The relationship of cold acclimation and extracellular ice formation to winter thermonasty in twoRhododendronspecies and their F₁hybrid

Abstract

Premise: Thermonastic leaf movements in evergreen Rhododendron species have been used to study plant strategies for winter photoprotection. To add to the current fundamental understanding of this behavior, we addressed the following questions:(1) Is the cold-acclimated (CA) state necessary for thermonasty, and do cold-induced leaf movements also occur in non-acclimated (NA) plants? (2) Which of the two movements, leaf rolling versus curling, is more responsive to freezing, if any, in a nonthermonastic species? (3) What is the temporal relationship between extracellular freezing and thermonasty? (4) What genetic inferences can be drawn from leaf movement in an F 1 hybrid relative to its parents? Methods: A temperature-controlled, gradual cooling regime was used to quantify freeze-induced leaf movements. Infrared thermography was used to confirm extracellular ice-formation in leaves. Results: Both NA and CA plants of thermonastic species exhibited thermonasty, but leaf rolling/curling increased significantly in CA plants. In the cold-acclimated condition, a non-thermonastic species showed almost no rolling during freezing, while the thermonastic species and F 1 hybrid did, the latter exhibiting a response intermediate to the parents. Freezing-induced leaf curling in the non-thermonastic species and the F 1 hybrid was equivalent and significantly less than the degree of curling in the thermonastic species. Conclusions: Milder thermonasty in NA than CA leaves could be associated with differential anisotropy in the rolling forces and/or response of aquaporins to freezing. Leaf movements in the hybrid suggest that leaf rolling and curling are additive and dominant genetic traits, respectively. Infrared thermography confirms that ice formation in tissues precedes cold-induced thermonasty in R. catawbiense.

Infrared thermography of in situ natural freezing and mechanism of winter‐thermonasty in Rhododendron maximum

“…It has been suggested that thermonasty could serve as a photoprotective strategy in overwintering rhododendrons, which harvest more radiation than their capacity to utilize it due to sluggish photosynthetic biochemistry under freezing temperatures. Surplus excitation energy can potentially cause photo‐damage, whereas thermonasty would reduce the absorbance of excess light (Arora et al, 2021; Bao & Nilsen, 1988; Russell et al, 2009; Wang et al, 2009; Wei et al, 2005). Additionally, desiccation‐avoidance, reduced heat load, protection from mechanical damage due to the weight of ice/snow, and avoidance of potential rapid‐thaw induced damage by slowing the thaw rate have also been proposed to be of adaptive significances of thermonasty during freezing cold (Nilsen, 1992).…”

“…During freezing that is above the killing temperature, cells' membrane semi‐permeability is intact, and they can regulate the movement of water in and out, presumably by the action of aquaporins, so leaf rolling and unrolling is likely a result of intact cells contracting and expanding, respectively (Arora et al, 2021). However, when the leaf is dead, it still rolls and unrolls, even though cells are incapable of active water transport.…”

“…However, at one or two degrees colder than the freeze‐initiation temperature, leaves began to roll inward, with the mid‐rib serving as the central axis (Figure 4G,H and Video S4). Shortly after the start of leaf‐rolling, the petiole of each leaf began to curl downward markedly beyond the initial, modest curling that typically occurs during cold acclimation (Arora et al, 2021). The entire leaf then became oriented in a position that was more vertical than it was prior to freezing (Figure 4H).…”

“…cv. Olga) (Arora et al, 2021; Wisniewski et al, 1997), whereas one study involved in situ freezing of a relatively small‐leaved, dwarf rhododendron shrub ( Rhododendron ferrugineum ) from Alpine timberline (Hacker & Neuner, 2008). No studies have been conducted on freezing response (preferred sites of ice‐nucleation and propagation) in field‐established, large‐leaved rhododendrons under natural freezing events.…”

“…Cold‐induced leaf movement, termed “thermonasty,” can reduce the amount of leaf area exposed to solar radiation and has been associated with photoprotection in some overwintering rhododendrons (Bao & Nilsen, 1988; Russell et al, 2009); thermonasty in rhododendrons includes both petiole curling (change in leaf angle) and inward rolling of the leaf lamina, thus enclosing the abaxial surface (Arora et al, 2021; Nilsen, 1992). Nilsen et al have extensively studied the eco‐physiological aspects of thermonasty in rhododendrons (Nilsen et al, 2014 and references therein), but the presumptive relationship of leaf curling and rolling in nature with confirmation of in situ ice‐formation in tissues has not been documented.…”