IMPROVING PECAN PRODUCTION BY INVESTIGATING PHYSIOLOGICAL AND ENVIRONMENTAL CONSTRAINTS ON FLOWERING

• Sponsoring Institution: National Institute of Food and Agriculture
• Project Status: COMPLETE
• Funding Source: HATCH
• Reporting Frequency: Annual
• Accession No.: 1019501
• Project Start Date: Oct 1, 2019
• Project End Date: Sep 30, 2024
• Program Code: [(N/A)]- (N/A)

Recipient Organization
OKLAHOMA STATE UNIVERSITY
(N/A)
STILLWATER,OK 74078

Performing Department
Horticulture & Landscape Architecture

Non Technical Summary
Alternate bearing and spring freeze is one of the most severe problems threatening pecan bloom and production in Oklahoma.Pecan has irregular crop yield patterns producing heavy crop loads one year ("on" year) and followed by one or more years of light crop loads or no production ("off" year). The phenomenon of alternate bearing may be caused by a general lack of carbohydrate at about the time of bud break. Carbohydrate reserves in pecan trees may be insufficient at bud break and bloom because its late harvest trait - most assimilate has been depleted for fruiting at previous growth season. Furthermore, in pecans, pistillate flower differentiation occurs in early spring rather than synchronically with fruiting in the past year for most nut crops. This fruiting habit exacerbates the poor carbohydrate pool reserved for spring return bloom, affecting tree energy and vigor for development of pistillate flowers. In addition, pecan female flowers are usually positioned terminally in new spring shoots where the flush and leaf expansion are in keen competition for assimilating. Subsequently, carbohydrate allocation dynamics may be essential in studying return bloom and AB characteristic of pecans. Our project will facilitate the development of plant physiological tools to mitigate AB phenomenon of trees and thereafter alleviate year by year fluctuations in pecan marketing. This proposed research will focus on three main objectives: 1) determine carbohydrate transport and relocation in spring for pistillate flower differentiation, bud break and flowering; 2) investigate carbohydrate demand for individual female flower or shoot and provide techniques such as girdling, pruning and growth regulators to manage bloom; 3) develop a model for tree carbohydrate analysis that includes bud vessel formation, flower qualities (e.g. stigma activity, pollen tube kinetics and ovule viability), fertilization, and pistillate abortion to guide future research. Complementally, endogenous hormone levels including Gibberellic acids (GA3 and GA4), auxins (IAA), cytokinins (kinetin or zeatin) and ethylene will be explored to document hormonal modulation of pistillate flower initiation and nut retention associated with carbohydrate status to better understand AB and ultimately produce a stable and higher yield of pecans.In 2018, the late April freezes across Oklahoma's pecan production areas damaged up to 70% of growers' crops. It is not fully clear, however, the threshold temperatures and tolerant period of pecan flowers (both male and female) to spring freeze. Uniquely, some pecans cultivars' secondary buds will develop into healthy flowers if the primary buds are impaired, which functions as a fail-safe to guarantee the yield. However, previous reports demonstrated some other cultivars failed to produce, or set abnormal flowers, from secondary buds. This trait was also correlated to rootstocks, but the mechanism behind this flowering characteristic is unknown. To resolve spring freeze tolerance, this project will focus on three objectives: 1) Evaluate the development and qualities of flowers exposed to different freeze temperature ranges and durations; 2) Investigate the correlations between the bud break and bloom of secondary buds, and the physiological determinants at carbohydrate and endogenous hormone levels; 3) Assess the influence of rootstocks on flower differentiation and development correlated to carbohydrate status and hormone balance. The intended outcome of this project is to provide growers effective knowledge and tools at aspects of environmental control, genotype selections (both cultivars and rootstocks) and physiological models, to ultimately reduce the flower and production shortfalls of pecans experiencing Oklahoma's spring freeze.

Animal Health Component

40%

Research Effort Categories

Basic

30%

Applied

40%

Developmental

30%

Classification

Knowledge Area (KA)	Subject of Investigation (SOI)	Field of Science (FOS)	Percent
102	1211	1020	100%

Knowledge Area
102 - Soil, Plant, Water, Nutrient Relationships;

Subject Of Investigation
1211 - Pecan;

Field Of Science
1020 - Physiology;

Keywords

pecan

flowering

alternate bearing

spring freeze

carbohydrates

endogenous hormones

Goals / Objectives
The major goal of this project is to reduce the loss of flowers and yield caused by both the inner factor - alternate bearing and outer factorspring freeze and mainly study the mechanism behind alternate bearing and secondary bud triggered by freezing temperatures at physiological levels (carbohydrates and hormones). The final purpose of this project is to create a stable pecan production and marketing environment in Oklahoma.

