DPD (Diffusion Pressure Deficit) refers to:
B fits best: Difference in diffusion pressure from pure water. Keep the WATER POTENTIAL basics straight and Difference in diffusion pressure from pure water stands out as the precise answer. Avoid: A) Total water potential; C) Osmotic pressure; D) Turgor pressure. Light and flowering items often hinge on which photoreceptor or day-length rule is in play.
Ref: Best CSIR NET Plant Physiology books: Master Unit 6 with Taiz & Zeiger and Salisbury & Ross. Crack Part C experimental questions with top textbooks.