Three of 6 antitranspirants significantly reduced winter injury 1 of 5 years when applied 1 and 2 times to Ilex × attenuata Ashe ‘Foster's No. 2’. No reduction of winter injury was observed during 1979 and 1980 even though leaf water potentials recorded during winter stress periods indicated that Exhalt 4-10, Vapor Gard, Wilt Pruf, Folicote, and Foligard significantly reduced transpirational loss of water in midwinter. Two applications of Folicote and Exhalt 4-10 increased water potential more than the single fall application, but did not reduce winter injury significantly. Clear Spray did not increase leaf water potential and appeared to crack and peel within 3 weeks aftef application. Scanning electron micrographs indicated better coverage of leaf surfaces by dipping them by spraying and verified the rapid cracking of Clear Spray.
A study was conducted to investigate environmental factors which affect leaf water potential (LWP) response of chrysanthemums (Chrysanthemum × morifolium Ramat). Meteorological parameters, including air temperature (TEMP), relative humidity (HUM), total solar radiation (RAD), and photosynthetically active radiation (PAR) were measured simultaneously as LWP determinations were made diurnally for plants grown with 5 different irrigation rates. Stepwise multiple regression analyses using the meteorological parameters as independent variables and LWP as the dependent variable showed that models developed for each irrigation rate included TEMP, HUM, and PAR as statistically significant (P = 5%) independent variables. Coefficients of determination (R2) for the models ranged from 0.83–0.87. A combined model, including irrigation rate (R) as an independent variable along with the meteorological parameters, revealed that TEMP, PAR, HUM, and R were statistically significant at P = 1% and had an R2 = 0.84. Results reveal environmental factors which must be considered in studies involving LWP measurements for chrysanthemums in order to avoid misinterpretation of data.
Spraying 7-year-old citrus trees with film-forming antitranspirants before transplanting increased leaf water potential, thereby reducing “transplant shock.” Leaf water potential decreased rapidly after transplanting, by as much as 21 atm in unsprayed, and as little as 6 atm in sprayed trees. There was little benefit from transplanting in late afternoon rather than the morning.
The relationship between stomatal conductance and leaf water potential in field-grown apple trees (Malus domestica Borkh.) was determined throughout one growing season. Between May and September the leaf water potential required to close stomates decreased (became more negative) by about 25 bars, indicating decreasing sensitivity of the stomates to leaf water stress. A good linear correlation was found between stomatal conductance and net photosynthesis in trees grown under a wide range of water stress conditions. In September net photosynthesis of excised leaves of field trees was not reduced to zero until leaf water potentials reached −50 to −60 bars. The results emphasize the importance of pre-conditioning and time of season in plant water relations studies.
Measurements of leaf water potential for 5 plant species were made with the J-14 leaf press and a Scholander-type pressure bomb. Although a significant linear relationship was found between the readings from both instruments, the variability in the J-14 readings at a particular pressure bomb value prevents reliable predictions of the pressure bomb valve from the J-14 measurements.
The relationship between pea (Pisum sativum L.) leaf water potential (ψ) and crop color was investigated as water stress developed in the field. A technique was developed to measure pea leaflet ψ using a Scholander-type pressure chamber. The pressure chamber gave values similar to those of the screen-caged thermocouple psychrometer against which it was calibrated. The pressure chamber measured the decrease in associated with the green-to-blue change in crop color that occurs with the development of water stress. Use of crop color change offers an easily identifiable indicator of the onset of water stress that could, with further testing, be used in crop water management.
Three-year-old peach [Prunus persica (L.) Batsch] trees in a growth room under well-watered conditions were subjected to photosynthetically active radiation (PAR) levels (400-700 nm) of 444, 287, 144, 61, and 19 μmol°s−1m−2. At very low PAR levels (19 and 61 μmol°s−1m−2), the photosynthetic rate (P) declined with increasing temperature. Multiple regression analyses for the 3 highest PAR levels showed that an increase in PAR increased the optimum temperature for both P and stomatal conductance (gs). The highest leaf water potential (ψL) was found at 21°C in all 3 PAR levels. The regression equations also showed that PAR was the main determinant of P and gs, but that temperature was the main determinant of ψL.
Tomato plants (Lycopersicon esculentum Mill.) were grown under 4 irrigation treatments. In the first three an attempt was made to maintain soil water at a different level for each treatment. The fourth treatment was an approximation of the varied moisture levels that would be encountered with furrow irrigation. Leaf water potential (ΨL) was affected more during a day by atmospheric factors than by soil water availability. Hourly changes in ΨL and air water potential (Ψ air) were highly correlated (range .94*** and .99***). ΨL decreased as the plant aged, apparently due to decreasing soil water availability, decreasing root activity, and increasing resistance to water flow in stems and leaves. The stress factor, (which is an integration of the area below −6 bars; a critical level for tomatoes) was determined from the second degree polynomial regression of time versus ΨL and is proposed as a useful integration of ΨL. By using stepwise regression it was found that plant water status as evaluated by tensiometer reading, stress factor, the ratio of soil water/soil water at field capacity, and daily pan evaporation had the greatest effect on yield and total soluble solids content of tomato fruit.
Regression models of potato (Solanum tuberosum L. cv. Russet Burbank) leaf water potential (ψleaf) were developed for irrigated conditions using concurrent measurements of total solar irradiance (Rs), air vapor pressure deficit (VPD), air temperature (Ta) and windspeed. Estimates of potential evaporation (Ep) also were related to ψleaf. The data were collected over a 3-year period from two locations in southern Idaho. Of the variables tested, Rs had the highest simple correlation with ψleaf (r = 0.93), although a slightly higher correlation was obtained for log-transformed values of VPD (1n VPD). Potato ψleaf was best described by the equation ψleaf = -0.3672 - 0.1959 In VPD – 0.0005 Rs, where ψleaf, VPD, and Rs are expressed in units of MPa, kPa and W·m–2, respectively. The model accounted for 95% of the variation in ψleaf for well-watered ‘Russet Burbank’ potatoes. When the model was tested on an independent data set, it estimated diurnal changes in ψleaf for several different cultivars to within ±0.1 MPa of the measured values. The relationship between ψleaf and Ep was nonlinear and was described by an exponential function. Estimates for the Ep model were nearly identical to those for the VPD Rs model when ψleaf values were below -0.3 MPa.
Stomatal conductance, transpiration, and photosynthesis declined steadily with decreasing leaf water potential in seedlings of peach [Prunus persica (L.) Batsch] grown in large pots containing about 84 kg of steam-sterilized sandy loam soil under controlled environmental conditions. Growth and transpiration were reduced mainly through the effect of stomatal closure as soon as water stress commenced.