Mason T. MacDonald

Understanding the Physiology of Postharvest Needle Abscission in Balsam Fir.

Balsam fir (Abies balsamea) trees are commonly used as a specialty horticultural species for Christmas trees and associated greenery in eastern Canada and United States. Postharvest needle abscission has always been a problem, but is becoming an even bigger challenge in recent years presumably due to increased autumn temperatures and earlier harvesting practices. An increased understanding of postharvest abscission physiology in balsam fir may benefit the Christmas tree industry while simultaneously advancing our knowledge in senescence and abscission of conifers in general. Our paper describes the dynamics of needle abscission in balsam fir while identifying key factors that modify abscission patterns. Concepts such as genotypic abscission resistance, nutrition, environmental factors, and postharvest changes in water conductance and hormone evolution are discussed as they relate to our understanding of the balsam fir abscission physiology. Our paper ultimately proposes a pathway for needle abscission via ethylene and also suggests other potential alternative pathways based on our current understanding.

Seasonal changes in balsam fir needle abscission patterns and links to environmental factors

ABSTRACT Several studies have suggested that postharvest needle retention increases in autumn, likely due to cold acclimation. But some of the dynamics of the phenomenon have yet to be studied. The objectives of this study are to (1) describe seasonal changes in postharvest abscission, water uptake, fluorescence, and moisture content of balsam fir, (2) determine the relationship of needle abscission with water uptake, fluorescence, and moisture content, and (3) link the postharvest changes to certain environmental factors. Branches were collected from 18 trees each month and needle abscission, fluorescence, water uptake, and water content were monitored for 12 weeks. Fluorescence, water uptake, and water content were all correlated with needle abscission. Further, all the above factors improved in autumn months compared to spring or summer months and significantly related to changes in photoperiod and temperature. In addition, fluorescence and moisture content are valuable indicators of postharvest quality.

Environmental and Hormonal Physiology of Postharvest Needle Abscission in Christmas Trees

ABSTRACT Several of the conifer species are increasingly adopted as Christmas trees worldwide. These species have become integral parts of the horticultural economies of North American and European countries. Postharvest characteristics such as needle abscission/retention, color, fragrance and rehydration abilities vary with species and these complex physiological traits are strongly modulated by hormonal and environmental factors. A large body of research indicates that prevalence of low temperature before harvest evokes cold acclimation responses that involve an increase in complex sugar concentrations, alterations in membrane structures and enhancements in scavenging abilities promoting postharvest needle retention. Adverse postharvest environmental factors, for example, high temperature and vapor pressure deficit are found to increase water stress, cause dehydration and accelerate needle abscission and/or discoloration. Postharvest water stress/cellular dehydration is one of the fundamental biophysical signals that triggers a cascade of hormonal changes, leading to needle abscission. Abscissic acid levels increase during cold acclimation as well as prior to abscission indicating a complicated and paradoxical role in abscission. Ethylene levels increase before abscission and are well proven to instigate the needle fall. Concentrations of cytokinins, auxins and polyamines decline postharvest. However, their interactive roles with other phytohormones orchestrating the abscission process still remain elusive. This review presents and discusses our current knowledge of the physiological aspects of pre-and postharvest environmental factors on needle abscission.

Carrot Yield and Quality as Influenced by Nitrogen Application in Cut-and-Peel Carrots

Root bulking, quality, and uniformity in cut-and-peel carrots (Daucus carota) are paramount for optimizing marketable yield and quality. Root bulking is an ecophysiological manifestation in response to inputs such as fertilizers. Understanding this ecophysioloical interaction will help to optimize yield, quality, and amount of inputs used. Three years of field trials were conducted in Kings County, Nova Scotia, to investigate the effects of varying levels of nitrogen (N) fertilizer on yield, recovery, and root and tissue N of two cut-and-peel varieties, Sugarsnax and TopCut. Seven levels of ammonium nitrate (34–0–0; 0, 50, 100, 150, 200, 300, and 400 kg N h−1) were hand broadcast in a split (60% pre-emergence and 40% 8 weeks after emergence) application. No significant interactive effects of N and variety in terms of gross yield or recovery were observed, though Sugarsnax total yields were 12.7% greater than those of TopCut. Overall, optimum yields were achieved at N rates of 150 kg N h−1 and further addition did not significantly improve yield or quality. Increased N significantly increased root and tissue N, but N concentration in both tissues peaked at the 300 kg N h−1 rate. However, neither root nor leaf tissue N had any effect on marketable or total yield. These results show that root bulking is not modulated by altering N applications, and the results also suggest that carrots may have high N-use efficiency or harness N from deeper zones.

Seasonal changes in soil and tissue nutrition in balsam fir and influence on postharvest needle abscission

ABSTRACT Postharvest needle abscission in balsam fir varies based on time of harvest, but little is known about the effect of soil and tree nutrition on postharvest needle abscission. From April to November, 18 balsam fir samples were obtained each month and evaluated for postharvest needle abscission characteristics. Soil samples were collected simultaneously and evaluated for moisture, pH, organic matter, cation exchange capacity, and mineral nutrition. Needle tissue samples were also evaluated for mineral nutrition. Many parameters were found to vary based on month throughout the study, but not all were associated with needle abscission. Soil organic matter, cation exchange capacity, Ca, and Mn all had a significant (P < .01) positive relationship with needle retention; soil Al had a significant (P < .01) negative relationship with needle retention. Needle N, Ca, and Fe were also associated with needle abscission. The strongest positive relationship was found between needle Ca and needle retention (R2 > .50), suggesting that higher concentrations of tissue Ca were linked to superior needle retention in balsam fir. Overall, this study provides evidence that changes to soil management, such as the inclusion of organic matter and certain minerals, may improve needle retention in balsam fir.

Changes in Endogenous Hormone Levels Explains Seasonal Variation in Balsam Fir Needle Abscission Patterns

Postharvest needle abscission in balsam fir varies considerably depending on the season and may be dependent on endogenous hormones of roots or shoots. Samples were obtained from three trees each month from April 2013 to November 2013 and evaluated for retention. In addition, root and needle tissues were analyzed for abscisic acid, cytokinins, auxins, and gibberellins and their conjugates. There was a 3.5-fold increase in time until needle abscission commencement and 1.75-fold increase in time until complete abscission in autumn compared to spring. There were several changes to root and needle hormones with respect to harvest time, but fewer hormones were linked to postharvest needle retention. Among all hormones analyzed, both isopentenyladenine and isopentenyladenosine in roots were strongly linked to needle retention. When isopentenyladenine was present in roots, needle abscission took 114% longer to begin and 58% longer to finish. Isopentenyladenosine in roots was the strongest hormonal predictor of needle retention (R2 = 79%). Dihydrozeatin riboside (R2 = 37%), trans-abscisic acid (R2 = 50%), and isopentenyladenosine (R2 = 59%) in the needles were also strong predictors of needle retention. The relative effectiveness of certain hormones in predicting needle retention and speculation pertaining to the roles these hormones could play are discussed.