Effect Of Diurnal Temperature Range Research Articles

An Australian national panel study of diurnal temperature range and children's respiratory health

BackgroundIt is still uncertain whether diurnal temperature range (DTR) affects children's respiratory function. ObjectiveTo examine the effects of DTR on lung function and respiratory symptoms for school children with asthma in Australia. MethodsA panel of 270 children (ages 7-12 years) with asthma living in 6 Australian cities was recruited. They were asked to perform 3 successive forced expiratory maneuvers using a portable electronic peak flow meter twice daily for 4 weeks. The highest values for peak expiratory flow (PEF) were stored for each session. At the same time, they were asked to record their respiratory symptoms (eg, cough and/or phlegm and wheeze and/or chest tightness) every day in the morning (for nighttime symptoms) and evening (for daytime symptoms). Daily data on different metrics of ambient temperature and air pollution were obtained from fixed monitors nearby. Relative humidity data were downloaded from the Weather Underground website. Mixed models, adjusting for children's individual characteristics and air pollution, were used to examine the effects of DTR on PEF and respiratory symptoms. ResultsDTR had linear effects on PEF and respiratory symptoms. An increase in DTR induced a reduction in PEF and increased the occurrence of respiratory symptoms. In general, the effects lasted for 3 days (lag, 0-2 days). The effects occurred for both boys and girls. ConclusionOur findings provide evidence that DTR had significant effects on lung function and respiratory symptoms for children with asthma. These results indicate that it is important and necessary to protect children with asthma from the effect of unstable weather.

Effects of diurnal temperature range and seasonal temperature pattern on the agronomic traits of fibre flax (Linum usitatissimum L.)

Yang, F., Liu, F.-h. and Rowland, G. 2013. Effects of diurnal temperature range and seasonal temperature pattern on the agronomic traits of fibre flax ( Linum usitatissimum L.). Can. J. Plant Sci. 93: 1249–1255. Three cultivars of fibre flax (Linum usitatissimum L.), Viking, Argos and Ariane, were used to study the effect on various agronomic traits of diurnal temperature ranges (DIF) and seasonal temperature patterns (STP) applied throughout the crop growth period. Diurnal temperature ranges were set at 5, 10 and 15°C with the same daily mean temperature and accumulated growing degree days (GDDa), under a parabolic STP. Seasonal temperature patterns were set following parabolic, anti-parabolic and quasi-horizontal configurations with DIF set at 10°C. The results showed that DIF or cultivar had a significant influence on the biomass, plant height (PH), stem diameter (SD), length of technical stem (LTS), weight of technical stem (WTS), fibre weight (FW), fibre content (FC) and seed weight per pot (SWp). Seasonal temperature pattern also had a significant influence on these traits except for WTS and FW. Significant interactions were observed between DIF and cultivar, affecting all traits with the exception of FW, and between STP and cultivar, affecting biomass, PH, SD and WTS. Diurnal temperature range set at 10°C was optimal for biomass, PH, SD, LTS, WTS, and FW, while DIF set at 5°C benefited FC and SWp. Among the different STP configurations, anti-parabolic pattern was optimal for biomass, PH, SD, LTS and FC, and the quasi-horizontal pattern best fit SWp. In conclusion, the combination of DIF set at 10°C and anti-parabollic STP provided the most favorable temperature conditions for fibre flax growth, which is the situation with winter flax production in Yunnan, China.

Is Greater Temperature Change Within a Day Associated With Increased Emergency Hospital Admissions for Heart Failure?

Although the seasonal variation and the effect of cold temperature on heart failure (HF) morbidity have been well documented, it is unknown whether the temperature variation within a day, that is, diurnal temperature range (DTR), is an independent risk factor for HF. We hypothesized that large DTR might be a source of additional environmental stress and, therefore, a risk factor for HF exacerbation. We aimed to test the association between DTR and HF hospitalization and to examine the effect modifiers, such as age, sex, and season. We collected daily meteorologic data and emergency HF hospital admissions from 2000 to 2007 in Hong Kong. We used Poisson regression models to fit the relationship between daily DTR and emergency HF hospitalizations, after adjusting for the time trend, seasonality, mean temperature, humidity, and levels of outdoor air pollution. We confirmed the seasonal variation of HF with peak hospital admissions in winter in Hong Kong. The adverse effects of DTR on emergency HF admissions were observed on the current day and lasted for the following several days. Every 1°C increase of DTR at lag0 corresponded to 0.86% (95% confidence interval, 0.31%-1.43%) increment of emergency hospital admissions for HF. DTR exhibited significantly greater effect in the cool season, and on female and elderly patients. Greater temperature change within a day was associated with increased emergency hospital admissions for HF. Health policymakers and hospitals may want to take into account the increased demand of specific facilities for susceptive population in winter with greater daily temperature variations.

