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IAWA Journal, Vol. 31 (2), 2010: 203–216 OCCURRENCE OF REACTION XYLEM IN THE PEDUNCLE OF COUROUPITA GUIANENSIS AND KIGELIA PINNATA Pramod Sivan, Preeti Mishra and K. S. Rao* BR Doshi School of Biosciences, Sardar Patel University, Vallabh Vidyanagar 388 120, Gujarat, India *Corresponding author [E-mail: [email protected]] SUMMARY The anatomy of the secondary xylem and distribution pattern of gelati- nous fibres (G-fibres) have been studied in the developing and heavy fruit bearing mature peduncles of Kigelia pinnata and Couroupita guianensis. The peduncle in both the plants developed reaction xylem as a result of growth stresses caused by development of large fruits. In Couroupita peduncles which are originally horizontal, G-fibre distribution was uni- lateral and similar to that of typical tension wood whereas the hanging Kigelia peduncles have uniformly distributed gelatinous fibres throughout the xylem. The tension xylem severity was higher in the basal region and decreased towards the terminal region of the current year’s peduncle but after fruit development a drastic increase in tension wood severity was observed in the terminal region. In the Kigelia peduncles, tension wood severity in terms of G-layer proportion to lignified wall was found to be less than in Couroupita. The abundance of vessels decreased with high frequency of gelatinous fibres inCouroupita . The peduncle of Kigelia is characterized by high vessel frequency, thin normal fibre walls, and thick outer walls with thin gelatinous layer in tension wood fibres. Dimensional variations were also noticed in the mechanical and conducting elements varying with tension wood severity. Key words: Reaction xylem, peduncle, Kigelia pinnata, Couroupita guia- nensis, G-fibres. INTRODUCTION Tree stems maintain their orientation (vertical for trunks, oblique for branches) by gen- erating asymmetrical (from one side of the stem to the other) stresses in wood, during cell wall maturation, i.e. formation of secondary cell wall and lignification (Archer 1986; Fournier et al. 1994). Angiosperms generate stronger tension stresses on the upper side of stem (Wardrop 1964; Fisher & Stevenson 1981) leading to the formation of tension wood on the upper part of leaning or bending branches. The most distinguishing feature of tension wood is the occurrence of fibres with a particular morphology and chemi- cal composition due to the development of so-called gelatinous layer or the G-layer (Evert 2006). This layer is essentially made up of strongly crystalline cellulose (Côté et al. 1969), with a very low microfibril angle (Chaffey 2000). G-fibres are commonly found on the upper side of the leaning stems and branches. In species where tension wood exhibits a typical G-layer, its occurrence is always correlated with high tensile Downloaded from Brill.com09/30/2021 01:04:16AM via free access 204 IAWA Journal, Vol. 31 (2), 2010 growth stress. (Clair et al. 2003; Washusen et al. 2003). On the other hand, tension wood may also develop in stems free from any bending stress (Berlyn 1961). The occur- rence of tension wood has been studied widely in stems, branches and leaves of many dicots (Patel 1964; Fisher & Stevenson 1981) and in non vascular tissues of monocot leaves (Staff 1974). However, the information available on tension wood formation in the peduncles (flower or fruit stalks) of tropical trees is meagre. The present work, therefore, was undertaken to study the distribution of tension wood in peduncles of two fruit bearing tropical tree species, Couroupita guianensis and Kigelia pinnata. In the former peduncles grow horizontally from the surface of the main stem, while in the latter peduncles hang vertically downward from the tips of branches, exhibiting positive gravitropism. MateriAlS AnD MeTHoDS Plant materials Peduncles showing different developmental stages were collected from the trees of Kigelia and Couroupita growing at the University Botanical garden, Sardar Patel University, Gujarat, India. Peduncles representing three developmental stages of Couroupita, i.e.: from young inflorescences, from mature inflorescence without, and with heavy fruit (c. 1 kg), and two developmental stages of Kigelia, i.e.: from young inflorescences, and from mature inflorescences with heavy fruit (2–5 kg) were selected. For each developmental stage, three peduncles were collected from each of the species. Samples were collected from basal, middle and terminal regions of each peduncle. Samples were also collected from current year’s and one year old branches with di- ameters similar to those of peduncles, and growing at an angle of about 60° from each tree trunk. All material was fixed along in FAA (Berlyn & Miksche 1976). Light microscopy Transverse sections of 8–10 μm thick were cut on a sliding microtome (leica SM 2000R) and stained overnight in safranine O and counter-stained with