HERBERTIA 45(1&2): 50-55. 1989
THERMOMORPHOGENESIS IN BULBOUS PLANTS
W.J. DE MUNK
BULB RESEARCH CENTER
POSTBUS 85, LISSE, 2160 AB, THE NETHERLANDS
THERMOMORPHOGENESIS is the periodical growth pattern of bulbous plants under the influence of temperature. Both normal and aberrant morphological phenomena are briefly reviewed.
Bulbous plants show periodical growth patterns. The plants flower at specific times of the year, then produce subterranean storage organs and finally above-ground organs die off. Regrowth starts again in a specific time after a period of apparent rest. Batches of the same variety, growing within certain geographic limits, show an almost synchronous development. It is known that the variation in temperature, according to seasonal conditions, is the most important factor for the control of the periodical growth pattern. The succession of temperature conditions is mostly from high to low to high, corresponding with summer, winter, and spring.
The physiological status of a tissue determines the type of response to the temperature. This means that the exposure of an intact bulb to a certain temperature evokes several effects, since it is composed of bulb scales, one or more generative and/or vegetative buds in different positions, disc tissue and root primordia. The net result of these effects is the periodical development. The characteristic growth pattern of bulbous and tuberous plants has been called thermoperiodicity (TP) (Hartsema 1961).
Recently there has been serious comment on the use of the term thermoperiodicity. Salisbury (1986 and 1987) stated that TP is the phenomenon in which growth and/or development is promoted by alternating day and night temperature on the analogy of photoperiodicity. He suggested to distinguish between a day-type and stage type TP or to find another name.
A better name for the response of plants to the influence of alternating high and low temperatures according to seasonal variation could be thermomorphogenesis. In agreement with the definition of Salisbury, the term TP can be used for those phenomena which occur under the influence of alternating day and night temperatures. From experiments by Kamerbeek (1969), it is known that high night temperatures and lower day temperatures in the greenhouse after planting of the iris bulbs promote the development of the flower bud. A reversed temperature regime results in growth stagnation of the flower bud and leafy plants. Such effects in bulbous plants can be called TP-effects.
Effects of TP on bulbs during dry storage has never been investigated, because of the impracticality on a commercial scale.
The type of response of plant tissues to external factors will be determined by its own physiological status. The reaction of meristematic tissue will be an effect on cell division to a certain extent. We know that high temperatures promote leaf and flower formation due to this process. After the formation of new plant organs the temperature exerts its influence on other tissues that are at an incipient stage when a single apex is present. The subsequent response will be an inhibition of the formation of new cells by the apex. The plant system is becoming more and more complex and requires another signal before it can complete its total life cycle That other signal will he given by another (lower) temperature.
Low temperature will retard cell division, but many biochemical processes will continue thus preparing the plant system for the susceptibility of a new stimulus. A high temperature is required for the second time for the elongation of 'resting' organs. It is not exactly known when the initiating processes are complete.
VARIOUS TEMPERATURE EFFECTS
A. High temperature
For tulips, hyacinths and irises, high temperatures after lifting of the bulbs in summer promote flower formation in the central bud. However in small bulbs the flower formation fails. It has been found that the critical size for flowering can be shifted to smaller bulbs by increasing the temperature and/or extending the period of exposure to higher temperatures.
Prolonged exposure of tulip bulbs to higher temperatures induces leaf and flower formation in axillary buds. Under normal seasonal conditions the temperature drops and cell division ceases before the apices in the axils of the bulb scales reach the stage to respond with flower formation. On the other hand, prolonged high temperature exposures promote the desiccation of flower primordia in the main shoot which senesce.
B. Low temperatures
In tulip bulbs various effects occur under the influence of a low temperature depending on the organ under consideration. First, it causes a preparation for elongation of stems in buds which bear a flower in primordial stage and, second it induces bulbification in vegetative buds (Blaauw et al. 1930; Le Nard 1981). This multiple response clearly demonstrates the different reactions of a complex plant system to an integrally administered factor.
