Spectral changes in metal halide and high-pressure sodium lamps equipped with electronic dimming

Electronic dimming of high-intensity discharge lamps offers control of photosynthetic photon flux (PPF) but is often characterized as causing significant spectral changes. Growth chambers with 400-W metal halide (MH) and high-pressure sodium (HPS) lamps were equipped with a dimmer system using silicon-controlled rectifiers (SCR) as high-speed switches. Phase control operation turned the line power off for some period of the alternating current cycle. At full power, the electrical input to HPS and MH lamps was 480 W (root mean squared) and could be decreased to 267 W and 428 W, respectively, before the arc was extinguished. Concomitant with this decrease in input power, PPF decreased by 60% in HPS and 50% in MH. The HPS lamp has characteristic spectral peaks at 589 and 595 nm. As power to the HPS lamps was decreased, the 589-nm peak remained constant while the 595-nm peak decreased, equaling the 589-nm peak at 345-W input, and the 589-nm peak was almost absent at 270-W input. The MH lamp has a broader spectral output but also has a peak at 589 nm and another smaller peak at 545 nm. As input power to the MH lamps decreased, the peak at 589 diminished to equal the 545-nm peak. As input power approached 428 W, the 589-nm peak shifted to 570 nm. While the spectrum changed as input power was decreased in the MH and HPS lamps, the phytochrome equilibrium ratio (Pfr : Ptot) remains unchanged for both lamp types.     

Modification of yield and chlorophyll content in leaf lettuce by HPS radiation and nitrogen treatments

This study evaluated the potential of high photosynthetic photon flux (PPF) from high-pressure sodium (HPS) lamps, alone or in combination with metal halide (MH) plus quartz iodide (QI) incandescent lamps, to support lettuce growth, with or without N supplementation. Varying exposures to radiation from combined HPS, MH, and QI lamps influenced dry weight gain and photosynthetic pigment content of hydroponically grown Black-Seeded Simpson' lettuce (Lactuca sativa L.) seedlings. Cumulative leaf dry weight declined with increasing exposure, up to 20 hours per day, to 660 micromoles m-2 s-1 of photosynthetically active radiation (PAR) from HPS lamps concomitant with constant 20 hours per day of 400 micromoles m-2 s-1 from MH + QI lamps. Leaves progressively yellowed with increasing exposure to radiation from the three-lamp combination, corresponding to lower specific chlorophyll content but not to specific carotenoid content. Lettuce grown under 20-hour photoperiods of 400, 473, or 668 micromoles m-2 s-1 from HPS radiation alone had the highest leaf dry weight at a PPF of 473 micromoles m-2 s-1. Chlorophyll, but not carotenoid specific content, decreased with each incremental increase in PPF from HPS lamps. Doubling the level of N in nutrient solution and supplying it as a combination of NH4+ and NO3- partially ameliorated adverse effects of high PPF on growth and pigment content relative to treatments using single-strength N as NO3-.     

Improving spinach,radish, and lettuce growth under red light-emitting diodes (LEDs) with blue light supplementation

Radish (Raphanus sativus L. cv. Cherriette), lettuce (Lactuca sativa L. cv. Waldmann's Green), and spinach (Spinacea oleracea L. cv. Nordic IV) plants were grown under 660-nm red light-emitting diodes (LEDs) and were compared at equal photosynthetic photon flux (PPF) with either plants grown under cool-white fluorescent lamps (CWF) or red LEDs supplemented with 10% (30 micromoles m-2 s-1) blue light (400-500 nm) from blue fluorescent (BF) lamps. At 21 days after planting (DAP), leaf photosynthetic rates and stomatal conductance were greater for plants grown under CWF light than for those grown under red LEDs, with or without supplemental blue light. At harvest (21 DAP), total dry-weight accumulation was significantly lower for all species tested when grown under red LEDs alone than when grown under CWF light or red LEDs + 10% BF light. Moreover, total dry weight for radish and spinach was significantly lower under red LEDs + 10% BF than under CWF light, suggesting that addition of blue light to the red LEDs was still insufficient for achieving maximal growth for these crops.     

