Lecture Notes taken from Plant Physiology text written by Taiz and Zeiger (2002)
Control of flowering
As we move into spring and summer, one may start to wonder why it is that some plant species flower early in the growing season while others flower later in the growing season. Also, some perennial plants will flower early in their life span while other will take several years before they flower.
We know that flowering is induced by internal developmental factors and environmental cues.
Humans undergo four developmental stages:
Can you name them?
IN a similar manner, there are three developmental stages in the life cycle of plants.
Adult reproductive phase
In animals, the entire organism undergoes changes as it passes from one developmental stage to the next. In plants, physiological and morphological changes occur only in the shoot apex.
In herbaceous annuals, there are not differences in the morphology of the leaves arising from the shoot apical meristems. Physiological conditions simply change in the shoot apex to allow flowering to occur.
In perennials, there are distinctive differences in the morphology of leaves arising from the shoot apical meristems during juvenile versus adult developmental stages. Obvious examples include English Ivy and Eastern Red cedars.
The juvenile period varies among species. It appears that the attainment of a certain size is more important than the plant’s chronological age in determining the transition to the adult phase.
The shoot apex must be competent and determined.
Competence: a group of cells is competent if it can respond in the expected manner when the appropriate developmental signal is provided. Genes are in a state to be turned on.
If a vegetative shoot is grafted onto a flowering stock, the scion flowers immediately. It means that is is competent. If it does not flower, it implies that he shoot apical meristem has not yet attained competence.
In plants that respond to photoperiod, the inductive photoperiod causes the leaves to export a floral stimulus to the rest of the plant that alters the developmental fate of the cells of the shoot apex. Once the meristem becomes committed to the new developmental program, it is said to be determined. The meristem is determined if it follows the same developmental program even after being removed from its normal physical environmental context.
For example, in tobacco, plants must attain a certain size to transmit a sufficient amount of the floral stimulus to the shoot apex. During this change in size, the shoot apex may undergo a phase transition that renders it more responsive to the floral stimulus.
Tobacco plants typically flower after producing 41 leaves.
Flowering tobaccos were decapitated just above the thirty fourth node (from the bottom). The axillary bud at the 34th node grew out and flowering occurred along this stem after it produced seven more leaves.
If the axillary bud at the 34th node was simply cut off and rooted or grafted to a stem without leaves, it produced a complete set of leaves before flowering.
What does this mean?
In another experiment, the tobacco plant was decapitated above the 37th leaf. The same treatments were imposed as before. This time the thirty 37th bud flowered after producing four leaves in all three situations (1. when simply allowed to grow out, 2. when the axillary bud was cut from the stems and rooted or 3. grafted to another stem).
What does this mean?
Circadian rhythms and leaf movements in plants
Plants are usually accustomed to cycles of light and darkness. Examples include leaf and petal movements, stomatal opening and closure, photosynthetic and respiration rate.
These processes maintain a periodicity close to 24 hours. For example, stomata will open every 24 hours. When plants are transferred from daily dark-light cycles to continuous dim light, the rhythms will continue to be expressed for several days. We can conclude that there must be an internal pacemaker that is controlling the physiological event and that presence or absence of light must simply be entraining the internal pacemaker.
Sleep movements (nyctinasty) in bean, Arabidposis and many legumes exhibit a circadian rhythm.
Based on what we know about the leaf movements in the touch sensitive plant, Mimosa penduca, the leaf opens and closes due to changes in turgor in the region at the base of the plant, called the pulvinus. IN order for the leaf to close, K+ moves out of the cells on the inside of the pulvinus and water follows allowing these cells to lose turgor. The leaflets close in this manner. It is thought that phytochrome plays a role in changing the permeability of the plasma membrane to potassium movement into and out of the cell. We know that blue light stimulates closed leaflets to open and red light followed by darkness, causes open leaflets to close. The leaflets begin to close within 5 minutes after being transferred to darkness and closure is complete in 30 minutes. The effect of red light can be canceled by far-red light. Therefore, phytochrome must be involved.