Cambial Activity in the Young Branches and Peduncles of Couroupita Guianensis (Lecythidaceae)

Rajput et al. – CambialIAWA Journal activity 35 in (3), peduncles 2014: 281–292 of the Cannon ball tree 281 CAMBIAL ACTIVITY IN THE YOUNG BRANCHES AND PEDUNCLES OF COUROUPITA GUIANENSIS (LECYTHIDACEAE) Kishore S. Rajput1,*, Amreen Saiyed1, Vidya S. Patil1 and K. S. Rao2 1Department of Botany, Faculty of Science, The Maharaja Sayajirao University of Baroda, Vadodara-390002, India 2Department of Biosciences, Sardar Patel. University, Vallabh Vidyanagar-388120, India *Corresponding author; e-mail: [email protected] AbstRact Peduncles of Couroupita guianensis Aubl. undergo extensive secondary growth, which is a rare and unexplored feature so far. In the present investigation seasonal behaviour of vascular cambium was studied in fruit-bearing peduncles and compared with the vegetative branches of similar diameter. In peduncles, the cambium remained active throughout the year. The number of cambium cells and differentiating xylem cells increased from May and reached a maximum in July-August. Although cambial growth occurred throughout the year, it was relatively sluggish in February despite the development of new leaves and on- going extension growth. In contrast, cambial cell division in young branches initiated in February, peaked in the same months as peduncle cambium while cambial cell division and differentiation of xylem remained suspended from October to January. Cessation of cambial cell division in the branches during this period may be correlated with the presence of mature leaves. In both (branches and peduncle), rapid cell division and increase in the number of differentiating xylem elements in April-May is positively correlated with the development of flower buds and new leaves. The present anatomical investigation revealed that cambial activity in both peduncle and vegetative branches are independent of phenology and climatic conditions. In conclusion, we believe that variations in the number of differentiating cambium derivatives in peduncles benefits from a dual source of growth hormone supply, i.e. from developing new leaves and flower buds. Keywords: Cambium, cauliflorous inflorescence, Cannon ball tree, secondary xylem, seasonal behaviour, young branches. INTRODUCTION The Cannon ball tree (Couroupita guianensis) is a unique South American tree species grown in tropical and subtropical regions throughout the world. In India, and other parts of the world, it is cultivated for its ornamental value and beautiful flowers and for the medicinal value of its leaves, bark, flowers and fruits, which have antibiotic, antifun- gal, antiseptic and analgesic properties (Al-Dhabi et al. 2012). It produces about one- © International Association of Wood Anatomists, 2014 DOI 10.1163/22941932-00000066 Published by Koninklijke Brill NV, Leiden Downloaded from Brill.com10/09/2021 08:25:14PM via free access 282 IAWA Journal 35 (3), 2014 metre-long cauliflorous racemes, that behave like short branches if they fail to develop fruits. The fruits and flowers are produced directly on the trunk; thus in older trees the whole lower part of the trunk is covered with several fruits. Each fruit can reach up to 20–25 cm diameter and ripen approximately in 12–18 months (Anonymous 2012). The peduncle may attain up to 1.5 m in length and bear 4–8 fruits. It undergoes secondary thickening by forming a complete ring of vascular cambium. If the inflorescence fails to develop fruits, the peduncle produces a cluster of leaves at the apex and behaves like a branch. Such peduncles may exceed 4–4.5 cm in diameter. There is no unanimous opinion about the leaf shedding and new flush of leaves. Available information indi- cates that trees shed their leaves two or three or several times in a year with spells of 15 to 18 days between leaf shedding and the development of new leaves. Cambial periodicity (i. e. alternation of the cambial activity and dormancy) in the stem axis is related to the alternations of cold and hot or dry and rainy seasons, exogenous and endogenous factors, photo period and supply of growth hormones from the young leaves (Rao & Rajput 2001; Callado et al. 2004; Marcati et al. 2008; Begum et al. 2012). Cambial anatomy in young branches of the Cannon ball tree was studied earlier by Rao et al. (1996), who found that cambial cell division and formation of secondary xylem continue for a major part of the year (i.e. February to October). However, like in tropical evergreen trees, correlations among extension growth, local climatic conditions and the cambial activity were not clear (Dave & Rao 1982). Cambial activity and development of secondary xylem are also reported in leaf petioles of both the temperate and tropical species (Ewers & Aloni 1987; Rajput & Rao 2011), but no information is available with reference to peduncles. Therefore, the main aim of the present study was to understand the seasonal behaviour of cambium in flowering branches (peduncles) and compare it with that in vegetative branches of similar diameter in order to investigate whether (a) the cambial activity in the