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James A. Zwack, William R. Graves, and Alden M. Townsend

Little is known about drought stress resistance of Freeman maples (Acer ×freemanii E. Murray), which are hybrids of red maples (A. rubrum L.) and silver maples (A. saccharinum L.). The objective of our study was to measure plant growth and leaf water relations of `D.T.R. 102' (Autumn Fantasy), `Celzam' (Celebration), and `Marmo' Freeman maples subjected to drought. Plants grown from rooted cuttings were subjected to four consecutive cycles of water deficit followed by irrigation to container capacity. Average stomatal conductance at container capacity for all cultivars was 255 mmol·s-1·m-2 in the first drought cycle and 43 mmol·s-1·m-2 during the fourth drought cycle. Predawn and midmorning leaf water potentials of droughted plants at the end of the fourth drought cycle were 1.16 and 0.82 MPa more negative than respective values for control plants. Osmotic potential of leaves at full turgor was -1.05 MPa for controls and -1.29 MPa for droughted plants, indicating an osmotic adjustment of 0.24 MPa. Root and shoot dry mass and leaf area were reduced similarly by drought for all cultivars, while Celebration exhibited the least stem elongation. `Marmo' treated with drought had the lowest root-to-shoot ratio and the greatest ratio of leaf surface area to root dry mass. Autumn Fantasy had the lowest ratio of leaf area to stem xylem diameter. Specific leaf mass of drought-stressed Autumn Fantasy was 1.89 mg·cm-2 greater than that of corresponding controls, whereas specific masses of Celebration and `Marmo' leaves were not affected by drought. Leaf thickness was similar among cultivars, but leaves of droughted plants were 9.6 μm thicker than leaves of controls. This initial characterization of responses to drought illustrates variation among Freeman maples and suggests that breeding and selection programs might produce superior genotypes for water-deficient sites in the landscape.

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Hava F. Rapoport, Giacomo Costagli, and Riccardo Gucci

Water deficit was applied between 4 and 9 weeks after full bloom by withholding irrigation from 3-year-old Olea europaea L. (`Leccino') plants grown in 2 L containers in a greenhouse. At 6, 8, and 22 weeks after full bloom (AFB), fruit were sampled for fresh weight and volume determinations, and then fixed for anatomical studies. Structural observations and measurements were performed on transverse sections at the point of widest fruit diameter using image analysis. Water deficit applied between 4 and 9 weeks AFB produced a significant decrease in predawn leaf water potential, which reached minimum values of -3.1 MPa. The applied water deficit reduced fruit fresh weight and volume at 8 and 22 weeks AFB. Fruit transverse area of the water deficit treatment was 50%, 33%, and 70% of the irrigated one at the 6-, 8-, and 22-week sampling dates, respectively. Mesocarp growth occurred for both irrigated and water deficit plants between 8 and 22 weeks AFB. At 22 weeks AFB differences between treatments were significant for mesocarp transverse area, but not for endocarp area. Mesocarp cell size, indicated by area, was significantly different between treatments at 8 and 22 weeks AFB. However, the mesocarp cell number was similar for both treatments at all times, and most mesocarp cells were produced by 6 weeks AFB. The growth of endocarp area showed the greatest shift in timing in response to the early water deficit. Ninety percent of endocarp growth had occurred by 8 weeks AFB in the irrigated treatment, but only 40% when the deficit irrigation treatment was imposed.

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Krishna S. Nemali and Marc W. van Iersel

