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Literature reports on the Q₁₀ for respiration vary widely, both within and among species. Plant size and metabolic activity may be responsible for some of this variation. To test this, respiration of whole lettuce plants was measured at temperatures ranging from 6 to 31 °C during a 24-h period. Subsequently, plant growth rate (in moles of carbon per day) was determined by measuring the CO₂ exchange rate of the same plants during a 24-h period. Environmental conditions during this 24-h period resembled those that the plants were exposed to in the greenhouse. The measured growth rate was then used to estimate the relative growth rate (RGR) of the plants. The respiratory Q₁₀ ranged from 1.4 for small plants to 1.75 for large plants. The increase in Q₁₀ with increasing plant size was highly significant, as was the decrease in Q₁₀ with increasing RGR. However, growth rate had little or no effect on the respiratory Q₁₀. One possible explanation for these findings is that the Q₁₀ depends on the ratio of growth to maintenance respiration (which is directly related to RGR). The growth respiration coefficient generally is considered to be temperature-insensitive, while the maintenance respiration coefficient normally increases with increasing temperature. Based on this concept, the Q₁₀ for the maintenance respiration coefficient can be estimated as the estimated Q₁₀ at a RGR of zero (i.e. no growth and thus no growth respiration), which was 1.65 in this experiment. Although the concept of dividing respiration into growth and maintenance fractions remains controversial, it is useful for explaining changes in respiratory Q₁₀ during plant development.

Do you accurately measure and report the growing conditions of your controlled environment experiments? Conditions in controlled environment plant growth rooms and chambers should be reported in detail. This is important to allow replication of experiments on plants, to compare results among facilities, and to avoid artefacts due to uncontrolled variables. The International Committee for Controlled Environment Guidelines, with representatives from the U.K. Controlled Environment Users' Group, the North American Committee on Controlled Environment Technology and Use (NCR-101), and Australasian Controlled Environment Working Group (ACEWG), has developed guidlines to report environmental conditions in controlled environment experiments. These guidelines include measurements of light, temperature, humidity, CO₂, air speed, and fertility. A brochure with these guidelines and a sample paragraph on how to include this information in a manuscript will be available.

Bedding plants are exposed to a wide range of environmental conditions, both during production and in the landscape. This research compared the effect of short-term temperature changes on the CO₂ exchange rates of four popular bedding plants species. Net photosynthesis (P_net) and dark respiration (R_dark) of geranium (Pelargonium ×hortorum L.H. Bail.), marigold (Tagetes patula L.), pansy (Viola ×wittrockiana Gams.), and petunia (Petunia ×hybrida Hort. Vilm.-Andr.) were measured at temperatures ranging from 8 to 38 °C (for P_net) and 6 to 36 °C (for R_dark). Net photosynthesis of all species was maximal at 14 to 15 °C, while R_dark of all four species increased exponentially with increasing temperature. Gross photosynthesis (P_gross) was estimated as the sum of P_net and R_dark, and was greater for petunia than for the other three species. Gross photosynthesis was less sensitive to temperature than either P_net or R_dark, suggesting that temperature effects on P_net were caused mainly by increased respiration at higher temperatures. Gas exchange-temperature response curves were not useful in determining the heat tolerance of these species. There were significant differences among species in the estimated R_dark at 0 °C and the Q₁₀ for R_dark. Differences in the Q₁₀ for R_dark were related to growth rate and plant size. Large plants had a greater Q₁₀ for R_dark, apparently because these plants had a higher ratio of maintenance to growth respiration than small plants. The Q₁₀ of the maintenance respiration coefficient was estimated from the correlation between the Q₁₀ and relative growth rate, and was found to be 2.5 to 2.6.

Efficient water use in nurseries is increasingly important. In recent years, new soil moisture sensors (ECH₂O probes) have become available, making it possible to monitor the moisture content of the growing medium in containers. One piece of information that is lacking for fully-automated irrigation systems is how much water actually needs to be present in the growing medium to prevent detrimental effects of drought on plants. We determined the effect of substrate moisture on photosynthesis and plant water relations of hydrangea and abelia. Growth rates of these species were measured during two subsequent drying cycles to determine how drought affects the growth rate of these species. Whole-plant photosynthesis, an indicator of growth rate, of both species remained stable as the volumetric moisture content of the substrate dropped from 25% to 15%, with pronounced decreases in photosynthesis at lower substrate moisture levels. Abelias and hydrangeas wilted when the substrate moisture level dropped to 6.3% and 8.3%, respectively. At wilting, abelias had lower leaf water potential (–3.7 MPa) than hydrangeas (–1.8 MPa). After the plants were watered at the end of the first drying cycle, the photosynthesis of the plants did not recover to pre-stress rates, indicating that the drought stress caused a long-term reduction in photosynthesis. Despite the more severe drought stress in the abelias (both a lower substrate water content and lower water potential at wilting), abelias recovered better from drought than hydrangeas. After the plants were watered at the end of the first drying cycle, the photosynthetic rate of abelias recovered to ≈70%, while the photosynthetic rate of the hydrangeas recovered to only 62% of the pre-stress rate.