The response of plant growth and leaf gas exchange to the speed of lamp movement in a greenhouse

High-pressure sodium (HPS) light supplementation during the low-light months has become quite common for high-light requiring crops at latitudes above 45°. Most common systems have fixed installations, while movable systems have been tried with various results in greenhouses. The concept is that fewer lamps are used on a track system, and that light intensity varies over time. In two trials, we determined whether the speed of the HPS lamp movement had any effect on leaf CO₂ exchange rate, growth and developments of various plants species. Plants (chrysanthemum, petunia, rose and tomato) were grown in a greenhouse supplemented with HPS lamps which moved at various speeds (0, 2, 8 and 20 mm s⁻¹) between 06:00 and 24:00 h daily for about 6–7 weeks. One trial started at the end of November and one started at the end of January. The light sum from the lamps were 0.212 ± 0.004 mol m⁻² h⁻¹ at bench level, and the supplemental lighting represented 55 and 35% of the total light received by the plants for the two trial dates, respectively. The growth (dry matter) was reduced for tomato only when grown under moving lights compared to those grown under the stationary system, while plant height was not affected. Light saturated CO₂ exchange rate on the youngest fully developed leaves increased with lamp speed for petunia and tomato, but not for chrysanthemums, while apparent quantum yield was not affected by lamp speed for any species. In situ measurements of net CO₂ exchange rate (NCER) with supplemental lighting only, showed that NCER decreased exponentially when lamp speed increased from 0 to 20 mm s⁻¹.     

Green-light supplementation for enhanced lettuce growth under red- and blue-light-emitting diodes

Plants will be an important component of future long-term space missions. Lighting systems for growing plants will need to be lightweight, reliable, and durable, and light-emitting diodes (LEDs) have these characteristics. Previous studies demonstrated that the combination of red and blue light was an effective light source for several crops. Yet the appearance of plants under red and blue lighting is purplish gray making visual assessment of any problems difficult. The addition of green light would make the plant leave appear green and normal similar to a natural setting under white light and may also offer a psychological benefit to the crew. Green supplemental lighting could also offer benefits, since green light can better penetrate the plant canopy and potentially increase plant growth by increasing photosynthesis from the leaves in the lower canopy. In this study, four light sources were tested: 1) red and blue LEDs (RB), 2) red and blue LEDs with green fluorescent lamps (RGB), 3) green fluorescent lamps (GF), and 4) cool-white fluorescent lamps (CWF), that provided 0%, 24%, 86%, and 51% of the total PPF in the green region of the spectrum, respectively. The addition of 24% green light (500 to 600 nm) to red and blue LEDs (RGB treatment) enhanced plant growth. The RGB treatment plants produced more biomass than the plants grown under the cool-white fluorescent lamps (CWF treatment), a commonly tested light source used as a broad-spectrum control.     

Effect of 16 and 24 hours daily radiation (light) on lettuce growth

A 50% increase in total radiation by extending the photoperiod from 16 to 24 hr doubled the weight of all cultivars of loose-leaf lettuce (Lactuca sativa L.) 'Grand Rapids Forcing', 'Waldmanns Green', 'Salad Bowl', and 'RubyConn', but not a Butterhead cultivar, 'Salina'. When total daily radiation (moles of photons) was the same, plants under continuous radiation weighed 30% to 50% more than plants under a 16 hr photoperiod. By using continuous radiation on loose-leaf lettuce, fewer lamp fixtures were required and yield was increased.     

Accuracy of quantum sensors measuring yield photon flux and photosynthetic photon flux

Photosynthesis is fundamentally driven by photon flux rather than energy flux, but not all absorbed photons yield equal amounts of photosynthesis. Thus, two measures of photosynthetically active radiation have emerged: photosynthetic photon flux (PPF), which values all photons from 400 to 700 nm equally, and yield photon flux (YPF), which weights photons in the range from 360 to 760 nm according to plant photosynthetic response. We selected seven common radiation sources and measured YPF and PPF from each source with a spectroradiometer. We then compared these measurements with measurements from three quantum sensors designed to measure YPF, and from six quantum sensors designed to measure PPF. There were few differences among sensors within a group (usually <5%), but YPF values from sensors were consistently lower (3% to 20%) than YPF values calculated from spectroradiometric measurements. Quantum sensor measurements of PPF also were consistently lower than PPF values calculated from spectroradiometric measurements, but the differences were <7% for all sources, except red-light-emitting diodes. The sensors were most accurate for broad-band sources and least accurate for narrow-band sources. According to spectroradiometric measurements, YPF sensors were significantly less accurate (>9% difference) than PPF sensors under metal halide, high-pressure sodium, and low-pressure sodium lamps. Both sensor types were inaccurate (>18% error) under red-light-emitting diodes. Because both YPF and PPF sensors are imperfect integrators, and because spectroradiometers can measure photosynthetically active radiation much more accurately, researchers should consider developing calibration factors from spectroradiometric data for some specific radiation sources to improve the accuracy of integrating sensors.     

