Environmental control of development
- Related Topics:
- plant
- biological development
The rate of maturation and the timing of the transition to the reproductive phase are sometimes governed by internal controls and thus are relatively insensitive to the environment, provided conditions are generally favourable for growth. Frequently, however, the developmental rate is affected profoundly by recurring cycles in the environment, particularly those of temperature and of day length. In effect, these cycles provide a timetable for the plant, thus adjusting flowering, fruiting, and seed dispersal to the season and increasing the chances for successful propagation.
The control of the developmental rate by temperature is especially evident in many herbaceous plants of temperate climates. These plants, as indicated earlier, often must experience cold, either as seeds or as young plants, before they can begin flower production; otherwise they undergo an excessively long period of leafy, or vegetative, growth. After the cold experience, which can be given artificially, the plant is said to have been vernalized, or brought to the spring condition. Again the response is akin to a determination, because the condition attained is transmitted through subsequent cell divisions. Furthermore, there are indications that vernalization induces a persistent modification in the metabolism of apical cells and their derivatives.
The annual cycle of changing day length obviously provides the best of all “clocks” for the regulation of plant development. The effect of day length (or rather length of continuous darkness) on the transition to flowering is part of the general phenomenon of photoperiodism. Certain plants, called short-day plants, grow vegetatively when the nights are shorter than a critical minimum period (days long); exposure to longer nights (days short), however, accelerates development and brings on early flowering. Conversely, long-day plants develop very slowly toward flowering during daily cycles with longer than a minimum of darkness (days short), and are accelerated by exposure to short nights (days long). Other plants either require days of intermediate length for flowering or respond to a sequence of different photoperiods.
The leaf, rather than the stem apex, is the light-receiving organ in the photoperiodic reaction, although it is at the apex that subsequent developmental changes occur. As in the case of vernalization, photoperiod undoubtedly affects the metabolism of the known plant hormones, and so influences many other developmental responses apart from flowering. The effect of the duration of illumination on the carbohydrate balance of the plant may also be important. Nutritional effects on flowering are well known in many species—certain fruit trees, for example.
Whether or not environmental factors influence the passage into a reproductive state of a plant, the transition must be viewed as part of the general development from juvenility to maturity: in this sense, flowering is not a radical alternative to vegetative growth but its culmination. Yet, entirely new organ types are produced at the flowering apex, presumably under the influence of genes inactive during vegetative growth.
Seasonal adaptations
Certain plants are perennial and survive from year to year by matching their growth to the progression of the seasons or by suspending growth altogether during unfavourable times, such as winter or a dry season.
In the temperate zone, sometime before winter begins, growth ceases in the shoots of woody plants, resting buds are formed, and deciduous trees lose their leaves. The resting bud consists of a short axis, with the stem apex surrounded by modified unexpanded leaves, which protect the stem, especially from drying. The cells show marked frost resistance, similar to that of the embryo of the seed. Corresponding changes occur in herbaceous plants, in which the preparation for winter may involve the dying back of aerial parts altogether, leaving protected organs at or below the soil surface.
Growth sometimes ceases, even under favourable conditions, as a result of internal changes in the plant. This is true for some trees, which cease growth in midsummer. The passage into winter dormancy, however, is often controlled by the shortening of day length at the end of the growing season; in some plants, decreasing night temperature also plays a part. Most temperate-zone trees cease growth and form resting buds when the day-length falls below a critical minimum.
Photoperiodic control seems to involve the formation of inhibitor compounds. In birches, for example, the leaf perceives the day-length “signal” and transmits inhibitory materials to the apex, thus bringing growth to a stop and inducing the formation of a resting bud. The dormancy hormone, abscisic acid, may be concerned in this response and also in stomata closure in response to water stress and in leaf abscission (the separation of the leaf from the plant); there is a great deal of debate among scientists concerning what role, if any, this hormone plays in abscission. Other signaling hormones, such as indole-3-acetic acid (an auxin) and ethylene, appear to play a more significant role in leaf abscission.
Budbreak in certain trees is controlled by photoperiod, growth resuming in the lengthening days of spring; light-perceptive organs are probably the young leaves inside the bud scales. Sometimes budbreak depends only on temperature increases that occur in spring, as in certain plants of Mediterranean climates.
The resumption of development in buds may result from a change in the balance of growth-inhibiting substances, such as abscisic acid, and growth promoters, notably the gibberellins. Buds can be caused to open prematurely by gibberellin treatment, which, as in the case of vernalization, can sometimes replace a cold experience; moreover, the gibberellin content in the buds of certain woody plants increases during chilling. Other hormones are probably also involved, however, for budbreak in plants such as the grapevine can be promoted by cytokinins, the plant cell-division factors.
An important general feature of adaptive periodicities is that the developmental changes anticipate the conditions for which they will ultimately provide the appropriate physiological or morphological adjustment. The ability of plants to utilize environmental indicators such as temperature and day-length changes is vital for the survival of plants. The production of such adaptive devices is made possible by the state of continuous embryogeny, already stressed as one of the most important characteristics of plant growth.
