Transpiration and water uptake play an important role in the growth of horticultural crops, such as tomatoes. Water uptake ensures the transport of nutrients. However, the transpiration rate is affected by the humidity level in the greenhouse. High levels of humidity restrict transpiration and lead to fungal diseases resulting in yield losses. Under northern latitudes, using more airtight structures combined with high levels of artificial lighting increase the humidity level inside the greenhouses. To decrease humidity, growers have to dehumidify by ventilating and heating at the same time, leading to increased energy consumption. However, to our knowledge, the literature does not report on the energy consumption needed to dehumidify. To evaluate this energy consumption, we used a greenhouse simulation model of heat and mass exchanges integrated into a general greenhouse control and management software system (GX). Evapotranspiration, condensation on the cladding, and infiltration and ventilation rates were taken into account for the water balance. Based on 1 year of climatic data, three sets of simulation were realized: 1) no dehumidification; 2) standard dehumidification by ventilation and heating; 3) dehumidification with heat exchangers. Results indicate that for an acceptable level of humidity within a greenhouse tomato crop (vapor pressure deficit >5 kPa), the energy consumptions with standard dehumidification and with heat exchangers are 25% and 15% higher, respectively, than without dehumidification. These results are being used to establish recommendations for the management of humidity under northern latitudes.
Damien de Halleux and Laurent Gauthier
William H. Rein, Robert D. Wright, and John R. Seiler
Stem cuttings of Blue Rug juniper (Juniperus horizontalis Moench `Wiltonii'), `Hino-Crimson' azalea [Rhododendron (Lindl.) P1anch `Hino-Crimson'], and `Helleri' holly (Ilex crenata Thunb. `Helleri') were propagated in 1 peat: 1 perlite (v/v) at one of five moisture levels based on medium dry weight (125%, 250%, 375%, 500%, or 625%). Cutting survival and percentage of rooted cuttings were highest at the highest medium moisture level in all three species. Incidence of cutting basal rot was not directly related to medium moisture level, but more to the growth stage of the stock plant. Midday xylem water potential (ψ) of cuttings for each species was highest in the wettest propagation medium and lowest in the driest medium. During propagation, stem cutting ψ below - 2.0 MPa occurred even in the wettest medium tested, and frequently reached - 4.0 MPa in cuttings in the driest treatment (125%). Basal water uptake by cuttings was highest in the wettest medium moisture level. Water uptake was highest during the first few days after insertion, and thereafter decreased until root emergence.
H. J. Hill and A. G. Taylor
Imbibed nonviable lettuce (Lactuca sativa L.) seeds have been shown to have lower density than imbibed control seeds. The purpose of this study was to investigate density differences associated with seed death. The relationship between endosperm integrity and the volume, density, and leakage of imbibed control and heat-killed ‘Montello’ lettuce seeds was studied. After an 8-hr soak, heat-killed seeds imbibed 23% more water than control seeds. The percentage of heat-killed seeds with density of 1.08 g·cm-3 was 2%, compared to 90% for the control. Mean electrical conductivity of the steep water was similar for heat-killed and control seeds. Seeds were punctured to rupture the endosperm layer surrounding the embryo. Puncturing the heat-killed seeds decreased total water uptake, as measured by decreased swelling, and increased density compared to intact heat-killed seeds. Leachate from punctured heat-killed seed had a 41% higher mean conductivity than that from punctured control seed. These data suggest that the undamaged endosperm restricted leakage of electrolytes from the embryo to the soak water. We speculate that the endosperm caused osmotically active solutes to accumulate in the extra-embryonic fluid of heat-killed seeds. This accumulation of solutes decreases the water potential inside the embryonic pouch, resulting in a greater uptake of water from the environment. The additional water uptake by heat-killed seeds would increase seed swelling and decrease seed density relative to control seeds.