peduncle is influenced by climatic conditions and phenology, (b) secondary xylem develops simultaneously in both, and (c) any changes occur in the structure of secondary xylem when the peduncle develops leaves. MATERIALS AND METHODS Sample collection site: Periodic collection of vegetative branches and peduncles of Couroupita guanensis Aubl. (Lecythidaceae), 18–20 mm thick, were collected from 12 different trees growing in the Maharaja Sayajirao University campus and arboretum, located in coordinates 22º 19' 15.9" N and 73º 10' 45.1" E at an altitude of 35.5 m. The natural vegetation in the region is tropical dry deciduous, with 450 to 750 mm annual rainfall. The monthly maximum mean temperature during day time is highest in May (43.8°C) and lowest in December-January (18.3°C), respectively. Sample collection and processing: Samples were collected in the second week of every month from January to December 2008. Each time 5–6 pieces of 50–60 mm in length were collected from five vegetative branches and peduncles that develop fruits. However, samples of peduncles that do not produce fruits were also studied but there is no difference in their cambial behaviour from fruiting peduncles; therefore, data are not included here. Immediately after collection, samples were fixed in FAA (Berlyn & Downloaded from Brill.com10/09/2021 08:25:14PM via free access Rajput et al. – Cambial activity in peduncles of the Cannon ball tree 283 Miksche 1976). After 24 hrs of fixation these samples were transferred in 70% alcohol for further processing and storage. Transverse and tangential longitudinal sections of 15–20 µm thickness were obtained with the help of a Reichert Jung sliding microtome and stained with safranin fast green combination (Johansen 1940). Sections were mounted in Dibutyl Phthalate Xylene (DPX) after passing through ethanol-xylene dehy- dration series. Important observations from stained sections were micro-photographed with a Leica DME 2000 research microscope. Seasonal changes in phenology such as initiation of new leaves, maturation, yel- lowing, defoliation, development of vegetative buds, flower buds, fruit development, maturation, dispersal were recorded at the time of each sample collection while the data on air temperature and rainfall were obtained from the Meteorological Centre, Department of Physics, The Maharaja Sayajirao University of Baroda, Vadodara. The term cambium is used to include the entire population of ray and fusiform cambial cells between the xylem and phloem. Activity of the cambium was determined by counting the number of undifferentiated layers of cambium cells lying between differentiating xylem and phloem in transverse sections. The terms cambial inactivity or rest are used to define the suspension of cell division activity anywhere within the cambial zone. RESULTS Morphological changes in the peduncle: A fully grown Cannon ball tree produces fruits and flowers directly on the trunk; thus in older trees the whole lower part of the trunk is covered with several large round fruits (Fig. 1A, B). The raceme inflorescence reaches about 1.5 metre in length and produces several flowers, out of which four to eight flowers develop into fruits (Fig. 1B) while others are shed. Some of the peduncles fail to produce fruits; such peduncles turn into vegetative branches by producing clusters of leaves and behave like vegetative branches (Fig. 1A). All the peduncles undergo secondary growth by forming a complete ring of vascular cambium. However, peduncles that behave like vegetative branches remain short and drooping. Structure of cambium: Cambium in both peduncles and branches is non-storied with vertically elongated fusiform cambial cells and more or less isodiametric cambial ray cells (Fig. 1C). Cambial rays are heterocellular, mostly uni- to biseriate but multi- seriate rays were also observed occasionally. The cambium contained two to three layers of cells in a radial file during the dormant state (Fig. 2A) and six layers in the active period (Fig. 2B). Active cambial cells showed thin radial walls, presence of cell plates or phragmoplasts and absence of beaded walls in tangential longitudinal cells, while dormant cells possessed thick radial walls in transverse view and beaded radial walls in tangential longitudinal view. The cambium, when dormant, was sandwiched between fully matured xylem and phloem elements. The length and width of fusiform cambial cells varied from 291 to 440 µm and 17 to 23 µm, respectively. Cambial rays were 165 to 361 mm and 47 to 87 mm in height and width, respectively. Cambial activity: In flowering branches (peduncles), cambial cell division and differentiation of xylem occurred throughout the year. In the radial files of cambium and differentiating xylem, the highest number of cells was observed in July and August Downloaded from Brill.com10/09/2021 08:25:14PM via free access 284 IAWA Journal 35 (3), 2014 Figure 1. Habit, fruits and tangential longitudinal view of a Couroupita guianensis branch. – A: Couroupita tree under cultivation at the University arboretum, Baroda.

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