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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.

The effect of shoot density and crop level on gas exchange and water relations of filed-grown Sauvignon blanc was studied. Ten and 44 shoots/vine and one and two clusters per shoot treatments were examined in a factorial design. The two-cluster treatments had higher stem water potential (Y_stem), assimilation rate, and stomatal (g_s) and nonstomatal (g_m) conductance. A quantitative analysis suggests that capacitance cannot account for the simultaneous increase in g_s and Y_stem in the two clusters treatment. The two-cluster treatment had higher Y_stem for similar transpiration rates (similar g_s) compared to the one-cluster treatment. The similar transpiration rate and lower stem to root water potential difference in the two-cluster treatment was explained by increased root permeability in the two-cluster treatment. The similar g_s–g_m, in spite of a meaningful decrease of gs with decreasing Ystem, suggests that g_s and g_m synchronize themselves to perturbations of g_m due to sink effect and g_s due to water stress.

Effects of two shoot densities (14 and 44 shoots/vine) and two crop levels (one and two fruit clusters per shoot) on yield, pruning weight, crop load, and juice and wine quality of field-grown `Sauvignon blanc' grape (Vitis vinifera L.) were studied in a factorial experiment over 3 years. Main shoot length, lateral shoot length and number, shoot diameter, leaf area per shoot, and specific leaf weight were greater at the lower compared with the higher shoot density for all years whereas pruning weight was significantly increased only in the third year. Crop yield increased proportionally with the number of clusters, up to 44 clusters per vine, by both shoot and cluster thinning; a lower rate of yield increase was apparent when the number of clusters per vine was increased further, probably because of increasing source limitation. Berry maturation was delayed in the 44 shoots per vine treatment. Unchanged soluble solids, higher total acidity, and lower pH in the 44-shoot vine treatment in the third year indicated that the effect of cluster number on the must quality was not due to delayed maturation. No effect of cluster number per shoot on vegetative parameters was apparent. Berry size and number were affected by cluster thinning only in the 44 shoot/vine treatment. Both the number of shoots per vine and the number of clusters per shoot affected wine sensory attributes. Herbaceous aroma scores increased with increasing pruning weight. The wine sensory evaluation score decreased with increasing crop load. Total wine sensory scores decreased with decreasing leaf area to fruit weight ratio below ≈18 cm²·g^-1, whereas a critical value of the crop to pruning weight ratio, for wine quality, was not apparent. Crop load expressed as crop to pruning weight ratio (kg·kg^-1) was highly correlated with fruit weight to leaf area ratio (g·cm^-2) (r ² = 0.86), providing a biological rationale for the relevance of crop load and wine quality relations.

The effect of fall irrigation level in `Mauritius' and `Floridian' lychee (Litchi chinensis Sonn.) on soil and plant water status, flowering intensity, and yield the following year was studied in a field during 2 consecutive years. At the end of the second vegetative flush after harvest (1 Oct. 1994 and 10 Oct. 1995), four irrigation treatments were initiated: 0.5, 0.25, 0.125, and 0 Class A pan evaporation coefficients designated 100%, 50%, 25%, and 0%. The three lower irrigation levels effectively stopped shoot growth, suggesting the 50% treatment to be the threshold for shoot growth cessation in both years. For both years, flowering intensity and yield in the 100% treatment were lower than those following the other three treatments. Soil and plant water-stress indicators responded to the water-stress irrigation treatments. However soil water-potential values were highly variable relative to plant water potentials. Stem water potential differed more markedly between treatments than leaf water potential. Midday stem water potential appeared to be the best water-stress indicator for irrigation control. Midday stem water potential in both years was correlated with midday vapor-pressure deficit, suggesting that the threshold for irrigation control should take into account evaporative demand.

The interactions between irrigation and crop level with respect to fruit size distribution and soil and stem water potentials were investigated in a nectarine (Prunus persica (L.) Batsch. `Fairlane') orchard located in a semiarid zone. Irrigation treatments during stage III of fruit growth ranged from 0.62 to 1.29 of potential evapotranspiration (ETp). Fruit were hand thinned to a wide range of fruit levels (200 to 1200 fruit/tree in the 555-tree/ha orchard). Total yield did not increase with increasing irrigation rate above 0.92 ETp in 1996 and maximum yield was found at 1.06 ETp in 1997. Fruit size distribution was shifted towards larger fruit with increasing irrigation level and with decreasing crop level. The two highest irrigation treatments had similar midday stem water potentials. Our findings indicate that highest yields and highest water use efficiency (yield/water consumption) are not always related to minimum water stress. Total yield and large fruit yield were highly and better correlated with midday stem water potential than with soil water potential. This confirms other reports that midday stem water potential is an accurate indicator of tree water stress and may have utility in irrigation scheduling.

