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Geoffrey Weaver and Marc W. van Iersel

Plant light use efficiency decreases as light intensity is increased, and a better understanding of crop-specific light responses can contribute to the development of more energy-efficient supplemental lighting control strategies for greenhouses. In this study, diurnal chlorophyll fluorescence monitoring was used to characterize the photochemical responses of ‘Green Towers’ lettuce (Lactuca sativa L.) to photosynthetic photon flux density (PPFD) and daily light integral (DLI) in a greenhouse during a production cycle. Plants were monitored continuously for 35 days, with chlorophyll fluorescence measurements collected once every 15 minutes. Quantum yield of photosystem II (ΦPSII) decreased exponentially with PPFD, whereas electron transport rate (ETR) increased asymptotically to 121 µmol·m–2·s–1. Daily photochemical integral (DPI) is defined as the integral of ETR over a 24-hour period; DPI increased asymptotically to 3.29 mol·m–2·d–1 with increasing DLI. No effects of plant age or prior day’s DLI and a negligible effect of PPFDs 15 or 30 minutes before measurements within days were observed. Simulations were conducted using the regression equation of ETR as a function of PPFD {ETR = 121[1 – exp(–0.00277PPFD)]} to illustrate methods of increasing photochemical light use efficiency for improved supplemental lighting control strategies. For a given DLI, DPI can be increased by providing light at lower PPFDs for a longer period of time, and can be maximized by providing light with a uniform PPFD throughout the entire photoperiod. Similarly, the DLI required to achieve a given DPI is reduced using these same methods.

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Krishna S. Nemali and Marc W. van Iersel

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 (Rd), quantum yield (α), the light compensation point (LCP), and maximum gross photosynthesis (Pg,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 Rd and Pg,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 Pg,max to efficiently utilize the available light, thereby maximizing carbon gain under both situations.

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Krishna Nemali and Marc W. van Iersel

Bedding plants are at increased risk for exposure to drought stress during production because they are grown in small containers. Physiological mechanisms of bedding plants at leaf and cellular scales that regulate whole-plant photosynthesis under drought conditions are not well understood. This information can be useful for screening bedding plant cultivars with improved drought-tolerance and generate guidelines to mitigate drought stress during production. We subjected drought-sensitive salvia (Salvia splendens ‘Bonfire Red’) and drought-tolerant vinca (Catharanthus roseus ‘Cooler Peppermint’) to gradual drought stress inside whole-plant gas exchange chambers. Substrate water content (Θ), whole-plant net photosynthesis (Pn_avg), whole-plant respiration (Rd_avg), and daily carbon gain (DCG) were measured continuously, whereas stomatal conductance (g S) to water, leaf water (ΨL), osmotic (ΨS), and turgor (ΨP) potentials were measured at the start and end of the drought phase. In addition, ΨS was measured before exposure to stress and after thoroughly rehydrating plants. Dark-adapted quantum efficiency (dark-adapted ΦPSII) was measured after rehydrating plants. The results indicated that, at whole-plant scale, vinca continued to uptake water at lower Θ levels than the Θ level that resulted in wilting of salvia. There were no differences in Rd_avg; however, Pn_avg and DCG of salvia decreased at a higher Θ level than that of vinca. This indicated that salvia experienced drought stress at a higher Θ level than did vinca. At the leaf scale, there were no differences in ΨL; however, a more negative ΨS (P = 0.06) and significantly higher ΨP were observed in vinca (compared to salvia) under drought conditions. In addition, ΨS was not different between species before exposure to drought, whereas ΨS of rehydrated leaves after exposure to drought in vinca was significantly lower than that in salvia. Moreover, ΨS of rehydrated leaves after exposure to drought was significantly lower than that observed before exposure to drought in vinca. This indicated osmotic adjustment (OA) in vinca under drought conditions. Dark-adapted ΦPSII was lower in salvia than in vinca after exposure to drought, indicating damage to photosynthetic mechanisms. Our results suggested that increased OA likely helped to maintain higher ΨP under drought conditions and continuation of water uptake at lower Θ in vinca compared to salvia. In addition, healthier photosynthetic mechanisms of vinca (compared to salvia) under drought conditions likely resulted in its higher Pn_avg and DCG at lower Θ. Screening for OA and dark-adapted ΦPSII may be useful for developing drought-tolerant bedding plant cultivars.

