Are trichomes involved in the biomechanical systems of Cucurbita leaf petioles?

Abstract

Trichomes are involved in petiole movement and likely function as a part of the plant biomechanical system serving as an additional reservoir of hydrostatic pressure. The large, non-glandular trichomes on Cucurbita petioles occur across collenchyma strands. Time-lapse imaging was used to study the leaf reorientation of Cucurbita maxima 'Bambino' plants placed in horizontal position. The experiment comprised four variants of the large non-glandular petiole trichomes: (1) intact, (2) mechanically removed, (3) dehydrated, and (4) intact but with longitudinally injured petioles. Isolated strands of collenchyma with intact epidermis or epidermis mechanically removed from the abaxial and adaxial sides of the petiole were subjected to breaking test. The stiffness of the non-isolated tissue with intact epidermis was measured using the micro-indentation method. Petioles without trichomes did not exhibit tropic response, and the dehydration of trichomes slowed and prevented complete leaf reorientation. Isolated strands of collenchyma showed no correlation between strength values and position on the petiole. However, strands of collenchyma with epidermis exhibited a significantly greater strength regardless of their position on the petiole. The indentation test showed that non-isolated collenchyma is stiffer on the abaxial side of the petiole. Trichomes from the abaxial side of the petiole were larger at their base. The application of the 'tensile triangles method' revealed that these trichomes had a biomechanically optimized shape in comparison to the adaxial side. We conclude that trichomes can be involved in plant biomechanical system and serve as an additional reservoir of hydrostatic pressure that is necessary for maintaining petioles in the prestressed state.

Keywords: Collenchyma; Epidermis; Plant biomechanics; Shape optimization; Tropic response.

Fig. 1

Morphology of the trichomes from the adaxial (a) and abaxial (b) petiole sides in C. maxima ‘Bambino’. Surface of the petioles before (c) and after (d) removing the apical part of trichomes. e–f Two examples of deformed trichomes after treatment with 7 % NaCl solution. Scale bars 500 µm

Fig. 2

Photographs of a longitudinally cut petioles of C. maxima ‘Bambino’ (a–b). Scale bar 10 mm

Fig. 3

a Schematic cross-section view of the petiole subjected to breaking stress analysis. Variant 1—six strands of collenchyma with epidermis (black triangles). Variant 2—four strands from the adaxial (circles) and four strands from the abaxial (squares) sides of the petiole; strands of collenchyma with epidermis (black circles or squares) and without epidermis (white circles or squares). Fragment of the petiole cross-section before (b) and after (c) isolation of the collenchyma with epidermis strand (white arrow). The sections were stained with safranin–alcian blue. Scale bars 500 µm

Fig. 4

Scheme of indentation measurement. a Three locations on the petiole from which the samples were collected. b The samples from the adaxial and abaxial sides of the petiole. c Scheme of the indenter operation, where h _max is the maximum deformation, and h ₀ is the deformation remaining after removal of the indenter

Fig. 5

Reorientation of leaves in horizontally placed C. maxima ‘Bambino’ plants. a The initial phase (0 h) of the reorientation of the leaf with petioles with or without trichomes. Point trajectory for the analyzed leaf with petioles deprived of trichomes (white circle), and the petiole with trichomes (red circle). b The final phase (5 h) of the experiment, representing the lack of tropic response for the petiole deprived of trichomes and tropic reorientation of the leaf with an intact petiole. c Comparison of the movement of leaves with petioles without (black line) and with (red line) trichomes, based on the analysis of a time-lapse movie using Tracker software

Fig. 6

a Cross-section of the petiole with trichomes on the adaxial and abaxial sides. b Cross-section through the fragment on the petiole of the abaxial side. Multicellular non-glandular trichomes (T) on collenchyma (C) strand, above vascular bundle (VB), and ground parenchyma (P) are observed. In the epidermis (E), glandular trichomes (GT) and small non-glandular (NGT) trichomes can be seen. c The trichome from the abaxial side of the petiole with plotted lines of ‘tensile triangles’, with a blue line of optimal shape. Images by scanning electron microscopy (b–c). Scale bar 1.5 mm for (a) and 500 µm for (b–c)

Fig. 7

Comparison of the cross-section area of collenchyma strands, cell wall thickness and cell lumen surface area in collenchyma cells between the adaxial and abaxial sides of the petiole. Differences between means of 44 collenchyma strands from 11 petioles are not statistically significant

Fig. 8

Local buckling on the adaxial side of the petiole. View from above epidermis (a) and longitudinal (b) and transverse (c) petiole sections. Buckling in the form of deep longitudinal wave-like grooves occurs in the tissue region where not vascular bundles (VB) are present. White arrows maxima of the wave-like pattern in (a) and (b), and the groove bottom in (c). Scale bars 1 mm

Fig. 9

Results of the breaking tests of the isolated strands of collenchyma (10 cm long) with epidermis from adaxial (triangle) and abaxial (circle) sides of the petiole. a Plot presenting maximum deformation versus breaking stress. b Plot showing cross-section area of the strand versus maximum tension strength. The regression equation (Y) and coefficient of determination (R ²) are shown for adaxial (dotted lines) and abaxial (solid lines) sides of the petiole. Data for 7 leaves with 3 strands on the adaxial and 3 on abaxial side of each petiole

Fig. 10

Results of the breaking test of isolated strands of collenchyma (10 cm long) with and without epidermis from adaxial (circles) and abaxial (squares) sides of the petiole; strands with intact epidermis (black circles and squares) and with removed epidermis (open circles and squares) are indicated. a Data for separate petioles from 6 leaves. b Average values for the petioles with and without epidermis are represented by two columns with error bars denoting ±SE (n = 24). Two collenchyma strands with epidermis and two strands without epidermis from adaxial and abaxial side of each petiole were tested

Fig. 11

Results of the indentation test of the strands of collenchyma with epidermis not isolated from the surrounding tissues of ground parenchyma and vascular bundles. a Load-penetration (P–h) curves for one sample petiole (leaf no. 2). Measurements were conducted on samples (10 mm × 5 mm) from abaxial and adaxial sides obtained from three locations on the petiole: basal (1), middle (2), and apical (3). The experiment was repeated five times with similar results. b Average values of stiffness for the non-isolated strands on the adaxial and abaxial sides of the petioles in five individual leaves. The values of stiffness were calculated as the ratio of maximum force to maximum penetration for the three locations on the petioles. Mean values for the adaxial and abaxial sides for all of the five petioles are represented by the last two columns with error bars denoting ±SE (n = 5)