The effects of the timing and severity of postharvest water stress on the productivity and fruit quality of field-grown nectarine [Prunus persica (L.) Batsch cv. Snow Queen] were studied for two consecutive years. Three levels of postharvest water status (midday stem water potentials of -1.2, -2.0, and -2.8 MPa) were examined. They were designated as High, Med, and Low, respectively. In the second year two additional treatments were examined in which Low and Med water status were interchanged on 1 Sept. 2002, and these treatments were designated as Low/Med and Med/Low. The percentages of double fruits and of those having a deep suture increased with decreasing postharvest midday stem water potential during the previous year, and most of these defects were stimulated by water deficits that occurred prior to 1 Sept. Postharvest water stress led to decreased crop yield in the subsequent year because there were fewer fruits per tree. Flower buds with double pistils were first noticed in mid-September, and by mid-November the ranking of double pistils in the various treatments were similar to the ranking of double fruits measured a month after bloom in the subsequent season. Postharvest water stress delayed flower bud development. The percentage of double fruits increased from 10% in 2002 to 40% in 2003 and the higher percentage in 2003 was associated with higher air temperatures during the reproductive bud development stage in 2002 than in 2001. Our data and others suggest that high temperatures create a potential for the occurrence of double fruits, but that the fulfillment of that potential is highly dependent on postharvest tree water status. The occurrence of double and deep suture fruits were highly correlated with midday stem water potential in August of the previous year, i.e., during the initial stages of flower bud development. The occurrence of double fruits was observed to increase sharply as the midday stem water potentials fell below -2.0 MPa, which suggests that a midday stem water potential of -2.0 MPa could serve as a threshold for postharvest irrigation scheduling.

The interactions between irrigation and crop level with respect to fruit size distribution and midday stem water potential were investigated for 3 years in a nectarine (Prunus persica L. `Fairlane') orchard located in a semi-arid zone. Wide ranges of crop loads and irrigation rates in stage III were employed, extending from practically nonlimiting to severely limiting levels. Irrigation during stage III of fruit growth ranged from 0.63 to 1.29 of potential evapotranspiration (ETp). Fruit were hand thinned to a wide range of fruit levels (300 to 2000) fruit/tree in the 555-tree/ha orchard. The yields and stem water potentials from 1996, 1997 and 1998 were combined together and the interrelations among yield, crop load and stem water potential were examined. Fruit <55 mm in diameter growing at 400 fruit per tree were the only ones not affected by irrigation level. The yield of fruit of 60 to 75 mm in diameter increased with irrigation level, but only a slight increase was observed when the irrigation rate rose above 1.01 ETp. A significant decrease in the yields of 60 to 65, 65 to 70, and 70 to 75-mm size grades occurred at crop levels greater than 1000, 800, and 400 fruit per tree, respectively. Midday stem water potential decreased with increasing crop level, and it is suggested that midday stem water potential responds to crop load rather than crop level. Relative yields of the various size grades were highly correlated with midday stem water potential. It was suggested that the midday stem water potential integrates the combined effects of water stress and crop load on nectarine fruit size.

The interaction between irrigation and crop load with respect to fruit size distribution was investigated in a `Golden Delicious' apple (Malus domestica Borkh) orchard located in a semi-arid zone. Irrigation levels during the main fruit expansion phase ranged from 0.42 to 1.06 of the Class A pan evaporation coefficient. Crop load was adjusted to 100 to 450 fruit/tree in the 1250 trees/ha orchard by hand thinning. Total yield was not affected by irrigation level up to a crop load of 200 fruit/tree. Yield of all grades >65 mm was affected by irrigation level for higher crop densities. The yield of fruit of diameter <75 mm was not affected by increasing the Class A pan evaporation coefficient above 0.75. Our data indicate that availability of assimilates may limit the size of fruit with potential to grow larger than 70 mm in diameter at all crop loads higher than 200 fruit/tree. This limitation increases with decreasing irrigation level. The volumetric relative growth rate (VRGR) increased with irrigation level and with decreasing crop load. VRGR was more affected by crop load than by irrigation level in the ranges studied.