Evaluation of Natural Spring Freeze Tolerance of Five Pecan Scion–Rootstock Combinations

Abstract

Pecan [Carya illinoinensis (Wangenh.) K. Koch] is a member of the Juglandaceae family. During spring, pecan trees break their bud dormancy and produce new leaves and flowers. Carbohydrates stored in roots and shoots are thought to support the bloom and early vegetative growth during this time until new leaves start the full photosynthetic activity. Spring freeze is known for its damaging effects on pecan bud and flower growth and development. Pecan shoots with leaves and flowers from five scion–rootstock combinations were collected hours before and after a recent spring freeze (below 0 °C for 6 hours, 21 Apr 2021, Perkins, OK, USA). Morphologies of the leaf, bud, and catkin were visually observed, and the morphologies of the anther and pollen in paraffin sections were investigated by light microscopy. Soluble sugar and starch from bark and wood were analyzed using the anthrone reagent method. The Kanza–Mount showed the maximum damage to terminal leaves, buds, and catkins, whereas Maramec–Colby had the minimum damage only to leaves. Pollen grains were shrunk and reduced in number in the anthers in the protandrous Pawnee scions, whereas no pollen damage was observed in the protogynous Kanza scion. Furthermore, bark soluble sugar levels increased in all the scion–rootstock combinations after the freeze, which may indicate a physiological response to the cold stress. Overall, the extent of spring freeze damage of pecans is affected by the growth stage, types of scion and rootstock, and the scion–rootstock interactions. Furthermore, in addition to low temperature, scion–rootstock interactions also affected the starch and soluble sugar contents in wood and bark tissues.

Dormant carbohydrate reserves enhance pecan tree spring freeze tolerance: controlled environment observations

“…Similar results were reported in peach after controlled freezing treatments; sugar content was lower after freezing temperature treatments ( Yun et al., 2014 ). However, in our samples from an earlier natural freeze experiment, soluble sugar content was increased after low-temperature conditions ( Kaur et al., 2023 ). This could be explained by the fact that excised branches in growth chambers are no longer attached to trees, limiting sugar translocation from lower plant parts to the more resource demanding shoot apex.…”

“…After drying, the samples were kept at room temperature until further processing. The fine powder (25–27 mg) bark and wood tissues were further processed for soluble sugars and starch analysis using colorimetric Anthrone regent method ( Kaur et al., 2023 ).…”

“…High levels of carbohydrates before bud burst in spring are indeed associated with a higher yield in almond (Prunus dulcis), pistachio (Pistacia vera), and walnut ( Zwieniecki et al., 2022 ) and a heightened occurrence of secondary bud burst in cases where primary buds were damaged by spring freeze in pecan ( Zhang et al., 2023 ). Our previous study, focused on an April 21, 2021, 6-hour freeze with temperatures from -1.6 to -0.5°C, indicated damage to terminal leaves, buds, and catkins varied among cultivars and demonstrated an increase in bark soluble sugar levels after the freeze ( Kaur et al., 2023 ). Understanding the role of carbohydrates in the low temperature responses of pecans at flowering could be a significant step towards a better freeze mitigation,…”