Project Methods
This project will specifically address the low crop yield issues interfered by alternate bearing or spring freeze through the study of pecan bud and flower development and qualities related to carbohydrates and hormones. To study the physiological mechanism behind the alternate bearing, carbohydrate transport for pistillate flower development will be investigated; and the correlations among carbohydrates, hormones and pistillate primordium differentiation and flower qualities will be evaluated. To handle spring freeze, three sub-projects will be developed: 1) Evaluate the development and qualities of flowers exposed to different freeze temperatures and durations. 2) Build the correlations between the bud break and bloom of secondary buds, and the carbohydrate and endogenous hormone levels. 3) Assess the influence of rootstocks on flower development correlated to carbohydrate status and hormone balance.Bud differentiation will be observed by using an electron scanning microscope; bud vessel structures will be evaluated through making paraffin sections and using a confocal microscope. Flower quality including pollen activity, stigma receptivity and ovule viability will be investigated.Pollen activity will be tested by AmpaZ30 (Amphasys AG), an impedance flow cytometer, using a lab-on-chip technology to measure pollen viability by imparting an electrical charge on single cells passing through the microfluidic channel on a semi-disposable chip. Stigma receptivity will be checked by using Perex-Test® kit. Tableting sections will be made to test ovule viability and slices will be observed using a confocal microscope. In this research, carbohydrates including sugar and starch in both xylem and phloem will be tested through using a microplate spectrophotometer. Endogenous hormone levels including gibberellic acids (GA3 and GA4), auxins (IAA), cytokinins (kinetin or zeatin) and abscisic acid (ABA) will be tested by using HPLC.All experiments will be appropriately designed and replicated, SAS program will be applied for statistical analysis. Several types of non-linear models, including Asymptotic Regression, Michaelis - Menten, 3-parameter Logistic and Gompertz models, will be coded in the R Statistics software and compared to forecast key phenological events.Stakeholder meetings for researchers, growers and other individuals interested in this research will be held for feedback of our progress and adjustments to meet project objectives.

Progress 10/01/19 to 09/30/20

Outputs
Target Audience:The target audience of this project is in-state and out-of-state pecan growers and researchers. The results will also be shared with university undergraduate and graduate students through the Course Hort3213 Fruit and Nut Production and multi-state ACCEPtS course Hort4133/5133 Temperature Stress Physiology. Changes/Problems: Nothing Reported What opportunities for training and professional development has the project provided?The project has trained one graduate student and 3 undergraduate students in studying flower damage caused by a spring freeze. The project also provided professional industry training with over 150 growers attending. How have the results been disseminated to communities of interest?The results of this project have been disseminated through the publication of 1 peer-reviewed paper, 2 scientific conference presentations, 1 industry conference with at least 150 growers attending, 2articles by industry magazines Oklahoma Pecan Growers' Quarterly and Pecan South. What do you plan to do during the next reporting period to accomplish the goals?I will continue the research focused on projects 1, 2, and 3 with some preliminary research on projects 4 and 5. Research implementation, data collection, and analysis, publication, and dissemination of the research results are the target of the following years.

Impacts
What was accomplished under these goals? For the research session 2019-2020, we mainly focused on research objective 1 and prepared preliminary research for objectives 2 and 3. Two pecan cultivars Pawnee and Kanza were investigated. The buds and flower samples along with shoots were collected at 2 different stages. The samples were subjected to a total of 12 treatments with 4 different temperatures (-6°C, -4°C, 0°C, and 4°C) for 4, 8, and 12 hours and one kept as a control. The results showed that -6°C and -4°C caused very poor development of flowers. Surprisingly, even the 4°C, though not freezing, had a significant effect on the flowering when given for 4 hours. It was observed in shoot samples at the early bud stage, male flowers were wilted, and no sprouting of the female flowers. Furthermore, 4°C treatment for 4 hours also caused the collapse of stigma cells at the later flowering stage, which plays an important role in pollen adhesion and germination. This research has figured out that 4°C for 4 hours is the critical temperature and duration causing flower damage in pecans. The results indicate that the pecan flower is more sensitive to low temperatures other than our traditional understanding. The preliminary study of project 2 studying the correlation between secondary buds, and carbohydrate and endogenous hormone levels showed that the secondary buds of the Kanza cultivar had better development potential than the Pawnee cultivar. The preliminary research of assessing project 3 the influence of rootstocks on the flower in spring freeze studies Kanza cultivar grafted on Gills, Colby, and Mount rootstocks. The results showed that after -10°C for 8 hours, there was no flower development on Colby and Mount rootstocks, but 10% on Gills rootstock. It reveals that the temperature needs to be adjusted to a higher setting for the second year study. But it also revealed the rootstock Gill has better potential for Kanza cultivar dealing with spring freeze.

Publications

• Type: Journal Articles Status: Published Year Published: 2020 Citation: Kaur A., L. Ferguson, N. Maness, B. Carroll, W. Reid, and L. Zhang. 2020. Spring Freeze Damage of Pecan Bloom: A Review. Horticulturae. 6(4), 82; https://doi.org/10.3390/horticulturae6040082
• Type: Journal Articles Status: Published Year Published: 2020 Citation: Zhang L., C. Rohla, N. Maness, B. Cheary, L. Costa. and Y. Ampatzidis Y. Know Your Nuts  From Flowering to Fruiting. January 2020. Pecan South.
• Type: Journal Articles Status: Published Year Published: 2019 Citation: Zhang L. and B. Cheary. Preliminary Study on the Influence of Spring Freeze on Pecan Bloom. 2019 by Oklahoma Pecan Growers Quarterly.
• Type: Conference Papers and Presentations Status: Published Year Published: 2020 Citation: Kaur A, L. Zhang L, N. Maness, and J. Moss. 2020 The Effect of Different Temperature Treatments on Pistillate and Staminate Pecan Flowers. 2020 American Society Horticultural Science Annual Conference. Virtual/Online.
• Type: Conference Papers and Presentations Status: Published Year Published: 2020 Citation: Zhang L., N. Maness, and J. Moss. Freezing Temperature Responses of Flower Stigmas on Pawnee, Kanza and Maramec Pecan Trees. 2020 Southern Region American Society Horticultural Science Annual Conference. Louisville, KY.