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Krishna Nemali and Marc W. van Iersel

drought in leaves and cells to whole-plant photosynthesis in bedding plants. This information can be useful for 1) developing fundamental knowledge regarding plant physiological responses to drought, 2) generating guidelines to mitigate drought stress

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Marc W. van Iersel and Sue Dove

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.

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R. Thomas Fernandez, Robert E. Schutzki, and Kelly J. Prevete

Responses of Magnolia ×soulangiana (Soul.-Bod.) `Jane' (`Jane' saucer magnolia) to consecutive short term pretransplant drought stresses and recovery after transplanting were evaluated beginning October 1997 and June 1998. Plants were subjected to one (mild) or two (moderate) 3-day drought stress periods or a two 3-day and one 4-day (severe) drought stress period, each separated by two rewatering periods over 24 hours. One day after each stress period, plants were transplanted into the field and well watered to monitor recovery from stress. Plant response was determined by measuring whole-plant CO2 assimilation, leaf gas exchange (CO2 assimilation, transpiration, stomatal conductance) and canopy growth throughout stress and recovery periods. Whole-plant and leaf CO2 assimilation were lower for the stressed treatments for most of the measurements taken during stress in the fall and spring. After release from stress and transplanting, leaf CO2 assimilation returned to control levels for mild and moderate fall stresses within 2 to 3 d by the next measurement, while it was over 3 weeks until recovery from the severe stress. There was no difference in leaf gas exchange following release from stress and transplanting during the spring stress. More rapid defoliation occurred for the severe fall-stressed plants compared to the controls after release from stress in the fall. Flower number was reduced in spring for the fall-stressed plants. At termination of the experiment, the growth index was lower for severe fall-stressed plants but there were no differences for other fall stress treatments. There was no increase in growth for control or stressed plants for the spring experiment.

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Joshua K. Craver, Krishna S. Nemali, and Roberto G. Lopez

, as α and A op were significantly lower under the radiation quality of R 50 :B 50 compared with R 90 :B 10 at both CO 2 concentrations ( Table 3 ). In a previous work that used whole-plant photosynthesis measurements on wax begonia ( Begonia

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E.D. Leonardos, M.J. Tsujita, B. Grodzinski, and T.J. Blom

Gas exchange (net photosynthesis Pn, dark respiration, transpiration, and stomatal resistance) of `Jaqueline' Alstroemeria, grown in pots in a greenhouse, were measured. Measurements were made under laboratory conditions using an open-flow infrared gas analysis system for leaf studies, and a semi-closed computer controlled whole plant photosynthesis system for whole plant studies.

Apical fully expanded leaves on non-flowering and flowering (at two stages) shoots had similar photosynthetic responses in respect to photosynthetically active radiation (PAR) and to CO2 concentration. Light saturation occurred at 600 umol/m2/s PAR with maximum leaf Pn rates ranging from 9 to 11 umol CO2/m2/s. CO2 saturation was estimated at approximately 1100 to 1200 ppm with maximum leaf Pn rates from 17 to 22 umol CO2/m2/s.

Whole plant Pn rates increased with increased PAR. Maximum rates 4 to 5 umol CO2/m2/s (half that of individual leaves) occurred at approximately 1000 to 1100 umol/m2/s PAR. CO2 saturation was estimated at 1100 to 1200 ppm, with maximum whole plant Pn rates ranging from 7 to 8 umol CO2/m2/s. These data will be discussed in relation to respiration and mutual shading at the leaf canopy.

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David C. Percival, John T.A. Proctor, and J.P. Privé

Rubus idaeus L. cv. Heritage raspberries were placed in controlled environment chambers (25°C, 14-hour photoperiod, 2.0 kPa vapor pressure deficit, CO2 concentration of 380 mol·m-2·s-1) to study the effects of drought stress on leaf gas exchange and stem water potential. Whole-plant photosynthesis (Pn) and transpiration were sensitive to drought stress and gradually decreased from the second day of the study until rehydration. Stomatal aperture feed-back regulation was present during the initial 48 hours of the study with transpiration rates dropping in response to a decrease in stem water potential. Spatial differences were also present with leaf Pn, and stomatal and CO2 conductance values of the younger, distal (i.e., closer to the apex) leaves decreasing at a faster rate than the older, proximal leaves (i.e., close to crown). Evidence of increased mesophyll resistance to drought stress was apparent with ci either remaining constant or increasing, while Pn and carboxylation efficiency simultaneously decreased. Protection of the underlying photochemistry was evident with parahelionastic leaf movements which resulted in a reduction in the effective leaf area and subsequent heat load. Therefore, an optimum balance between water loss and ci existed, and an alteration in these rates represented a stomatal conductance adjustment to match the intrinsic photosynthetic capacity rather than just a causal relationship.

