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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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Steven A. Weinbaum, Wesley P. Asai, David A. Goldhamer, Franz J.A. Niederholzer, and Tom T. Muraoka

There is legitimate concern that excessive fertilizer nitrogen (N) application rates adversely affect groundwater quality in the San Joaquin Valley of California. A 5-year study was conducted to assess the interrelationships between N fertilization rates, tree productivity, leaf [N], soil [NO 3], tree recovery of isotopically labeled fertilizer N, and NO 3 leaching. High N trees recovered <50% as much labeled fertilizer N in the crop as did trees previously receiving low to moderate fertilizer application rates. Our data suggest that the dilution of labeled N in the soil by high residual levels of NO 3 in the soil had a greater effect than tree N status (as expressed by leaf N concentration) on the relative recovery of fertilizer N.

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Steven A. Weinbaum, Wesley Asai, David Goldhamer, and Franz J.A. Niederholzer

A project to study the interrelationships between leaf N conc., relative tree yield (RTY), nitrate leaching and fertilizer N recovery was established in 1990. Collection of pretreatment baseline data was followed by differential rates of N fertilization. Significant differences in leaf N conc. and RTY were obtained in 1992 and 1993, respectively. RTY is defined as tree yield in 1993 expressed as a percentage of pretreatment (1990) yield. 15N-depleted (NH4)2SO4 was applied postharvest in 1993 to 17 trees differing in RTY and leaf N conc., and recovery of labelled N in the blossoms of these trees (March, 1994) will be discussed.

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Edwin J. Reidel, Patrick H. Brown, Roger A. Duncan, and Steven A. Weinbaum

Almond [Prunus dulcis (Mill.) D.A. Webb] yields have increased substantially since the 1961 publication of the Univ. of California (UC) guidelines for leaf potassium (K). Numerous growers and reputable analytical laboratories are concerned that the recommendations for leaf K are inadequate. A highly productive almond orchard with low leaf K was selected to reassess the leaf K critical value of 1.1% to1.4% and determine the relative sensitivity of various yield determinants to inadequate K availability. Baseline yields for 100 individual trees were measured in 1998 and four rates of potassium sulfate were applied under drip irrigation emitters to establish a range of July leaf K concentrations between 0.5% and 2.1%. No relationship was observed between leaf K and post-treatment yield measurements made in 1999. We also monitored individual limb units on trees from the treatment extremes for effects of low K availability on flower number, percentage fruit set, fruit size, spur mortality, and vegetative growth (potential fruiting sites in subsequent years). Those measurements indicated that although current-year yield determinants (percentage fruit set and fruit size) were not influenced by K deficiency, components of future yield were impacted negatively by low K availability: mortality of existing fruiting spurs was increased by K deficiency and growth of fruiting wood was reduced.

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Daniel S. Kirschbaum, Kirk D. Larson, Steven A. Weinbaum, and Theodore M. DeJong

The pattern of total nonstructural carbohydrate [starch and soluble sugars (TNC)] accumulation in strawberry (Fragaria ×ananassa Duch.) nursery runner plants, cv. Camarosa, was determined for three growing seasons. A similar study was conducted on `Selva', but for only one year. Growth, development and fruit production patterns of plants transplanted to growth chambers (GC) or fruiting fields were also evaluated. The experiments were carried out on plants propagated in high latitude (41°50' N) nurseries in California (Siskiyou County). Plants were sampled beginning late summer through early autumn and analyzed for dry mass (DM) and TNC. Plants from different digging dates were established in GC or fruit evaluation plots in Irvine, Calif. (33°39'N). Initial TNC concentration in storage tissues at the time of nursery digging increased steadily from the second week of September to the third week of October. Crown and root TNC concentration and content were correlated positively with the accumulation of chilling units (CU = hours ≤7.2 °C) in the nursery. Root TNC concentration consistently increased from 6% to 10% DM in `Camarosa' (a short-day cultivar), and from ∼4% to 14% DM in `Selva' (a day-neutral cultivar) from mid-September to the first week of October. The root TNC content increased ∼2.5 times in `Camarosa' and ∼3.7 times in `Selva' during the same period. Transplant growth, development, and fruiting pattern were affected by digging date. Root TNC concentration and content were more sensitive to CU accumulation than crown TNC concentration and content. Therefore, root sampling appeared to be more appropriate than crown sampling for assessing the carbohydrate status and optimal digging dates of strawberry nursery runner plants early in the fall.

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Timothy Spann, Robert H. Beede, Steven A. Weinbaum, and Theodore M. DeJong

Rootstock significantly alters the pattern of shoot growth of pistachio (Pistacia vera) cv. Kerman. Trees grown on P. atlantica typically produce a single flush of spring growth, whereas trees on P. integerrima selection PGI and P. atlantica × P. integerrima selection UCB-1 can produce multiple flushes during the season. We have shown that the spring flush is entirely preformed in the dormant bud for all three rootstocks, but later flushes are neoformed, that is, nodes are initiated and extended during the same season. Shoots producing both preformed and neoformed growth have lower yield efficiency than those producing only preformed growth. Additionally, yield components of the crop from shoots with both preformed and neoformed growth was different than for shoots producing only preformed growth. However, these differences do not appear to be significant at the whole tree level. These data suggest that neoformed growth can both compete with fruit growth for available resources (lower yield efficiency) and act as an additional source (altered yield components), depending on the factor being measured. Controlling neoformed growth may potentially increase pistachio yield through a shift to the more efficient preformed shoots while at the same time lowering orchard maintenance costs by reducing required pruning. We have data to indicate that regulated deficit irrigation and new pruning techniques may be viable methods for controlling neoformed growth in pistachio without affecting yield.

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Timothy M. Spann*, Robert H. Beede, Steven A. Weinbaum, and Theodore M. DeJong

Rootstock significantly alters the pattern of shoot growth of pistachio (Pistacia vera) cv. Kerman. Trees on P. atlantica typically produce a single flush of spring growth whereas trees on P. integerrima selection PGI and P. atlantica × P. integerrima selection UCB-1 can produce multiple flushes during the season. Terminal buds of shoots on all three rootstocks were dissected during the dormant season to determine the number of preformed nodes. Data indicate that there are 8-9 nodes preformed in the dormant terminal bud of shoots from Kerman trees and that this number is independent of rootstock, canopy location, crop load, and shoot carbohydrate concentration, suggesting genetic control. This number corresponds with the number of nodes typically found on a shoot at the end of the spring growth flush. Unlike the spring flush which is preformed in the dormant bud, later flushes are neoformed, that is, nodes are initiated and extended during the same season. Neoformed growth depends on current season photosynthates and may compete with fruit growth for available resources. Neoformed growth is sensitive to water stress and trees on all three rootstocks grown under two levels of regulated deficit irrigation showed a reduction in both the number and length of neoformed shoots. Preformed shoot growth did not appear to be reduced under water stress conditions, supporting the hypothesis that preformed shoots are more dependent on environmental conditions during the season they are initiated than during the season they are extended. Additionally, preformed shoots on well irrigated trees were similar in length for all rootstocks, further supporting the idea that preformed shoots are under genetic control and are not easily manipulated.