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Abstract

Ethylene and other olefinic compounds cause apples and other climacteric fruits to ripen. Propylene, which fruits do not produce, was employed to determine, 1) the stage of maturity apples must attain to autocatalytically produce ethylene, and 2) the effect of O₂ tension on autocatalysis. ‘Red Delicious’ apples harvested at developmental stages representing 52, 58, 65, and 75% of maturity were gassed with propylene at concentrations of 0, 10, 50, 100, 500, and 1000 ppm for 1 week at 20°C. Propylene induced ethylene synthesis at all stages of maturity. Its ability to stimulate ethylene production, however, increased progressively with fruit maturation, although rate of production following treatment with 500 ppm propylene was constant. A shorter lag time to the onset of autocatalytic production was observed in more mature fruits which reflects a natural increase in sensitivity. Propylene administered at 6.5% O₂ or less did not induce ethylene production, but an anaerobic atmosphere was necessary to completely inhibit ethylene synthesis in fruits once autocatalysis began.

Abstract

Soil moisture use, shoot growth, fruit size, and yield of apricot (Prunus armeniaca) trees (cv. Hatif de Colomer) were measured under flood water-spreading conditions in the desert. Root distribution was estimated from interpretation of soil moisture data. A highly significant correlation was found between pooled values of past and current seasons' water depletion values and current season's yield. Relative maximum water depletion (on a daily use basis) occurred during fruit maturation. Maximum trunk growth occurred in spring. In some seasons a second, small peak occurred in the autumn.

Shoot growth usually terminated by the beginning of June, and maximum trunk rate growth was usually (except in 1967) attained shortly thereafter.

Trees survived the extremely dry 1968 season (no flood, and only 80 mm of rain), with partial recovery in shoot growth and yield and full recovery in trunk growth during the subsequent year.

Moisture depletion under the trees, per 1000 m², during the active period of the trees (March to end of September) was only 26 mm in 1968, about 100 mm in 1966, 106 mm in 1967 and 146 mm in 1969. Yields per tree (26 trees per 1000 m²) ranged from 1.4 kg (in 1970) to 27.4 kg (in 1967).

The central cylinder around the tree (2 meters in diam) accounted for the highest relative use of water in comparison with other annuli extending around the tree.

Abstract

Maturity and storage characteristics of ‘Bartlett’ and ‘d’ Anjou’ fruits from trees planted in 1965 on 8 Pyrus rootstocks were studied in 1977, with all fruits of a cultivar harvested on the same date. Fruits of ‘Bartlett’/Old Home (OH) had high soluble solids, low weight loss, and rapid change of color from green to yellow after 3 months storage, indicating that these fruits were harvested at optimum maturity. Fruits of ‘d’ Anjou’/OH were also high in soluble solids at harvest and after storage for 3 and 4 months and changed color properly after storage. Fruits from ‘Bartlett’/P. betulaefolia Bunge, changed color slowly and had high weight loss and firmness after storage, indicating immaturity at harvest. Fruit of ‘d’ Anjou’/P. ussuriensis Maxim, were firmest at harvest but relatively soft after 3 and 4 months of storage. Fruit of ‘d’ Anjou’/P. betulaefolia were firm after storage with moderate weight loss, low soluble solids, and cork spot. Fruit of ‘d’ Anjou’/P. betulaefolia and P. ussuriensis were high in cork spot but low in scald. Some of these changes reflect an apparent rootstock influence on fruit maturation.

Pierce's Disease (PD) is a major factor limiting grape production in the southeast United State. This disease is caused by a bacterium, Xylella fastidiosa Wells et al., which is transmitted to the xylem system of the grapevines primarily by glassy-winged sharpshooters (Homalodisca coagulata Say). Once it is in the xylem, the X. fastidiosa will use the xylem sap as a nutrient source to multiply, colonize, and eventually plug the xylem vessels and cause the PD in susceptible cultivars. On the other hand, symptoms of PD in tolerant cultivars do not appear until fruit maturation, and symptoms are rarely observed in PD resistant cultivars. In order to understand the correlation between X. fastidiosa and PD symptom development, a study was initialed to monitor X. fastidiosa in xylem of resistant, tolerant, and susceptible vines on a monthly basis. Presence of X. fastidiosa was detected directly from xylem sap of field-grown vines by medium culture and confirmed by polymerase chain reaction (PCR). Xylella fastidiosa was detectable throughout the growing season in PD susceptible cultivar `Chardonnay', PD tolerant Florida hybrid grape `Blanc du Bois', and muscadine cultivar `Carlos'. The bacteria were also appeared in the dormant vines with high density in cultivars `Chardonnay' and `Blanc du Bios'. Although X. fastidiosa was also found in dormant canes of `Carlos', the density decreased throughout the late fall and winter months, and they were hardly found before June. The results indicated that X. fastidiosa were carried over from previous season in cultivars `Chardonnay' and `Blanc du Bois', while in PD tolerant cultivar `Carlos', they were newly acquired from the sharpshooter feedings during the growing season.