Optimal substrate volumetric water content (θ) and drought tolerance of impatiens, petunia, salvia, and vinca were investigated by growing plants under four constant levels of θ (0.09, 0.15, 0.22, and 0.32 m3·m-3). Gas exchange, quantum efficiency (ΦPSII), electron transport rate (ETR), non-photochemical quenching (NPQ), and leaf water potential (ϒ) were measured for all species, and response of photosynthesis (Pn) to internal CO2 concentration (Ci) was studied in petunia and salvia. Leaf photosynthesis (Pmax) was highest at a θ of 0.22 m3·m-3 for all species and did not differ between a θ of 0.15 and 0.22 m3·m-3 for vinca and petunia. The Pn-Ci response curves for petunia were almost identical at a θ of 0.22 and 0.15 m3·m-3. Regardless of species, ETR and ΦPSII were highest and NPQ was lowest at a θ of 0.22 m3·m-3. Based on these results, a θ of 0.22 m3·m-3 for salvia and impatiens and a slightly lower θ of 0.15 m3·m-3 for vinca and petunia, is optimal. Mean osmotic potential in all treatments was lower in vinca and salvia and resulted in higher turgor potential in these species than other species. Analysis of Pn-Ci response curves indicated that Pn at a θ of 0.09 m3·m-3 was limited by both gas phase (stomatal and boundary layer) and non-gas phase (mesophyll) resistance to CO2 transfer in salvia. At the lowest θ level, Pn in petunia was only limited by gas phase resistance, indicating that absence of mesophyll resistance during drought may play a role in the drought tolerance of petunia.

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Tessa M. Mills, M. Hossein Behboudian, and Brent E. Clothier

Information on fruit water relations is scant for apple trees, especially under deficit irrigation. Here we discuss plant and fruit responses to deficit irrigation. Three-year-old potted `Braeburn' trees were studied in a glasshouse. The treatments were: well-watered control (C), early deficit (D1), and late deficit (D2). The latter two were, respectively, water stressed during 61–183 and 109–183 days after full bloom (DAFB). The final harvest was at 183 DAFB. Photosynthesis, stomatal conductance, and trunk circumference were lower in D1 and D2 than in C. Leaf area and shoot growth was reduced only in D1. Root length remained the same for all treatments. Fruit were smaller in D1 than in C; however, fruit growth was less sensitive to deficit irrigation than was vegetative growth. Fruit growth in D2 was the same as in C. Fruit concentrations of K+, fructose, sorbitol, total sugars, and titratable acidity were higher in D1 than in D2 and C. Total soluble solids were higher in D1 and D2 than in C. Although fruit water potential was lower in D1 than in C, a concomitant lowering of osmotic potential in D1 fruit led to maintenance of turgor potential, indicating osmotic adjustment. This could have been effected, at least partially, through accumulation of K+ and soluble sugars. Water relations of D2 fruit were not affected by deficit irrigation, although leaf water potential was lower than in C. Fruit water relations and fruit growth are therefore less sensitive to deficit irrigation than are those of vegetative parts.

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James A. Zwack, William R. Graves, and Alden M. Townsend

Freeman maples (Acer × freemanii E. Murray) are marketed as stress-resistant alternatives to red maples (Acer rubrum L.), but few data from direct comparisons of these species are available. As a first step in comparing the stress resistance of red maple and Freeman maple, responses to drought were studied in Acer × freemanii `Autumn Fantasy', `Celebration', and `Marmo'. Plants grown from rooted cuttings were treated by withholding irrigation through four drought cycles of increasing severity that were separated by irrigation to container capacity. Drought reduced shoot dry mass, root dry mass, and height growth by 64%, 43%, and 79%, respectively, over all cultivars. Predawn leaf water potential was reduced by 1.16 MPa over all cultivars, and stomatal conductance data indicated water use was more conservative over all root-zone moisture contents after repeated cycles of drought. Specific mass of drought-stressed leaves increased by 25% for `Autumn Fantasy', and microscopy to determine leaf thickness and cellular anatomy is ongoing. `Autumn Fantasy' also had the lowest ratio of leaf surface area to xylem diameter, and `Autumn Fantasy' and `Celebration' had higher ratios of root to shoot mass than `Marmo'. Pressure-volume curve analysis revealed osmotic potential of drought-stressed plants at full turgor was 0.24 MPa more negative than controls, and droughted plants had a greater apoplastic water percentage than controls. Although osmotic adjustment during drought was similar among cultivars, differences in specific mass of leaves and in ratios of transpiring and conducting tissues suggest cultivars of Freeman maple vary in resistance to drought in the landscape.

