PP-03 — Ten-Sentence Master Summary

1. Plant growth is an irreversible permanent increase in size involving cell division, enlargement, and differentiation, proceeding through meristematic, elongation, and maturation phases.

2. Arithmetic growth (Lt = $L_{0}$ + rt) occurs when one daughter cell differentiates; geometric growth ($W_{1}$ = $W_{0}$eʳᵗ) occurs when both daughters continue dividing; in nature, growth follows a sigmoid (lag → log → stationary) curve.

3. Differentiation produces permanent specialized cells; dedifferentiation reverses this (parenchyma → cambium); redifferentiation produces new permanent tissue (cambium → secondary xylem).

4. Auxins (IAA, indole compounds) promote apical dominance and cell elongation; 2,4-D is a synthetic auxin used as a selective herbicide killing dicots but not monocots.

5. Gibberellins ($GA_{3}$, terpenoids, from Gibberella fujikuroi) cause bolting in rosette plants and induce alpha-amylase production in the aleurone layer of barley endosperm during germination.

6. Cytokinins (adenine derivatives, discovered by Skoog and Miller from herring sperm DNA) promote cell division and delay leaf senescence — the delay of senescence by cytokinins is the Richmond-Lang effect.

7. Ethylene ($C_{2}H_{4}$) is the only gaseous hormone; it promotes fruit ripening, abscission, and causes the triple response (inhibited elongation + radial swelling + horizontal growth) in etiolated seedlings.

8. Abscisic acid (ABA, a terpenoid) is the stress hormone — it promotes stomatal closure by causing $K^{+}$ efflux from guard cells, and maintains seed dormancy in direct antagonism with gibberellin.

9. Photoperiodism classifies plants as SDP (rice, chrysanthemum — need long nights), LDP (wheat, spinach — need short nights), or DNP (tomato, cucumber — independent); phytochrome's Pfr form promotes LDP flowering and inhibits SDP flowering.

10. Vernalization requires cold treatment at 0–5°C (needed by winter wheat and biennials), and works by epigenetically silencing the FLC floral repressor via H3K27me3, enabling subsequent gibberellin-driven bolting and flowering in spring.