Winter Nights during Summer Time: Stress Physiological Response to Ice and the Facilitation of Freezing Cytorrhysis by Elastic Cell Wall Components in the Leaves of a Nival Species

Abstract

Ranunculus glacialis grows and reproduces successfully, although the snow-free time period is short (2–3 months) and night frosts are frequent. At a nival site (3185 m a.s.l.), we disentangled the interplay between the atmospheric temperature, leaf temperatures, and leaf freezing frequency to assess the actual strain. For a comprehensive understanding, the freezing behavior from the whole plant to the leaf and cellular level and its physiological after-effects as well as cell wall chemistry were studied. The atmospheric temperatures did not mirror the leaf temperatures, which could be 9.3 °C lower. Leaf freezing occurred even when the air temperature was above 0 °C. Ice nucleation at on average −2.6 °C started usually independently in each leaf, as the shoot is deep-seated in unfrozen soil. All the mesophyll cells were subjected to freezing cytorrhysis. Huge ice masses formed in the intercellular spaces of the spongy parenchyma. After thawing, photosynthesis was unaffected regardless of whether ice had formed. The cell walls were pectin-rich and triglycerides occurred, particularly in the spongy parenchyma. At high elevations, atmospheric temperatures fail to predict plant freezing. Shoot burial prevents ice spreading, specific tissue architecture enables ice management, and the flexibility of cell walls allows recurrent freezing cytorrhysis. The peculiar patterning of triglycerides close to ice rewards further investigation.

Fusion of Mitochondria to 3-D Networks, Autophagy and Increased Organelle Contacts are Important Subcellular Hallmarks during Cold Stress in Plants

“…This shows a good recovery capacity of R. glacialis after freezing events. Rapid recovery seems important, as night frosts are frequent throughout the whole vegetation period in its nival habitat [ 57 , 58 , 59 ] and ice nucleation with consequent freezing cytorrhysis of mesophyll cells is already observed at −2.6 °C [ 50 ]. Mitochondrial fusion and aggregation in R. glacialis appeared during and directly after extracellular freezing stress at −5 °C.…”

“…+1 °C (day/night) in order to exclude artificial ultrastructural damage from excavation and transport. Based on previous studies [ 44 , 45 , 46 , 59 ], +10 °C was chosen as the control temperature for R. glacialis . During daylight but without direct sun exposure, plant canopy temperatures of nival plants of +10 °C are highly frequent [ 57 ].…”

Flower preformation in the nival plant Ranunculus glacialis L.: shoot architecture and impact of the growing season length on floral morphogenesis and developmental dynamics

“…The staggered preformation of architectural units creates a permanent belowground reserve pool of reproductive units which ensure the survival of an individual in case aboveground tissues get lost due to herbivory or abiotic disorders. In addition, R. glacialis is ice tolerant during all reproductive stages which helps to survive sudden cold spells in summer and to tolerate night frosts whose frequency increases with elevation (Ladinig et al 2013;Stegner et al 2020). All these attributes are highly advantageous in the harsh and unpredictable high-mountain climate and contribute to the colonization success of R. glacialis in nival environments.…”

Protective Role of Ice Barriers: How Reproductive Organs of Early Flowering and Mountain Plants Escape Frost Injuries

“…Frost survival of reproductive organs in all other alpine species investigated until now relied on avoidance of ice formation, i.e., a strategy termed supercooling. Vegetative plant parts usually show little supercooling capacity in nature and ice forms at mild subzero temperatures [8]. Ice spreading occurs fast, at rates of up to 27 cm•s −1 into all plant parts that are colder than 0 • C [9,10].…”

“…; [15]) and every reproductive shoot on the cushion needed an autonomous ice nucleation event to initiate freezing, which facilitates frost survival by supercooling. Similarly, burial of the interconnecting shoot several centimeters below the soil surface creates a thermal barrier against ice spread between single leaves and flowers [8]. Generally, little is known how herbaceous early-flowering and mountain species manage ice spread around and into reproductive organs.…”