R. Holubowicz, A.G. Taylor, M.C. Goffinet, and M.H. Dickson
During imbibition, water always follows the same pattern when entering the seed testa in semihard seeds (SHS) of snap bean (Phaseolus vulgaris L.). Water first enters the raphe and the chalazal region of the testa (R-CT), then migrates circumferentially along the midline of the seed, leaving the lateral faces the last to be fully imbibed. The R-CT region is the main site of primary uptake of both water vapor and liquid water by SHS. The hilum, micropyle, and strophiole play only a minor role in water uptake in SHS. In comparison to the readily permeable seeds of ‘Bush Blue Lake 47’, SHS have more total phenols in the osteosclereid cells and more pectic substances in the palisade cells of the CT. The presence of these compounds may account for the impermeable nature of SHS. Measurements made of palisade cell length and width in the R-CT region revealed that cell length increased and width decreased in the chalazal testa region (CT) as seed moisture content increased from 6% to 12%. It is proposed that semihardening of bean seeds is mainly a result of the reversible physical changes in the length and width of the palisade cells in the R-CT region. Seeds imbibe at high moisture content (12%) because the palisade cells have stretched, which allows water uptake. Seeds are impermeable at low moisture content (6%) because the palisade cells change in size and form a physical barrier to water movement.
Terril A. Nell, James E. Barrett, and Robert H. Stamps
Leatherleaf fern [Rumohra adiantiformis (G. Forst) Ching] fronds became desiccated rapidly after harvest with water potential decreasing from -0.45 to -1.75 MPa within 30 minutes. Partial closing of stomates 30 minutes after harvest accounted for a decrease in the rate of frond desiccation and frond water potential was -2.26 MPa after 180 minutes. Postharvest frond desiccation to water potentials of -1, -2, and -3 MPa in the field prior to water dip and cold storage resulted in frond curl of 32, 56, and 84%, respectively, when placed in postharvest rooms. Water uptake decreased during the first 4 days in postharvest rooms. Declining frond water potentials suggested reduced rate of water uptake was due to blockage of the xylem. However, no obstructions were observed at cut end of stipe from fern with frond curl or those not exhibiting frond curl. Frond water potentials were lower one hour after harvest than when undergoing normal postharvest senescence. Fronds did not exhibit normal drought-imposed wilt or frond curl during prestorage stress. Desiccation resulted in frond curl in some experiments but had little effect in others. These results indicate that frond curl can be triggered by desiccation stress but other factors are predisposing fronds to this disorder.
C. S. Tan, A. Cornelisse, and B. R. Buttery
Tomato (Lycopersicon esculentum Mill.) plants were grown in specially designed sectional treatment boxes which divided the root systems into 4 separate quadrants. Transpiration, photosynthesis, and stomatal conductance were determined in tomato plants with 4, 3, 2 and 1 quadrants of the root system supplied with water. The results suggested that there was no simple relationship between the percentage of root available for water uptake and transpiration rate. The shoot: root ratio of tomato plants increased as the proportions of roots supplied with water increased. The application of water to only 50% or 75% of the root system did not reduce transpiration, photosynthesis, stomatal conductance, or leaf surface area compared to a fully-watered plant. Where a substantial part of the root system (75%) was subject to moisture stress, only a small reduction in transpiration rate (20%) was observed. These results suggested that tomato roots had a greater relative absorption capacity for water uptake in response to the transpirational demand. The recovery of transpiration, photosynthesis, and stomatal conductance following the return to a fully watered state indicated that there had been no damage to the roots in the dry quadrants in any of the treatments.
Robert E. Paull and Theodore T. C. Goo
Anthurium flower (Anthurium andraeanum Andre) postharvest life was studied relative to water balance. Rate of water uptake declined to 20% of the harvest rate within 10 days. Silver nitrate pulsing reduced the water uptake rate decline and helped to maintain an increased rate 10 days after harvest. The spadix was the site of 50% to 60% of water loss while 20% to 40% of the loss occurred via the spathe and 10% to 20% via the stem. Water loss can be reduced by waxing with carnauba and other commerical fruit waxes, and this treatment can double postharvest life. The inhibitors Co2+ and amino-ethoxyvinylglycine significantly reduced the peak of ethylene produced, whereas inhibitors of ethylene response (Ag+, nitrogen and carbon dioxide bubbling) reduced autocatalytic ethylene production from cut anthurium flower stem sections. The cut stem ethylene production peaked 10 hr after cutting, then declined. Cytokinin also affected postharvest life, apparently via effects on water balance. Biocides (quaternary ammonium compound, streptomycin, nystatin, chloramphenicol, and a commercial flower preservative) had little effect on vase life. Wound ethylene-induced stem clogging and not microbial clogging of vascular tissue probably was the major factor limiting postharvest life, inducing water stress and senescence.