Physiological effects on plant growth caused by the plant hormone ethylene have been noted for many years. More than 100 years ago, workers noted that illuminating gas or broken gas mains had deleterious effects on surrounding trees or plants. It was not until the 1960s that scientists documented that plant growth may be manipulated by applying ethylene. Some of the biological effects since noted include premature defoliation, fruit maturation ripening, induction of flowering, stimulation of sprouting or germination, and shortening of plant height. These effects are noted on a wide variety of agricultural crops, including vegetables, field crops, tree crops, and ornamentals. Ethylene is a gas and dissipates rapidly, and, thus, does not lend itself to field application. In the 1960s, the product ethephon [(2-chloroethyl)phosphonic acid] was developed. When taken up by the plant, ethephon is converted to ethylene in the cells and becomes available for physiological interactions. Because ethephon precipitates a wide variety of biological reactions, application technology becomes extremely important. Factors such as plant growth stage, plant stress status, plant foliage spray coverage, ethephon rates, and environmental conditions determine the responses obtained. An example is provided by processing tomatoes, where the desired response is to maximize fruit maturity enhancement and minimize premature defoliation—both ethylene responses. We have selected five agricultural applications of ethephon as examples of how plant growth may be altered. These are: increased boll opening in cotton; enhanced pistillate flower induction in hybrid squash seed; accelerated fruit maturity in processing tomatoes; enhanced hull splitting in walnuts; and reduced lodging in wheat. Each of these applications, and others, are common in California agriculture. Brevity necessitates providing only a summary of relevant applied research activities, which are not intended to be complete or thorough. Details on specific ethephon applications may be obtained from that particular researcher.

Microcitrus is one of five genera that are partially sexually cross-compatible with the genus Citrus. The genus Microcitrus contains seven species with characteristics that may be valuable for breeding citrus scions and rootstocks, including zygotic embryony, short juvenile period, short fruit maturation time, and resistance to nematodes and Phytophthora. However, relatively few F₁ hybrids between Microcitrus and Citrus have been reported, and most of these have been pollen- and ovule-sterile. Some of these intergeneric hybrids have also been highly susceptible to cold damage. To create a genetic bridge for recombination of useful traits from Microcitrus into Citrus, two selections of Citrus ichangensis (an exceptionally cold hardy species with zygotic embryony and short juvenile period) were hybridized with Microcitrus warburgiana and two selections of Microcitrus inodora. Seed were collected from these crosses and germinated in a warm greenhouse. A total of 94 M. inodora xC. ichangensis hybrids and 34 M. warburgiana xC. ichangensis hybrids) were obtained and transplanted to 4-gallon pots in a screenhouse. At 33 months after planting the seed, 42% of the M. inodora × C. ichangensis hybrids and 67% of the M. warburgiana × C. ichangensis hybrids had flowered. Pollen germination tests on agar plates indicated that several hybrids produced large quantities of viable pollen. Numerous crosses were completed using some of these F₁ hybrids as pollen and seed parents. Several F₁ hybrids were confirmed to be highly fertile by recovery of healthy F₂ and backcross hybrids with Microcitrus sp., Citrus sp., Poncirus trifoliata, and other Microcitrus (C. ichangensis) selections.

Use of plastic mulch to increase rhizosphere temperatures is a common practice in spring production of vegetable crops. However, supraoptimal soil temperatures during the fruit maturation period in early summer can impair root function and reduce produce quality. The effects of colored plastic mulch on rhizosphere temperature and `Primo' muskmelon root respiration were investigated in the field during Fall (Aug.-Nov. 2002) and Spring (Mar.-May 2003) seasons. Rhizosphere temperatures (measured at 0.1 m below the soil surface with thermo-couples) and respiration under four plastic mulches (black, silver, white, and clear), and a bare ground control were studied. The soil warming properties of the different mulches differed between Spring and Fall. Bare ground rhizosphere temperatures declined from ≈33 to 21°C in the Fall and increased from 14 to 26 °C in Spring. In both studies, black and clear plastic mulches had the highest rhizosphere warming effects (3-8 °C) compared to bare ground. In the Fall, average midday soil temperatures under the white and silver mulches were 2-3 °C cooler than the bare ground treatment. Canopy establishment was accelerated by plastic mulches in Spring but not in Fall. Root + soil respiration was positively correlated with measured rhizosphere temperatures (r = 0.69), with the highest respiration rates recorded under the clear and black plastic mulches. More than 80% of fruits from the clear plastic treatment were deformed and unmarketable. The number of marketable fruit was similar among the black, white and silver mulch treatments and significantly greater (32% in Spring & 12% in Fall) than in the bare ground treatments.

