Lauren C. Garner

Response of evergreen perennial tree crops to gibberellic acid is crop load-dependent: II. GA₃ increases yield and fruit size of ‘Hass’ avocado only in the on-crop year of an alternate bearing orchard

Lauren C. Garner et al. — Fri, 17 Feb 2012 12:48:07 PST

Despite problems of low fruit set, small fruit size and alternate bearing, the Hass cultivar dominates commercial avocado production worldwide. To increase yield and fruit size, gibberellic acid (GA₃) (25 mg L⁻¹) was applied at different stages of ‘Hass’ avocado tree phenology: (i) mid–late April (flower abscission), end of June–beginning of July (fruit abscission and beginning of the exponential phase of fruit growth), and mid-January (beginning of pre-harvest fruit drop); (ii) end of June–beginning of July; and (iii) mid-September (near the end of the major fruit abscission period; period of exponential fruit growth). In both years of the research, applications of GA₃ in April and June–July were within the periods of intense flower and fruit abscission, respectively; fruit abscission was low in September and January. Maximum air temperature was not related to flower or fruit abscission. In the on-crop year (391 fruit per untreated control tree), a single application of GA₃ at the end of June–beginning of July significantly increased total yield (kilograms only) and yield of commercially valuable fruit (178–325 g/fruit) (as kilograms and number per tree) compared with the control (P < 0.0001). GA₃ applied in September increased total yield (kilograms only) and yield of commercially valuable fruit (kilograms and number per tree) to values intermediate to and not significantly different from all other treatments, except trees receiving multiple applications of GA₃. This treatment reduced total yield and yield of commercially valuable fruit (kilograms and number per tree) relative to all treatments (P ≤ 0.0002). In contrast, during the off-crop year (32 fruit per control tree), no GA₃ treatment had a significant effect on yield or fruit size compared with the control and all other GA₃ treatments. For ‘Hass’ avocado, there was no negative effect from applying GA₃ at the end of June–beginning of July in both the off- and on-crop years; 2-year cumulative total yield and yield of commercially valuable fruit were increased by 27 kg (128 fruit) and 22 kg (101 fruit) per tree, respectively, above the yield of untreated control trees (P < 0.0001).

Meeting Learning Objectives through Service-Learning: A Pomology Case Study

Lauren C. Garner — Wed, 04 May 2011 11:25:11 PDT

Undergraduate students enrolled in the introductory pomology course at California Polytechnic State University, San Luis Obispo, CA, from 2007 to 2010, participated in a service-learning project. Students helped the community organization, the California Rare Fruit Growers (CRFG), teach grafting skills to San Louis Obispo County high school students and community members. At the end of each quarter, pomology students completed evaluations of their experience. Results of these evaluations were used to improve teaching methodology and the experience in which the students participated. Self-reported and instructor evaluations of the service-learning project demonstrated that students increased their grafting knowledge and skills, their confidence in learning new skills, and their interest in fruit science and community involvement. The service-learning project enabled students to meet course learning objectives of understanding and becoming experienced in horticultural techniques, such as grafting, and to meet university learning objectives of developing critical thinking and communication skills and increasing community involvement.

Brushing Pansy (Viola tricolor L.) Transplants: A Flexible, Effective Method for Controlling Plant Size

Lauren C. Garner et al. — Thu, 15 Jul 2010 17:07:19 PDT

Though brushing is an effective method for controlling excessive elongation in many species, its adoption by the commercial plug transplant industry will depend on the ease and flexibility of its application. Brushing was applied to pansy (Viola tricolor L.) seedlings growing at a density of 1500 plants m^-2 by daily stroking with 20/20 gauge netting. In dose response experiments, final petiole length appeared to approach a lower asymptote as number of brush strokes increased, and the number of strokes required to give near full reduction in length increased under environmental conditions favouring extension growth. Ten or 20 daily strokes, typically gave a 25-30% reduction in petiole elongation without causing plant damage or affecting subsequent flowering. Reductions in shoot dry weight tended to parallel those for petiole length. Increasing the interval between strokes by up to 10 rain resulted in similar reductions as continuous brushing. There was no difference between brushing the plants at 09:00 or 16:00 h, or between brushing 1 or 2 times day^-1. Plants required at least 5 days treatment week^-1 for significant size control. Brushing is an effective, flexible method for controlling petiole length in pansy and it should find application in the commercial transplant industry.

