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S. Poni, E. Magnanini, and B. Rebucci

The reported system interfaces a commercially available portable infrared gas analyzer with a measurement and control module for continuous and automated measurements of whole-canopy gas exchange. Readings were taken for several days, under mostly sunny or partly cloudy conditions, on two potted vines (total leaf area per vine of ≈1.3 m2) enclosed in inflated polyethylene chambers. The air flow rate through the chambers was provided by a centrifugal blower and set at 5 L·s-1 by a butterfly valve. It prevented ΔCO2 from dropping below –40 mL·L-1. Switching of the two CO2 analysis channels to the infrared gas analyzer (operated in a differential mode) was achieved by solenoid valves, whereas wet and dry-bulb temperatures at chambers' inlet and outlet were measured by low-cost, custom-made thermocouple psychrometers. Whole-vine assimilation rate (WVA) and whole-vine transpiration rate were calculated from the inlet—outlet differences in CO2 and absolute humidity. When compared to assimilation measured on single leaves (SLA) under saturating light at equivalent times, the WVA reduction (area basis) was ≈50%, suggesting that whole-canopy photosynthetic efficiency based on SLA readings can be greatly overestimated.

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Sung Kyeom Kim, Duk Jun Yu, Ro Na Bae, Hee Jae Lee, and Changhoo Chun

Grafted transplants are widely used for watermelon culture in Korea mainly to reduce the yield and quality losses caused by soil-borne diseases. It is normal practice to cure the grafted transplants under high relative humidity (RH) and low photosynthetic photon flux (PPF) conditions for a few days after grafting to prevent the wilting of the transplants. Transpiration rate (TR) and net photosynthetic rate (NPR), however, could be suppressed under those environmental conditions. In the present study, TR and NPR of the grafted watermelon transplants were compared during graft union formation under 18 environmental conditions combining two air temperatures (20 and 28 °C), three RHs (60%, 80%, and 100%), and three PPF s (0, 100, and 200 μmol·m-2·s-1). Percentages of graft union formation and survival were also evaluated. TR and NPR dramatically decreased just after grafting but slowly recovered 2 to 3 days after grafting at 28 °C. The recovery was clearer at higher PPF and lower RH. On the other hand, the recovery of TR and NPR was not observed in 7 days after grafting at 20 °C. Differences in TR and NPR affected by RH were nonsignificant. Percentage of graft union formation was 98% when air temperature, RH, and PPF were 28 °C, 100%, and 100 μmol·m-2·s-1, respectively, which was the highest among all the treatments. Percentage of survival was over 90% when air temperature was 28 °C and RH was higher than 80% (when vapor pressure deficit was lower than 0.76 kPa). In addition, higher PPF enhanced TR and NPR and promoted rooting and subsequent growth of grafted transplants. Results suggest that the acclimation process for grafted watermelon transplants can be omitted by properly manipulating environmental factors during graft union formation.

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Marc van Iersel

Mechanical conditioning can be used to control the height of vegetable and ornamental transplants. Previous research indicated that brushing plants increases cuticular water loss from detached leaves, which may be an indication of decreased drought resistance. This might decrease post-transplant survival of the plants. The objectives of this study were to determine the effect of brushing on growth and gas exchange by tomato (Lycopersicon esculentum Mill.) and quantify whole-plant water use during a slow dry-down period. Tomato plants were grown from seed in a greenhouse during Fall 1995. The brushing treatment started 11 days after seeding and consisted of 40 strokes, twice each day. After 39 days of treatment, brushing reduced height 32%, leaf area 34%, and shoot dry mass 29% compared to control plants. Brushing did not affect leaf gas exchange. Brushed plants had a higher stem water flux than control plants during the first 3 days of a 6-day dry-down period. Stem water flux was lower than that of control plants later in the cycle, presumably because brushed plants used more of the available water during the first 3 days. On the third day of the dry-down period, leaf conductance of brushed plants was 35% higher than that of control plants, resulting in a 10% higher transpiration rate per unit leaf area. Because brushed plants had less leaf area than controls, differences in whole-plant water use were small. Time to wilting was similar for the brushed and unbrushed plants (6 days after withholding water). It seems unlikely that brushing would have a major effect on drought tolerance of plants.

