Environmental Plasticity and Heterophylly

Introduction

Plant species may respond to variations in environmental conditions. Many plant species can alter their leaf morphology in response to changes in environmental conditions. This phenomenon is known as heterophylly and it is widespread among land plants. In many cases, heterophylly is an adaptive mechanism that permits plants to respond to environmental heterogeneity. Recently, many studies and research have been done to investigate the occurrence of heterophylly in various plants. Many studies have found that heterophylly in plants is a result of the action of phytohormones.

Plasticity is the manner in which plants follow various pathways in response to the external environment. It has been found that phenotypic plasticity may play a decisive role in the adaptation of plants and other organisms to changing environmental conditions. Heterophylly is a special case of plasticity which refers to the ability of semi−aquatic plants to have different types of leaves below and above water.

Submerged leaves are thin and do not have both a cuticle and stomata, however, aerial leaves bear stomata and they are thicker, and cutinized. The striking and notable variety in the in−water, floating, and aerial leaves of heterophyllous aquatics are considered a great example of adaptive phenotypic plasticity. It has been found in studies that heterophylly is quite often controlled by matching environmental cues across diverse taxa, which may indicate a common underlying mechanism.

Patterns of plasticity in plants in response to environmental conditions in the laboratory suggest either convergent or parallel evolution of plant variations from terrestrial ancestors. However, meaningful evaluations of the ecological and evolutionary significance of this trait are hard to find.

Heterophylly− Meaning and its Features

Plants have the capacity to alter their own morphology and adapt to their environmental conditions. This phenomenon which is unique and limited to only some plant species is known as phenotypic plasticity. The phenotypic plasticity of leaf form alteration for environmental conditions such as intensity of light and quality, ambient temperature, and availability of water is called heterophylly.

The original definition of heterophylly was devoid of environmental control. However, now, heterophylly is referred to the leaf form alteration in response to environmental cues. This occurrence is different from heteroblasty, which refers to morphological changes in leaves throughout the lifecycle of plants.

• Heterophylly differs from anisophylly. The latter is a special case of shoot symmetry in which leaves inserted on different sides of the stem differ in size and shape. Heteroblasty and anisophylly are not the results of morphological changes induced by environmental stimuli. On the other hand, heterophylly is the environmentally induced adaptation between more than one leaf morphology in the same plant.

Image 1: Cucumber leaf

• Heterophylly is shown by various land plants which include terrestrial and aquatic species. In the case of angiosperms, heterophylly takes place in diverse taxa. Heterophylly studies have shown that this trait has evolved many times during plant evolution among numerous unrelated taxa. In many cases, heterophylly is considered to be an adaptive mechanism that lets plants optimally respond to environmental heterogeneity.

Image 2: Watermelon plant bearing fruit and flowers

• The adaptive plasticity hypothesis proposes that plants that exhibit heterophylly in leaf architecture in response to heterogeneous environmental conditions are expected to have better fitness compared to other plants. However, the adaptive significance of heterophylly is limited in terms of information. Moreover, studies have indicated that the occurrence of heterophylly may be constrained by the genetic cues and limits of plasticity. Hence, it is not proven whether heterophylly has evolved as an adaptive response to environmental cues.

• Heterophylly is unique in nature and hence it is a matter of many studies. Meanwhile, studies on the molecular mechanisms that help heterophylly to occur have suggested that various phytohormones affect heterophylly. Therefore, it is worthwhile to notice the relationships and interactions between heterophylly and phytohormones.

Types of Heterophylly

Although there are various and wide−ranging phenomena of heterophylly that exist in various plant species, the forms of heterophylly may be divided into three major categories which are the following:

• Habitual heterophylly − In the case of habitual heterophylly, the leaves are borne in different shapes on different parts of the plant. The leaves of an aquatic plant may have different types of leaves in different parts of the plant in such kinds of heterophylly. However, this kind of heterophylly is also present in terrestrial plants. An example of habitual heterophylly is found in an herb in Eastern India where the stem has ovate leaves and the branch has acicular leaves.

• Developmental heterophylly − In developmental heterophylly, the leaves of a plant have different shapes and sizes at different developmental phases of the plant. The leaves in such kind of heterophylly arise in different periods of growth. Examples are cotton and coriander leaves in the juvenile and mature stages.

• Environmental heterophylly − Environmental heterophylly is found in plants that adapt to environmental conditions by having different types of leaves that are present in different conditions. This heterophylly is usually found in aquatics. An example is a buttercup where the aerial leaves and floating leaves are different.

Key Notes about Plasticity and Heterophylly

• Plasticity is the method by which plants follow numerous pathways responding to the external environment.

• Plants like cotton, larkspur, coriander, and buttercup have leaves in the juvenile phase which are completely different from the mature phase are known as heterophylly.

• There are three types of Heterophylly: Habitual heterophylly, Developmental heterophylly, and Environmental heterophylly.

• In Habitual heterophylly, the leaves are borne in different shapes on different parts of the plant.

• In Developmental heterophylly, the leaves in different periods of growth are different in shapes, sizes, and nature.

• In Environmental heterophylly: the leaves in different environmental conditions are different in types.

• Environmental heterophylly is usually found in aquatic plants.

• The phenomenon by which plants alter their morphology to adapt to environmental conditions is known as plasticity.

• Heterophylly is a special type of plasticity that is limited to the leaves of plants.

Conclusion

Hetyerophylly and plasticity are unique characteristics limited to some plant species and it is worthwhile to research why such occurrence takes place. Researchers have been interested in finding the reasons why heterophylly takes place and they are inquisitive about why only some plants show heterophylly. The research is interesting as well as important because it can reveal the nature of plants and their responding mechanism to adapt to environmental conditions. This can reveal many secrets about plant species and their morphologies.

Frequently Asked Questions

Qns 1. What is meant by heterophylly?

Ans. Plant species may respond to variations in environmental conditions. Many plant species can alter their leaf morphology in response to changes in environmental conditions. This phenomenon is known as heterophylly and it is widespread among land plants.

Qns 2. How many types of heterophylly are there?

Ans. There are three types of Heterophylly: Habitual heterophylly, Developmental heterophylly, and Environmental heterophylly.

Qns 3. What is developmental heterophylly?

Ans. In developmental heterophylly, the leaves of a plant have different shapes and sizes at different developmental phases of the plant. The leaves in such kind of heterophylly arise in different periods of growth. Examples are cotton and coriander leaves in the juvenile and mature stages.