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Krishna S. Nemali and Marc W van Iersel

Efficient use of irrigation water is increasingly important in the production of bedding plants. Two approaches to efficient water use include reducing irrigation water wastage during production by growing plants at the optimal substrate water content (θ) and growing species with high water-use efficiency (WUE). However, there is little information on the effects of different θ levels on leaf physiology of bedding plants and variation in WUE among different species of bedding plants. The objectives of this study were to determine the effects of θ on leaf water relations, gas exchange, chlorophyll fluorescence, and WUE of bedding plants and to identify the physiological basis for differences in WUE between two bedding plant species. We grew salvia ‘Bonfire Red’ (Salvia splendens Sellow ex Roemer & J.A. Schultes), vinca ‘Cooler Peppermint’ [Catharanthus roseus (L.) G. Don.], petunia ‘Lavender White’ (Petunia × hybrida Hort ex. Vilm.), and impatiens ‘Cherry’ (Impatiens walleriana Hook F.) at four constant levels of θ (0.09, 0.15, 0.22, and 0.32 m3·m−3) using an automated irrigation controller. Regardless of species, leaf water potential (Ψw) and leaf photosynthesis (A) of all four species were lower at a θ of 0.09 m3·m−3 and were not different among the other θ levels, but stomatal conductance to H2O (g S) was lower at 0.09 than at 0.15 and 0.22 m3·m−3 and highest at 0.32 m3·m−3. WUE of bedding plants at different θ levels depended on species. The WUE of petunia was unaffected by θ, whereas for the other three species, WUE was higher at a θ of 0.09 m3·m−3 than at 0.32 m3·m−3. Differences in WUE between petunia and salvia were partly from differences in photosynthetic capacity between the two species. Based on the response of A to leaf internal CO2 concentration (Ci), mesophyll conductance to CO2 [gm (a measure of photosynthetic capacity)] was higher in petunia than salvia, whereas gas phase conductance to CO2 (gCO2) was similar for these two species, which resulted in higher WUE in petunia than salvia.

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Shuyang Zhen and Marc W. van Iersel

Photosynthetic responses to light are dependent on light intensity, vary among species, and can be affected by acclimation to different light environments (e.g., light intensity, spectrum, and photoperiod). Understanding how these factors affect photochemistry is important for improving supplemental lighting efficiency in controlled-environment agriculture. We used chlorophyll fluorescence to determine photochemical light response curves of three horticultural crops with contrasting light requirements [sweetpotato (Ipomea batatas), lettuce (Lactuca sativa), and pothos (Epipremnum aureum)]. We also quantified how these responses were affected by acclimation to three shading treatments-full sun, 44% shade, and 75% shade. The quantum yield of photosystem II (ΦPSII), a measure of photochemical efficiency, decreased exponentially with increasing photosynthetic photon flux (PPF) in all three species. By contrast, linear electron transport rate (ETR) increased asymptotically with increasing PPF. Within each shading level, the high-light-adapted species sweetpotato used high light more efficiently for electron transport than light-intermediate lettuce and shade-tolerant pothos. Within a species, plants acclimated to high light (full sun) tended to have higher ΦPSII and ETR than those acclimated to low light (44% or 75% shade). Nonphotochemical quenching (NPQ) (an indicator of the amount of absorbed light energy that is dissipated as heat) was upregulated with increasing PPF; faster upregulation was observed in pothos as well as in plants grown under 75% shade. Our results have implications for supplemental lighting: supplemental light is used more efficiently and results in a greater increase in ETR when provided at low ambient PPF. In addition, high-light-adapted crops and crops grown under relatively high ambient light can use supplemental light more efficiently than low-light-adapted crops or those grown under low ambient light.

