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Wen-Hao Sun and Roy K. Nishimoto

The effect of single and daily fluctuating temperature cycles on budbreak of purple nutsedge (Cyperus rotundus L.) tubers was characterized to determine whether the budbreak response was a trigger or rhythmic action. Glasshouse-grown tubers were incubated at 20 °C for 2 weeks, and the nonsprouted tubers were used in experiments. A single 30-minute temperature pulse from 20 to 35 °C caused 80% budbreak, while tubers without the thermal pulse had only 25% budbreak. Increasing the duration of the single 35 °C temperature pulse from 30 minutes to 12 hours did not increase budbreak. Even a 3-minute 35 °C pulse caused 63% budbreak. A single gradual shift (0.02 °C per minute) from 20 to 35 °C was as effective as a rapid shift (0.5 °C per minute) in stimulating budbreak. Nonsprouted tubers preincubated at 25 °C were stimulated to cause budbreak by exposure at 35 °C for 12 hours, but not at 15 °C for 12 hours. A single temperature pulse of at least 10 °C differential (from 20 to 30, 35, or 40 °C) stimulated budbreak, while a 5 °C differential (from 20 to 25 °C) did not; all pulses had the same heat unit (°C > 20 °C × time (minutes) = 3600 °C/minute). These results suggest that budbreak of purple nutsedge tubers responds to high temperature as a trigger action.

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Wen-Hao Sun and Roy K. Nishimoto

The effect of single and daily alternating temperature cycles on elongation of emerged buds of purple nutsedge (Cyperus rotundus L.) was characterized to determine whether shoot elongation responded to alternating temperature as a thermoperiodic function. Glasshouse-grown tubers with emerged buds of 2 to 5 mm in length were used in experiments. Shoot extension increased at 35 °C after 7 days, but no significant shoot extension occurred at all other constant temperatures of 20, 25, 30, 40, and 45 °C. However, 2- to 8-fold increases in shoot extension occurred at alternating temperatures of 25/15, 30/20, 35/25, 40/30, 41/35, 42/38, and 45/35 °C (12/12 hours) as compared to the respective mean constant temperatures. Daily temperature differences of 2 and 4 °C did not stimulate shoot elongation, while temperature differences of 8 and 12 °C caused an 8-fold shoot stimulation when compared to the 24 °C constant temperature. Shoot elongation increased with increasing numbers of alternating temperature cycles. The optimal duration of the lower and upper temperature phases differed depending on temperature regimes; at 40/30 °C, maximal elongation occurred with daily exposures of 40 °C for 1 to 3 hours and 30 °C for 23 to 21 hours respectively, while at 30/20 °C, maximal elongation occurred with daily exposures of 30 °C for 15 hours and 20 °C for 9 hours. These results suggest that elongation of purple nutsedge tuber buds responds to alternating temperature as a thermoperiodic function.