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N.S. Lang, R. Smithyman, L. Mills, R.L. Wample, J. Silbernagel, and E.M. Perry

Blackleaf (a.k.a. chocolate leaf) is of worldwide concern in Vitis due to its negative impact on fruit ripening, yield reduction and overall stress on grapevines. Research suggests blackleaf is induced by high levels of UV radiation and overall light intensity, which induce color changes (purple-brown-black) in exposed leaves, resulting in >50% reduction in photosynthesis. The ability to detect blackleaf symptoms before expression can provide insight into metabolic stresses and the possibility of the use and/or timing of management practices to reduce its impact. Remotely sensed imagery and spatial analysis may elucidate reflectance-related processes and symptoms not apparent to the un-aided eye. In this research we mapped canopy growth (leaves/shoot and shoots/vine), metabolic triggers (photosynthesis, leaf water potential, soil moisture), and percent blackleaf expression within vineyards using global positioning system (GPS), infrared gas analyzer, and digital remotely-sensed images. Each image and data record was stored as an attribute associated with specific vine location within a geographical information system (GIS). Spatial maps were created from the GIS coverages to graphically present the progression of blackleaf across vineyards throughout the season. Analysis included summary statistics such as minimum, maximum, and variation of green reflectance, within a vineyard by image capture date. Additionally, geostatistics were used to model the degree of similarity between blackleaf values as a function of their spatial location. Continuing research will be aimed at identifying spectral characteristics of early season stresses due to UV light, water stress, and reduced photosynthetic capacity. Spatial relationships between early season stress and later blackleaf expression will be assessed using joint spatial dependence measures. Overall, information obtained through digital image and spatial analysis will supplement site level information for growers.

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Lenore J. Nash and William R. Graves

Responses of five bottomland tree taxa to drought and flooding were studied to identify those adapted to urban environments. During one experiment, containerized `Franksred' red maple [Acer rubrum L. `Franksred' (trademark = Red Sunset)], sweetbay magnolia (Magnolia virginiana L.), black tupelo (Nyssa sylvatica Marsh.), bald cypress [Taxodium distichum (L.) Rich.], and pawpaw [Asimina triloba (L.) Dunal.] were treated with various irrigation regimes for up to 118 days. Net assimilation rate (NAR) and relative growth rate (RGR) were reduced more by flooding than by drought for plants of all taxa, except pawpaw, which showed similar NAR and RGR during flooding and drought. Only sweetbay magnolia and bald cypress maintained positive NAR and RGR during flooding, and sweetbay magnolia was the only taxon that did not produce significantly less leaf surface area, shoot dry mass, and root dry mass during flooding and drought. Apparent morphological mechanisms of stress resistance included an increase in specific mass of leaves (mg·cm-2) during drought for red maple and bald cypress and a 385% increase in the root: shoot mass ratio for droughted plants of pawpaw. Leaf water relations of drought- and flood-stressed `Franksred' red maple and sweetbay magnolia were determined in a second experiment. Predawn and mid-day leaf water potential (ψ) decreased with decreasing root-zone matric potential for both taxa, and transpiration rate was reduced by drought and flooding. Pressure-volume analysis showed that leaves of `Franksred' red maple responded to drought by shifting symplastic water to the apoplast. Leaves of drought-stressed sweetbay magnolia adjusted osmotically by reducing osmotic potential (ψπ) at full turgor by 0.26 MPa. Our results suggest that sweetbay magnolia and bald cypress will perform well at urban planting sites where episodes of drought and flooding regularly occur.

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Thayne Montague and Lindsey Fox

Recent droughts and depleted water tables across many regions have elevated the necessity to irrigate field-grown (FG) nursery trees. At the same time, ordinances restricting nursery irrigation volume (often without regard to plant water requirements) have been implemented. This research investigated gas exchange and growth of two FG maple tree species (Acer × freemanii `Autumn Blaze' and A. truncatum) subjected to three reference evapotranspiration (ETo) irrigation regimes (100%, 60%, and 30% of ETo) in a semi-arid climate. During Spring 2002, nine containerized (11.3 L) trees of each species were field planted in a randomized block design. Each year trees were irrigated through a drip irrigation system. During the first growing season, all trees were irrigated at 100% ETo. Irrigation treatments began Spring of 2003. Gas exchange data (pre-dawn leaf water potential and midday stomatal conductance) were collected during the 2003 and 2004 growing seasons and growth data (shoot elongation, caliper increase, and leaf area) were collected at the end of each growing season. For each species, yearly data indicates irrigation regime influenced gas exchange and growth of these FG trees. However, it is interesting to note gas exchange and growth of these FG maple trees were not necessarily associated with trees receiving the high irrigation treatment. In addition, it appears the influence of irrigation volume on the growth of these FG trees is plant structure and species specific. Our data suggests irrigation of FG trees based upon local ETo measurements and soil surface root area may be a means to conserve irrigation water and produce FG trees with adequate growth. However, continued research on the influence of reduced irrigation on FG tree species is needed.