Senescence in plants
The growth of the vascular plant depends upon the activity of meristems, which are, in a sense, always embryonic. Continued indefinitely, this mode of growth could mean immortality; indeed, the longest lived individual organisms ever to have existed on earth have been certain species of trees. Plants and plant parts, however, do die, and death is often not the consequence of accident or environmental stress but of physiological decline—aging, or senescence.
Various kinds of physiological senescence and death occur and may affect particular cells, tissues, organs, or the whole plant. In the formation of the vessels of the xylem, cells conclude their differentiation by dying and contribute their empty walls to the conducting tissue. Individual organs such as leaves usually have a limited life span. Entire shoot systems may gradually die back in the aerial parts of perennial plants, which overwinter underground. And, finally, the whole plant may die after a limited period of growth and the completion of reproduction. This behaviour is found in many annual plants, which complete their life cycle in a single growing season. The life span may extend to two years, as in biennial plants, or longer, as in banana and certain bamboos, which die after flowering and fruiting.
In the examples cited above, the death of cells, organs, or individual plants appears to be “programmed” and, in some sense, adaptive. This is clearly so with the death of individual cells during differentiation, when residual products contribute to the effective function of the entire plant body. The death of leaves and of shoot systems is part of the plant’s adaptation to the cycle of the seasons. In annual species, the death of the whole individual may be viewed in a similar way. The succession of generations in this case is carried on by seeds; the sacrifice of the parent plant may, in fact, contribute to the success of the seedling by making available to the seed a pool of reserves derived from the breakdown of parent tissues.
Certain features characterize the onset of senescence. The cells show degenerative changes often associated with the accumulation of breakdown products. Metabolic changes accompany the degeneration. Respiration may increase for a period, but the rate ultimately declines as the cellular apparatus degenerates. Synthesis of proteins and nucleic acids ceases, and, in some instances, disintegration of cells has been associated with the release of enzymes through the disruption of membrane-bounded bodies called lysosomes.
The death of individual cells in tissues such as the xylem appears to be governed by internal factors, but senescence often depends upon interaction of tissues and organs. The presence of young developing leaves often accelerates the aging of older leaves; removal of the younger leaves retards the senescence of the older ones, suggesting control by competition for nutrients. A similar effect is seen in annual plants, in which the development of fruits and seeds is associated with the senescence and, ultimately, the death of the rest of the plant; the removal of reproductive structures slows the rate of aging. In these instances competition obviously has some effect, but it does not sufficiently explain why older, mature organs suffer in competition with those still in active development. The link may lie partly in the capacity of developing organs to draw nutrients to themselves, even from older parts of the plant. Developing organs thus provide “sinks” toward which nutrients tend to move. The senescence of organs drained in this way could result from the progressive loss of certain key constituents; should leaf protein, for example, turn over by breakdown of proteins to their amino acid constituents and then be resynthesized, a steady drain of amino acids from the leaf would progressively deplete the proteins in the leaf.
“Sinks” can be only part of the explanation, however, for in detached leaves of plants such as tobacco, protein synthesis decreases, and protein content falls, while the amino acid content actually rises. Senescence in such instances can hardly depend on the withdrawal of nutrients. Furthermore, leaf senescence can be retarded locally by the application of cytokinins, hormones that stimulate plant cell division. Parallel effects have been demonstrated with growth substances of the auxin type in other plant systems. In the same way that active buds and fruits form sinks for nutrients from elsewhere in the plant, a cytokinin-treated area of a leaf attracts nutrients from other parts of the leaf. Although the metabolism of isolated leaves may differ in many respects from that of attached leaves, leaf senescence probably does not result only from nutrient drainage but also from the synthetic activity of leaf tissues, which may be under hormonal control from other parts of the plant. The root may be important, for roots are known to export cytokinins to the shoot.
Environmental factors, primarily photoperiod (daily length of darkness) and temperature, play important parts in governing senescence and death in plants. In annual plants, death is the natural conclusion of development; thus, conditions accelerating development automatically advance senescence. This is readily seen in short-day plants, in which precocious reproduction upon exposure to long dark periods is followed by early death. Senescence may be retarded in these cases, however, by hormonal treatments of the kind known to delay degeneration and death in detached leaves. Competition for nutrients between vegetative and reproductive structures cannot be the primary cause of death, for, in species such as hemp, the male plants—which do not produce seeds—die earlier than the females under short-day (long-night) conditions.
In perennial plants, leaf fall is associated with approaching winter dormancy. In many trees leaf senescence is brought about by declining day length and falling temperature toward the end of the growing season. Chlorophyll, the green pigment in plants, is lost; yellow and orange pigments called carotenoids become more conspicuous; and, in some species, anthocyanin pigments accumulate. These changes are responsible for the autumn colours of leaves. There are some indications that day length may control leaf senescence in deciduous trees through its effect on hormone metabolism, for both gibberellins and auxins have been shown to retard leaf fall and to preserve the greenness of leaves under the short-day conditions of autumn.
From the foregoing it may be seen that senescence and death are important in the general economy of plants. The paradox that death contributes to survival is resolved when it is understood that the death of the part contributes to the better adaptation of the whole—whether organ, individual, or species. Viewed in this way, death is no more than another—albeit the ultimate—manifestation of development.
John Heslop-Harrison