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Steven A. Weinbaum

Our understanding of the quantities and seasonal patterns of nutrient uptake by mature fruit trees has been limited by the difficulties in working with the large woody biomass of these organisms, tree-to-tree variability, and the resolution to distinguish between recently acquired nutrient from the nutrient background of the tree. We have coupled the use of stable isotopes of nitrogen (N) with periodic whole-tree excavations and nutrient analyses during the year. Vegetative growth, reproductive growth, and nutrient storage in perennial tree parts during tree quiescence represent nutrient sinks. Data obtained using mature pistachio, prune, and walnut trees indicate that macronutrient accumulation in metabolic sinks is associated with increases in tree macronutrient uptake. These data are consistent with the concept that sink removal of phloem-mobile nutrients from vascular circulation may provide the stimulus to further uptake of the nutrient(s) sequestered. We propose that the recognition of those patterns can be used to increase the efficiency of tree nutrient recovery and utilization.

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Thomas M. Gradziel and Steven A. Weinbaum

The regulation of anther dehiscence by relative humidity (RH) was assessed for detached anthers and detached whole flowers from a limited selection of apricot (Prunus armeniaca L.), peach [P. persica (L.) Batsch], and almond [P. dulcis (Mill.) D.A. Webb, syn. P. amygdalus Batsch; P. communis (L.) Arcangeli, non Huds.] genotypes, as well as an almond X peach F2 progeny. Dehiscence was evaluated at 33, 64, 87, 93 and 97% RH for detached anthers, and at 33, 64 and 97% RH for whole detached flowers. Anther dehiscence was suppressed with increasing RH for all genotypes. Apricot anthers showed the greatest dehiscence at low RH and measurable dehiscence at high RH even when detached. Anther dehiscence in almond appeared more suppressed than in apricot at all RH levels tested, being completely suppressed by high RH in detached anthers. Peach genotypes exhibited the full range of variability between apricot and almond patterns. Evidence for transgressive segregation of RH-controlled anther dehiscence was observed in the occurrence of cleistogamy in an almond × peach F2 progeny. Rates of anther dehiscence were approximately linear with change in RH in detached anthers but exhibited a more buffered, step-wise response when detached whole flowers were tested. Results are consistent with field observations, and highlight the low but measurable risk of cleistogamy in these species, as well as opportunities to modify the breeding systems and crossing environments to facilitate controlled hybridization, and to reduce pollination vulnerability to adverse environments.

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Steven A. Weinbaum and T.T. Muraoka

An average of >20% seedless (blank) fruit are produced annually in Pistacia vera cv. Kerman. The degree of blank production was reportedly not related to individual tree yields and, therefore, was not thought to be resource limited (Crane, J.C., 1973. HortSci. 8:388-390). In two crop years, we studied the variability in percentage blanking among individual shoots characterized by widely varying leaf area to fruit (L/F) ratios. L/F ratios were related inversely to the percentage of blank fruit produced. Thus, individual branches behaved somewhat autonomously with respect to blanking. Our data are consistent with the view that embryo development was resource-limited. Although `Kerman' exhibits the potentiality for parthenocarpic fruit set, the hissed distribution of seedless fruit within the tree presumably indicates that blanking is an example of stenospermocarpy. Blanking does not result primarily from inadequate pollination under typical field conditions.

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Holly A. Johnson*, Steven A. Weinbaum, and Theodore M. DeJong

The effects of low and high crop loads in 2002 on floral development (Summer 2002), pistil size at anthesis (Spring 2003), and subsequent season fruit size at maturity (Summer 2003) were studied. Trees were all thinned to the same crop load in 2003. Three peach cultivars (Elegant Lady, O'Henry and Fairtime) with different ripening times (mid-July, mid-August, and early-September, respectively) were used to assess the effects of current season crop on floral development for the subsequent season. Based on previous literature, we reasoned that the maximum competition for carbohydrates between maturing fruit and developing buds is likely to occur at fruit maturity, especially under heavy crop loads. In 2003, individual fruit were harvested and weighed at maturity. In all three cultivars, a heavy crop load reduced the percentage of floral buds initiated and delayed floral differentiation. A heavy crop load also reduced pistil size at anthesis and fruit size at maturity in the subsequent season. These data support the practice of vigorous pruning to annually renew fruiting wood in peach to minimize the influence of crop in the previous season on the subsequent season's fruit and maintain large fruit sizes.

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Oswaldo A. Rubio, Patrick H. Brown, and Steven A. Weinbaum

Leaf N concentrations (% dry wt) appear relatively insensitive to high levels of applied fertilizer N (Weinbaum et al, HortTechnology 1992). This insensitivity may be attributable to growth dilation, lack of additional tree N uptake, a finite capacity of leaves to accumulate additional N or our inhability to resolve a limited increment. Our objective was to asses the relative accumulation of mobile forms of N (NO3, NH4 and amino acids) relative to a total N over a range of fertilizer N application rates in 3 year old, field-grown “Fantasia” nectarine trees. Between 0 and 136 Kg N/Ha/Yr we observed a linear relationship between N supply and all N fractions. Above 136 Kg N/Ha/Yr leaf concentrations of amino acids and total N remined constant, but NO3 and NH4 accumulation continued. These results suggest that leaf concentration of NO3 and NH4 are more sensitive indicators of soil N availability and tree N uptake than was total leaf N concentration.

