How do plants primarily respond to gravity?

Plants primarily respond to gravity through a phenomenon known as gravitropism. This response is critical for their growth and survival, allowing them to orient themselves correctly in their environment. In gravitropism, roots exhibit positive gravitropism, meaning they grow downward in response to gravitational pull. This downward growth helps the roots anchor the plant in the soil and enables them to access water and essential nutrients from the ground. Meanwhile, stems typically show negative gravitropism by growing upward, away from the gravitational force, which allows the plant to position its leaves towards sunlight for optimal photosynthesis.

The other options do not directly relate to how plants respond to gravity. Closing stomata pertains to gas exchange and regulation of water loss rather than a gravitational response. Adjustments in photosynthesis do not specifically indicate a direct reaction to gravity, as photosynthesis primarily depends on light availability. Similarly, increasing transpiration rates is related to moisture loss and does not reflect the plant's response to gravity. Thus, gravitropism is the primary and most relevant response mechanism for plants when dealing with gravitational forces.