2.5Proliferation and Growth of Organisms and the Environment
The proliferation and growth of organisms are greatly influenced by the environment. Light is an important environmental factor in the regulation of the growth and activity of most organisms. Plants derive energy from sunlight by photosynthesis. Moreover, the use of light as environmental information (Fig. 2-5) is widely known as follows; a phenomenon in which plants orient their direction of growth toward light (phototropism) and greening of plants. Plants that grow in dark turn yellow, and their leaves curl inward instead of growing outward (Fig. 2-5B). When exposed to light, the leaves turn green and unfurl, which is a phenomenon different from photosynthesis occurring with comparatively smaller amounts of light. Algae also have a tendency to move towards light (phototaxis). Even many microbes that do not photosynthesize may either move towards light or avoid light (Fig. 2-5D).
Fig. 2-5 Plant Reactions to Light
(A) Phototropism in corn coleoptiles. Blue light is absorbed by phototropin and leads to phototropism. (B) Greening of pea leaves. (1) Seedlings on day 6 after germination in shade. (2) Seedlings on day 14 after germination in light. In shade, leaves do not unfurl and do not develop green color. Both blue and red light are involved in the development of greening of the plants; cryptochrome and phytochrome act as light receptors, respectively. (C) Photorelocation movement of chloroplasts. When strong blue light is focused on the leaves, the chloroplasts in that portion escape out (pattern in the center; explanations on the right). In reaction to intense light, chloroplasts escape to and line up along the side walls, therefore the leaf area is pale green in color from above. (D) Phototaxis of Chlamydomonas. (1) A microscopic view of the cells; they have two flagella and swim upward. A protein similar to human visual substances called rhodopsin is present in the eye spot, which senses green light and causes cell movement to light. (A, C, D: modified from drawings of “the Exhibition of the World of Light for Schoolchildren” held at Museum of Natural History, the University of Tokyo).
For organisms originating in warm regions, low temperatures cause harm. Chilling injury is the damage from low temperatures above the freezing point and is distinct from freezing injury. The rice crop does not form pollen when it encounters low temperatures in the summer, and even if pollination occurs, it is unable to bear fruit. In fact, continuous high temperatures above a certain point are very important for the growth of the rice crop, which is not greatly affected by day length as described later. Therefore, in warm climates, double-cropping is possible. Conversely, low temperatures are also needed sometimes. Plant seeds and bulbs often cannot germinate without being exposed to the cold of winter. This is known as dormancy, and the low temperature is required to break this dormancy.
The switch from vegetative growth to reproductive growth involves seasonal changes. Plants switch from the development of leaves only to the development of flowers. In animals, developmental inhibition of the gonads and dormancy are affected by the four seasons. Many organisms perceive seasonal variation from the changes in light or temperature. Artificially promoting the formation of flower buds by lowering temperatures is called vernalization. In case of the wheat crop, original winter wheat does not produce ears when not exposed to the winter cold and needs to pass through winter. However, wheat cannot be cultivated in places where the winter cold is too intense. Spring wheat has lost the characteristic of cold requirement and therefore can be cultivated from spring to summer even in cold regions. Photoperiodism is the ability of organisms to sense day length of a single day. In case of plants, pigment proteins called phytochromes are known to be the sensors of day length. When the days are short, buds are formed (flowering) in short-day plants, and the opposite is observed in long-day plants. To regulate the growth and activity according to day length, the plants must recognize the length of a single day. The rhythm of activity over the course of approximately one day, called the circadian rhythm (Fig. 2-6), is a mechanism found in almost all organisms. Recently, this mechanism has even been observed in prokaryotes. In many cases, light functions to assign a “reset signal” (a phase regulator signal) (see Chapters 20 and 22).
Fig. 2-6 Circadian Rhythm
(A) Circadian rhythm is observed as periodic gene expression. When cells previously cultured in light and dark cycles are placed under continuous bright conditions, the rhythm of gene expression continues for approximately one day. (B) Circadian rhythm is established by the presence of oscillators consisting of a combination of mutually regulated components within an organism (or a cell). The time is set by environmental stimuli such as light. The state of the oscillators is represented by the rhythm of expression of various genes and is the rhythm of biological activity.
For humans, the circadian rhythm refers to sleeping at night and being awake during the day, but this rhythm is reset on a daily basis according to dawn light. In birds and amphibians, the pineal gland senses light, while in mammals, the hypothalamus senses light. Eyes do not function as light sensors in either case. In humans, seasons have no influence on reproduction, but in many animals, seasonal changes influence offspring production. In such cases, photoperiodism plays an important role.
Why Do All Cherry Trees Blossom at the Same Time?
The cherry tree “Someiyoshino” (Prunus x yedoensis), blooms in spring, and all cherry trees blossom at the same time. Cherry blossom viewing is an activity that is unique to Japan. However, many of these cherry blossom trees all bloom simultaneously. Although this blooming can occur slightly earlier by sun exposure, it mostly occurs at the same time. However, it is not considered a mystery, possibly because this happens naturally,.
Let us start by figuring out the secret to this amazing phenomenon. “Someiyoshino” is said to have been propagated during the Edo period by a gardener in Somei (Komagome, Toshima-ku, Tokyo) by breeding “Oshima zakura” (P. speciosa) and “Edohigan” (P. pendula f. ascendens.) Unlike other cherry trees, “Someiyoshino” trees blossom before leaves appear. This crossbreeding has also been confirmed by genetic research. Because such a horticultural variety is generated by crossbreeding, the seeds produce offspring with various, occasionally dissimilar, properties, even if it sets seeds (see Chapter 3). However, when a tree is propagated by cutting and grafting a branch, the same type of tree can be grown. In other words, the cherry tree undergoes asexual reproduction. In this manner, all “Someiyoshino” trees in Japan and even the rest of the world were grown from the same tree (although some would call this activity “cloning,” grafting has been known and used for a longer period than cloning). Accordingly, all cherry trees have the same blossoming properties, and in the same environment, they all blossom at the same time. Therefore, this property of the cherry tree can be used as the index of the climate and for the cherry blossom forecast, which is a hot topic every year.
Column Fig. 2-3 How Long has “Someiyoshino” been So Loved?
The tree referred to in Kino Tomonori’s poem “Why, with ceaseless, restless haste falls the cherry’s new-blown bloom?” is not “Someiyoshino”. Does not this considerably change the imagery of this poem?
Crossreeding can often be used to obtain different horticultural and crop varieties. After eating delicious apples and pears, some people sow seeds to grow fruit-bearing trees, but this is often unsuccessful. For example, Japanese pear cultivar Nijusseiki, meaning the 20th century, is a mutant found by chance in Matsudo, Chiba. The Nijusseiki pears grown in all over Japan are grafted from that tree, and currently, it has become the specialty of other places such as Tottori. Because this tree is not capable of self-fertilization, the tree is grown by artificial fertilization with pollen of other pear trees. Consequently, only a hybrid can be obtained by planting seeds of the Nijusseiki pears.
Conversely, occasionally they can be grown from seeds. Pea plants, which are described in Chapter 3, are capable of self-fertilization, and therefore, the plants grown from seeds are always identical to the parent plants. Rice is also self-fertilization. Thus, some amount of rice harvested by farmers is maintained as seed rice and used in the coming year to cultivate more rice. Sexual and asexual reproduction play an important role in living organisms.