Drooping of Gerbera flower heads: mechanical and structural studies of a well-known phenomenon

Abstract

Gerbera , one of the most loved cut flowers, is (in)famous for the drooping of its flower heads under dehydration. This effect has been quantified by analysing both fully turgescent and wilting peduncles of Gerbera jamesonii ‘Nuance’. Wilting peduncles display pronounced bending in the region directly below the inflorescence after 24 h of dehydration, while the rest of the peduncle remains upright. Using anatomical measurements and three-point bending tests, we have analysed whether this phenomenon is caused by mechanical and/or geometrical alterations. We have found that both the flexural rigidity and the axial second moment of area are significantly decreased in the apical part of wilting peduncles, whereas the bending elastic modulus shows no significant change. Moreover, cross-sections of wilting peduncles ovalize significantly more than those of turgescent peduncles and exhibit considerable shrinkage of the parenchyma, taking up the majority of the cross-sectional area. Generally, the drooping of wilting Gerbera flowers can be regarded as a temporary instability of a rod-shaped cellular solid caused by anatomical differences (tissue arrangement, existence or the absence of a pith cavity) and geometrical changes (the decrease of axial second moment of area, cross-sectional ovalization, shrinkage of tissues) between the apical and basal regions of their peduncles.

Twist-to-Bend Ratios and Safety Factors of Petioles Having Various Geometries, Sizes and Shapes

“…Fully turgescent peduncles of the dicotyledonous Gerbera jamesonii ‘Nuance’ have a safety factor of 1.42. This is different in wilted peduncles, which have a safety factor of 0.95 and cause pronounced drooping of their flower heads ( Lehmann et al, 2019 ).…”

Longevity of System Functions in Biology and Biomimetics: A Matter of Robustness and Resilience

“…This well known phenomenon was analysed with respect to the safety factor of Gerbera jamesonii peduncles ( Figure 3 c). The safety factor was found to be 1.42 for fresh, fully turgescent plants, whereas in wilted plants with drooping flower heads, it decreased to 0.95 within 24 h [ 65 ].…”

“…The safety factor is 0.95 for wilted peduncles. On increasing the turgescence of the tissues, the safety factor increases to 1.42 [ 65 ]. ( d ) Since the technical plant stem is a biomimetic lightweight construction, its functionality is defined as mechanical performance in relation to sample weight.…”

Rigidity control mechanism by turgor pressure in plants

“…In herbaceous plants, when the inflow of water exceeds the outflow of water from inside a cell, turgor pressure is generated to equalize the pressure inside and outside the cell. However, when a loss of water occurs as a result of transpiration caused by, for example, stem and branch cutting, or drying, the bending rigidity and the tension force both decrease owing to cross-sectional shrinkage 33 and turgor pressure, respectively; a large deflection occurs instantly 34 . This is thought to be caused not only by a decrease in the bending rigidity of the plant itself because of its drying but also by the loss of the plant’s “geometric rigidity”, which is maintained by the tension forces associated with turgor pressure.…”