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Part 9 of a “Basic Botany for Bonsai” Series

You will recall that our series on basic botany for bonsai continues with a discussion of the role of hormones and light in plant growth and flowering. This is the ninth and next to last in the series which will culminate next month in a look at senescence and dormancy. This information is based on work in a college biology text by John Posthlethwait and Janet Hopson. The book is entitled The Nature of Life and was published by McGraw-Hill in 1995. Considering our discussion on hormones and their actions last month, it’s clear that hormones mediate growth responses as well as adaptive responses to pruning which is one of our major points of interest. By understanding these responses we can minimize trauma and utilize the plant’s natural growth habits to achieve our design goals more effectively. Hormones also influence flowering and fruit formation as well as seed germination. Serious consideration has been given to encouraging farmers to plow their fields at night rather than during the day to avoid exposing weed seeds to light which would stimulate their germination.

Plant movement is ordinarily (but not always) regarded as a slow process, permanent and the orientation is generally called tropism. Very simply, tropism is a plant’s response to external stimuli as well as internal stimuli and results in changes such as bending, turning and directional growth based on the stimulus. One of the primary tropisms we have to keep in mind is the basic response to light which is a positive tropism resulting in movement toward light. Other positive tropisms include hydrotropism (growth toward water) and thigmotropism which is the characteristic of some vining plants to wrap themselves around a nearby object for support. Wisteria is an excellent example of thigmotropism in action as the tendrils reach for stability which also apparently enhances their growth rate. Gravitropism is the response of a plant to turn toward gravity and influences a tree’s roots to grow downward. (I knew there was a reason for that).

Interestingly, plants that are laid horizontally display an interesting phenomenon with their root formation. The roots use a tiny starch granule that acts like little rocks to pull the roots down into the soil. Cells on the upper side of the root become longer while cells on the lower side remain the same length . This results in the root bending downward and the starch granule (amyloplasts) sinking to the bottom of the cells in response to gravity. They may also cause auxin to move to the lower side of the root to block elongation of lower cells. Auxins apparently bind to cell receptors in cell membranes the same way hormones in animals do and mutant plants that grow horizontally rather than vertically lack these cell receptors.

Charles Darwin and son published one of the first studies on phototropism in plants in 1881. Their main question was where the controlling factor for plants bending toward light was. The bending response toward light had been observed but noone knew for sure how it worked and exactly what influenced it. They very simply covered the tip of a growing plant and exposed the bending part of the plant and then reversed the conditions. Thus, they determined that the seedling’s tip sends a signal of some sort to it’s lower portion causing it to bend in the appropriate direction. Subsequently, research showed that auxin coming from the tip of the plant accumulated on the side away from the light which caused a drop in pH inside the cells which increases enzyme activity. The enzyme weakens the cellulose in the cell walls which then stretch and enable the cell to elongate. That’s the long reason why a plant bends toward light and may be useful next time you want to impress a newcomer to your club. Just tell them to turn their trees every now and then and save everyone lots of time.

There are plant movements other than tropisms. We’ve all watched Mimosa leaves droop when touched anywhere along their length and observed Venus flytraps as their leaves close rapidly to trap a small insect. These responses that are independent of stimulus are called nastic responses and are not based on cell growth. Flytraps close rapidly and reopen later and their movement is not permanent like a bend in a trunk. Other plants display ‘sleep’ movements resulting in leaves drooping or closing at night (Texas Ebony, Brazilian Rain Tree) and reopening during daylight. Biologists believe that these responses are internally regulated by ‘biological clocks’ because they tend to respond the same way for a few days if suddenly moved into complete darkness. These sleep movements are believed to be hormonally regulated.

In the previous article of this series, I mentioned phytochrome along with the hormones even though it is actually a pigment. Phytochrome is a light-sensitive blue-green pigment that allows plants to detect differing amounts of light in the environment. It’s not classified as a hormone because it doesn’t move from cell to cell. Seeds contain phytochrome which enables them to determine when they can germinate and flowering is influenced by phytochrome so that flowers emerge at the proper time to enhance reproduction of the plant. Understanding this process helps us to understand why Satsuki azaleas bloom so predictably in some areas and sooner in other areas with more light earlier in the year. It also explains why azalea growers suggest placing the plants in full sun early in the year to enhance blooming and to move them into semi-shade after flowering to conserve moisture and prepare buds for the next flowering season.

Phytochrome responds to light of different wavelengths the same way that rhodopsin does in the cone cells of human eyes . Visible light consists of a broad spectrum of colors that can be separated with a prism or through water droplets. Plants respond more to red and far-red light and chlorophyll absorbs red light but reflects green light (ever wondered why plants appear to be green?). The change in phytochromes from red to far-red (types of phytochrome) cause the plant to grow green and lush and is reversed in low light or darkness.

