress or aging using hand-held spectrometers like CI-710s SpectraVue Leaf Spectrometer on the field. Remote imagery can record leaf color changes or leaf loss at large scales. Spectrometers and cameras used for acquiring remote imagery will measure the unique spectral waves created due to light interaction with pigments to measure their concentrations. Spectrometers are commonly used during research, and remote imagery is used in precision agriculture research and practice.

Canopy Analysis is another way to measure leaf senescence and crown thinning due to stress. CID Bio-Science’s CI-110 Plant Canopy Imager is a standard research tool used by scientists to record changes in the crop canopy. The tool has a camera with hemispherical lenses that capture canopy photographs, which are analyzed to show gaps in the canopy and Leaf Area Index.

The two methods are non-destructive and give quantitative analysis results in real-time.

Sources

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Schippers, J.H.M., Schmidt, R., Wagstaff, C., Jing, H-C. (2015). Living to Die and Dying to Live: The Survival Strategy behind Leaf Senescence. Plant Physiology, 169 (2), 914–930. https://doi.org/10.1104/pp.15.00498

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