Soil pH effects on yield and fruit size of two rabbiteye blueberry cultivars

The effects of soil pH on yield and average berry weight of rabbiteye blueberry cvs Delite and Tifblue were investigated. Over a three year period, the higher soil pH decreased from 6.5 to 5.9, but the total yield of both cultivars decreased more than 60% as soil pH increased from 4.5 to 6.5 in the first two years and to 5.9 during the last year. Percent marketable yield was less influenced by the change in soil pH than was total yield. ‘Tifblue’ produced more fruit and had higher marketable yields, but the fruit size determined by average berry weight was smaller than ‘Delite’. When the soil pH was at a more desirable range for maximum yield (4.5 to 5.3), ripening was more uniform for both cultivars. This study illustrated that: (a) two rabbiteye blueberry cultivars differed in tolerance to alkaline growing conditions, (b) the response of the two cultivars to the change in soil pH varied from year to year, and (c) total yield of both cultivars was maximized when soil pH was below 5.0.     

Low Tunnels as a Strawberry Breeding Tool and Season-Extending Production System

Strawberries (Fragaria ×ananassa Duchesne ex Rozier) are economically valuable and popular with consumers who want to buy them all year round. However, a typical harvest season in most U.S. states is limited to only 3 to 4 weeks. Repeat-fruiting cultivars grown in these states could help meet that demand, especially if bred to be adapted to those states. To develop new repeat-fruiting cultivars that meet modern standards when grown in the U.S. Mid-Atlantic and similar regions, a production system utilizing low tunnels over raised white-plastic-covered beds was developed as a breeding tool to enable meaningful selection. The system also showed promise as a commercial production system. Fruit production using repeat-fruiting strawberries was compared with and without low tunnels. Season length was extended to 9 months. Total yield and marketable yield under low tunnels was 188% greater and 313% greater, respectively, compared with open beds. With no fumigation or fungicides, losses to fruit rots under low tunnels was 12% less than in open beds. The cost of materials to construct low tunnels was determined to be recoverable from the increased yield. Management strategies and applications of the findings from this study towards breeding repeat-fruiting strawberry cultivars are discussed.     

Bulb Formation in Onions as Affected by Photoperiod and Spectral Quality of Light

In glasshouse experiments bulb formation occurred in 5 north-temperate onion cultivars in photoperiods of 14–15 h and longer. Some response was obtained by giving 14 long days, but a greater response was obtained when the plants were kept in continuous long days. Three cultivars in the Rijnsburger group all differed from each other in the extent of their bulb development in all of several photoperiodic regimes tried, but there was no cultivar × photoperiod interaction. Thus it is possible that cultivars could usefully be compared in the glasshouse for earliness by providing any one of a wide range of photoperiodic treatments.

When grown in controlled environments with artificial light, little or no bulb formation occurred in cv. Rijnsburger with warm-white fluorescent lamps, but with de-luxe warm-white lamps it occurred when the photoperiods were 17 or more hours long. These lamps appeared to be elfective because they emitted a greater proportion of far-red light than the warm-white lamps, although not as much as sunlight. Because it is possible that the sensitivity of cultivars to the spectral composition of light may vary, it is concluded that attention should be given to this in comparative studies of the photoperiodic requirements of different cultivars.     

贵州地方辣椒品种比较试验

以遵椒2号作对照,对7个贵州地方常规干制辣椒品种进行多点比较试验。试验结果表明,有6个品种比对照增产3．01％-37．77％,其中增幅达到显著或极显著水平的品种有4个;有1个品种比对照减产7．38％,减幅差异不显著。其中表现最好的品种是224,不仅产量高,而且稳产性最好,适宜推广种植;其次是204,产量较高。     

Effects of interlighting on yield and external fruit quality in year-round cultivated cucumber