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Francesco Montesano and Marc W. van Iersel

The availability of good-quality irrigation water is decreasing worldwide, and salinity is an increasingly important agricultural problem. To determine whether detrimental effects of NaCl on plant growth and leaf physiology can be minimized by additional Ca2+ supply, tomato (Solanum lycopersicum L.) ‘Supersweet 100’ was grown hydroponically. The basic nutrient solution contained 11.1 mm NO3 and 2.8 mm Ca2+. Three levels of NaCl (14.1, 44.4, and 70.4 mm) were added to the basic solution to determine Na+ effects on leaf physiology and growth. To determine if Ca2+ could alleviate the toxic effects of Na+, treatments with 10 or 20 mm Ca2+ combined with 44.4 or 70 mm NaCl were included as well. To distinguish between osmotic and ion-specific effects, there were three treatments in which all nutrient concentrations were increased (without NaCl) to obtain electrical conductivity (EC) levels similar to those of the NaCl treatments. Nutrient solutions with 70.4 mm NaCl reduced leaf photosynthesis, chlorophyll content, gas-phase conductance, carboxylation efficiency, and dark-adapted quantum yield. Inclusion of 20 mm Ca2+ prevented these effects of NaCl. NaCl also decreased leaf length and elongation rate. This could not be prevented by adding extra Ca2+ to the solution; reductions in leaf elongation were due to osmotic effects rather than to Na+ specifically. Likewise, plant dry weight was negatively correlated with solution EC, suggesting an osmotic effect. Leaf area development apparently was more important for dry matter accumulation than leaf photosynthesis. Adding 20 mm Ca2+ to the 70 mm NaCl solution reduced the Na+ concentration in the leaf from 79 to 24 mg·g−1.

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Marc W. van Iersel and Orville M. Lindstrom

Temperature-response curves for photosynthesis and respiration are useful in predicting the ability of plants to perform under different environmental conditions. Whole crop CO2 exchange rates of three magnolia (Magnolia grandiflora L.) cultivars (`MGTIG', `Little Gem', and `Claudia Wannamaker') were measured over a 25 °C temperature range. Plants were exposed to cool temperatures (13 °C day, 3 °C night) temperatures before the measurements. Net photosynthesis (Pnet) of all three cultivars increased from 3 to 15 °C and decreased again at higher temperatures. `MGTIG' had the highest and `Little Gem' the lowest Pnet, irrespective of temperature. The Q10 (relative increase in the rate of a process with a 10 °C increase in temperature) for Pnet of all three cultivars decreased over the entire temperature range. `MGTIG' had the lowest Q10 at low temperatures (1.4 at 8 °C), while `Little Gem' had the lowest Q10 for Pnet at temperatures >17 °C and a negative Q10 > 23 °C. This indicates a rapid decline in Pnet of `Little Gem' at high temperatures. The decrease in Pnet of all three cultivars at temperatures >15 °C was caused mainly by an exponential increase in dark respiration (Rdark) with increasing temperature. `Little Gem' had a lower Rdark (per unit fresh mass) than `MGTIG' or `Claudia Wannamaker', but all three cultivars had a similar Q10 (2.46). Gross photosynthesis (Pgross) was less sensitive to temperature than Pnet and Rdark. The optimal temperature for Pgross of `MGTIG' was lower (19 °C) than those of `Little Gem' (21 °C) and `Claudia Wannamaker' (22 °C). The Q10 for Pgross decreased with increasing temperature, and was lower for `MGTIG' than for `Little Gem' and `Claudia Wannamaker'. All three cultivars had the same optimal temperature (11 °C) for net assimilation rate (NAR), and NAR was not very sensitive to temperature changes from 3 to 17 °C. This indicates that the plants were well-adapted to their environmental conditions. The results suggest that respiration rate may limit magnolia growth when temperatures get high in winter time.