I. Gergely, R. F. Korcak, and M. Faust
Polyethylene glycol (PEG)-induced water stress in nutrient solutions decreased both water consumption and 45Ca uptake by apple seedlings (Malus domestica Borkh.) The decrease in water uptake was more severe than the decrease in 45Ca uptake. When 45Ca uptake was calculated on the basis of water consumption, it was found that 45Ca uptake was not dependent on water uptake although water was necessary for movement of 45Ca. In split-root experiments, PEG and 45Ca were either applied to the same half of the root or to separate halves. Calcium uptake decreased in plants subjected to water stress. The results indicated that the site of this decrease was at the root, not the aerial portion of the plant which, indirectly, may affect root function and thus 45Ca uptake. Split-root experiments also indicated that the unstressed half of the root cannot fully compensate for the stressed half of the root in either water or 45Ca uptake. Water use of plants with half of their root under a −5.0 bar water stress was decreased by 30%. Urea-nitrogen pretreatment did not modify the effect of osmotic stress on leaf Ca, Mg and K concentrations, water use or dry matter production during the period of applied water stress. All these parameters decreased with increasing solution osmotic stress.
Carlos A. Parera, Daniel J. Cantliffe, D.R. McCarty, and L. Curtis Hannah
The shrunken-2 (sh2) mutant of maize (Zea mays L.) increases sucrose and reduces starch in developing endosperm. An associated trait is poor seed and seedling vigor in seeds containing the mutation. The specific effects of sh2 mutant endosperm on embryo and seedling vigor were determined by analyzing seeds that contained either concordant wild-type or nonconcordant combinations of mutant and wild-type embryo and endosperm genotypes. The nonconcordant seed types that contained a wild-type embryo in association with a sh2 mutant endosperm or a sh2 mutant embryo in association with a wild-type endosperm were generated using the TB-3La translocation chromosome in which a wild-type Sh2 gene is attached to the centromeric portion of a B chromosome. Under stress conditions (complex stress vigor test), the seeds with mutant endosperm had lower germination, seedling fresh and dry weight, and index of conductivity than seeds with wild-type endosperm. Mutant endosperm and embryos excised from mutant endosperm imbibed more water than wild-type endosperm or embryos excised from wild-type endosperm. Because of the high concentration of osmotic solutes in the mutant endosperm, a rapid water uptake may induce a membrane disorganization. Leachate conductivities of seeds with mutant endosperm were higher than seeds with wild-type endosperm. In addition, a higher sucrose content and a lower raffinose to sucrose ratio were measured in the wild-type embryos associated with mutant endosperms than in the normal embryos excised from concordant wild-type seeds. These results suggest that a high rate of water uptake caused by the elevated concentration of osmotic solutes in seeds with mutant endosperms may affect membrane integrity during imbibition. Alternatively, the lower raffinose to sucrose ratio present in the mutant endosperm class might affect stabilization of cell membranes during seed desiccation. Embryos cultured in media containing 10% starch or no carbohydrate produced smaller seedlings than embryos cultured in 5% or 10% sucrose. Wild-type embryos excised from mutant endosperms exhibited lower germination in 0% and 5% sucrose media than embryos from concordant seed, indicating that reduced water uptake rates associated with lower external osmotic potential (10% sucrose) can improve vigor of embryos associated with sh2 mutant endosperm. The reduced vigor of embryos and seedlings that develop in association with sh2 mutant endosperm can be traced to the physiological and biochemical effects of the elevated sucrose levels present during seed formation and imbibition.
Juan C. Díaz, Kenneth Shackel, and Ellen Sutter
The contribution of in vitro-formed roots to the water status of tissue culture plants was studied by observing the stomatal responses of rooted and unrooted apple shoots. Stomatal conductance was measured on whole plants with a modified steady state porometer in a temperature-controlled room. The porometer was maintained at a steady 90% RH and conductance was measured every 30 seconds. Plants were kept in the gas exchange system for about 28 h. Steady state values of stomatal conductance for rooted and unrooted shoots were 220 (S.E= 19) and 163 (S.E=23) mmol m-2 s-1, respectively. When the plants were exposed to a light stimulus (1200 μmol m-2 s-1), rooted shoots showed an increase of about 64% in stomatal conductance. In the absence of roots, no response to light was observed. These results suggest that the presence of the roots improved, at least partially, water uptake and plant water status.