Influence of Humidity and Temperature on Postharvest Needle Abscission in Balsam Fir in the Presence and Absence of Exogenous Ethylene

Ethylene accumulation increases after harvest and culminates in needle abscission in balsam fir [Abies balsamea (L.) Mill.]. We hypothesize that water deficit induces ethylene evolution, thus triggering abscission. The purpose of this research was to investigate the role of temperature and humidity on postharvest needle abscission in the presence and absence of exogenous ethylene and link vapor pressure deficit (VPD) to postharvest needle abscission in balsam fir. In the first experiment, branches were exposed to 30%, 60%, or 90% humidity while maintained at 19.7 °C (VPD of 1.59, 0.91, or 0.23 kPa, respectively); in the second experiment, branches were exposed to 5, 15, or 25 °C (VPD of 0.35, 0.68, or 1.26 kPa, respectively) while maintained at 60% relative humidity. Needle retention duration, average water use, xylem pressure potential relative water content, and ethylene evolution were response variables. Reducing water loss or xylem tension by changing temperature or humidity effectively delayed needle abscission, although the 90% humidity treatment had the most profound effects. In the absence of exogenous ethylene, branches placed in 90% humidity had a fivefold increase in needle retention, 67% decrease in average water use, and had a final xylem pressure potential of –0.09 MPa. There was a near perfect relationship between VPD and needle retention (R2 = 0.99). These findings suggest that increasing xylem tension or decreasing water status may trigger ethylene synthesis and needle abscission. In addition, these findings demonstrate an effective means of controlling postharvest needle abscission by modifying temperature and/or relative humidity.

Ethylene Exposure Duration Affects Postharvest Needle Abscission in Balsam Fir (Abies balsamea L.)

Needle loss after harvest is a major problem for Atlantic Canada's Christmas tree and greenery industry. Ethylene is a signal for abscission in balsam fir, but preliminary studies have suggested that the role of ethylene may be influenced by length of exposure. Short-term and long-term ethylene exposure experiments were conducted. Branches were exposed to ethylene for 24 h (short-term) or continuously (long-term) at concentrations of 0 to 1000 ppm. The response variables measured were needle retention duration (NRD), average water use (AWU), and xylem pressure potential (XPP). Short-term exposure to any concentration of ethylene delayed needle abscission by 30 to 40 days. In contrast, long-term exposure to all concentrations of ethylene accelerated abscission, most evident by a 21-day decrease in NRD at 1000 ppm ethylene. There was a 60% decrease in NRD, 160% decrease (more negative) in XPP, and 80% increase in AWU as a result of long-term exposure to ethylene. Overall, our results demonstrate an opposite effect of short-term and long-term ethylene exposure, which suggests that short-term exposure to ethylene might help to precondition balsam fir and delay needle abscission during postharvest handling.

The physiological responses of oak seedlings to warm storage

The effects of warm storage (15 °C) on the physiological responses and subsequent field performance of 2-year-old oak (Quercus robur L.) seedlings that had been cold (1–2 °C) stored until May were assessed. Shoot xylem pressure potential, shoot water content, shoot tip dry mass, root electrolyte leakage, and root growth potential were evaluated at regular intervals during the 21-day warm storage period. The date of lifting to cold storage had little effect on the physiological responses to warm storage. Root electrolyte leakage increased after 7 days of storage, despite the fact that root growth potential and survival were unaffected. Values then increased further before decreasing again by the end of the 21-day storage period to values similar to those recorded after 0 or 7 days of storage. Storage for up to 21 days greatly reduced root growth potential and shoot quality after planting in the field. The shoot xylem pressure potential of seedlings declined during the early stage of warm storage, but then increased to values similar to those recorded after cold storage. Shoot water content increased during the early storage period, then declined slightly, but values were higher after 21 days of storage than at the time of placement in warm storage. While most physiological responses fluctuated in a consistent manner, these changes were not necessarily predictable. Root growth potential was a good indicator of plant field performance potential, but the other physiological parameters were less useful to this end.

Postharvest Characteristics of Canaan Fir and Fraser Fir Christmas Trees

Needle retention, xylem pressure potential and overall quality of canaan fir (Abies balsamea var. phanerolepis (L.) Mill.) and fraser fir (Abies fraseri (Pursh) Poir.) Christmas trees were evaluated over a 40-day display period. Cut trees were stored outdoors for 24, 48, or 96 hours, and half the trees had a 2.5-cm section trimmed from the basal portion of the stem, before placement in water at an indoor display room. Controls were placed in water immediately after harvest. Xylem pressure potentials and overall quality were similar for both species except for trees stored 96 hours. Untrimmed canaan fir dried to -2.4 MPa and was rated below average by the end of the display period compared to -1.3 MPa and a good quality rating for fraser fir. Needle retention and color characteristics were excellent across all treatments for fraser fir during the entire display period. Needle loss for canaan fir began relatively soon during display, generally increased across all treatments, and was highly variable. In addition, quality of some canaan fir trees decreased as needles turned brown, but did not shed during the display period. Tree water status alone did not completely account for loss of needles and quality in canaan fir; the need exists to identify seed sources with better postharvest characteristics.

