O Christmas Tree: Managing Postharvest Needle Abscission through Stomatal Conductance
Postharvest needle abscission poses the largest threat in the balsam fir Christmas tree industry. Abscission is the removal of a plant organ either by stress or the environment. There are numerous factors that contribute to postharvest needle abscission in balsam fir. One of the most important factors is the water properties of the plant. Water uptake and needle abscission show a relationship where higher water uptake levels correspond to increased needle retention. After harvest the amount of water taken up by the plant is solely dictated through transpiration. Transpiration is the process of moisture being taken up by the plant controlled by the roots however, after the root system is detached stomatal conductance controls water uptake and moisture content.
Stomatal conductance is the rate of transpiration through the plant determined by stomata closure and aperture. Stomata are small pores on the underside of the needle which open and close to allow for higher water uptake. Therefore, through the manipulation of stomata needle abscission should be able to be regulated. The study described below was conducted to determine if this is plausible.
The objectives of the study were to (1) determine if stomatal conductance can be manipulated through the use of chemical treatments, (2) determine if increased stomatal conductance can increase water uptake and moisture content and (3) to determine if stomatal aperture encourages needle retention and if stomatal closure encourages needle abscission.
In order to determine if stomatal conductance has an effect on needle abscission 70 balsam fir branches were collected and placed in six different chemical treatments and water serving as a control. Three treatments were hypothesized to increase stomatal conductance: Fluoridone, 6-Benzylaminopurine (BAP) and Potassium Nitrate (KNO3). The other three treatments were hypothesized to decrease stomatal conductance: Abscisic acid (ABA), Theophylline and Dopamine. Trees were left in treatments until all needles fell off and measurements were taken three times per week. Needle loss was measured using a ‘finger run test’ where the thumb and forefinger were brushed over the branches to dislodge any loose needles. Stomatal conductance, water uptake and moisture content were also measured.
Abscisic acid was shown to successfully decrease stomatal conductance compared to the control. All other treatments showed similar results to the control and therefore did not manipulate the stomata. ABA showed decreases in water uptake compared to all other treatments however, moisture content of branches in ABA showed increases. There were no differences in needle abscission measured as percent needle loss between treatments.
Stomatal conductance can be successfully manipulated through the use of chemical treatments as shown by ABA. Stomatal closure is directly related to decreases in water uptake however, not moisture content. This may be due to the tendency of abscisic acid to conserve water within the plant after it has been taken up. The results showed that the manipulation of stomatal conductance has no direct effect on needle abscission. The lack of effect on needle abscission may be due to inadequate concentrations of treatments and further analysis can be done using different concentrations.