The effects of interlighting and of the proportion of interlight on the yield and fruit quality of year-round cultivated cucumber (Cucumis sativus L. cv. Cumuli) were investigated for this study. Artificial lighting was provided by high pressure sodium (HPS) lamps and the lighting regimes included top lighting (TL), top + interlighting 24% (T + IL24) and top + interlighting 48% (T + IL48). In TL, all of the lamps were mounted above the canopy. In T + IL24 and T + IL48, top lamps covered 76 and 52% of the lighting, respectively, while 24 and 48% of the lighting came from interlighting lamps which were mounted vertically 1.3 m above the ground between the single plant rows. The outdoor daily light integral (DLI) varied greatly during the cultivation periods; the mean values were 36.8, 5.3 and 19.9 mol m⁻² day⁻¹ for the summer, autumn–winter and spring stands, respectively. Lighting regime affected both yield and external fruit quality. Interlighting increased first class yield and decreased unmarketable yield, both in weight and number. The increase in the annual first class yield in weight was 15% in the two T + IL regimes. Interlighting improved energy use efficiency in lighting, being for the whole year 120, 130 and 127 g total yield kW h⁻¹ in TL, T + IL24 and T + IL48, respectively. Interlighting increased the fruit skin chlorophyll concentration in all seasons, but had only a minor effect on the fruit dry matter concentration. The mean total chlorophyll concentration in fruit skin was 70.8, 76.7 and 82.2 μg cm⁻² in TL, T + IL24 and T + IL48, respectively. In addition, interlighting extended the post-harvest shelf life of cucumber fruits in spring. Besides interlighting per se, also the higher proportion of interlight tended to further improve the fruit quality. For example, the fruit skin chlorophyll concentration increased along with the higher proportion of interlighting. In general, the effects of lighting regime were more prominent in lower natural light conditions in winter and spring. It is concluded that interlighting is a recommendable lighting method in cucumber cultivation, especially in lower natural light conditions.     

Effects of supplementary light to mother plants on adventitious shoot formation in flower peduncle segments of Begonia × hiemalis Fotsch in vitro

Plants of two Begonia × hiemalis cultivars (‘Schwabenland’ and ‘Nixe’) were grown in a greenhouse and continuously illuminated (24 h a day) from late October to early March, with light from warm white fluorescent lamps (L“WW”), high pressure mercury lamps (HPI/T) or high pressure sodium lamps (SON/T), each at three irradiance levels (5, 10 or 15 W m^?2). The control received light continuously (0.8 W m^?2) from incandescent lamps (INC). Every third week, plant material (flower peduncles) was collected for in vitro culture. The adventitious shoot formation was strongly dependent on cultivar, length of mother plant illumination and light source. The use of L“WW”, HPI/T or SON/T improved the regeneration ability of flower peduncle segments compared to control. Prolonged irradiation lowered the regeneration in ‘Schwabenland’, especially when HPI/T lamps were used.     

郑州地区春保护地鲜食番茄品比试验

为提高郑州地区春保护地番茄的产量和品质,以金棚1号为对照,对来自全国4个保护地番茄新品种在河南省的适应性、产量表现及经济价值进行了品种比较试验,以期筛选出适合郑州地区栽培的品种。试验结果表明,粉宝高抗病毒病,畸裂果率偏低;生育期181 d,表现为早熟;前期产量最高,较对照增产45%,总产量较对照增产27.5%,达到丰产。铁将军畸裂果率最低,生育期186 d,表现为中早熟;前期产量较对照增产6.6%,总产量较对照增产21.2%。以上2个品种综合表现较好,具有较强的市场潜力,适合郑州地区大面积推广。仙客1号和粉钻2个品种综合表现也不错,但在栽培期间注意防治病毒病,粉钻花芽分化期注意温度管理,防止畸形花。农户可以根据栽培需要,选择不同的适宜品种种植。     

福州地区高产优质结球甘蓝品种引进试验

为缓解福州市夏季蔬菜淡季,以钢球50为对照,综合比较8个引进结球甘蓝新品种的农艺性状、叶球特征特性、产量及抗病性,以期筛选出适合福州地区栽培的品种。试验结果表明:格林、翠绿58这2个品种的产量高,分别比对照钢球50增产24.23%、29.23%,软腐病抗性强,适宜于福州地区夏季栽培;美味107商品性好,产量略低于对照,但外叶数少,可通过合理密植提高产量,可作为补充品种推广应用。     

Optimizing environmental conditions for mass application of mechano-dwarfing stimuli to Arabidopsis.

