22-Agric-A1 Applied Plant, Animal or Human Physiology · May 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. 04-Agric-A1 Applied Plant Physiology, National Exams May 2015 — a three-hour closed-book examination; one of two approved calculator models (Casio or Sharp) is permitted. All six (6) printed questions constitute a complete exam paper totaling 100 marks, and all six are worked here.
Reference texts. L. Taiz, E. Zeiger, I.M. Møller and A. Murphy, Plant Physiology and Development, 6th ed. (plant tissue types, photosynthesis, water potential and osmosis, phytochrome-mediated photoperiodism); E. Runkle, Daily Light Integral: A Useful Tool for Greenhouse Growers, Michigan State University Extension (DLI, greenhouse light transmission, supplemental lighting design); ASABE Standards (American Society of Agricultural and Biological Engineers) (greenhouse environment engineering).
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
Photoperiodism is a plant's developmental response to the relative lengths of day and night, and it is the main environmental cue many species use to time flowering to the season. Despite the name, plants actually measure the length of the uninterrupted dark period, not the length of daylight — the operative variable is the critical night length. Short-day plants (SDP) (e.g. chrysanthemum, poinsettia, rice) flower only when the uninterrupted night is longer than their critical night length, so they flower as nights lengthen in late summer/autumn. Long-day plants (LDP) (e.g. spinach, wheat, many spring bulbs) flower only when the uninterrupted night is shorter than their critical night length, so they flower as nights shorten in spring/early summer. Day-neutral plants (e.g. tomato, corn) flower on an internal developmental schedule regardless of photoperiod.
The night-length sensor is the pigment phytochrome, which exists in two photo-interconvertible forms: Pr (red-light-absorbing, biologically inactive) and Pfr (far-red-absorbing, the biologically active form that triggers the flowering signal). Red light (≈660 nm) converts Pr → Pfr; far-red light (≈730 nm) converts Pfr back to Pr; and in darkness, Pfr slowly reverts to Pr on its own. A long, uninterrupted night therefore lets Pfr fall to a low level by dawn, while a night broken partway through by a flash of red light drives Pfr back up — effectively resetting the plant's perceived night to two short nights instead of one long one. Because Pfr is the active signal, this "night-break" has opposite effects on the two plant classes: interrupting a long night with red light prevents flowering in a short-day plant (Pfr is no longer allowed to fall low enough for the required duration) but induces flowering in a long-day plant (the elevated Pfr is exactly the signal an LDP needs). The effect is fully reversible by phytochrome's own photochemistry: following the red flash immediately with a flash of far-red light converts Pfr back to Pr and cancels the interruption, restoring the plant's response to what it would have been under the original uninterrupted night — the classic experimental proof that phytochrome, not red light energy itself, is the flowering switch.
