Exponential Nature of Heat Exposure Duration Relative to Temperature Change in the Curing and Flowering of Bulbous Iris1

Abstract

A semi-log plot of time and temp, representing treatment conditions successfully used for the heat curing of ‘Wedgwood’ iris bulbs, resulted in a wedge shaped distribution of points. A line passing through the lower side of this distribution was interpreted as the locus of min time values at given temp for the successful heat treatment of iris bulbs. Several time-temp treatment combinations from points along this line and other combinations were selected for forcing trials with ‘Ideal’ iris. After subtraction of correction values based on a 2-day period of delayed bulb response, the corrected equivalent units showed positive correlation with the earliness of the 50% flowering date, the decreasing lengths of first and fifth leaves, and the decreasing number of leafy plants. The per day effect of treatment at 32°C declined beyond the 10th day. The equivalent unit method is a new procedure for calculating the heat curing response for bulbous iris,

Iris Bulb Heat Curing Calculations for the Minimum Number of Curing Days by the Equivalent Unit Method1

“…Tsukamoto and Ando (10) described heat curing as a process for the removal of growth inhibition, "an essen tial factor to termination of dormancy and, successively, to the promotion of flower development." There was a dis tinct acceleration of flowering first re- (5,7).…”

“…It was based on the curing of iris bulbs at 32.2°C and temper atures other than 32.2°C in conjunction with different time intervals and express ing the results by a common unit, the 32.2°C day equivalent unit (EU). One EU or equivalent unit has previously been defined (5) as 1 day at 32.2°C or its equivalent at another time-interval and temperature combination that effects a similar curing response in the iris bulb. Thus, an easily interpreted common unit, the EU, can be applied both to field and chamber curing with the result that the deficit in heat units can be calculated at any given time prior to or after the bulbs are lifted by a summation of the EU values.…”

“…The fact that the speed of the bulb cur ing process increases with temperature (5) suggests that enzymatic processes are involved. The decreasing effectiveness of applied heat beyond the 10th day at 32.2°C (5), moreover, suggests a limited quantity of an unknown substrate upon which the enzymes act.…”

“…EU values were correlated with bulb performance in 1970 (5). Bulbs were lift ed on July 8 with an accrued EU of 6 units (measured at WWREC) and re ceived further chamber curing at 32.2°C for 0, 5 (+ 5 EU), 10 (+ 10 EU), and 15 (+15 EU) days for performance evalua tion.…”

“…X EU values for temperatures below 21°C were ob tained from an extrapolation of time-interval vs. tem perature plot(5), the locus of minimum curing time values at given temperatures. Points on the plot were considered to have equal curing effects on the bulb, specifically, 10 EU per the number of days at the giv en temperature.…”