The Effect of Sunlight on Grass

The Processes

• The storage (mesophyll) cells in grass blades, commonly known as chloroplasts, contain chlorophyll- a substance that absorbs the red and blue light from the sun. We see the grass as green because the sun's green light is not absorbed, but rather reflected to be perceived by receptor cones in our eyes. Photosynthesis is a two part process. The first, the light reaction, changes light energy into chemical energy. The second, the dark reaction, converts carbon dioxide into sugar.

The Light Reaction

• The light reaction is so- called because the chemical reaction must occur in the presence of sunlight. Light is absorbed by clusters of pigments and directed to the central chlorophyll molecule where photosynthesis takes place. The energy produced is held in the plant cells with the help of adenosine triphosphate (ATP), a chemical compound used by plant cells for energy storage. In ATP, adenine is bonded to a ribose sugar that is bonded in turn to three phosphates.

The Dark Reaction

• The dark reaction does not require sunlight but does need the ATP produced by the light reaction. In this reaction, carbon dioxide and energy from ATP are combined to create molecules of glucose. This process is known as the Calvin cycle. Photosynthesis ceases to occur during hot, dry weather when carbon dioxide levels drop due to the excess evaporation of water from the surface of the leaves. The grass turns brown and becomes dormant until weather conditions allow the light and dark reactions to resume.

The Role of the Roots

• The sun's energy can only be captured if the grass roots supply enough water and minerals from the soil to the blades of grass. The supply process, known as transpiration, also shuts down during hot, dry spells. This protective measure allows the grass to store scarce water and mineral supplies deep in the roots until the weather improves. Stressed lawns may appear brown, dry and dead but most grasses will emerge from dormancy when growing conditions improve.