Plant growth depends upon the expansion of meristematic tissues near the tips of roots and stems. Left unchecked, this type of growth signifies unlimited vitality—a veritable fountain of youth-—and there would be no need for evolution and Darwin’s theory of survival of the fittest. But nature, as always, has a way and senescence, or the decline of the vibrant growth period due to aging, sets the stage for the dawn of a new generation of seedlings.
A plant’s death is not the end—it is often the beginning of new life and it is nature’s way of letting genetics advance and adapt to life here on Earth. However, the key to unlocking the true potential of a crop’s yield is often found in increasing the growth curve of a plant’s fruit. Delaying or prolonging senescence hinders the death spiral that harms yields. It is an age-old challenge that is the grower’s most elusive enemy.
There are numerous types of senescence. It may occur in only certain forms of cells, tissues and limbs, or it can take place in the entire plant. Xylem cells may begin to constrict pathways by dying and slowing or halting the importation of vital fluids sent to the fruit. Leaves typically have a maximum life span.
Whole root or shoot systems may slowly die back as the plant reaches its natural maturity. Ultimately, the plant may die after a seasonal period of growth has resulted in the completion of seed production and its procreation has been ensured. This characteristic plays out in many types of annuals living out their life cycle in a single, brief growing season.
The demise of leaves and the plant’s shoot systems is considered a normal part of a plant’s acclimatization to the cycle of the seasons and to the completion of life itself. The surrender of the parent to senescence contributes to the future success of the next seedling by fostering and sequestering away essential nutrients that may be required to start another life. The next generation is then carried on by seeds inside the developing fruit.
Slowing this downward trend is a critical factor in boosting harvest yields, as the longer a period of time a particular organism can continue to actively grow often provides greater fruit potential. Here are some tips to help delay the aging process and boost yields.
Cytokinins and Ascorbic Acids in Plants
Cytokines—concentrated amounts of plant growth regulators that stimulate plant cell division—can somewhat control plant senescence. Foliar applications of these products have been shown to delay the onset of senescence and promote shoot and root growth, which can provide for an expanding plant architecture that is more resilient to change.
Kelp and other sea plants contain relatively concentrated amounts of plant cytokinins, auxins and other growth regulators. When applied regularly, these products can extend life or lengthen the effective growth curve and drive up yields. Another option is kinetin, a type of cytokinin sold commercially under many trade names in mostly synthetic forms and usually applied to blossoms to induce fruit set. Kinetin increases the yields of annual fruits and vegetables, often producing seedless fruits, and increases the budding of herbs.
Foliar sprays of ascorbic acids are also important since they contain a vital anti-oxidant enzyme called superoxide dismutase (SOD), an enzyme thought to disarm DNA-damaging free radicals. Damaged DNA causes replication errors in cells and plant tissues and can hasten cell death, increasing the risk of plant disease. SOD is thought to be a powerful ally in reducing the effect of free radicals. These age-causing mutations damage DNA within the cells, often shutting down plants early and stunting growth.
Maintaining a Healthy Root Zone
We have seen how important it is to control the above-ground conditions while supplying plant growth regulators, but enhancing the rhizosphere, or root zone, has huge benefits as well in delaying senescence. Growers who ensure adequate availability of moisture and solubility of nutrients in soil solutions can delay senescence, as excessive moisture levels in the root zone reduces oxygen concentrations and can hinder the exchange of nutrients.
Saturated soil promotes roots that shelter themselves from the elements as they form a waxy-like surface upon them called suberin. The cork-like layer shields the damaging effects of increased moisture stress, lowering yields dramatically.
Reducing Environmental Stressors
The deaths of individual cells in tissues such as the xylem appear to be controlled by internal factors, but senescence often depends on a steady state. To keep producing, plants need a constant microclimate harmonious to life. The redistribution of nutrients to actively growing parts of the plant is greatly affected by the climate of the day.
Plants in drought stress react by reducing the function of xylem channels, hindering the transport of water and nutrients, limiting plant productivity and greatly reducing yields. Buds and fruits exposed to these conditions rush headlong towards death in their hurry to complete the life cycle and ensure the viability of the next generation.
Not only is it critical to control these cumulative conditions, the effect of even moderate drought stressors can damage xylems and often leads to fruit poor set, cracking, pitting, splitting and blossom end rot as well as poor overall fruit quality and taste.
Plants can use a variety of means to avoid, survive or function in drought stress. For example, annuals can mature quickly in a fast-paced, shortened life cycle that becomes compressed. High rates of growth during the early warm season may allow them to avoid drought entirely. Stomata may close tightly to reduce evaporative water loss, which increases root growth in search of moisture.
Decreasing the size of the canopy leaf structure reduces growth of new, above-ground shoots and this slows the increasing demand for water. These happenstances, however, will accelerate leaf and plant senescence.
In most plants under drought stress, the xylem pathways leading to fruit development will close by the fruit’s half-life. This is thought to protect the developing seeds from the damaging effects of drought. However, if xylem function decreases ahead of this time period, the result is fruits that are left with a meager supply of nutrients like calcium and boron that are commonly transported only in the transpiration stream. The blackened, shriveled and splitting distal ends of tomato fruit are prime examples of plants left without adequate xylem distribution pathways.
Limiting Exposure to Ethylene Gas
Ethylene gas is a significant natural plant growth hormone that can shorten the growth of plants. It is used in agriculture to enhance and force the ripening of fruits and certain vegetables. It operates at low parts per million trace levels during the life of the plant. Ethylene promotes the ripening of fruit, the timing of flowering and the shedding of leaves as well as the senescence of vegetative tissues.
The production of ethylene gas in plants can also be induced by a large range of factors, from temperature extremes to external wounding and other environmental stresses, including excessive wind damage. Compost piles produce ethylene and should be located at a distance from tender crops so as not to encourage the early onset of senescence. Limiting exposure to the gas helps ensure the life of the plant is prolonged.
Every plant’s challenge is to complete its life cycle while fully achieving the future vitality and vigor of its offspring. For the grower, the challenge is to extend the effective growth curve of the plant. Delaying the inevitable onset of senescence slows the plant’s urgency to complete its life cycle. Growers who employ these control factors and harmonize the growth environment of the plant are well on their way to achieving maximum yields.