Is diurnal temperature range a risk factor for childhood diarrhea?

BackgroundPrevious studies have found that high and cold temperatures increase the risk of childhood diarrhea. However, little is known about whether the within-day variation of temperature has any effect on childhood diarrhea.MethodsA Poisson generalized linear regression model combined with a distributed lag non-linear model was used to examine the relationship between diurnal temperature range and emergency department admissions for diarrhea among children under five years in Brisbane, from 1st January 2003 to 31st December 2009.ResultsThere was a statistically significant relationship between diurnal temperature range and childhood diarrhea. The effect of diurnal temperature range on childhood diarrhea was the greatest at one day lag, with a 3% (95% confidence interval: 2%–5%) increase of emergency department admissions per 1°C increment of diurnal temperature range.ConclusionWithin-day variation of temperature appeared to be a risk factor for childhood diarrhea. The incidence of childhood diarrhea may increase if climate variability increases as predicted.

The association between diurnal temperature range and emergency room admissions for cardiovascular, respiratory, digestive and genitourinary disease among the elderly: A time series study

ObjectivesTo evaluate the short-term effect of diurnal temperature range (DTR) on emergency room (ER) admissions among elderly adults in Beijing. MethodsAfter controlling the long-time and seasonal trend, weather, air pollution and other confounding factors, a semi-parametric generalized additive model (GAM) was used to analyze the exposure–effect relationship between DTR and daily ER admissions among elderly adults with different lag structures from 2009 to 2011 in Beijing. We examined the effects of DTR for stratified groups by age and gender, and conducted the modifying effect of season on DTR to test the possible interaction. ResultsSignificant associations were found between DTR and four major causes of daily ER admissions among elderly adults in Beijing. A 1°C increase in the 8-day moving average of DTR (lag 07) corresponded to an increase of 2.08% (95% CI: 0.88%–3.29%) in respiratory ER admissions and 2.14% (95% CI: 0.71%–3.59%) in digestive ER admissions. A 1°C increase in the 3-day and 6-day moving average of DTR (lag 02 and lag 05) corresponded to a 0.76% (95% CI: 0.07%–1.46%) increase in cardiovascular ER admissions, and 1.81% (95% CI: 0.21%–3.45%) increase in genitourinary ER admissions, respectively. The people aged 75years and older were associated more strongly with DTR than the 65–74 age group. The modifying effect of season on DTR was observed and it was various in four causes. ConclusionsThis study strengthens the evidence that DTR is an independent risk factor for ER admissions among elderly persons. Some prevention programs that target the elderly and other high risk subgroups for impending large temperature changes may reduce the impact of DTR on people's health.

Lagged effect of diurnal temperature range on mortality in a subtropical megacity of China.

BackgroundMany studies have found extreme temperature can increase the risk of mortality. However, it is not clear whether extreme diurnal temperature range (DTR) is associated with daily disease-specific mortality, and how season might modify any association.ObjectivesTo better understand the acute effect of DTR on mortality and identify whether season is a modifier of the DTR effect.MethodsThe distributed lag nonlinear model (DLNM) was applied to assess the non-linear and delayed effects of DTR on deaths (non-accidental mortality (NAD), cardiovascular disease (CVD), respiratory disease (RD) and cerebrovascular disease (CBD)) in the full year, the cold season and the warm season.ResultsA non-linear relationship was consistently found between extreme DTR and mortality. Immediate effects of extreme low DTR on all types of mortality were stronger than those of extreme high DTR in the full year. The cumulative effects of extreme DTRs increased with the increment of lag days for all types of mortality in cold season, and they were greater for extreme high DTRs than those of extreme low DTRs. In hot season, the cumulative effects for extreme low DTRs increased with the increment of lag days, but for extreme high DTR they reached maxima at a lag of 13 days for all types of mortality except for CBD(at lag6 days), and then decreased.ConclusionsOur findings suggest that extreme DTR is an independent risk factor of daily mortality, and season is a modifier of the association of DTR with daily mortality.