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Geno A. Picchioni, Steven A. Weinbaum, and Patrick H. Brown

Factors affecting the phloem mobility of foliar-applied B have received little study. The purpose of this experiment was to evaluate foliar retention of B solutions, foliar uptake kinetics, and phloem mobility of foliar-applied B among four tree fruit species. Leaves on current-year shoots of nonbearing 'Red Delicious' apple, 'Bartlett' pear, 'French' prune, and 'Bing' cherry were immersed in 1000 mg/liter B solutions (supplied as 10B-enriched boric acid) in midsummer. Export of the applied label from leaves was monitored between 0 and 24 h, and throughout the following 20 days by ICP-mass spectrometry. Uptake by leaves increased steadily in all species between 0 and 24 h, and reached 80% to 95% of the applied quantity within 24 h. By 24 h, 62% to 75% of the applied label, depending on species, had been exported from the treated leaves. Apple leaves retained, absorbed, and exported over twice the amount of labelled B as prune and pear leaves, and nearly four times the amount of cherry leaves. Foliar retention largely controlled the capacity for uptake and export.

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Steven A. Weinbaum, R. Scott Johnson, and Theodore M. DeJong

Over-fertilization (i.e., the application of fertilizer nitrogen (N) in excess of the tree or vine capacity to use it for optimum productivity) is associated with high levels of residual nitrate in the soil, which potentially contribute to groundwater and atmospheric pollution as a result of leaching, denitrification, etc. Overfert-ilization also may adversely affect productivity and fruit quality because of both direct (i.e., N) and indirect (i.e., shading) effects on flowering, fruit set, and fruit growth resulting from vegetative vigor. Pathological and physiological disorders as well as susceptibility to disease and insect pests also are influenced by the rate of applied N. Over-fertilization appears to be more serious in orchard crops than in many other crop species. The perennial growth habit of deciduous trees and vines is associated with an increased likelihood of fertilizer N application (and losses) during the dormant period. The large woody biomass increases the difficulty in assessing the kinetics and magnitude of annual N requirement. In mature trees, the N content of the harvested fruit appears to represent a large percentage of annual N uptake. Overfertilization is supported by a) the lack of integration of fertilizer and irrigation management, b) failure to consider nonfertilizer sources of plant-available N in the accounting of fertilizer needs, c) failure to conduct annual diagnosis of the N status, and d) the insensitivity of leaf analysis to over-fertilization. The diversity of orchard sites (with climatic, soil type, and management variables) precludes the general applicability of specific fertilization recommendations. The lack of regulatory and economic penalties encourage excessive application of fertilizer N, and it appears unlikely that the majority of growers will embrace recommended fertilizer management strategies voluntarily.

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Vito S. Polito, Steven A. Weinbaum, and Tom T. Muraoka

Experiments were conducted to determine if differential responses of walnut pollen germination to temperature, previously observed to occur among genotypes, were genetically fixed or expressions of phenotypic plasticity representing adaptive responses to temperatures experienced during pollen development. Individual branches of a single walnut (Juglans regia L. cv. Serr) tree were warmed above ambient conditions during the final stages of pollen differentiation by directing a stream of moist, heated air into polyethylene enclosures, each containing an individual branch unit. Pollen was collected at staminate anthesis and incubated in germination medium on a temperature gradient apparatus. Model curves fitted to the in vitro pollen germination data were used to determine optimum germination temperatures. We found adaptive responses of pollen germination to temperatures experienced during pollen development. The optimum temperature for in vitro germination for pollen from branches maintained under ambient conditions was lower than that of pollen from branches with elevated temperature, and optimum germination temperature increased as a log function of integrated daily temperature (degree-days) experienced during pollen development.

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Richard J. Heerema*, Ted M. De Jong, and Steven A. Weinbaum

Spurs are the primary bearing unit in mature `Nonpareil' almond (Prunus dulcis (Mill.) D.A. Webb) trees. Our objective was to determine whether almond spurs behave autonomously with respect to various biological activities throughout the season. If autonomous, a spur's carbohydrate demands are met primarily by its own leaves and, therefore, the sink to source ratio of the spur itself is expected to be closely linked to its growth and development. In these experiments almond spurs differing in leaf area and/or fruit number were monitored for leaf development, fruit set, floral initiation, spur survival and carbohydrate storage. Previous-season spur leaf area had no relation to the number of leaves preformed within the dormant vegetative bud or final spur leaf area in the current season, but spurs which fruited in the previous season began spring leaf expansion later and current-season spur fruiting was associated with lower spur leaf area. There was little or no relationship between final percentage fruit set at the spur level and spur leaf area in either the current or previous seasons. Current-season spur leaf area was positively related to both spur flower bud number and spur winter survival. Carbohydrate storage in dormant spurs increased with increasing previous-season spur leaf area. These data are consistent with the concept of spur autonomy especially with regards to spur activities late in the season. The relationships of some of these same spur parameters to spur light exposure are currently being investigated.

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Steven A. Weinbaum, Theodore M. DeJong, and John Maki

In a simple, yet elegant experiment conducted 30 years ago, Chan and Cain (1967) using 'Spencer Seedless', a facultatively parthenocarpic apple (Malus×domestica Borkh.) cultivar, proposed that seeds inhibited flowering and accentuated biennial bearing in apple. Their conclusions have been extrapolated widely to include apple and other species. We have tested the universality of their conclusions using 'Bartlett' pear (Pyrus communis L.), a commercially important, facultatively parthenocarpic cultivar. Unlike 'Spencer Seedless' apples and seedless 'Bartlett' pear grown in France, California-grown seedless 'Bartlett' pear fruit strongly inhibited flowering the following year. However, the presence of seeds increased 'Bartlett' pear fruit size relative to seedless fruit by 13% and 20% in nonthinned and heavily-thinned pear trees, respectively, indicating that seeds increased fruit sink strength.