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Pattern of Photoassimilate Partitioning in Pseudobulbous and Rhizomatous Terrestrial Orchids

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Pattern of Photoassimilate Partitioning in Pseudobulbous and Rhizomatous Terrestrial Orchids Environmental and Experimental Botany 40 (1998) 93–104 Pattern of photoassimilate partitioning in pseudobulbous and rhizomatous terrestrial orchids C.S. Hew *, K.T. Koh, G.H. Khoo School of Biological Sciences, The National Uni6ersity of Singapore, Lower Kent Ridge Road, Singapore 119260, Singapore Accepted 22 October 1997 Abstract Photoassimilate partitioning patterns in two terrestrial orchids, Spathoglottis unguiculata and Bromheadia finlayso- 14 niana, were mapped using a CO2 dosing technique. A highly integrated source-sink photoassimilate partitioning pattern was observed in both orchids. In S. unguiculata, a pseudobulbous terrestrial orchid, all test leaves supplied similar percentages of 14C-assimilates to all plant parts on a single shoot. In both orchids, the inflorescence during the reproductive stage and the axillary bud during the vegetative stage had the highest sink activity and percentage distribution of 14C-assimilates. A high percentage of 14C-assimilates was imported by the pseudobulb of S. unguiculata. The pseudobulb accumulated a higher percentage of 14C-assimilates (44%) during the vegetative stage than during the flowering stage (21–30%). In B. finlaysoniana, a rhizomatous terrestrial orchid, a high percentage of 14C-photoassimilates was observed in the stem internodes at all three developmental growth stages (the vegetative stage (stage 1), the flowering stage (stage 2) and the fruiting stage (stage 4)). Sink activities of the rhizome were higher at stages 1 and 4 than at stage 2. A similar pattern of photoassimilate partitioning was observed for B. finlaysoniana grown naturally in its habitat. A polar movement of 14C-assimilates towards the major sink (inflorescence) was observed in the current shoot of field plants with a competing sink (axillary bud). © 1998 Published by Elsevier Science B.V. All rights reserved. Keywords: Photoassimilate partitioning; Pseudobulb; Stem internode; Rhizome; Tropical terrestrial orchids; Spathoglottis unguiculata; Bromheadia finlaysoniana 1. Introduction epiphytic orchids. Studies on tropical thick-leaved (Aranda and Dendrobium) (Clifford et al., 1992, The pattern of photoassimilate partitioning in 1995; Wadasinghe and Hew, 1995) and thin- orchids has attracted considerable interest in re- leaved (Oncidium) (Yong and Hew, 1995a,b,c) cent years. However, most information on pho- epiphytic orchids which are CAM and C3 respec- toassimilate partitioning is limited in tropical tively (Hew, 1989) revealed two salient features in the photoassimilate partitioning pattern. Firstly, * Corresponding author. Tel.: +65 77 22713; fax: +65 77 photoassimilate partitioning is highly integrated 95671; e-mail: [email protected] in which both major (inflorescence, axillary bud S0098-8472/98/$19.00 © 1998 Published by Elsevier Science B.V. All rights reserved. PII S0098-8472(98)00024-0 94 C.S. Hew et al. / En6ironmental and Experimental Botany 40 (1998) 93–104 and vegetative apex) and minor sinks (leaves, phology of terrestrial orchids is more variable. stems and roots) received 14C-assimilates from all The pseudobulb, rhizome or corm can be found in leaves. Secondly, mature leaves in these orchids terrestrial orchids. The underground rhizome is a import significant amounts of 14C-assimilates compound organ, made up of the basal sections from other mature leaves. The observation of an of successive shoots (Dressler, 1981). In addition, import in mature leaf contradicts the current view the internodal length of the shoot of terrestrial that mature leaves only export assimilates in most orchids is much more elongated than the epi- cases and young and developing leaves only im- phytic orchids (Goh and Kluge, 1989). The physi- port assimilates (Wardlaw, 1968, 1990; Kursanov, ological role of such structures in terrestrial 1984). orchids in relation to photoassimilate partitioning To date, we have very little information on the remains unknown. pattern of partitioning of assimilates in terrestrial In this paper, we study the pattern of pho- orchids. Ecologically, the epiphytic and terrestrial toassimilate partitioning in two tropical terrestrial orchids are two different entities. The physiology orchids, Spathoglottis unguiculata and Bromheadia of plants in these diverse habitats can be very finlaysoniana which have a different morphology. different. The cost of reproduction in epiphytic S. unguiculata is pseudobulbous (with a pseudob- orchids is thought to be higher than in the terres- ulb) while B. finlaysoniana does not possess a trial orchids because nutrient availability for epi- pseudobulb but is rhizomatous instead. We inves- phytes is expected to vary with time and space tigated the hypothesis whether the assimilates al- (Benzing, 1973). This supports the observation of location to the reproduction tissues in the tropical a difference in flowering periods of the epiphytic terrestrial orchids with different storage organs is and terrestrial Mexican orchids that differ in different. In addition, the carbon allocation pat- drought adaptation ability (Sahagun-Godinez, tern in the field B. finlaysoniana was studied. 