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.

Keywords: biomechanics; cross-sectional ovalization; drought stress; flexural rigidity; functional morphology; safety factor.

Figure 1.

Schematic drawing of the experimental set-up of the three-point bending tests carried out on G. jamesonii ‘Nuance’ peduncles. The ROI ranged from 1.0 cm b.i. to 12.0 cm b.i. and contained the samples for both the anatomical (black squares) and relative water content (RWC) (grey squares) measurements. During testing, the bending supports were kept at a fixed position in direct contact with the peduncle (not shown), whereas the force sensor was moved towards the inflorescence stalk (6.5 cm b.i.). The peduncle was loosely attached to the hanging fixation with a crossed string.

Figure 2.

Morphological and anatomical characterization of turgescent and wilting peduncles of G. jamesonii ‘Nuance’. Turgescent peduncles (a) are upright, whereas wilting peduncles (h) bend just below the inflorescence. (b–g) Cross-sectional images stained by toluidine blue (TB) at various positions along the turgescent peduncle and (i–n) along the wilting stalk. (a,h) The original locations of all cross-sections are highlighted in their respective overview images and arrows mark the apex of the pith cavity. ep, epidermis; pa, parenchyma; pc, pith cavity; vt, vascular tissues.