Thermoperiodicity in Shoot Elongation of Purple Nutsedge

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. Sprouting of single tubers of purple nutsedge is thermally regulated (Holt and Orcutt, 1996; Miles 1991; Miles et al., 1996; Tripathi 1967; Ueki 1969). Miles et al. (1996) studied the effect of alternating temperatures on purple nutsedge tuber sprouting and showed that all tubers sprouted under alternating temperatures (35/ 25 ˚C, 12/12 h), while only 66% sprouted within 4 weeks at the constant mean temperature (30 ˚C). They defined sprouting as budbreak and shoot elongation to ≥10 mm. Purple nutsedge tuber sprouting probably involves two steps: budbreak and elongation of the emerged bud (Nishimoto et al., 1995). Budbreak occurred within 2 weeks for ≈75% of the tubers at constant 20 ˚C, or at relatively constant temperatures of ≈23 ˚C in the laboratory, but only ≈10% of the emerged buds elongated >10 mm (Nishimoto et al., 1995). Nearly all of the remaining dormant tubers were induced to the budbreak stage by a single 30- min 35 ˚C pulse (Sun and Nishimoto, 1997). Thus, the budbreak process occurs readily and can be triggered by high temperature. However, there is no information on thermal regulation of bud elongation of purple nutsedge. This research focused on the response of the elongation of the emerged bud to alternating temperatures and whether the response is thermoperiodic. Thermoperiodicity or thermoperiodism refer to a physiological response of an organism to the alternation of high and low temperatures. The term thermoperiodicity in plants was coined by Went (1944). Went (1944) and other researchers (Dale, 1964; Friend and Helson, 1976; Wellensiek, 1957) indicated that the thermoperiodic effect could stimulate plant growth and develop- ment; their definition assumed that light and associated growth was important during part of the daily cycle. The thermoperiodic- ity concept provides a framework for characterizing the elongation response of purple nutsedge buds. Thermoperiodicity is the plant response to alternating tempera- ture rather than to different absolute temperatures. It is difficult to separate effects of alternating temperature from effects of the different temperatures themselves. The use of constant upper, lower (Went, 1944), or mean temperature of the alternation cycle (Haroon et al., 1972; Lionakis and Schwabe, 1984; Wellensiek, 1957) as a reference temperature is not sufficient to demonstrate thermoperiodicity because the growth response to constant tem- perature is not linear over a wide temperature range. The choice of suboptimal or supraoptimal temperatures for experiments can cause inconsistent effects over a wide temperature range (Friend and Helson, 1976). Thus, Dale (1964) and Friend and Helson (1976) tested for thermoperiodicity by assessing growth (dry weight, stem elongation, etc.) at the constant temperature where optimal growth occurred and compared that with growth at the alternating temperature regime with the same diurnal mean. Thermoperiodism is not a phenomenon common to all species. Using Dale's definition of thermoperiodism, no evidence was found for a thermoperiodic response in dry weight of bean ( Phaseo- lus vulgaris L.), cucumber (Cucumis sativus L.), maize (Zea mays L.), oat (Avena sativa L.), pea (Pisum sativum L.), tomato (Lyco- persicon esculentum Mill) and wheat (Triticum aestivum L.) (Dale, 1964; Friend and Helson, 1976). A number of studies showed that growth of sugar cane (Saccharum officinarum L. hybrid 'Pintar') (Glasziou et al., 1965), sugar beet ( Beta vulgaris