Physiological Mechanism and Genotypic Variation in Drought Tolerance of Processing Carrots

Processing carrots are mainly grown under rain-fed conditions in Nova Scotia, and thus become vulnerable to frequent periods of drought. Prolonged drought results in significant reductions in the yield and quality of carrot crops. Resistance to water deficit is gene controlled and it is essential to identify the genotypes that withstand water stress. It is equally important to understand the physiological mechanism(s) that contribute to drought tolerance. Physiological measurements were made on eight carrot varieties exposed to natural drought in a controlled greenhouse. Measurements were made on net photosynthesis, soil moisture, relative water content, membrane injury index, xylem pressure potential, and stem elongation. Overall, the slicer variety Bergen sustained normal plant functions under drought stress better than any of the other varieties. Bergen maintained stem elongation, photosynthetic activity, membrane function, and relative water content under droughted conditions. Another study was conducted to identify carrot varieties that are naturally resistant to drought. A mass screening of 85 slicer, dicer, and cut and peel varieties was conducted under greenhouse conditions. Two-week-old seedlings were exposed to gradual water deficit and observed for visual symptoms of wilting each day as soil moisture declined. Each variety was assigned a wilting score based upon the number of days it withstood drought (min = 1, max = 5). The dicer variety, Prodigy, was the only variety to score a 5 indicating the most drought resistance. Other varieties that withstood drought well were `Caropak', `Interceptor', `Oranza', and `Berlanda'. Varieties such as KC713126, Cello712113, and Cello711411 were more sensitive to water deficit and began to wilt 8 days after drought was imposed.

Interactive effects of fertilization and throughfall exclusion on the physiological responses and whole-tree carbon uptake of mature loblolly pine

Few studies have examined the combined effects of nutrition and water exclusion on the canopy physiology of mature loblolly pine (Pinus taeda L.). Understanding the impacts of forest management on plantation productivity requires extensive research on the relationship between silvicultural treatments and environmental constraints to growth. We studied the physiological responses of 18-year-old loblolly pine trees exposed to a combination of fertilization (fertilizer or no fertilizer) and throughfall (normal throughfall or throughfall exclusion). Gas exchange variables were measured in the upper and lower crown between 0900 and 1700 h from May to November in 1999. Needle fall was collected to estimate foliage mass and leaf area. Summer drought and throughfall exclusion significantly decreased predawn xylem pressure potential. Needle-level photosynthesis, transpiration, and stomatal conductance declined during the drought and were significantly lower in the throughfall exclusion treatment. Throughfall exclusion also reduced annual foliage mass and daily whole-crown photosynthesis and transpiration. In the normal throughfall treatment, fertilization had no effect on needle-level physiology, but increased annual foliage mass and whole-crown photosynthesis by 26% and 41%, respectively. With the exclusion of throughfall, however, annual foliage mass and daily whole-crown photosynthesis exhibited little response to fertilization. We conclude that greater nutrient availability enhances the carbon uptake of mature loblolly pine trees by stimulating foliage production, but the positive effects of fertilization on leaf area and carbon fixation are limited by low water availability.Key words: foliage mass, photosynthesis, Pinus taeda, seasonal trend, transpiration, xylem pressure potential.

Seed pre-treatment using a derivative of 5-hydroxybenzimidazole (AMBIOL) pre-acclimates carrot seedlings to drought

The possibility that AMBIOL, a derivative of 5-hydroxybenzimidazole, may promote drought acclimation of carrot seedlings was studied. Carrot seeds were pre-treated by soaking in AMBIOL solutions of 0, 0.1, 1 or 10 mg L–1 for 24 h and germinated. Fifteen-day-old seedlings were exposed to a 7-d drought by withholding water until soil moisture content declined to one-third of the initial values. Drought caused a significant reduction in xylem pressure potential, elongation growth, leaf area expansion, root growth and dry matter production. Membrane capacitance, net photosynthesis, stomatal conductance, transpiration and water use efficiency all declined in all plants under drought. However, seed pre-treatment using AMBIOL 0.1 and 10 mg L–1 completely alleviated the drought-induced reduction in shoot dry matter production. Seed preconditioning using AMBIOL 10 mg L–1 promoted dry matter production, which was 214% higher than in the untreated droughted plants and was even 26% higher than that of the untreated, unstressed controls. AMBIOL appeared to have induced drought acclimation through root adjustments that enhanced root growth, possibly supplying root-derived essential factors. Key words: Acclimation, antioxidant, carrot, drought, dry matter production, 5-hydroxybenzimidazole, growth, membrane capacitance

Water use of interior Douglas-fir

Water use of individual Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco) trees was measured in two plots at a forest site in southern British Columbia, Canada. Average daily early summer water use by trees with diameters of 7.5-70 cm varied from 1.8 to 166 L. Sap flux density (cm3 water/cm2 sapwood per hour) was linearly related to shoot xylem pressure potential and was found to increase with increasing vapour pressure deficit (VPD) and short-wave irradiance (I), reaching maximum rates with VPD > 0.6 kPa and I > 200 W·m-2. Daily sap flux density varied among trees but was not related to tree diameter, so an average value of 1137.4 L·m-2 sapwood area was used to estimate average early summer stand transpiration for the two plots of 1.08 and 1.5 mm·d-1. A close curvilinear relationship (r2 = 0.85) was found between stem cross-sectional area increment and sapwood area. The relationship was only slightly better (r2 = 0.89) between area increment and early summer individual tree water use. Stand volume growth for 1988-1998 for the two plots was 36-47 m3·ha-1. Stem volume relative growth rate over this 10-year period is estimated at 0.027 and 0.029 m3·m-3·a-1.