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Marc van Iersel and Jong-Goo Kang

Subirrigation is an economically attractive irrigation method for producing bedding plants. Because excess fertilizer salts are not leached from the growing medium, salts can accumulate in the growing medium. Fertilizer guidelines developed for overhead irrigation may not be appropriate for subirrigation systems. Our objective was to quantify the effect of the fertilizer concentration (N at 0, 135, 285, and 440 mg·L–1) on whole-plant CO2 exchange and growth of subirrigated pansies. Whole plant CO2 exchange rate (net photosynthesis and dark respiration) was measured once every 10 min for 31 days. Whole-plant photosynthesis, dark respiration, and carbon use efficiency increased during the experiment. Fertilizer concentration started to affect the growth rate of the plants after approximately 7 days. Maximum photosynthesis and growth were achieved with N at about 280 mg·L–1 in the fertilizer solution [electrical conductivity = 2 dS·m–1]. Growth was reduced by ≈10% when the plants were fertilized with N at 135 and 440 mg·L–1 compared to 280 mg·L–1. Growth of plants watered without any fertilizer was greatly reduced, and plants showed symptoms of N and K deficiency. The size of the root system decreased and the shoot: root ratio increased with increasing fertilizer concentration, but the size of the root system was adequate in all treatments. These results indicate that subirrigated pansies can tolerate a wide range of fertilizer concentrations with relatively little effect on plant growth.

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D. Schwarz, H.-P. Kläring, M.W. van Iersel, and K.T. Ingram

An increase in nutrient solution concentration to produce high-quality fruit vegetables, such as tomatoes, may reduce growth and yield. One reason might be inhibition of photosynthesis, but results of photosynthesis studies in the literature are inconsistent. In this study, we investigated growth and photosynthesis of whole `Celebrity' and `Counter' tomato [Lycopersicon esculentum (L.) Mill.] plants in response to nutrient solution concentration, measured as electrical conductivity (EC). The effects of two levels of photosynthetic photon flux density (PPF = 400 or 625 μmol·m-2·s-1) on plant response to nutrient solution EC in a range between 1.25 to 8.75 dS·m-1 in a series of four experiments in gas exchange chambers placed in larger growth chambers were examined. Increasing PPF enhanced tomato growth and photosynthesis but increasing EC diminished them. Reduction of dry weight was 1.9% to 7.3%, while plant photosynthesis was reduced between 1.7% and 4.5% for each 1 dS·m-1. Increasing EC did not decrease dry matter content and leaf photosynthesis. Mean plant dry matter content ranged between 70 and 95 g·kg-1, and net leaf photosynthesis on the last measurement day was between 7.5 and 11.3 μmol·m-2·s-1, depending on experiment. The decrease in whole plant photosynthesis with an increase in EC was caused by decreased leaf area but not by a decrease in leaf photosynthesis.

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Qiyuan Pan and Bruno Quebedeaux

Low CO2 concentrations ([CO2]) frequently occur in dense crop canopy. To determine plant performance under sub-atmospheric [CO2], young `Gala' apple plants were phytotron-grown at 928 mmole m-2s-1 light intensity. Whole-plant photosynthesis and respiration under [CO2] between 0 and the ambient level (382 to 460 ml 1-1) were measured by monitoring [CO2] of the air entering and coming out of a 38-1 clear plexiglass gas exchange chamber at either 3.4 or 6.2 1 min-1. The chamber seals two plants of up to 77 cm height for long-term experiments. There was a linear relationship between [CO2] and net photosynthesis (Pn), with the R2 being as high as 0.99. The increase of Pn with increased [CO2] was 51% greater for the high air flow than for the low air flow. At the ambient CO2 level Pn at the high flow rate was 49% higher than that at the low flow rate. CO2 compensation points were 57.6 and 58.5 ml 1-1 at the high and low flow rates, respectively. The relationship between [CO2] and dark respiration was linear. Dark respiration decreased by 20% on average as the [CO2] increased from 0 to the ambient level, and it was 11% higher at the high flow rate than at the low flow rate. These results suggest that wind may act to reduce Pn depression in dense crop canopy by both reducing leaf resistance and atmospheric [CO2] gradient outside the boundary layer.

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Marc van Iersel

Poinsettias (Euphorbia pulcherrima Willd. ex Klotzsch) were grown in pots filled with 1.5 L of soilless growing medium and subirrigated daily with a fertilizer solution containing N at 210 mg·L-1 [electrical conductivity (EC) = 1.5 dS·m-1] for 128 days. After production, plants were placed in a whole-plant photosynthesis system and the effects of applying different volumes of water (0, 0.75, 1.5, and 3 L) to the top of the pots were quantified. Leaching with 0.75, 1.5, or 3 L of water reduced the EC in the top and middle layers of the growing medium. Applications of 0.75 or 1.5 L of water significantly increased the EC in the bottom third of the pots, where most of the root growth occurred. However, even in these treatments the EC in the bottom layer was only 2.6 dS·m-1 (saturated medium extraction method), which is well within the recommended range. The 0.75- and 1.5-L treatments also reduced the respiration rate of the plants by 20%, but none of the treatments had a significant effect on the photosynthesis of the plants. Regression analysis indicated a negative correlation between the EC of the bottom layer of the growing medium and dark respiration, while the EC of the top and middle layer had no significant effect on respiration. Although top watering can increase the EC in the bottom layer of the growing medium, this effect is unlikely to be large enough to cause significant plant stress and damage.