Obtaining uniform mechano-dwarfing of Arabidopsis thaliana (L.) Heynh. seedlings within dense plantings is problematic. Alternative forms of mechano-stimulation were applied to seedlings in effort to obtain uniform growth reduction compared with undisturbed controls in both greenhouse and controlled growth environments. Arabidopsis grown under low photosynthetic photon flux (PPF) artificial light grew upright with limited leaf expansion, which enhanced mechano-responsiveness compared to that of rosette-growing plants under filtered sunlight or high PPF artificial light. Hypocotyls of seedlings grown at PPFs > 60 micromoles m-2 s-1 elongated less and had 6% less sensitivity to mechanical stress than seedlings grown at PPFs < 60 micromoles m-2 s-1. Fluorescent lamps alone (F) or fluorescent plus incandescent (F+I) lamps were compared for seedling responses to mechanical stress. Under F lighting, hypocotyl elongation was reduced 25% to 40% by twice-daily brush or plate treatments, and brushed seedlings exhibited more growth reduction than did plate treatments. Seedlings grown under F+I lamps exhibited similar stress-induced growth reduction compared to seedlings grown under F only, but stressed F+I seedlings lodged to a greater extent due to excessive hypocotyl elongation. Temperature-response studies using standardized F-only lighting indicated increased hypocotyl elongation but decreased leaf expansion, and decreased mechano-responsivity to brushing over the temperature range from 20 to 28 degrees C. Daylength studies indicated similar degrees of mechano-inhibition of hypocotyl elongation over the daylength range of 12, 16, 20, and 24 hours, whereas fresh weight of stressed seedling shoots declined compared to controls. A combination of environmental growth parameters that give repeatable, visual mechanical dwarfing of Arabidopsis include low-PPF fluorescent lighting from 55 to 60 micromoles m-2 s-1, ambient temperatures from 22 to 25 degrees C, and twice-daily brush treatments.     

Plant spacing and cultivar affect melon growth and yield components

The objective of this study was to evaluate the effect of cultivars and in-row spacing on vegetative growth and yield components in melon (Cucumis melo L.). The cultivars ‘Early Dawn’ (ED), ‘Superstar’ (SS) and ‘Medoro’ (ME) were grown in rows 1.5 m apart at plant spacing 0.6, 0.9, 1.2 or 1.5 m. Cultivars ED and SS were grown in 1998 and 1999, while ED and ME were tested in 2000. The main vine length, diameter, number of leaves and lateral branches were highest at 1.5 m in-row spacing in 1999 and 2000, while there was no influence of plant spacing on observed growth parameters in 1998. ED performed better compared with other two varieties, mostly due to more fruit per ha and per plant. Yield was significantly decreased with increased plant spacing in all 3 years of research. At in-row spacing of 1.5 m yield was in average 25% lower than at 0.6 m spacing. Average fruit weight slightly increased with plant spacing up to 1.2 m, even though the differences were significant only in 1999. Small differences among cultivars were also found. Number of fruit per ha decreased linearly while number of fruit per plant increased linearly as plant spacing increased in all cultivars except SS in 1999.     

Use of a cyclic high-pressure sodium lamp to inhibit flowering of chrysanthemum and velvet sage

Photoperiod is commonly controlled in the commercial production of ornamental crops to induce or prevent flowering. Flower induction in short-day (SD) plants can be prevented or delayed when the natural daylength is short by providing low-intensity lighting during the dark period. A stationary high-pressure sodium (HPS) lamp with an oscillating aluminum parabolic reflector (cyclic HPS) has been developed to provide intermittent lighting to greenhouse crops. We determined the efficacy of a cyclic HPS lamp at preventing flowering in SD plants garden chrysanthemum [Chrysanthemum × grandiflorum (Ramat.) Kitam.] ‘Bianca’, pot chrysanthemum ‘Auburn’, and velvet sage (Salvia leucantha L.) relative to traditional night interruption (NI) lighting strategies. Plants were grown in a glass-glazed greenhouse at a mean daily temperature of 19.5–20.7 °C with natural SD photoperiods. NI lighting was delivered during the middle of the night (2230–0230 h) from a 600 W cyclic HPS lamp mounted at one gable end of the greenhouse or from incandescent (INC) lamps that were illuminated for the entire 4 h (CONT INC) or for 6 min every 30 min for 4 h. Plants under cyclic HPS were grown at lateral distances of 1, 4, 7, 10, or 13 m from under the lamp. Control plants were grown under an uninterrupted 15 h skotoperiod. As the distance from the cyclic HPS lamp increased from 1 to 13 m, the maximum irradiance measured during the NI decreased from 25.4 to 0.3 μmol m⁻² s⁻¹ and time to visible inflorescence (VI) and the number of nodes at VI decreased. All species had a VI within 54 d, but ≤10% of plants flowered when grown at a lateral distance of 1 or 4 m from the cyclic HPS lamp or under CONT INC. Plants grown without NI had a VI 2 to 15 d earlier and flowered 7 to 24 d earlier than plants grown at 10 or 13 m from the cyclic HPS. All garden chrysanthemums flowered under cyclic INC, whereas velvet sage and pot chrysanthemum had 15% and 35% flowering, respectively. These results indicate that a cyclic HPS lamp can be used effectively to delay flower induction and prevent flowering in these species when NI is delivered at ≥2.4 μmol m⁻² s⁻¹.     

