What is Abiotic stress?

A plant cannot escape, so it hardens. Drought: falling water potential in the roots triggers ABA synthesis; ABA closes stomata within minutes, and over days switches on genes for osmotic adjustment (proline, sugars and other compatible solutes accumulate, lowering the cell’s water potential so it keeps drawing water), for dehydrins that protect proteins, and for a smaller leaf area and a deeper root. Salt is drought plus poison: sodium enters through potassium channels and competes with potassium; tolerant plants pump it back out of roots (the SOS pathway), into vacuoles (NHX exchangers), or excrete it from leaf glands, and their halophyte extreme lives in sea water. Cold: chilling stiffens membranes and slows enzymes; freezing draws water from cells into extracellular ice and desiccates them. Cold acclimation — a week of cool days — induces the CBF transcription factors, which turn on hundreds of genes for membrane-fluidising lipids, sugars, antifreeze proteins and dehydrins, and a winter rye that would die at $-5\,{}^{\circ}\mathrm{C}$ in August survives $-30\,{}^{\circ}\mathrm{C}$ in January. Heat unfolds proteins; within minutes of a rise a heat-shock factor releases heat-shock proteins, chaperones that refold or dispose of them, the same families as in every organism. Flooding starves roots of oxygen; rice makes air channels (aerenchyma) and elongates to keep its leaves above water, ethylene being the signal that accumulates when it cannot escape into the water. Many of these programmes overlap — ABA, reactive oxygen species and calcium spikes are shared second messengers — and a plant acclimated to one stress is often partly protected against another.