Juvenility

A plant goes through different stages in its life cycle. These stages are embryonic growth, juvenility, maturity, senescence, and death. Juvenility is a developmental stage in a plant during which it is unable to induce flower. The length of juvenility varies among plant species. Annual plants have very short and perennial plants have a long juvenility period.

Table – Juvenile period of some plants

Characteristics of Juvenility

1. Leaf Form – The leaf is vigorous in its juvenile. The morphology of the leaf also differs in the juvenile stage of some plants, for example, Acacia sp.
2. Growth Form – Young branch appears like a whip. The branch grows parallel to the main stem. Such a branch is known as a water sprout, eg. Citrus, Guava, etc.
3. Presence of thorns- In some fruit plants thorns are present in the young or juvenile stage and these thorns disappear when they reach the adult stage. Example- Apple, Pear, Citrus, etc.
4. Leaf Retention – Plants do not shed leaves throughout the year in the juvenile stage. Whereas, it is shed in the adult stage or mature stage.
5. Root emergence – When juvenile parts of the plant are used in propagation, the root emerges rapidly.

Flower Bud Differentiation

Bud – The bud is an immature shoot system that is often surrounded by protective scale leaves. The bud develops into a lateral branch, a flower, or an inflorescence. Whether these buds develop as flowers or in inflorescences is a very desirable aspect of fruit production. The stages of development in a flower are as follows-  

1. Flower bud differentiation- It is also known as flower induction. Enhanced cell division occurs below the apical part of the meristem in the central region for bud differentiation to occur. Due to cell division, parenchyma cells differentiate into flower primordia that surround the meristem.
2. Initiation – Flower formation takes place in the second initiation.
3. Development of Flowers- This includes the period from induction to bloom. During this period the flower is usually receptive to pollination. The opening of the flower (anthesis) is the final stage of development.

A flower is a modified reproductive branch, basically a stem with an apical meristem that gives rise to the leaf primordia.

Table – Flower bud differentiation and flowering in some important fruits

Types of buds

1. Simple Bud – It develops in vegetative shoots. It is also known as leaf bud.
2. Mixed bud- It develops in the form of a shoot with flowers. It is also known as a flower bud.
3. Compound bud – It develops in both leaves and flowers.

Factors affecting flower bud differentiation

The signals leading to flower induction include the following endogenous and external factors:

1. Endogenous factors

a) Carbon: Nitrogen Ratio (C: N Ratio)- High ratio of nitrogen to carbohydrate increases flowering. In young plants, heavy use of nitrogen which lowers the C:N ratio delays flowering. Plants that are old, weak, and with a high C:N ratio tend to flower heavily. With regard to the C:N ratio, there can be four situations, namely:

i) High nitrogen and low carbohydrate – flowering

ii) High nitrogen and sufficient carbohydrate-flowering

iii) Moderate nitrogen as well as carbohydrate – good growth and abundant flowering.

iv) Low nitrogen and high carbohydrate – only a few buds formed.

When there is no growth in the plant, photosynthesis results in the accumulation of carbohydrates that help maintain the proper balance of C:N. During the phase of active growth, photosynthesis takes place, changing the C:N ratio and, accordingly, affecting flowering.

b) Genes – When plants undergo a phase change from juvenile to adult, synthesize the gene efl. This gene is responsible for reducing the effect of the juvenile stage which induces early flowering. In contrast, the HST gene promotes juvenility in plants.

2. External factors

A) Environmental factor

Temperature

Temperature is the main environmental factor associated with flowering. It is a very common thing that there are abundant flowers in the spring. Due to the availability of a particular temperature in the spring, the flowers appear at the same time every year. Exposure of the plant to lower temperatures before spring helps it flower.

Biennial plants like carrots, celery, cauliflower, cabbage, turnips, etc. start flowering when exposed to temperatures from 0 to 10⁰C. This is known as vernalization. In these plants, the low-temperature requirement is commonly known as the chilling requirement. They require chilling below 7⁰ C for about 4-60 days for bud blooms in spring.

Table – the chilling requirement of some temperate fruits

Photoperiod – Some plants flower only when their photoperiodic requirement is fulfilled. Plants are classified into three categories according to their light period:-

• Long day plant (LDP): – Flowering occurs only when the day length is more than 12 hours or more than 12 hours.
• Short-day plant (SDP): – These plants flower when the length of the day is less than 12 hours.
• Day Neutral Plant (DNP): – Flowers in these plants are not controlled by the length of the day.

Light

Light intensity, duration, as well as quality, affect bud differentiation and flowering. Higher light intensity is more helpful in flowering than low light. This is the reason that the outer branches of a fruit tree bear more fruit than the inner branches.

Red light (sunlight) promotes flowering while far-red light inhibits it. Therefore, the quality of light also affects flowering.

B) Managemental Factor

1. Nutrients – Nutrients determine the vegetative or reproductive growth of the plant. Nitrogen enhances the vegetative growth of the plant. And it aids in the carbohydrate utilization of the plant. Phosphorus acts as protein synthesis, cell division in the plant; Potash in the transfer of sugars. Other nutrients are also involved in flowering and bud differentiation.
2. Moisture- The unavailability of moisture prolongs the time of flower buds differentiation. Flowering primordia is less formed in case of lack of water.