fast green FCF (Berlyn & Miksche 1976). G- fibres in the reaction xylem of peduncles were identified by their red-stained lignified walls and green-stained gelatinous layer. After dehydration in ethanol-xylene series the sections were mounted in DPX. Tension wood severity at different regions of the peduncle was determined according to Washusen and Evans (2001). The proportion of gelatinous wall area to lignified wall area in transverse section was measured with an ocular micrometer. Thickness of the normal fibre walls was also measured and vessel frequency was counted with a grid ocular micrometer. Percent- age of eccentricity was measured from transverse sections of peduncles of different ages. The radial extent of secondary xylem on upper (UX) and lower side (LX) of the xylem cylinder was measured using an ocular micrometer at a 10× magnification. The percentage of eccentricity was then calculated using the formula, % of eccentricity = UX–LX + UX+ LX × 100, Where UX = Radial extent of secondary xylem on upper side of xylem cylinder and LX = Radial extent of secondary xylem on the lower side of the xylem cylinder. Downloaded from Brill.com09/30/2021 01:04:16AM via free access Pramod, Mishra & rao — Reaction wood in developing fruit stalks 205 Maceration Peduncle xylem from each developmental stage was macerated by using Jeffrey’s fluid (Berlyn & Miksche 1976). The length and width of fibres and vessel elements were recorded. One hundred random measurements were taken for each dimension to obtain mean values. Scanning Electron Microscopy Samples were cut into blocks of 2–3 mm thickness, dried and coated with gold using planner magnetron sputtering unit (Model 12 MSPT) and observed with a Philips XL 30 ESEM (Philips). Statistical analysis Analysis of variance (AnoVA) was used to determine statistically significant dif- ferences of anatomical parameters at a 0.05 confidence level using Sigmastat software (Version 3.5, San Jose, CA, USA). RESULTS Both Couroupita guianensis and Kigelia pinnata are cultivated in India and semi- deciduous in the local climate of alternating dry and wet season. In Couroupita, one year old mature peduncles were about one metre long bearing 6–8 round fruits each weighing about 1 kg (Fig. 1A). In Kigelia, one year old peduncles were about 3.5 m long carrying 1–2 oblong fruits, each with a weight ranging from 2 to 5 kg (Fig. 1B). The young peduncles from both species show similarity in length to that of the mature peduncles and carry an inflorescence with more than 10 flowers. Figure 1. – A: Fruits of Couroupita guianensis hanging from the main trunk. – B: Pendulous long peduncles with fruit in Kigelia pinnata. Downloaded from Brill.com09/30/2021 01:04:16AM via free access 206 IAWA Journal, Vol. 31 (2), 2010 Table 1. Percentage of eccentricity in different regions of peduncles in Couroupita and Kigelia. Species Age Region Radial extent of UX + LX UX + LX %*) secondary xylem Upper (UX) Lower (LX) CYi Basal 55 36 91 19 21 Middle 24 21 45 3 6 Terminal 21 16 37 5 13 oYi Basal 101 71 172 30 17 Couroupita Middle 62 50 112 12 11 Terminal 37 25 62 12 19 oYF Basal 210 130 340 80 24 Middle 135 98 233 37 16 Terminal 118 40 158 78 49 CYi Basal 34 31 65 3 5 Middle 32 30 62 2 3 Kigelia Terminal 25 24 49 1 2 oYF Basal 78 62 140 16 11 Middle 58 52 110 6 5 Terminal 56 42 98 14 14 *) = Percentage of eccentricity (UX – LX + UX + LX × 100). Note: Measurements are given in µm. CYi, current year’s peduncle bearing inflorescence; oYi, one year old peduncle bearing inflorescence; oYF, one year old peduncle bearing fruit. Reaction xylem in the peduncle of Couroupita guianensis Tension wood was more strongly developed in the fruit bearing peduncles than in young inflorescences with no fruits. Within the same peduncle the distribution of G- fibres varies from the basal to terminal region. Peduncles do show eccentric secondary growth with more xylem on the upper side (Table 1). In both current and one year old peduncles, G-fibres were more abundant in the basal region (about 60% of the total radius of secondary xylem) closer to the main trunk. in this area, G-fibre distribution extended radially from the periphery to the centre of the xylem cylinder (Fig. 2A). At the middle region of peduncles, G-fibres were less abundant (about 30% of the total radius) representing as 2–3 tangential bands (Fig. 2B) separated by a wide zone of normal fibres. The terminal region of the peduncles showed heterogeneity in G-fibre distribution at various development stages. G-fibres were found to be less abundant in the current year’s peduncle whereas the one year old peduncles have a narrow band of gelatinous fibres near the cambial zone (Fig. 2D) and their abundance was maximal after fruit development (Fig. 2C). The vessels distributed in the basal and middle region of the peduncle were mostly solitary (Fig. 2A & B) whereas the terminal region showed radial multiples of 2–5 vessels (Fig.