Radicle length and osmotic stress affect the chilling sensitivity of cucumber radicles

Cold soil and air temperatures reduce germination, seedling growth, and stand establishment of many important agronomic and horticultural crops. Chilling tolerance of many seeds is lost as they germinate and grow into young seedlings. Cucumber (Cucumis sativus cv. Poinsett 76) seedling radicles lose chilling tolerance as they emerge and elongate. Chilling at 2.5 degrees C for 72 h reduced subsequent elongation at 25 degrees C by 12, 41, and 77% for 1-, 10-, and 20-mm-long radicles, respectively. Radicle elongation followed an exponential decline with increasing mannitol concentration (r2 > 0.9) for radicles initially 1 or 10 mm in length. Chilling 1-mm radicles treated for 24 h in 0.3 or 0.6 M mannitol inhibited elongation 90 or 40%, respectively. During mannitol treatment, 0.3 and 0.6 M treated radicles increased 12.5 and 0.7 mm in length. Chilling 10-mm radicles treated for 24 h in 0.3 or 0.6 M mannitol inhibited elongation 99 or 33%, respectively. During mannitol treatment, 0.3 and 0.6 M treated radicles increased 23.8 and 1.1 mm in length. The percentage inhibition of chilling-induced radicle elongation was related to the initial radicle length when chilled by a curve of the form C(L) = C(o) x (1 - e(-KL)) that describes many biological reactions where the rate of change is constant and proportional to the amount of reactants present. The increase in chilling-induced inhibition of radicle elongation with increasing radicle length was consistent with the progressive loss of protective compounds, possibly through dilution as the tissue expanded in volume during radicle elongation.