Diurnal temperature range and childhood asthma: a time-series study

BackgroundHot and cold temperatures have been associated with childhood asthma. However, the relationship between daily temperature variation and childhood asthma is not well understood. This study aimed to examine the relationship between diurnal temperature range (DTR) and childhood asthma.MethodsA Poisson generalized linear model combined with a distributed lag non-linear model was used to examine the relationship between DTR and emergency department admissions for childhood asthma in Brisbane, from January 1st 2003 to December 31st 2009.ResultsThere was a statistically significant relationship between DTR and childhood asthma. The DTR effect on childhood asthma increased above a DTR of 10°C. The effect of DTR on childhood asthma was the greatest for lag 0–9 days, with a 31% (95% confidence interval: 11% – 58%) increase of emergency department admissions per 5°C increment of DTR. Male children and children aged 5–9 years appeared to be more vulnerable to the DTR effect than others.ConclusionsLarge DTR may trigger childhood asthma. Future measures to control and prevent childhood asthma should include taking temperature variability into account. More protective measures should be taken after a day of DTR above10°C.

Global climate change: Impact of diurnal temperature range on mortality in Guangzhou, China

Diurnal temperature range (DTR) is an important meteorological indicator associated with global climate change, but little is known about the effects of DTR on mortality. We examined the effects of DTR on cause-/age-/education-specific mortality in Guangzhou, a subtropical city in China during 2003–2010. A quasi-Poisson regression model combined with distributed lag non-linear model was used to examine the effects of DTR, after controlling for daily mean temperature, air pollutants, season and day of the week. A 1 °C increase in DTR at lag 0–4 days was associated with a 0.47% (95% confidence interval: 0.01%–0.93%) increase in non-accidental mortality. Stroke mortality was most sensitive to DTR. Female, the elderly and those with low education were more susceptible to DTR than male, the youth and those with high education, respectively. Our findings suggest that vulnerable subpopulations should pay more attention to protect themselves from unstable daily weather.

Effect of diurnal temperature range on cardiovascular markers in the elderly in Seoul, Korea

While diurnal temperature range (DTR) has been found to be a risk factor for mortality, evaluation of the underlying mechanisms involved in this association are lacking. To explain the association between DTR and health effects, we investigated how cardiovascular markers responded to DTR. Data was obtained from 560 participants who regularly attended a community elderly welfare center located in Seoul, Korea. Data collection was conducted a total of five times over a 3-year period beginning in August, 2008. We examined systolic and diastolic blood pressure (BP), heart rate (HR), and heart rate variability (HRV). Mixed-effects models and generalized additive mixed models were used to assess the relationship of DTR with BP, HR, and HRV. BP was not associated significantly with rapid temperature changes during the day. While HR was associated linearly with increments of DTR, the relationship between DTR and HRV showed nonlinear associations, or the presence of a cutoff around median DTR. At the cutoff level of DTR determined by an inflection point in the graph, standard deviation of normal-to-normal intervals (SDNN) and root mean square successive difference (RMSSD) were peaked, whereas the low frequency:high frequency (LF:HF) ratio was elevated with decreasing DTR below the cutoff level. The study demonstrated that HR increases with increasing temperature range during the day, and that HRV is reduced at small or large DTR, which suggests minimal cardiovascular stress around the median level of temperature range during the day.

Effects of diurnal temperature range on cardiovascular and respiratory hospital admissions in Korea

The effects of heat and cold waves have been studied as risk factors for cardiovascular and respiratory diseases. However, few studies have examined the effect of diurnal temperature changes on health. We hypothesized that the diurnal temperature range (DTR) may affect the rate of hospital admissions for cardiovascular- and respiratory-related diseases, and therefore investigated the risk of hospital admissions of cardiovascular (stroke, myocardial infarction, ischemic heart disease, cardiac failure, cardiac disease, and arrhythmia) and respiratory (asthma, chronic obstructive pulmonary disease, and pneumonia) diseases attributable to DTR in four metropolitan areas in Korea during 2003–2006. The area-combined effects of DTR on some cardiovascular and respiratory diseases were significantly increased by an increment of DTR. In particular, the effects on cardiac failure and asthma were significant with the percentage change of hospital admissions per 1°C increment of DTR at 3.0% (95% CI, 1.4–4.6) and 1.1% (95% CI, 0.1–2.0), respectively, among 9 diseases. For those 75years and older, the DTR effect on asthma admissions was greater than in those aged under 75years. These results support the hypothesis of a positive association between DTR and cardiovascular and respiratory hospital admission.

