Energy Distribution in Plants

Annuals are plants that go through their entire life cycle within a single year.Annuals begin their life cycles in the spring when seeds that survived the winter germinate (begin to grow).In regions with distinct dry and wet seasons,germination occurs with the onset of the rainy season.Because it has only one growing season,an annual has to distribute its photosynthates (energy-rich molecules)first to leaves.Leaves,in turn,become involved in photosynthesis,which replenishes the supply of photosynthates and increases overall plant biomass.At the time of flowering,the plant decreases the amount of energy distributed to leaves and diverts most of its photosynthate to reproduction.For example,in the sunflower,the biomass of leaves declines from approximately 60 percent of the total plant weight during the period of growth to 10 to 20 percent by the time the seeds are ripe.When in bloom,the sunflower distributes 90 percent of its photosynthate to the flower head and the remainder to the leaves, stem,and roots.

Perennial plants maintain a vegetative structure over several years.Once established,they distribute their energy in a very different manner from annuals.Before perennials expend any energy on reproduction,they divert photosynthate to the roots.This distribution to roots is in excess of that required for the development of roots for the uptake of nutrients and water from the soil.In some species,such as the skunk cabbage,the roots develop into large storage organs.Energy stored in the roots makes up a reserve upon which the plants draw when they begin growth the following growing season.When they are ready to flower,perennials divert energy from storage to the production of flowers and fruit.As the flowers fade and the fruits ripen,the plant once more sends photosynthate to the roots to build up the reserves it will need for the following spring.

Trees and woody shrubs live a long time,which greatly influences the manner in which they distribute energy.Early in life,leaves make up more than one-half of their biomass;however,as trees age,they accumulate more woody growth.Trunks and stems become thicker and heavier,and the ratio of leaves to woody tissue changes. Eventually leaves account for only 1 to 5 percent of the total mass of the tree.The production system (the leaf mass)that supplies the energy is considerably less than the rest of biomass it supports.Thus, as the woody plant grows,much of the energy goes into support and maintenance,which increases as the plant ages.

When deciduous trees (trees that lose their foliage in winter) produce leaves again in the spring,they expend up to one-third of their reserve energy on the growth and expansion of leaves.This expenditure is repaid as the leaves carry out photosynthesis during the spring and summer.After leaves,trees give preference to flowers; then tissues that transport nutrients and water,new leaf buds,and deposits of starch in roots and bark;and finally,new flower buds.

Evergreen trees have a somewhat different approach.Many have fine,sharp-pointed leaves called“needles.”Because the photosynthetic tissues in these needles can function year-round when temperature and moisture conditions permit,they do not need to draw on root reserves for new growth in the spring.They can afford to wait until later in the growing season to produce new growth.Then evergreens can draw upon energy built up earlier in the spring.For the same reason,new growth develops rapidly and matures within a few weeks.

Reproduction and vegetative growth compete for energy allotments.If photosynthesis is limited,vegetative growth gets first claim.Because the energy that reproduction demands is high-up to 15 percent in pines,20 percent in deciduous trees,and 35 percent or more in fruit trees-trees can afford an abundance of fruit only periodically,once every two to three years in deciduous trees and two to six years in evergreens.

The proportionate distribution of net production to above-ground and below-ground biomass tells much about different ecosystems. Low light conditions favor the distribution of energy to the production of leaves and stems at the expense of roots.A reduction in water or nutrient availability favors the distribution of energy to the roots.?

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?Annuals are plants that go through their entire life cycle within a single year.Annuals begin their life cycles in the spring when seeds that survived the winter germinate (begin to grow).In regions with distinct dry and wet seasons,germination occurs with the onset of the rainy season.Because it has only one growing season,an annual has to distribute its photosynthates (energy-rich molecules)first to leaves.Leaves,in turn,become involved in photosynthesis,which replenishes the supply of photosynthates and increases overall plant biomass.At the time of flowering,the plant decreases the amount of energy distributed to leaves and diverts most of its photosynthate to reproduction.For example,in the sunflower,the biomass of leaves declines from approximately 60 percent of the total plant weight during the period of growth to 10 to 20 percent by the time the seeds are ripe.When in bloom,the sunflower distributes 90 percent of its photosynthate to the flower head and the remainder to the leaves, stem,and roots.?

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