We have developed a completely automated irrigation system that measures and maintains substrate volumetric water content (θ) at a target level for any length of time. Advantages of this system include complete automation of irrigation and simulation of precise levels of drought stress for potted plants. This system uses ECH₂O moisture sensors interfaced with a CR10X datalogger and solenoid valves connected to the datalogger by a SDM CD16 AC/DC controller. The datalogger measures the θ of the substrate hourly. When the θ of the substrate drops below the set point, the datalogger opens the solenoid valves, which results in irrigation. Substrate θ is maintained at a constant level as the datalogger is programmed to increase θ by 2% to 3% during each irrigation. When the system was validated for its accuracy, we determined that the θ measured in the substrate within the range of 0.15 to 0.35 m³·m^-3 was close (2% to 3%) to the θ determined by the conventional volumetric analysis. The daily average θ maintained in the substrate was slightly higher (within 3%) than the target level. Using this system, we were able to maintain four distinct levels of substrate θ for a prolonged period (40 days), regardless of differences in plant size and environmental conditions. Significant increases in number of irrigations, total water-use, and transpiration rate of impatiens, salvia, vinca, and petunia were noticed with increasing target θ of the substrate. For all species, highest and lowest water-use efficiency (WUE) were seen at 0.09 and 0.32 m³·m^-3, respectively, while WUE was not different between 0.15 and 0.22 m³·m^-3.

Physiological acclimation of plants to light has been studied mostly at the leaf level; however whole-plant responses are more relevant in relation to crop growth. To examine the physiological changes associated with different daily light integrals (DLI) during growth of wax begonia (Begonia semperflorens-cultorum Hort.), we grew plants under DLI of 5.3, 9.5, 14.4, and 19.4 mol·m^-2·d^-1 in a whole-plant gas exchange system. Photosynthesis-light response curves of groups of 12 plants were determined after 25 d of growth. Physiological parameters were estimated per m² ground area and per m² leaf area. On a ground area basis, significant increases in dark respiration (R_d), quantum yield (α), the light compensation point (LCP), and maximum gross photosynthesis (P_g,max) were seen with increasing DLI. Variations in physiological parameters among different treatments were small when corrected for differences in leaf area. On a leaf area basis, α, LCP, and the light saturation point (LSP) did not change significantly, whereas significant increases in R_d and P_g,max were seen with increasing DLI. There was a small decrease in leaf chlorophyll concentration (6.3%, measured in SPAD units) with increasing DLI. This study indicates that wax begonias acclimate to low DLI by increasing their leaf chlorophyll concentration, presumably to more efficiently capture the available light, and to high DLI by increasing P_g,max to efficiently utilize the available light, thereby maximizing carbon gain under both situations.

Dibutylurea (DBU), a breakdown product of benomyl, may be partially responsible for the previously reported phytotoxicity of the fungicide Benlate DF. We quantified the effect of DBU on the growth of two popular bedding plant species, petunia (Petunia × hybrida) and impatiens (Impatiens wallerana Hook. f.). DBU reduced photosynthesis of both species, and its effect strongly depended on the amount of DBU applied. The effects of DBU were most apparent 2 to 4 days after treatment, at which time 1.20 g·m^-2 (corresponding to 10% DBU in Benlate DF at maximum labeled drench rate) inhibited photosynthesis completely. DBU also decreased flower number and caused marginal necrosis. DBU effects were more pronounced in low relative humidity. Benlate DF containing 3.1% DBU and an equivalent amount of reagent grade DBU had similar effects on photosynthesis and petunia necrosis. Our results showed that DBU is responsible for at least part of the phytotoxic symptoms that can be caused by Benlate DF. However, other ingredients or breakdown products may also contribute to the phytotoxic symptoms of Benlate DF.

