Plant Biology

Plant Biology Shoot architecture enhances photosynthesis largely by maximizing the ability of the plant to get sunlight. Obviously light is the crucial resource in photosynthesis, but different plants obtain the resource in different ways. For example, the basic arrangement of leaves on the plant stem (known as "phyllotaxy") is different for every species of plant -- however, in all of these cases the plant has evolved so that the emergence of leaves does not block the available light for leaves above or below.

The evolutionary reason for this should be obvious: as leaves exist purely to facilitate photosynthesis, the production of leaves that underperform due to inadequate access to sunlight would be a waste of the plant's resources. We can also see the evolutionary imperative expressed in leaf size: in cold or dry environments without much access to liquid water, the leaf size is drastically shrunken in an attempt to balance the necessity of photosynthesis with the risk of losing precious water resources via evaporation.

Larger surface area of a leaf may mean more sunlight, but it also means more evaporative loss of water. Likewise plants have also evolved the ability to self-prune, which is to effectively remove those leaves which have grown in a way to have limited access to sunlight, and thus expend more resources on respiration than they acquire by photosynthesis. However other more complicated forms of shoot architecture are also geared toward the basic function of maximizing the plant's access to sunlight and thus enhancing the plant's photosynthetic productiveness.

As a result, plants that have evolved for height (like a sunflower) or to spread out in branching structures (like a white oak) are essentially engaged in competitive strategies to increase access to sunlight: the sunflower will grow taller than its nearer competitors, whereas the white oak will spread wider to maximize its own photosynthetic capabilities. Overall the basic evolutionary rationale behind all shoot architecture is to maximize light absorption. Question 2. (381 words) Evolution does not occur in a vacuum -- as plants evolved, various other organisms have evolved alongside them.

In many cases, this has resulted in symbiotic relationships, in which a mutually beneficial arrangement evolves between the plant and the other organism. In terms of plant root systems, for example, soil teems with various bacteria and fungi. In numerous cases, these have evolved together, so that the bacteria or fungi assist the plant and in return increase their own nutritional intake.

With bacteria, the process is generally known as "nitrogen fixation." The bacteria that grow around the root systems provide a source of fixed nitrogen which is used by the plant in the formation of chemical compounds. The process occurred when the bacteria, which are anaerobic and thus undergo respiration in an environment with no oxygen -- the bacteria provide the plant with nitrogen while in turn the plant protects the bacteria from oxygen. A different sort of symbiotic relationship occurs between plant root systems and fungi.

Mycorrhizae -- which literally means "fungus roots" -- are in essence a symbiotic arrangement of root systems with underground fungus. The fungus has a safe place to live and is supplied with sugar by the host plant, but in return the fungus is beneficial for the plant with which it has the relationship. The fungus spreads and grows in a way that expands the plants overall area for water absorption and ultimately can select specific chemicals (like phosphates).