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David A. Goldhamer, Elias Fereres, Merce Mata, Joan Girona, and Moshe Cohen

To characterize tree responses to water deficits in shallow and deep rooted conditions, parameters developed using daily oscillations from continuously measured soil water content and trunk diameter were compared with traditional discrete monitoring of soil and plant water status in lysimeter and field-grown peach trees [Prunus persica (L.) Batsch `O'Henry']. Evaluation occurred during the imposition of deficit irrigation for 21 days followed by full irrigation for 17 days. The maximum daily available soil water content fluctuations (MXAWCF) taken at any of the four monitored root zone depths responded most rapidly to the deficit irrigation. The depth of the MXAWCF increased with time during the deficit irrigation. Differences relative to a fully irrigated control were greater in the lysimeter than the field-grown trees. Minimum daily trunk diameter (MNTD) and maximum daily trunk shrinkage (MDS) responded sooner than midday stem water potential (stem Ψ), predawn or midday leaf water potential (predawn leaf Ψ and leaf Ψ), or photosynthesis (A). Parameters based on trunk diameter monitoring, including maximum daily trunk diameter (MXTD), correlated well with established physiological parameters of tree water status. Statistical analysis of the differences in the measured parameters relative to fully irrigated trees during the first 10 days of deficit irrigation ranked the sensitivity of the parameters in the lysimeter as MXAWCF > MNTD > MDS > MXTD > stem Ψ = A = predawn leaf Ψ = leaf Ψ. Equivalent analysis with the field-grown trees ranked the sensitivity of the parameters as MXAWCF > MNTD > MDS > stem Ψ = leaf Ψ = MXTD = predawn leaf Ψ > A. Following a return to full irrigation in the lysimeter, MDS and all the discrete measurements except A quickly returned to predeficit irrigation levels. Tree recovery in the field-grown trees was slower and incomplete due to inadequate filling of the root zone. Fruit size was significantly reduced in the lysimeter while being minimally affected in the field-grown trees. Parameters only available from continuous monitoring hold promise for improving the precision of irrigation decision-making over the use of discrete measurements.

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Sebastiano Delfine, Francesco Loreto, and Arturo Alvino

Physiological characteristics, growth, and biomass production of rainfed and irrigated bell pepper [Capsicum annuum L. var. anuum (Grossum Group) `Quadrato d'Asti'] plants were measured in the semiarid conditions of a Mediterranean summer to determine if drought stress effects are transient and do not affect plant growth and crop yield or are persistent and adversely affect plant growth and crop yield. A low midday leaf water potential indicated the occurrence of transient drought stress episodes in rainfed plants during the first 2 months of the study. Later on, predawn water potential also increased, indicating a persistent drought stress condition despite the occurrence of some rainfall. Photosynthesis was reduced when stress conditions developed, but the reduction was transient and limited to the central part of the day during the first 2 months. As plants aged, however, the impact of drought stress on photosynthesis was not relieved during the overnight recovery period. Stomatal conductance was reduced both during transient and permanent stress conditions while CO2 transfer conductance (i.e., conductance to CO2 inside the leaf) was only reduced when photosynthesis inhibition was unrecoverable. However, chloroplast CO2 concentration was higher in rainfed than in irrigated leaves indicating that CO2 availability was not limiting photosynthesis. Nonphotochemical quenching of fluorescence increased significantly in rainfed leaves exposed to permanent stress indicating the likely impairment of ATP synthesis. Transient inhibition of photosynthesis did not significantly affect leaf area index and biomass production, but growth was significantly reduced when photosynthesis was permanently inhibited. Fruit dry weight was even higher in rainfed plants compared to irrigated plants until drought stress and photosynthesis reduction became permanent. It is suggested that bell pepper growth without supplemental irrigation over the first part of the vegetative cycle does not impair plant growth and may even be useful to improve yield of early fruit.