Another factor influencing plant growth is the interaction of plant hormones which determines the eventual shape of a tree. Most conifers exhibit strong apical dominance which can be viewed as a measure of their concentration of auxin in the growing tips. (See Hormone Chart in previous issue for listing of effects of interactions of hormones). Briefly, auxin travels down the shoot from the growing tip and inhibits growth of lateral buds below it. Auxin is broken down as it moves from the growing tip down the stem resulting in higher concentrations closer to apices. Branches farthest from the tip of the plant are the least inhibited by auxin and are also the oldest and largest branches on the tree. Youngest branches closest to the growing tip are most inhibited by auxin which results in a conical shape. (I knew there was a reason for that). In addition, cytokinin flows upward from the roots and counteracts the effects of auxin which causes lateral buds to grow stronger.

So what about the typical shape of Maples, for example, in nature? They don’t grow in natural conical shapes without outside influence. We can presume that trees that grow in more spherical shapes have less concentrated auxin in the growing tips and maybe more cytokinin moving upward through the roots. The result is lower branching with stronger branches and branches that are less sensitive to auxin inhibition. Branches grow both high and low on the trunk and tend to become bushier (spelled ramification) when the apical buds are removed resulting in less concentrated apical dominance. The same effect can be achieved with conifers (pines for example) by judicious pruning at the correct time of the year. Removing pine needles from strongly growing areas decreases the amount of auxin produced in that area and allows cytokinin to have it’s effect of producing lateral bud growth.

External factors such as temperature, moisture and hours of sunlight in a day become important when considering flowering and fruiting trees. Trees respond to these external regulators to enable them to pollinate, set fruit, develop seeds and disperse seeds to help the plant species survive.

Vernalization is a process that describes exposure to cold that enables some plants to flower and is necessary for germination of many seeds. This alternation between extended cold followed by warmth indicates to the plant that winter has passed and that the optimum time for germination is at hand. By germinating early, seedlings have a longer growing season which ordinarily works to their advantage and enables them to prepare for the next winter.

Temperature alone can give false signals however, so many plants adapt to a photoperiod method for tracking seasonal changes. Generally, plants are recognized as either short-day plants or long-day plants. The absolute length of daylight is not the prime factor but is based on a critical number of hours required by each type of plant. Some plants require days with sunlight less than, for example, 14 hours per day while others require sunlight of greater than a given length in order to flower. If they are ‘short-day plants’ they will not flower if sunlight is longer than their specified requirement and if they are ‘long-day plants’ they won’t flower with less than their required period of sunlight. The hours required vary from species to species, but generally long-day plants blossom in late spring or early summer while short day plants bloom in late summer or fall as the days become shorter. For example, poinsettias are short-day plants that will flower only when days are less than 10 hours long and nights are at least 14 hours long. Flower shops know that they have to keep poinsettias in a dark closet for at least 14 hours per day in fall in order to force them to bloom for Christmas.

Plants are exquisitely sensitive to their light requirements because even a short flash of light will interrupt flowering which leads scientists to believe that plants actually measure night closer than they do daylight. Short-day plants are actually long-night plants. Long-day (short-night) plants that are interrupted by a flash of light in the middle of the night will flower more readily. Phytochrome sensitivity is so accurate that individual plants of a given species can flower within a day or two of each other which increases chances of fertilization of flowers. In addition, the photoperiods of particular plants often coincide with the most active food-seeking portion of a pollinator’s life cycle which is an indication of their interdependence.

One of the questions facing scientists was whether phytochrome in the individual flower buds measure day-night cycles or if the signal comes from elsewhere within the plant. Accordingly, they exposed buds of some plants to one light regimen and leaves to a different light cycle and observed flowering patterns. Results showed that leaves and not buds are responsible for sensing photoperiods. Scientists speculate that there is a mystery hormone (tentatively called florigen = flower generator) that moves from leaves to buds. Since florigen has yet to be isolated, many scientists believe that the signal is mediated by combinations of other known hormones and most agree that phytochrome in combination with hormones causes flowering at optimum times of the year.

Hormones are actively involved in fruit development also. Seeds apparently secrete auxin in some plants that controls development of fruit including ripening. As long as auxin is present the fruit develops normally, but if the seeds are displaced or removed, normal fruit development stops. After fruits have developed, ethylene (a gas) causes ripening. Ethylene is responsible for converting the starches to sugars, increasing fragrance of fruits and changes in color of fruits through the formation of pigments such as carotene.

This brings to an end our discussion of plant hormones and their effects on growth and reproduction. In our next article, we will take a long look at aging, senescence and dormancy to bring the series full circle through a plant’s life cycle. There will also be an examination of mechanisms that plants use for protecting themselves and healing wounds to finish the series. Have fun.

 

This article has been reprinted with permission from:
Paul Ringo and the “Bonsai News” of the Lake Charles Bonsai Society

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