Diurnal Temperature Range as a Novel Risk Factor for Sudden Infant Death

Objective To assess the relationship between diurnal temperature range (DTR) and sudden infant death (SID) between 2001 and 2004 in Shanghai, China. Methods We conducted a time-stratified case-crossover analysis to estimate the percent increase of SID associated with changes in DTR after adjustment for daily weather conditions (temperature and relative humidity) and outdoor air pollution. Results DTR was significantly associated with daily SID. An increase of 1 °C in the current-day (L0) and in the 2-day moving average (L01) DTR corresponds to a 1.56% (95% CI: 0.97%, 2.15%) and a 1.89% (95% CI: 1.17%, 2.60%) increase in SID, respectively. Conclusion An increased DTR was associated with an increased risk of SID in Shanghai. More studies are needed to understand the effect of DTR on infant deaths.

NONLINEAR ASSOCIATION OF DIURNAL TEMPERATURE RANGE WITH OXIDATIVE AND PHYSIOLOGIC STRESS MARKERS IN THE KOREAN ELDERLY ENVIRONMENTAL PANEL STUDY (KEEPS)

Background and Aims: While diurnal temperature range (DTR) has been found to be a risk factor for mortality in a previous study with an assumption of linearity, some disease mortality, such as that associated with rheumatic heart disease (RHD), showed a threshold effect for DTR. To define the nonlinear relationship pathway between DTR and health effects, we investigated how physiologic and oxidative stress markers responded to DTR. Methods: This study was part of the Korean Elderly Environmental Panel Study (KEEPS). Data was obtained from 569 participants who regularly attended a community elderly care center located in Seoul, Korea. The study was conducted a total of five times over a three years period beginning in August, 2008. We examined physiologic stress markers including heart rate variability (HRV) and indices such as mean heart rate, standard deviation of normal-to-normal (SDNN), normalized low frequency (LF) and high frequency (HF), and low/high frequency ratio (LF:HF). Urinary malondialdehyde (MDA) was evaluated as an oxidative stress marker. Effects of DTR below or above the median level were estimated using a generalized linear mixed model. Results: Significant differences in effects of DTR above and below the median level indicated the presence of a threshold effect or a nonlinear association between DTR and HRV markers. Estimated effects for log-transformed SDNN were 0.211 (standard error [SE] 0.069, P =0.0021) below and -0.093 (SE 0.038, P =0.0129) above the median DTR. While the mean heart rate slope above median DTR was significant, slopes of normalized HF and LF:HF ratio below median DTR were statistically significant. In addition, log-transformed MDA showed a pattern similar to HRV indices. Conclusions: The study demonstrated that physiologic and oxidative stress increased at low and high DTR with minimal stress levels around the median level of DTR indicating a physiologic comfort point.

Effects of diurnal temperature range on adult size and emergence times from diapausing pupae in Papilio glaucus and P. canadensis (Papilionidae)

Abstract With recent climate warming trends, both the increase in thermal variance (i.e., diurnal temperature range; DTR) as well as increased mean temperature may impact many different organisms, especially poikilothermic invertebrates. Predictions of insect developmental rates using degree-days (thermal unit accumulations above the developmental base threshold of the insect) are based on daily mean temperatures, regardless of DTR. However, non-linearity and variance in the means and extremes are often ignored. The role of thermal variance (e.g., daily temperature extremes and DTR) was evaluated experimentally for two swallowtail butterfly sister species using a common day/night photoperiod of 18: 6 h photo: scoto-phase and corresponding daytime thermophase and nighttime cryophase periods of 22: 22°C (constant 22°C), 24: 16°C, and 26: 10°C (all three treatments had the same daily mean and the same degree-day accumulations). Although developmental rates of post-diapause pupae were largely unaffected for both species, our results show that sizes in P. canadensis females (but not males) were smaller in the treatments with more variance (26°C: 10°C) compared to constant 22°C. Such potentially significant impacts of size reduction in P. canadensis females were not observed in P. glaucus males or females under the same series of thermo-period treatments.

