When a plant is deprived of nutrient availability in the root zone, it will generally prioritize the allocation of available nutrients to essential functions such as growth and development. In this scenario, it is unlikely that the plant will actively extract nutrients from the leaves and translocate them to the buds.
However, it's important to note that plants can undergo nutrient remobilization processes under specific conditions. When a plant is approaching senescence (the natural aging and death of plant tissue), it may mobilize nutrients from older leaves and redistribute them to newer growth areas, such as developing buds or fruits. This process occurs naturally and is regulated by the plant's physiology.
In summary, while plants have mechanisms for nutrient remobilization, the specific process you mentioned, where plants actively extract nutrients from leaves when the roots no longer receive nutrients, is not scientifically supported in the context of C3 plants.