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  • Author or Editor: M. Stevenson x
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Whole-tree effects of alternate bearing on the reproductive growth, vegetative growth, and carbohydrate storage in mature pistachio (scion Pistacia vera L., rootstock Pistacia atlantica Desf.) were examined. Although it is generally accepted that the “off” year in pistachio may be used to accumulate carbohydrate reserves, it was found that starch and sugars from the “off” year equaled only 8% of the construction cost of the “on” year's fruit load. The dry mass increase in perennial woody tissue of roots, trunk and branches in the “off” year (20.2 kg/tree), was 75% of the mass of fruit in “on” trees (26.4 kg/tree). When construction cost of wood and fruit is considered, “off” year woody tissue growth is 64% of “on” year fruit growth. Since previous studies have shown that leaf dry mass is generally 30% greater in “off” than “on” pistachio trees, we conclude that the overall production of biomass in “off” and “on” year pistachio is similar. Therefore, we suggest that the “off” year is not a period of substantial storage carbohydrate accumulation, but rather a switch in allocation from reproductive to vegetative growth. This is consistent with ecologically based theories of the adaptive advantage of masting behavior, and suggests that withholding water and fertilizer during the “off” year may not harm “on” year yield.

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Pistachio (Pistacia vera L.) is known to strongly exhibit alternate bearing. Over 19,500 individual shoots were measured on eight alternate bearing `Kerman' pistachio trees on P. atlantica Desf. rootstock. Average length of “on” year (5.4 cm) and “off” year new shoots (5.6 cm) were not significantly different. New shoot length distribution was skewed toward the shorter length categories, with a mode of 2 and 4 cm in “on” and “off” trees respectively. These results contrast with previous studies which have shown that “on” year new shoots of pistachio are much longer than “off” year new shoots. There were about twice as many “on” year new shoots in the 18 to 30 cm class as compared to “off” new shoots, and fruiting wood length was associated positively with fruit number. However, ≈80% of tree yield occurred on fruiting wood that was <10 cm long, with shoots longer than 15 cm contributing <5% to yield. We suggest that results from earlier studies regarding the bud abscission process in very long pistachio shoots should be confirmed on shorter shoots, which contribute significantly to yield. Shoots >30 cm in length may be important in establishing vegetative buds in a position above the main tree crown for canopy expansion during the following “off” year.

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Color, ethylene production and respiration of broccoli (Brassica oleracea L. var. italica) dipped in hot water (45 °C, 10 minutes; 47 °C, 7.5 minutes; and 20 °C, 10 minutes as control) were measured. Hot-water treatment (HWT) delayed yellowing. Compared to the control, ethylene production and respiration in broccoli dipped at 45 °C decreased but recovered, and rates of both were enhanced after 24 and 48 hours, respectively, at 20 °C in darkness. There was no recovery of ethylene production or respiration in broccoli dipped at 47 °C. Following HWT of 47 °C for 7.5 minutes, respiration, starch, sucrose, and soluble protein content of florets and stems decreased dramatically during the first 10 to 24 hours after harvest. At the same time, fructose contents in florets and stems increased. Glucose increased in the florets but decreased within 24 hours in stems. Thereafter, glucose and fructose in florets and stems decreased. Sucrose content in florets and stems increased dramatically within a short period of treatment (<10 hours) and then declined. Protein in HWT florets and stems decreased during the first 24 hours and then increased until 72 hours. Ammonia content was lower in HWT broccoli during the first 24 hours and then increased above the level in the controls.

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To be useful for indicating plant water needs, any measure of plant stress should be closely related to some of the known short- and medium-term plant stress responses, such as stomatal closure and reduced rates of expansive growth. Midday stem water potential has proven to be a useful index of stress in a number of fruit tree species. Day-to-day fluctuations in stem water potential under well-irrigated conditions are well correlated with midday vapor-pressure deficit, and, hence, a nonstressed baseline can be predicted. Measuring stem water potential helped explain the results of a 3-year deficit irrigation study in mature prunes, which showed that deficit irrigation could have either positive or negative impacts on tree productivity, depending on soil conditions. Mild to moderate water stress was economically beneficial. In almond, stem water potential was closely related to overall tree growth as measured by increases in trunk cross-sectional area. In cherry, stem water potential was correlated with leaf stomatal conductance and rates of shoot growth, with shoot growth essentially stopping once stem water potential dropped to between −1.5 to −1.7 MPa. In pear, fruit size and other fruit quality attributes (soluble solids, color) were all closely associated with stem water potential. In many of these field studies, systematic tree-to-tree differences in water status were large enough to obscure irrigation treatment effects. Hence, in the absence of a plant-based measure of water stress, it may be difficult to determine whether the lack of an irrigation treatment effect indicates the lack of a physiological response to plant water status, or rather is due to treatment ineffectiveness in influencing plant water status. These data indicate that stem water potential can be used to quantify stress reliably and guide irrigation decisions on a site-specific basis.

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