宁夏引黄灌区氮肥减量对温室辣椒光合特性、产量及品质的影响

针对宁夏引黄灌区温室辣椒氮肥用量过高的问题,以“长剑73”为试材,通过田间试验,研究了常规施肥(N 300.0kg·hm-2)、减氮10%(N 270.0kg·hm-2)、减氮20%(N 240.0kg·hm-2)、减氮30%(N 210.0kg·hm-2)及不施氮肥对辣椒生长发育、关键生育期光合特征、产量、品质以及经济效益的影响。结果表明:与常规施肥相比,氮肥减量10%并没有降低辣椒苗生长质量。减量施氮各处理在关键生育期内株高、茎粗均有所下降。辣椒盛果期氮肥减量10%处理SPAD最高,与常规氮肥相比提高了1.6%,继续减氮叶绿素含量开始下降。氮肥减量10%在辣椒花期、盛果期冠幅比常规施氮提高5.8%、5.6%。辣椒盛果期氮肥减量10%叶片气孔导度、胞间CO2浓度和光合速率比常规施肥处理提高9.4%、0.6%、5.3%。减氮10%处理辣椒产量比常规施氮增产1.1%,减氮20%辣椒产量略低于常规处理,但差异并不显著。氮肥减量降低了辣椒硝酸盐含量,同时提高了辣椒维生素C含量增幅范围为2.9%~8.8%。氮肥减量10%处理比常规氮肥收入提高1.5%,产投比最高为7.3。从辣椒生长发育、光合特征、产量、品质以及经济效益角度来考虑,氮肥投入不低于240kg·hm-2,不高于270kg·hm-2时是较为理想的氮肥减施方案,可在该地区推广应用。     

Growth of Rhododendron cultivars as affected by temperature and light

Summary

The purpose of this study was to investigate the adaptation of four Rhododendron cultivars to contrasting light and temperature conditions. Two evergreen rhododendron cultivars and two deciduous azaleas were grown for 112 d under short day (14 h) and long day (20 h) photoperiods combined with temperatures of 15 and 24°C. Additionally, these cultivars were compared for daylength extension at 24°C/long day under two irradiation treatments (incandescent lamps and fluorescent tubular lamps). The number of flushes of growth increased with increasing photoperiod and temperature in both evergreen cultivars and in R. canadense; azalea #89132 made only one flush in all treatments. In the evergreen cultivars the number of leaves per shoot in the first flush did not differ significantly between treatments, indicating that this character was predetermined by conditions during bud development. The number of leaves in later flushes increased with increasing photoperiod and temperature. The elongation growth of most flushes was also enhanced by longer photoperiod and higher temperature. High irradiation during photoperiodic extension further enhanced the growth. Azalea #89132 made more flower buds under high than low irradiation. The two evergreen cultivars differed in their growth habit. ‘Pohjola’s Daughter’ tended to continue growth in long days or at very high temperatures, and is thus predicted to thrive best in a maritime or semi-maritime cool climate. ‘Helsinki University’ responded to short daylength by ceasing growth regardless of temperature, and could be expected to perform successfully also in continental climates at latitudes around 45° N. R. canadense seemed to do best in a cool climate, but azalea #89132 should in time acclimatize in all kinds of climates within the limits of this study.