Split-Root Nutrition of Sweetpotato in Hydroponic Systems

M. A. Sherif et al. — Thu, 15 Jul 2010 17:07:19 PDT

Nutrient film technique (NFT) and deep water culture (DWC) hydroponic systems were used in a split-root study of the effect of four treatments on sweetpotato yield, the translocation of assimilates, and microbial population count. ‘TU-155’ cuttings (15 cm) were prerooted for 30 days in sand using deionized water and a modified half-Hoagland (MHH) solution. After 30 days, the plants were removed, and the roots of each were cleaned and split evenly between two sides of a channel (each 15 cm deep by 15 cm wide by 1.2 m long), four plants per channel. Replicated treatments were: MHH/MHH; MHH/Air, MHH/deionized water (DIW); and monovalent/divalent anions and cations (Mono/Dival). The entire experiment was repeated. Plants were harvested after growing for 120 days in a glasshouse. Storage roots, when produced, were similar in nutritive components. However, no storage roots were produced in Air or Mono channels and only a few in DIW suggesting inhibition of assimilate translocation. Fresh and dry weights for storage roots and foliage were highest in MHH/MHH in both NFT and DWC in both experiments. Solution samples were collected at 14-day intervals for microbial population profiling. Microbial counts (4.20–7.49 log cfu/ml) were highest in Dival channels. The counts indicated that solution composition influenced population size, and they were relatively high in both systems.

Spontaneous Hybridization between Maize and Teosinte

Norman C. Ellstrand et al. — Thu, 15 Jul 2010 17:07:18 PDT

The closest wild relatives of maize, Zea mays ssp. mays are various Zea taxa known as ‘‘teosinte.’’ Hybrids between maize and the teosinte taxon, Zea mays ssp. mexicana, often occur when the 2 are sympatric in Mexico. Measuring the spontaneous hybridization rate of the 2 taxa would shed light on the mechanisms contributing to the evolution and persistence of these hybrid swarms. We conducted a series of field experiments in Riverside, CA, to measure the natural hybridization rates between maize and 2 teosinte taxa, Z. m. ssp. mexicana and Zea mays ssp. parviglumis. We planted teosinte within and near maize plantations. Hybrids were identified by progeny testing for a maize-specific herbicide resistance allele and a teosinte-specific allozyme allele. Hybridity was confirmed by growing putative hybrid progeny to maturity to evaluate whether they had the characteristic morphology of maize X teosinte hybrids.We found that maize and Z. m. ssp. mexicana naturally hybridize at a low rate (<1%), whereas Z. m. ssp. parviglumis hybridizes with the crop at a high rate (>>50%).

Commercial Adaptations of Mechanical Stimulation for the Control of Transplant Growth

Lauren C. Garner et al. — Thu, 15 Jul 2010 17:07:18 PDT

The commercial use of mechanical stimulation to control transplant growth is quite limited. To be commercially successful, the technique must be simple and flexible, and must not reduce plant quality. Brushing was applied to tomato (Lycopersicon esculentum) seedlings growing at a density of 2100 plants m^-2 by daily stroking with a piece of polystyrene. Ten or twenty daily strokes typically provided significant height control without plant damage. Increasing the interval between strokes to up to 10 minutes resulted in the same amount of height control as continuous brushing. There were typically no differences between treating the plants at 0800 or at 1700 hours. Significant height control was achieved if treatments were begun at the first or second true leaf stage, but treatments begun at a later stage of development resulted in leaf damage. Brushing before transplanting to the field resulted in a significant decrease in stem elongation (~20%). However, there were no significant differences between brushed and unbrushed plants in long-term growth or final fruit yield after transplanting to the field. Preliminary studies were also conducted on four bedding plant species: geranium (Pelargonium x hortorum), impatiens (Impatiens holstii), petunia (Petunia hybrida) and pansy (Viola tricolor). Brushing was applied to seedlings growing at a density of 1500 plants m^-2 using either polythene or plastic netting. For geranium and impatiens, any reductions in plant stature were associated with significant plant damage. Brushing induced a prostrate growth habit and leaf distortion in petunia and significantly reduced leaf area and shoot dry weight. Pansy plants were undamaged by 10 or 20 brush strokes per day and showed a significant (17–22%) decrease in petiole length, typically without significant effects on leaf area, dry weight or days to flower. Brushing provides a flexible, effective method for controlling tomato and pansy transplant size without adversely affecting quality or long-term growth.