Modifiers of diurnal temperature range and mortality association in six Korean cities

Rapid temperature changes within a single day may be critical for populations vulnerable to thermal stress who have difficulty adjusting themselves behaviorally and physiologically. We hypothesized that diurnal temperature range (DTR) is associated with mortality, and that this association is modified by season and socioeconomic status (SES). We evaluated meteorological and mortality data from six metropolitan areas in Korea from 1992 to 2007. We applied generalized linear models (GLM) for quantifying the estimated effects of DTR on mortality after adjusting for mean temperature, dew point temperature, day of the week, and seasonal and long-term trends. Most areas showed a linear DTR-mortality relationship, with evidence of increasing mortality with increasing DTR. Deaths among the elderly (75 years or older), females, the less educated, and the non-hospital population were associated more strongly with DTR than with the corresponding categories. DTR was the greatest threat to vulnerable study populations, with greater influence in the fall season. DTR was found to be a predictor of mortality, and this relationship was modified by season and SES.

Diurnal Temperature Range and Cause-specific Cardiovascular Hospital Admission in Seoul, Korea—Time-series Analysis and Temperature-Matched Case-crossover Design

O-29A1-3 Background/Aims: Rapid temperature change within a day may increase blood pressure, clot, or cardiovascular work load. Hence, diurnal temperature range (DTR) may cause higher risk of cardiovascular disease (CVD) for population vulnerable to thermal stress. In this study, we hypothesized an increase of DTR which was associated with cardiovascular-related hospital admission in Seoul, South Korea. Methods: Data of daily hospital admission counts of 6 cause-specific CVD diseases in Seoul were collected from Korea National Health Insurance Corporation during 2003–2006. CVD diseases were arrhythmia, cardiac disease, cardiac failure, ischemic heart disease, myocardial infarction, and stroke. Results of time-series analysis and temperature-matched case-crossover design were compared. Associations in time-series analysis were evaluated using generalized linear model (glm) adjusted for confounding factors. In temperature-matched case-crossover design, control days were selected with temperature within ±1°C. Models included the first day of the month, day of the week, mean pressure, PM10, influenza, and relative humidity. Cause-specific effect of DTR was presented in percent change of risk. Results: Case-crossover design showed similar or greater effect than time-series analysis did. For time-series analysis, hospital admission risk was increased by 0.4%, 0.5%, 0.7%, and 1.4% for cardiac disease, stroke, myocardial infarction, and cardiac failure, respectively, with an increase of 1°C DTR. Significant risk of CVD-related hospital admission occurred after 5–8 days of higher DTR, depending on the symptoms. Conclusion: Case-crossover design and time-series analysis showed that DTR was a risk factor for cause-specific CVD hospital admission in Seoul.

Diurnal Temperature Range is a Risk Factor for Coronary Heart Disease Death

BackgroundAlthough the relation between day-to-day temperature change and coronary heart disease (CHD) mortality is well established, it is unknown whether temperature variation within 1 day, ie, diurnal temperature range (DTR), is an independent risk factor for acute CHD death.MethodsWe used time-series and case-crossover approaches to assess the relation between DTR and daily CHD mortality between 2001 and 2004 in Shanghai, China. Specifically, we used exposures averaged over periods varying from 1 to 5 days to assess the effects of DTR on CHD mortality. We estimated the percent increase in the number of daily deaths related to CHD that were associated with DTR, after adjustment for daily meteorologic conditions (temperature and relative humidity) and levels of outdoor air pollutants.ResultsBoth time-series and case-crossover analyses showed that DTR was significantly associated with the number of daily deaths related to CHD. A 1 °C increase in 2-day lagged DTR corresponded to a 2.46% (95% CI, 1.76% to 3.16%) increase in CHD mortality on time-series analysis, a 3.21% (95% CI, 2.23% to 4.19%) increase on unidirectional case-crossover analysis, and a 2.13% (95% CI, 1.04% to 3.22%) increase on bidirectional case-crossover analysis.ConclusionsOur findings suggest that DTR is an independent risk factor for acute CHD death.