Seasonal fluctuations of carbohydrate levels and compositions and the activities of related enzymes of three cultivated tomato (Lycopersicon esculentum Mill. cv. Lady First, Momotaro, and Minicarol) cultivars were examined at 45-days interval with seven different sowing in the relatively warm climate of Japan. Fruits picked on early winter to spring seasons had higher sugar concentrations compared to hot season. Fructose and glucose in nearly equal amounts were the predominant sugar in all the seasons. Sucrose was present in trace quantities, but cherry cultivar Minicarol accumulated higher levels than the other two large-fruited types.

Acid invertase (EC 3.2.1.26) was highest at red stage during December to April, while fruit matured during May to August had lowest activity. The activity levels of soluble invertase were predominant compared to cell wall-bound fraction. The sucrose synthase (EC 2.4.1.13) showed highest activity in rapidly growing fruits followed by a very low activity with fruit maturation. Sucrose synthase showed the higher activity during November to February, and almost low activity during all the experimental periods. The sucrose phosphate synthase (EC 2.4.1.14) also showed higher activity during October to February, but the activity levels did not change drastically throughout the fruit development. The results substantiate the conclusion that, in all the planting seasons, acid invertase is a principal enzyme in the process of tomato fruit ripening and during early stage of tomato fruit development, sucrose synthase is the dominant enzyme, which, in turn, plays a part in regulating the translocation of sucrose into the fruit.

ReTain™, a commercial derivative of aminoethoxyvinylglycine, was applied as a single application at 124 g·ha^-1 a.i. to `Bartlett' pear (Pyrus communis L.) trees 28, 21, 14, or 7 days prior to initial commercial harvest and at 62 g·ha^-1 a.i. in combination with naphthaleneacetic acid (NAA) at 92 g·ha^-1 a.i. 14 days prior to initial commercial harvest. Maturity and quality of treated fruits at harvest and following storage were compared with those of nontreated pears in 1996 and 1997. Ethylene production by mature green pears at harvest was not significantly affected by ReTain™ treatments, although softening, loss of chlorophyll, and starch clearance were usually inhibited by the 14- or 7-day treatment. ReTain™ suppressed ethylene production, softening and loss of chlorophyll in ripening pears and mature green pears cold-stored for 4 months, although loss of chlorophyll did not differ in the cold-stored fruit in 1997. ReTain™ had little effect on softening during a ripening period of 6 days after 4 months of cold storage. Application at 14 or 7 days prior to initial harvest appeared most effective, often with little difference between the two timings, and the 28- or 21-day treatment or combined ReTain™ and NAA treatment were seldom more effective. ReTain™ applied 14 or 7 days before initial harvest delayed fruit maturation by 4-10 days depending on the maturity index. The maturity or ripeness of pears from the combined ReTain™ and NAA, NAA only, and control treatments was often similar or differed only slightly. Premature ripening, prevalent in 1997, was dramatically suppressed in fruit treated with ReTain™. Ripening of both ReTain™- and non-ReTain™-treated fruit with ethylene reduced premature ripening by ≈50%.

A 4-year study was conducted to establish the effects of drip irrigation configuration and rate on fruit yield and quality of young highbush blueberry plants (Vaccinium corymbosum L. ‘Duke’). Plants were grown in a silt loam soil on raised beds and were non-irrigated or irrigated using either one or two lines of suspended drip tape. Each line configuration had in-line emitters spaced every 0.3 or 0.45 m for a total of four drip configurations. Water was applied by each drip configuration at two rates, a moderate rate of 5 L/plant per irrigation event, and a heavy rate of 10 L/plant. The frequency of irrigation was guided by measurements of soil matric potential. Irrigation was applied each year, and plants were cropped beginning the second year after planting. Rainfall was above normal in the first 2 years of the study, and differences in soil moisture were most evident in the last 2 years, in which soil matric potential increased with irrigation volume. Neither the number of irrigation lines nor emitter spacing had an effect on yield or fruit quality. Yield was unaffected by irrigation rate until the fourth year after planting and was only higher when 5 L/plant was applied. The yield increase was the result of differences in fruit weight during the second of two harvests and was associated with delays in fruit maturation. Irrigation affected plant mineral concentrations but leaves and berries responded differently; affected minerals tended to decrease in leaves but increase in the fruit. Many irrigation-induced changes in fruit quality were evident 1 or 2 years before changes in yield. Higher irrigation volume increased fruit size and water content but reduced fruit firmness and soluble solids. Irrigation reduced fruit water loss during storage and thereby promoted longer shelf life. Irrigation also resulted in a change in anthocyanin composition in the fruit but did not affect antioxidants or total anthocyanin content.