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Rangjian Qiu, Taisheng Du, Shaozhong Kang, Renqiang Chen, and Laosheng Wu

information for precision irrigation and N management. Water deficit has an adverse effect on transpiration, which has been confirmed in sap flow studies for many woody tree species such as olive [ Olea europaea ( Fernández et al., 2001 ; Tognetti et al

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Hui-lian Xu, Jean Caron, and André Gosselin

Water potential at soil–root interface (ψ s-r) indicates soil water availability to the plants. It is related to plant water potential and transpiration. To know the characteristics of ψ s-r, in the plants under a subirrigation system, hysteresis of ψ s-r, as well as xylem water potential (ψ x) and transpiration were examined in response to soil dehydration for Prunus × cistena grown in three soil mixes: mix 1-composted bark, peat, and sand; mix 2—peat, bark, sand, and compost; and mix 3—peat, sawdust, and sand. When the soil mixes were dried from high to low water potential (ψ s), plants grown in mix 2 maintained higher ψ s-r, as well as higher ψ x and higher transpiration. However, when the soil mixes were dehydrated from the bottom, the relationships of ψ s-r, ψ x, and transpiration to ψ s showed strong hysteresis effect. ψ s-r was always lower at a given ψ s when soil was rewetted from dry to wet conditions than when soil was dried from wet conditions. ψ x and transpiration also showed hysteresis in response to soil dehydration. The extent of hysteresis was the largest in mix 2 and the smallest in mix 3. Hysteresis of ψ X or transpiration showed a similar trend to that of ψ s-r. This suggests that ψ s-r is a good indicator of soil water availability to the plants and more directly related to ψ X and transpiration than is ψ s. The difference in hysteresis of ψ s-r among soil mixes might be related to the properties of hydraulic conductance, which are determined by the soil texture. Hence, further study is needed to elucidate the mechanism of the hysteresis phenomenon.

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Sanliang Gu, Sunghee Guak, Leslie H. Fuchigami, and Charles H. Shin

Seedling plugs of `Better Boy' tomato plants (Lycopersicon esculentum Mill.) were potted in processed fiber:perlite (60:40% by volume) media amended or nonamended with either crystalline or powdered hydrophilic polymer (2.4 kg·m–3), and treated with one of the several concentrations (0, 2.5, 5, 7.5, and 10%) of antitranspirant GLK-8924, at the four true-leaf stage. Plants were either well-irrigated or subjected to short-term water stress, water withholding for 3 days, after antitranspirant GLK-8924 application. Leaf stomatal conductance, transpiration rate, whole plant transpirational water loss, and growth were depressed by short-term water stress and antitranspirant GLK-8924. In contrast, hydrophilic polymer amendment increased plant growth, resulting in higher transpirational water loss. The depression of stomatal conductance and transpiration rate by short-term water stress was reversed completely in 2 days after rewatering while the reduction of plant growth rate diminished immediately. The effects of antitranspirant GLK-8924 were nearly proportional to its concentration and lasted 8 days on stomatal conductance and transpiration rate, 4 days on plant growth rate, and throughout the experimental period on plant height and transpirational water loss. Plant growth was reduced by antitranspirant GLK-8924 possibly by closing leaf stomata. In contrast, hydrophilic polymer amendment resulted in larger plants by factors other than influences attributed to stomatal status. Hydrophilic polymer amendment did not interact with antitranspirant GLK-8924 on all variables measured. The application of antitranspirant GLK-8924 was demonstrated to be useful for regulating plant water status, plant growth and protecting plants from short-term water stress.

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Sanliang Gu, Leslie H. Fuchigami, Sung H. Guak, and Charles Shin

