Which group of herbicides primarily causes symptoms along the leaf margins and tips of older leaves?

The group of herbicides that primarily causes symptoms along the leaf margins and tips of older leaves is indeed photosynthesis inhibitors. These herbicides disrupt the process of photosynthesis, leading to a reduction in the plant's ability to produce energy. As a result, the older leaves, which are already at the end of their life cycle and rely heavily on photosynthesis for sustenance, begin to show symptoms of stress first.

The marginal chlorosis or necrosis (yellowing or browning) seen in older leaves is a classic symptom of photosynthesis inhibitors. This discoloration often begins at the leaf tips and edges, as these areas are typically the furthest from the main vascular tissues that transport nutrients and water. The impact on older leaves is important because they cannot replace their energy reserves as effectively as new leaves.

In contrast, seedling growth inhibitors generally affect the root and shoot development of seedlings, while root growth stimulators promote root growth, and contact pesticides act on the parts of plants they physically touch, causing damage that typically does not specifically manifest at the leaf margins or tips. Understanding the modes of action for these different herbicide groups helps in diagnosing plant health issues and enhancing effective weed management practices.