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E.W. Pavel and E. Fereres

Responses to low soil temperatures at winter days of high evaporative demand were studied in 20-year-old (fi eld) and 1-year-old potted (controlled conditions) olive (Olea europaea L. cv. Picual) trees in 1996 and 1997. Low soil temperatures apparently affected tree water status as evidenced by low water potentials and stomatal conductance. Low night (2 and 5°C) but ambient day (above 10°C) temperatures did not affect stomatal conductance (gl), leaf (ψl), and xylem (ψx) water potentials of potted olive trees. Tree ψl and ψx decreased when exposed to low night and day temperatures (8°C), but gl was not affected. Water potential of those trees recovered very rapidly when the soil temperature was raised above 10°C at midday. When the trees were exposed to soil temperatures below 8°C for 3 days, ψl, ψx, and gl immediately decreased. After the first day, gl and ψx started to recover while gl was maintained at low levels, thus allowing for tree rehydration. Root hydraulic resistance (rroot)—a major part of whole plant resistance—increased immediately in response to soil temperatures below 10°C relative to that of control trees. The relationship between ψx and rroot indicated that the root system apparently plays a mayor role in the control of tree water status in response to low soil temperatures. During the winter months, olive tree water uptake seems to be primarily limited by low soil temperatures, even though soil water content is normally adequate due to high seasonal rainfall.

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E.W. PAVEL and T.M. DEJONG

The fruit growth of three peach (Prunus persica (L.) Batsch cvs. `Spring Lady', `Flamecrest', `Cal Red') and two apple cultivars (Malus domestica Borkh. cvs. `Cox Orange', `Golden Delicious') was measured weekly during the 1988 growing period. Seasonal patterns of fruit relative growth rate calculated on a dry weight basis were very similar for both species. Changes in nonstructural carbohydrate composition of peach mesocarp and apple pericarp were correlated with the two physiological phases of sink-activity of the relative growth rates Changes in sucrose concentrations seemed to coincide with increasing dry matter accumulation for both species, even though fructose was a dominant sugar in apples.

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E.W. Pavel and T.M. DeJong

Peach [Prunus persica (L.) Batsch] fruit thinning was used to reduce the competition for assimilates among peach fruits and to identify periods of source- and sink-limited growth during development. Individual fruit size, based on diameter or calculated dry matter accumulation, increased in trees with lower crop loads compared to fruits of unthinned trees in three peach cultivars. Relative growth rate analysis indicated that peach fruit growth was apparently limited by the assimilate supply (source-limited) or by its genetic growth potential (sink-limited) during specific growth periods. In stage I and at the beginning of stage III of the double-sigmoid growth curve, periods of source-limited growth occurred in the later-maturing cultivars Flamecrest and Cal Red. Peach fruit growth was apparently sink-limited during stage II of the growth curve when fruit relative growth rates were similar for the thinning treatments. Fruit growth in `Spring Lady', an early maturing cultivar, appeared to be primarily source-limited during the season. Although total fruit dry matter production was reduced by thinning, individual fruit dry weight on thinned trees was higher than that on trees with a heavy crop load. This typical thinning response was apparently caused by the differences in the amount of time that fruits grew under sink-vs. source-limited conditions with different crop loads. Final crop yield depended on fruit count per tree and on the available assimilate supply, and was affected by the individual fruit growth potential.

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E.W. Pavel and T.M. DeJong

Dry weights of whole fruit and of different fruit tissues, such as the mesocarp (with exocarp) and the endocarp (with seed), were accumulated on early (`Spring Lady'), midseason (`Flamecrest'), and late-maturing (`Cal Red') peach [Prunus persica (L.) Batsch] cultivars during the 1988 growing season. Seasonal relative growth rate (RGR) patterns of whole fruit showed two distinct phases for `Flamecrest' and `Cal Red'; however, `Spring Lady' did not exhibit two distinct RGR phases. The shift from phase I to phase II of the whole fruit RGR curve was related to an intersection of mesocarp and endocarp RGR curves, indicating a change of physiological sink activities in those fruit tissues in the later-maturing cultivars, but not in the early cultivar. Nonstructural carbohydrate compositional changes in concentration or content were similar in the three peach cultivars. Sucrose accounted for most of the seasonal increase in mesocarp nonstructural carbohydrate concentration. A sudden rise of sucrose was associated with the phase shift of the fruit RGR curves of the midseason and late-maturing cultivars, but not of the early maturing cultivar; however, in the early maturing cultivar, mesocarp compositional carbohydrate changes and, particularly, the sucrose increase, indicate that the physiological processes normally associated with the two phases exist in very early maturing fruit but are not associated distinctly with two separate RGR phases.