Effect of heat shock on the chilling sensitivity of trichomes and petioles of African violet (Saintpaulia ionantha)

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Abstract
Chilling at 6°C caused an immediate cessation of protoplasmic streaming in trichomes from African violets (Saintpaulia ionantha), and a slower aggregation of chloroplasts in the cells. Streaming slowly recovered upon warming to 20°C, reaching fairly stable rates after 4, 15, 25 and 35 min for tissue chilled for 2 min and for 2, 14 and 24 h, respectively. The rate of ion leakage from excised petioles into an isotonic 0.2 M mannitol solution increased after 12 h of chilling and reached a maximum after 3 days of chilling. A heat shock at 45°C for 6 min reduced chilling‐induced rates of ion leakage from excised 1‐cm petiole segments by over 50%, namely to levels near that from non‐chilled control tissue. Heat‐shock treatments themselves had no effect on the rate of ion leakage from non‐chilled petiole segments. Protoplasmic streaming was stopped by 1 min of heat shock at 45°C, but slowly recovered to normal levels after about 30 min Chloroplasts aggregation was prevented by a 1 or 2 min 45°C heat‐shock treatment administered 1.5 h before chilling, but heat‐shock treatments up to 6 min only slightly delayed the reduction in protoplasmic streaming caused by chilling. Tradescantia virginiana did not exhibit symptoms associated with chilling injury in sensitive species (i.e. cessation of protoplasmic streaming in stamen hairs and increased ion leakage from leaf tissue).