1996). Since assimilate partitioning in plants de- termines both the efficiency with which assimilates are used and the extent of its investment in differ- 2. Materials and methods ent developmental processes such as growth and reproduction (Marshall, 1990), it would be of 2.1. Plant materials interest to study the pattern of photoassimilate partitioning of tropical terrestrial orchids. There Spathoglottis and Bromheadia are C3 thin- are few reports on carbon allocation in terrestrial leaved terrestrial orchids (Hew, 1989) with clonal orchids such as Tipularia discolor and Dacty- growth. S. unguiculata (Dr T. Yam, personal com- lorhiza fuchsii but these were made in relation to munication) is characterized by the presence of an seasonal changes in the temperate zone (Leeson et ovoid pseudobulb bearing a few plicate leaves. al., 1991; Zimmermann and Whigman, 1992; Tis- The base of each Spathoglottis leaf forms a sheath sue et al., 1995). In Singapore, the climate remains which extends round the pseudobulb (Holttum, constant throughout the year. Whether the pat- 1964). The inflorescence arises from a basal leaf- terns of photoassimilate partitioning in the tropi- axil with the rachis bearing a succession of many cal epiphytic and terrestrial orchids are the same deep purple flowers. B. finlaysoniana Rchb. F. is is not known. commonly found in open country sites and devel- Most epiphytic orchids possess a prominent, oping belukar (secondary heath forest) (Holttum, enlarged bulbous structure at the base of their 1964; Sim et al., 1992). It is characterized by a leaves, termed a pseudobulb (Dressler, 1981). The long (commonly 1 m, sometimes up to 2 m), physiological role of the pseudobulb in relation to slender stem. The basal portion of the stem is photoassimilate partitioning has been studied covered with green sheaths, above which are 6 or (Yong and Hew, 1995c). It seems to act as a more pairs of leaves (oblong blade) with a sheath- storage organ for photoassimilates which support ing base (leaf sheath). The inflorescence is termi- inflorescence development. However, the mor- nal and bears ephemeral flower(s). The C.S. Hew et al. / En6ironmental and Experimental Botany 40 (1998) 93–104 95 Fig. 1. Distribution of 14C-assimilates in the various plant parts and sink activity of different plant parts within a single shoot of 14 S. unguiculata with an axillary bud (stage 1) at 6, 12, 24, 48 and 72 h after CO2 feeding of the test leaf, L4. L, leaf; Pb, pseudobulb; Rt, root; Rh, rhizome; Ab, axillary bud (n=4or5,9S.E.). 96 C.S. Hew et al. / En6ironmental and Experimental Botany 40 (1998) 93–104 Fig. 2. Distribution of 14C-assimilates in the various plant parts and sink activity of different plant parts within a single shoot of 14 B. finlaysoniana with an axillary bud (stage 1) at 6, 12, 33 and 72 h after CO2 feeding of test leaf, L4. L, leaf; In, stem internode; Rt, root; Rh, rhizome; Ab, axillary bud (n=4or5,9S.E.). underground rhizome often gives rise to an axil- and dry mass, leaf area and chlorophyll content) lary shoot. Adult plants of S. unguiculata were (data not shown) and it was used for all subse- 14 obtained from Multico Orchids while that of B. quent CO2 feeding experiments in both orchids. finlaysoniana were obtained from the garden of The developmental growth stages of the plants the School of Biological Sciences, National Uni- were arbitrarily assigned. Stage 1, a shoot with an versity of Singapore. axillary bud; stage 2, a shoot with a developing Leaves were denoted as L1 to L6 counting from inflorescence (or a flower bud, in the case of B. the shoot apex. L4 was considered as a mature finlaysoniana); stage 3, a shoot with a mature leaf based on various growth parameters (fresh inflorescence; and stage 4, a shoot with a mature C.S. Hew et al. / En6ironmental and Experimental Botany 40 (1998) 93–104 97 Fig. 3. Percentage distribution of 14C-assimilates and sink activity of the leaf blade and leaf sheath of the leaf above (L3) (A and C) and the leaf below (L5) the fed leaf, L4, within a single shoot of S. unguiculata with an axillary bud (stage 1) (n=4or5,9S.E.). fruit capsule. Single shoots at suitable develop- midity (maximum of 99%, minimum of 63%); mental stages (1, 2, 3 or 4) were potted in steril- maximum daylight intensity at noon (600 to 700 ized soil mixture containing sand, coarse mmol m−2 s−1). The plants were watered daily vermiculite and garden soil (1:1:1) and acclima- and fertilized twice weekly (Foliar Fertilizer 67, tized for a week prior to the feeding experiments. Blue Sky Agricultural Supplies, Singapore; 1.6 g For field grown B. finlaysoniana, two similar adult l−1; N:P:K of 13.5:27:27) (Yong and Hew, plants within the Adinandra Belukar of Kent 1995a). The light intensity on the day of the field Ridge (Sim et al., 1992) were chosen. The plants experiment was 240–260 mmol m−2 s−1. Four or within the selected sites 1 and 2, were arbitrarily five plants were used for each experiment con- labeled as plant 1 (2 m tall) and plant 2 (1 m tall), ducted in the laboratory.
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