In hyacinths the same sort of effects as in tulips can be observed: cooling induces the elongation of the scape and the leaves. Furthermore, low temperatures can reduce the number of leaves, when leaf primordia differentiate to bulb scales instead to foliar leaves. This effect can be observed if the bulbs are exposed to a rather low temperature (3.5¡C.) for long periods after an initial heat treatment at 35¡C. for 3 weeks. The cooling requirement is, however, less than for tulips: shorter periods and less lower temperatures are necessary for normal development.
In irises a high temperature also promotes cell division resulting in an increase of the leaf number. An exposure to temperatures below 20¡C is necessary for the actual flower formation. If the number of leaf primordia does not exceed the number of three, cooling will not result in flower formation, but a new bulb will originate in the center of the mother bulb. The smaller a bulb, the stronger the bulbification response will be. In other words, small bulbs require more heat to reach the generative state.
Comparing the temperature effects in tulips and irises, we may conclude that at the time of lifting bulbs in summer, the apical meristem of a tulip is in a more advanced physiological stale that of an iris. The transition of the apex of a tulip from the vegetative to a potentially generative stage has taken place under field conditions before lifting. The transition in the apex of irises occurs after lifting during the dry storage.
|Figure 1. Promotive effect of high night and low day temperatures on the development of flower buds of Iris cv, Wedgewood plants (Kamerbeek 1969).|
|Figure 2. Reduction of the number of leaves and increase of the number of bulb scales of Hyacinthus cv. 'L'Innocence' under the influence of a 27-week period of storage at low temperature (3.5 C) after a heat treatment at 35 C for 3 weeks (Beijer, unpublished).|
|Figure 3. Fifth-level transformed iris flower as a result of the exposure of Iris cv. 'H. C. van Vliet' plants to -2¡ C at the stage of flower formation.|
Table 1. Percentage of Hyacinths of cv. 'L'Innocence' with flat scapes (fasciations) and average number of flowers per inflorescence
as a result of the exposure of the bulbs to 20¡ C. for 10 days at the onset of flower formation after lifting of the bulbs (Beijer 1936).
|Flat scapes||Round scapes|
35¡ C. + 20¡ C.
20¡ C. + 30¡ C.
MORPHOLOGICAL ABERRATIONS UNDER THE INFLUENCE OF THE TEMPERATURE
As mentioned earlier, prolonged exposure of tulip bulbs to high temperatures results in flowering of axillary buds which normally produce daughter bulbs. Abnormal thermal conditions in a specific stage of morphogenesis may cause aberrations of the normal shape. In certain varieties it has been observed that a lower temperature during the process of flower formation results in a higher number of floral organs. The usual number is 15 (6 perianth leaves, 6 stamens and 3 fertile leaves), but up to 24 floral parts were found (Hartsema 1961). Similar phenomena can occur in irises when the temperature drops below zero at the time of flower formation. Fourth, fifth, and sixth level transformation flowers have been found (Figure 3). Pronounced effects have been found in hyacinths after exposure of the bulbs to a 10¡C. lower temperature at the onset of flower formation for two weeks. A fasciated scape will be the result with coincidently increase of the flower number per inflorescence (Table 1.). If at the same developmental stage of the apex the temperature drops to a very low level (0.5¡C) for a long period a branched inflorescence can be formed (Figure 4).
|Figure 4. Branched inflorescence of Hyacinthus cv. 'L'Innocence' as a result of exposure of the bulbs to 0.5¡ C for 24 weeks during dry storage beginning when the apex was at the transition to flower formation (Beijer, unpublished).|
OTHER MORPHOGENETIC FACTORS
Focusing on the morphological phenomena, the designation of thermomorphogenesis is useful to distinguish from photomorphogenesis and chemomorphogenesis. The similarity between these phenomena is that they are responses to external factors. After exposure of the plant material to external factors the tissue will translate the signals into internal triggers which in turn will activate particular receptors. It is very likely that plant hormones function as such internal triggers. A further analysis of that field will improve our understanding of the morphogenesis of bulbous plants. On the other hand the application of plant growth substances might be useful to replace several temperature or light-induced conditions.