Question

1. the absence of which substance is most responsible for abscission in plants?

chlorophyll
nitrogen
phytochromes
salt

Explanation:

Phytochromes are light - sensing pigments in plants. Chlorophyll is for photosynthesis, nitrogen is a nutrient, and salt is an environmental factor. Abscission (shedding of leaves, etc.) is related to phytochrome - mediated responses to light changes (like shorter days in autumn).

Answer:

phytochromes