Diurnal temperature range and daily mortality in Shanghai, China

Although the relationship between temperature level and mortality outcomes has been well established, it is still unknown whether within-day variation in temperature, e.g. diurnal temperature range (DTR), is a risk factor for death independent of the corresponding temperature. Moreover, DTR is a meteorological indicator associated with global climate change which may be related to a variety of health outcomes. We hypothesized that large diurnal temperature change might be a source of additional environmental stress and therefore a risk factor for death. We used daily weather and mortality data from Shanghai, China to test this hypothesis. We conducted a time-series study to examine the association between DTR and mortality outcomes from 2001 to 2004. A semi-parametric generalized additive model (GAM) was used to assess the acute effect of DTR on mortality after controlling for covariates including time trend, day of the week (DOW), temperature, humidity, and outdoor air pollution. We found a strong association between DTR and daily mortality after adjustment for those potential confounders. A 1 °C increment of the 3-day moving average of DTR corresponded to a 1.37% (95% CI 1.08–1.65%) increase in total non-accidental mortality, a 1.86% (95% CI 1.40–2.32%) increase in cardiovascular mortality, and a 1.29% (95% CI 0.49–2.09%) increase in respiratory mortality. The effects of DTR on total non-accidental and cardiovascular mortality were significant on both “cold” (below 23 °C) and “warm” (at least 23 °C) days, although respiratory mortality was only significantly associated with DTR on “cold” days. This study suggests within-day variation in temperature may be a novel risk factor for death.

Mo-P4:286 Seasonal variation in the occurrence of stroke in Northern Greece

Stroke is the second leading cause of death in the world. It has multiple risk factors of which some, such as ambient temperature, are less well documented.We aimed to examine the association between diurnal temperature range (DTR) and stroke mortality, and to test the possible effect modification of this association according to gender, age and educational level.Daily data on weather and stroke mortality from 16 provincial capital cities in China for the years 2007–2013 were obtained, with a total of 788,783 deaths from stroke. A quasi-Poisson generalized linear regression combined with a distributed lag non-linear model was used to examine the city-specific DTR effect on stroke mortality. The pooled effects of DTR on stroke mortality were then obtained using a meta-analysis, which was based on restricted maximum likelihood estimation.The DTR impacts were generally limited to a period of eight days, while significant effects during lag 0–8 days were only found in the cities of Beijing, Zhengzhou, Nanjing, Hefei, Chongqing and Changsha. The DTR effects were significantly and negatively associated with latitudes at lag 0–10 days (rs = − 0.640, P = 0.008). An increase of 1 °C in DTR was associated with pooled estimate of 0.66% (95%CI: 0.28–1.05%), 0.12% (− 0.26% to 0.51%) and 0.67% (0.26–1.07%) increases in stroke mortality at lag 0–10 days during the total, hot and cold days, respectively. The impact of DTR was much higher in southern China than in northern China [1.02% (0.62% to 1.43%) versus 0.10% (-0.27% to 0.47%) ]. For the individual characteristics, only females, the elderly aged ≥ 65 years, and those with lower educational attainment were vulnerable to DTR.DTR has considerable effects on risk of mortality from stroke in various cities in China, especially among the elderly, females, those with low educational level, and people living in southern China. The results can inform decisions on developing programs to protect vulnerable subpopulations from adverse impacts of DTR.

Response of corn (zea mays l.) in the pre-tassel initiation period to temperature and photoperiod

Controlled-environment facilities were used to examine the effects of temperature and photoperiod on tassel-initiation time, leaf number and rate of leaf initiation of two corn hybrids. The effect of diurnal temperature range on tassel-initiation time and leaf number was also investigated. There was a consistent decrease in time to tassel initiation as temperature was increased from 15° to 25°C regardless of photoperiod. There was no difference in tassel initiation time at 25° and 30°C under all photoperiods for both hybrids. Photoperiod increased the time to tassel initiation at all temperatures, however, the change in time was more pronounced at low temperature (15°C). Temperatures of 15, 20 and 25°C had little effect on changing the number of leaves. However, the 30°C-treatment resulted in an appreciable increase in leaf number over that of other temperature treatments. As photoperiod was extended, leaf number increased at all temperatures for both hybrids. Average leaf number increased by 2.1 for each hybrid by lengthening photoperiod from 10 to 20 h. The rate of leaf initiation was nearly doubled for United 108 and more than doubled for Guelph GX 122 when temperature was increased from 15° to 30°C. The two hybrids exhibited a pronounced difference in leaf-initiation rate at high temperature (30°C). There was a small change in the rate of leaf initialtion with photoperiod. It is concluded that the increase in leaf number with increasing photoperiod resulted from the prolongation of the time to tassel initiation. At low mean temperature (15°C), there was an appreciable delay in tassel-initiation time as the diurnal range was increased from 0.0 to 17.2°C. The delay at these ranges became smaller at the 20°C-mean temperature. At 25°C-mean temperature, there was a pronounced delay with an increase of range from 8.6 to 17.2°C. The number of leaves increased with mean temperature and diurnal range. However, the effect of temperature range became less pronounced at high mean temperature.