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Interrelations between water potential and fruit size, crop load, and stomatal conductance were studied in drip-irrigated `Spadona' pear (Pyrus communis L) grafted on quince C (Cydonia oblonga L.) rootstock and growing in a semi-arid zone. Five irrigation rates were applied in the main fruit growth phase: rates of 0.25, 0.40, 0.60, 0.80, and 1.00 of “Class A” pan evaporation rate. The crop in each irrigation treatment was adjusted to four levels (200 to 1200 fruit per tree) by hand thinning at the beginning of June 1999. The crop was harvested on 1 Aug. 1999, and fruit size was determined by means of a commercial sorting machine. Soil, stem, and leaf water potentials and stomatal conductance were measured during the season. Crop yield was highly correlated with stem and soil water potentials. The highest midday stem water potential was lower than values commonly reported for nonstressed trees, and was accompanied by high soil water potential, indicating that the maximal water absorption rate of the root system under those particular soil conditions was limited. Stomatal conductance was highly correlated with leaf water potential (r ² = 0.54), but a much better correlation was found with stem water potential (r ² = 0.80). Stomatal conductance decreased at stem water potentials less than -2.1 MPa. Both stem water potential and stomatal conductance were unaffected by crop load under a wide range of irrigation rates.

Water availability and crop load (number of fruit per tree) affect the fruit size of apple (Malus ×domestica Borkh.), but their interaction in relation to fruit size is not well understood. The objective of the present study was to explore the effect of crop load on water consumption of `Golden Delicious' apple in relation to fruit size. A wide range of irrigation rates and crop loads was applied to mature, field-grown `Golden Delicious' apple trees for two consecutive years, 1995 and 1996. The number of fruit, crop yield, and average fruit diameter were determined for each tree. A model was proposed to describe the combined effect of crop yield and irrigation rate on fruit size. In the model, irrigation waters were divided between two uses: vegetative water use (U_V), which enables the tree to produce a steady, long-term yield; and reproductive water use (U_R), which supports the production of the dry mass of commercial-size fruit. Potential fruit diameters were 77.1 and 72.2 mm for 1995 and 1996, respectively. Calculated vegetative use values were 300.2 and 323.4 mm for 1995 and 1996, respectively. The response of fruit diameter to reproductive water use per ton fresh weight (specific reproductive water use; U_RT) was fitted by a hyperbolic model in which the fruit diameter increases with increasing specific reproductive water use and approaches the yearly potential diameter at 60 to 70 m³·t^-1, irrespective of the potential fruit diameter. In both years, fruit diameter showed a closer correlation with the specific reproductive water use than with either crop load or irrigation rate. In conclusion, the crop yield and the potential fruit size determine the irrigation rate required to achieve a certain average fruit diameter. The year's potential fruit diameter does not affect the total tree water use or its components. The proposed model can be used by growers for supporting decisions on irrigation and thinning strategies in commercial orchards.

The sensitivity of leaf (ψ_leaf) and stem (ψ_stem) water potential and stomatal conductance (g_s) to soil moisture availability in apple (Malus domestics Borkh.) trees and their correlation with yield components were studied in a field experiment. Two drip irrigation treatments, 440 mm (H) and 210 mm (L), were applied to a `Golden Delicious' apple orchard during cell enlargement stage (55-173 days after full bloom). Data collected included ψ_stem, y leaf, g_s, and soil water potential at 25 (ψ_soil-25) and 50 cm (ψ_soil-50). No differences in midday ψ_leaf's were found between irrigation treatments. Stem water potential was higher in the H treatment than in the L treatment in diurnal measurements, and at midday throughout the season. Stomatal conductance of the H treatment was higher than the L treatment throughout the day. Stomatal conductance between 0930 and 1530 hr were highly correlated with ψ_stem. The H treatment increased the percentage of fruit >65 mm, and increased the proportion of earlier harvested fruit reaching marketable size compared to the L treatment. Fruit size in the first harvest and the total yield were highly correlated with ψ_stem. The degree of correlation between plant water stress indicators and yield component decreased in the following order: ψ_stem>ψ_soil-25,>ψ_soil-50>ψ_leaf. The data suggest that midday ψ_stem may serve as a preferable plant water stress indicator with respect to fruit size.

The relative contribution of various temperatures to dormancy completion of lateral vegetative apple [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.] buds was studied quantitatively on whole container-grown trees. Trees were exposed continuously to 10 different temperatures and also to daily alternating temperatures in a 24-hour cycle. In addition, fully chilled vertically and horizontally positioned shoots were compared under forcing conditions. No budbreak occurred in shoots chilled above 12.5 °C. There was a steep increase in budbreak as the chilling temperature fell from 12.5 to 7.5 °C. There was little difference in the level of budbreak on shoots chilled between 7.5 and 0 °C. The relative contribution of temperature to chilling accumulation in apple found in our study differs from what has been proposed for stone fruit and for apple in previous studies, especially at temperatures <6 °C. The length of exposure to forcing conditions required to initiate budbreak diminished as the chilling temperature was reduced. No additional bud-break was apparent on shoots chilled longer than 2100 chilling hours. The chilling requirement found here for lateral vegetative buds is much higher than that needed for terminal vegetative and flower buds. Trees that were exposed to daily alternating temperatures had lower levels of budbreak when the high temperature in the diurnal cycle was greater than 14 °C. Practically no budbreak was apparent on trees that were exposed to diurnal cycles with a high temperature of 20 °C for 8 hours. Budbreak on horizontally positioned trees was more than twice that on the vertically positioned trees, emphasizing the magnitude of the apical dominance effect and its strong masking of the chilling effect on lateral buds in vertically grown apple trees. Based on the data collected here we propose a new response curve for vegetative budbreak in `Golden Delicious·apple, within a temperature range between 0 to 15 °C.