Seedlings of Petunia hybrida cv. Snow Cloud were subjected to root zone temperatures at the bottom surface of the pots of 15.6° to 19.4°C (NT) or 21° to 35° (HT) and photoperiods of 9 (SD) or 13 hr (LD) for 25 days in a eontrolled-environment chamber with air temperatures of 21° for 9 hr and 15.6° for 15 hr. HT × LD plants produced the largest total leaf area, largest main stem leaves, and most dry weight of all treatments; they were tallest and bloomed first, but had the fewest lateral branches. HT × SD plants developed the most lateral branches at the fastest rate and had a total leaf area, dry weight gain, and root development comparable to those of the LD treatments. NT × SD plants were the smallest. Crop productivity efficiency was determined to be NT × SD = 2.9%, HT × SD = 3.4%, NT × LD = 3.7%, and HT × LD = 3.9%.
Geranium seedlings (Pelargonium × hortorum L.H. Bailey, ‘Mustang’) grown in 13 hr photoperiods were 23% taller due to stem and petiole elongation, had larger leaves, and prior to canopy closure had a higher crop productivity efficiency (CPE) than seedlings grown under 9 hr photoperiods. In general, the tallest plants were produced when grown with soil temperatures of 18°C. The highest weekly CPE attained was 3.8%.
Crop productivity efficiencies (CPE) of around 8% (the ratio of the dry weight gain of the crop to the potential to produce dry weight), were realized with petunias (Petunia hybrida Villm.), provided that the crop canopy was essentially closed at the beginning of the 9- to 12-day experimental periods and that there were many branches (sinks). This was found at either long or short photoperiods or at either a normal (15.6°C) or reduced (7.2°) temperature for the 16-hour night periods. Long photoperiods resulted in significantly increased CPE through increased size of the leaves before the crop canopy was closed. Elevated root temperature increased CPE after a sizeable number of lateral branches had formed.
Seedlings of Petunia hybrida ‘Snow Cloud’ and Pelargonium × hortorum ‘Red Elite’ and ‘Cardinal Orbit’ were grown to anthesis at day air temperatures of 27° ± 3°C (9 hr) and either 7° ± 3° or 18° ± 3° night air temperatures (15 hr). Petunia crop productivity (CP, grams of dry matter produced per square meter of crop) and crop productivity efficiency (CPE, percentage of photosynthetic photon flux incident on the crop stored in the form of crop dry matter) were the same at both temperature regimes from canopy closure to anthesis, but anthesis was delayed 10 days at 7°. Petunias grown at 7° had four more basal branches and were only one-third the height of petunias grown at 18° (12 vs. 37 cm). CP and CPE were 20% lower for geraniums grown at 7° compared to CP and CPE for geraniums grown at 18°. The geraniums grown at 7° flowered 3 weeks later, were more compact, and were 16 to 19 cm shorter than geraniums grown at 18°.
Seedlings of Begonia × semperflorens-cultorum Hort. ‘Scarletta’ were grown in a greenhouse at a plant density of 193 plants/m2. Crop productivity (grams of dry matter produced per day per square meter of crop) and crop productivity efficiency (percentage of the photosynthetic photon flux incident on the crop that is stored in the form of crop dry matter as energy of combustion) did not increase when the photoperiod was extended from 9 to 13 hr with incandescent lights. However, stem and petiole length did increase under 13- compared to 9-hr photoperiods. Crop productivity of begonia was less than maximum values reported for some other bedding plants. However, when crop growth was expressed in terms of fresh weight rather than dry weight, begonia crop growth exceeded that reported for other bedding plants. This increased growth seemed to be due to the low dry weight to fresh weight ratio in wax begonia of 0.03.