Benzimidazoles are effective and widely used fungicides, but they may be phytotoxic. We studied the effects of a single drench application of six benzimidazoles and one acetanilide fungicide on photosynthetic gas exchange, growth, development, and nutrient levels of four species of bedding plants in twenty growth-chamber and four greenhouse studies. Daily carbon gain and carbon-use efficiency were calculated from continuous crop gas-exchange measurements in the growth chambers. The maximum labeled rate of Benlate DF caused a 7- to 10-day decrease in net photosynthesis and daily carbon gain in transplants of all species. It also caused pronounced interveinal chlorosis and a 2- to 3-day delay in flowering. Growth of Benlate DF-treated plants was reduced more at high (90%) than at low (60% to 80%) relative humidity. Benlate DF had severe effects on 2-week-old petunia (Petunia ×hybrida) seedlings in plug flats, reducing photosynthesis 25% to 57%. Cleary's 3336 WP decreased photosynthesis in some trials. Benlate DF reduced photosynthesis within 24 hours, but 3336 WP effects did not become apparent until 1 week after the treatment. This suggests different modes of inhibition. 3336 WP also caused leaf-tip and marginal chlorosis in impatiens (Impatiens wallerana). Mertect 340-F was extremely phytotoxic but is not labeled for drench applications (it was included because of its chemical similarity to other benzimidazoles). The only benzimidazole fungicide that did not reduce photosynthesis was Derosal, but it caused slight interveinal chlorosis in some studies with petunia. Benlate DF and Derosal decreased leaf Ca levels. Subdue (or metalaxyl), an acetanilide fungicide, did not affect photosynthesis or cause any visual symptoms. Our results indicate that some benzimidazole fungicides can cause growth reductions and visual damage in bedding plants.

Several probes have been been recently developed that can be inserted directly into the growing medium of container-grown crops to get electrical conductivity (EC) or pH measurements. However, for many floriculture and greenhouse crops, EC interpretation ranges are based on substrate solution extraction methods such as the 1:2 v/v dilution, saturated media extract (SME), and more recently, the pour-through. We tested the sensitivity and accuracy of four in situ EC probes at a range of substrate moisture content and fertilizer concentrations. We also compared results from in situ probes with currently used methods of EC measurement. Concerning the effects of substrate volumetric water content (VWC) on the in situ probes, our results indicate little differences exist among probes when VWC exceeds 0.50, though drier substrates yielded differences depending on the measurement method. The SigmaProbe and W.E.T Probe measure the EC of the pore water specifically and show a decrease in EC with increasing water content, as the fertilizer ions in the pore water becomes more diluted as VWC increases. Results with the Hanna and FieldScout probes increased with increasing water content as the added water helps conduct the current of these meters. The EC measured with the various in situ probes differed slightly among the probes, but was highly and positively correlated with all three of the solution extraction methods over the range of fertilizer concentrations. It would be possible to convert substrate EC guidelines that have been established for any of the laboratory methods for use with the in situ probes, though our results indicate the substrate VMC must be above 0.35 for the interpretation to be valid.

The 4R nutrient stewardship framework presents four concepts to consider when applying fertilizers in a responsible matter; the “right source” of nutrients should be applied at the “right rate” during the “right time” and supplied to the “right place” to ensure their uptake. In this article, we provide ideas to consider when attempting to provide nutrients at the right time. When nutrients are applied at a time when they are not required by the plant, the result can be economic and environmental losses. Oversupply relative to plant demand can result in losses of applied nutrients because of leaching or volatilization. Undersupply relative to demand, especially in the case of phloem-immobile nutrients, may limit plant growth and yield. Several factors interact to affect plant nutrient demand such as growth stage, life history (annual vs. perennial), environmental conditions, and plant health. Techniques such as soil and tissue testing, isotopic labeling, and spectral reflectance have been used with varying degrees of success and expense to measure plant nutrient demand and guide fertilizer decisions. Besides knowledge of plant nutrient demand, efficient nutrient supply also depends on systems that allow precise spatial and temporal delivery of nutrients. Future improvements to the timing of nutrient delivery will depend on improvement in knowledge of plant nutrient demands. For example, targeted gene expression chips show promise for use in rapidly assessing plant status for a broad suite of nutrients. Future developments that allow more precise nutrient delivery or more robust agroecosystems that scavenge available nutrients before they are lost to the environment will also help producers use nutrients more efficiently.