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Marc W. van Iersel and David Gianino

Supplemental lighting in greenhouses is often needed for year-round production of high-quality crops. However, the electricity needed for supplemental lighting can account for a substantial part of overall production costs. Our objective was to develop more efficient control methods for supplemental lighting, taking advantage of the dimmability of light-emitting diode (LED) grow lights. We compared 14 hours per day of full power supplemental LED lighting to two other treatments: 1) turning the LEDs on, at full power, only when the ambient photosynthetic photon flux (PPF) dropped below a specific threshold, and 2) adjusting the duty cycle of the LEDs so that the LED lights provided only enough supplemental PPF to reach a preset threshold PPF. This threshold PPF was adjusted daily from 50 to 250 μmol·m−2·s−1. Turning the LED lights on at full power and off based on a PPF threshold was not practical since this at times resulted in the lights going on and off frequently. Adjusting the duty cycle of the LED lights based on PPF measurements underneath the light bar provided excellent control of PPF, with 5-minute averages typically being within 0.2 μmol·m−2·s−1 of the threshold PPF. Continuously adjusting the duty cycle of the LED lights reduced electricity use by 20% to 92%, depending on the PPF threshold and daily light integral (DLI) from sunlight. Simulations based on net photosynthesis (An) − PPF response curves indicated that there are large differences among species in how efficiently supplemental PPF stimulates An. When there is little or no sunlight, An of Heuchera americana is expected to increase more than that of Campanula portenschlagiana when a low level of supplemental light is provided. Conversely, when ambient PPF >200 μmol·m−2·s−1, supplemental lighting will have little impact on An of H. americana, but can still results in significant increases in An of C. portenschlagiana (1.7 to 6.1 μmol·m−2·s−1 as supplemental PPF increases from 50 to 250 μmol·m−2·s−1). Adjusting the duty cycle of the LEDs based on PPF levels assures that supplemental light is provided when plants can use that supplemental light most efficiently. Implementing automated duty cycle control of LED grow lights is simple and low cost. This approach can increase the cost effectiveness of supplemental lighting, because of the associated energy savings.

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Neil S. Mattson and Marc W. van Iersel

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.

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Francesco Montesano and Marc W. van Iersel

The availability of good quality irrigation water is decreasing worldwide, and salinity is an increasingly important agricultural problem. To determine whether detrimental effects of NaCl can be minimized by additional Ca2+, tomato (Lycopersicon esculentum Mill.) `Super-sweet 100' was grown hydroponically. The basic nutrient solution contained 11.9 mM NO - 3 and 3.2 mM Ca 2+. We added 14.1, 44.4 and 70.4 mM of NaCl to this solution to determine the effect of NaCl and there were treatments with 70 mM NaCl and 10 or 20 mM Ca2+ to look at Ca2+ effects. We also included three treatments in which all nutrient concentrations were increased (without NaCl) to distinguish between osmotic and ion-specific effects. 70.4 mM NaCl reduced leaf photosynthesis, chlorophyll content, gas phase conductance for CO2 diffusion, carboxylation efficiency, and dark-adapted quantum yield of photosystem II. The inclusion of 20 mM Ca2+ prevented these effects of NaCl. NaCl also decreased leaf size and elongation rate, but this could not be prevented by adding extra Ca2+ to the nutrient solution; these were caused by osmotic effects, rather than Na+ specifically. Likewise, plant dry mass was negatively correlated with solution EC, suggesting an osmotic effect. Our results indicate that leaf area development, which was inhibited by high EC, is more important for dry matter accumulation than leaf photosynthesis, which was inhibited by high Na+. Adding 20 mM Ca2+ to the 70 mM NaCl solution reduced the Na+ concentration in the leaf from 79 to 24 mg·g-1, which may explain why Ca2+ alleviates Na+ toxicity.