Seedling plugs of `Better Boy' tomato plants (Lycopersicon esculentum Mill.) were potted in 60% processed fiber: 40% perlite (by volume) media amended or nonamended with either crystalline or powdered hydrophilic polymer (2.4 kg·m-3), and treated with one of several concentrations (0%, 2.5%, 5%, 7.5%, and 10%) of antitranspirant GLK-8924, at the four true-leaf stage. Plants were either well-irrigated or subjected to short-term water stress, withholding water for 3 days, after antitranspirant GLK-8924 application. Leaf stomatal conductance, transpiration rate, whole-plant transpirational water loss, and growth were depressed by short-term water stress and antitranspirant GLK-8924. In contrast, hydrophilic polymer amendment increased plant growth, resulting in higher transpirational water loss. The depression of stomatal conductance and transpiration rate by short-term water stress was reversed completely in 2 days after rewatering while the reduction of plant growth rate diminished immediately. The effects of antitranspirant GLK-8924 were nearly proportional to its concentration and lasted 8 days on stomatal conductance and transpiration rate, 4 days on plant growth rate, and throughout the experimental period on plant height and transpirational water loss. Plant growth was reduced by antitranspirant GLK-8924 possibly by closing leaf stomata. In contrast, hydrophilic polymer amendment resulted in larger plants by factors other than influences attributed to stomatal status. Hydrophilic polymer amendment did not interact with antitranspirant GLK-8924 on all variables measured. The application of antitranspirant GLK-8924 was demonstrated to be useful for regulating plant water status, plant growth, and protecting plants from short-term water stress.

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W.F. Whitehead and B.P. Singh

Two studies were conducted to assess the effects of leaf aging on gas exchange in okra [Abelmoschus esculentus (L.) Moench] leaves. Gas exchange was measured at 6- to 10-day intervals starting 15 days after leaf emergence (DFE) and continuing until senescence at 50 DFE. Rates of transpiration (E), stomatal conductance (gs) and CO2 exchange (CER) increased as leaves matured up to ≈25 DFE, about full leaf expansion. Transpiration rate, gs, and CER declined after 25 DFE and as leaves aged further. Internal leaf CO2 concentration (Ci) was higher in old than young leaves. This study suggests that the most efficient okra canopy would maximize exposure of 25-day-old leaves to sunlight.

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A. Naor and S. Cohen

The sensitivity of water stress indicators to changing moisture availability, and their variability, determine the number of measurements that should be taken in order to represent properly plant water status in a certain orchard. In the present study we examined the sensitivity and variability of maximum daily trunk shrinkage, midday stem water potential, and daily transpiration rate in their responses to withholding irrigation from field-grown drip-irrigated `Golden delicious' apple trees in a commercial orchard. Irrigation was withheld from the stressed trees for 17 days starting in mid-July, and the control trees were irrigated daily at 100% of the “Class A” pan evaporation rate. The courses of daily transpiration rate, maximum trunk shrinkage, and midday stem water potential before and 10 days after the drying period were similar in the control and the stressed trees. Highly significant differences between the stressed and the control trees in their midday stem water potentials were apparent from the early stages of the stress period. Daily transpiration rate and maximum daily shrinkage were more variable than midday stem water potential, and differences between treatments became significant only after measurements were expressed relative to the initial values before irrigation was witheld. Differences between treatments (as percentages of the values obtained for the control trees) increased after irrigation stopped where these differences were greatest for maximum daily shrinkage, which reached 90%; moderate for stem water potential (60%); and least for daily transpiration rate, for which the differences remained below 20%. Our data show that the choice of a certain water stress indicator should be based on both the sensitivity to changing moisture availability and the degree of variability. Possible reasons for the different sensitivity to moisture availability and the different variability between the water stress indicators under study are discussed.

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Craig D. Green, Ann Stodola, and Robert M. Augé

Mycorrhizal colonization can alter stomatal behavior of host leaves during drought. This may be related to an altered production or reception of a chemical signal of soil drying. We tested whether intact root systems were required to observe a mycorrhizal effect on leaf transpiration (E), or whether some residual mycorrhizal influence on leaves could affect E of foliage detached from root systems. Transpiration assays were performed in the presence of several possible candidates for a chemical signal of soil drying. In detached leaves of Vigna unguiculata (cowpea), colonization interacted significantly with ABA and pH in regulating transpiration. Colonization affected E of detached Rosa hybrida (rose) leaves but had no effect on E of detached leaves of Pelargonium hortorum (geranium). In each species tested, increasing the ABA concentration decreased E. In cowpea, calcium appeared to alter stomatal sensitivity to ABA, as well as regulate stomatal activity directly. The pH of the feeding solution affected E in rose, but did not change E independently in cowpea or geranium. Adding phosphorus to the feeding solution did not alter E or the apparent sensitivity of stomata to ABA in any of the test species. Colonization of roots by mycorrhizal fungi can result in residual effects in detached leaves, that can alter the stomatal reception of chemical signals in both rose and cowpea.