Annals of Botany 97: 39–45, 2006 doi:10.1093/aob/mcj008, available online at www.aob.oxfordjournals.org
Short Photoperiod Induces Dormancy in Lotus (Nelumbo nucifera) J U N - I C H I R O M A S U D A 1,*, T O S H I H I R O U R A K A W A 1, Y U K I O O Z A K I 2 and H I R O S H I O K U B O 1 1 Laboratory of Horticultural Science, Faculty of Agriculture, Kyushu University, 812-8581 Fukuoka, Japan and 2Laboratory of Agricultural Ecology, Faculty of Agriculture, Kyushu University, Kasuya, 811-2307 Fukuoka, Japan Received: 29 June 2005 Returned for revision: 15 August 2005 Accepted: 27 September 2005 Published electronically: 15 November 2005
Background and Aims Lotus (Nelumbo nucifera) has been cultivated as an ornamental and food plant in Japan for more than 1000 years. As large areas are required for its cultivation (approximately 2 m2 per plant), physiological research, such as into the effect of environmental factors on dormancy, has not been well studied until recently. In this paper, seedlings were used to examine environmental factors affecting dormancy induction. Methods In a first experiment, seeds were sown from 6 April to 6 October at 2-month intervals, and cultivated for 2 months in an unheated greenhouse. In a second experiment, seeds were prepared for germination on 16 November and 16 May and the seedlings were grown at 25 or 30 C under natural daylength in phytotron growth rooms. After 1 month, the seedlings were cultivated at 20, 25 or 30 C for a further month. The number of leaves and rhizome branches on the main stem were counted, and growth of rhizomes on the main stem was calculated using a rhizome enlargement index (= maximum internode diameter/internode length) after 2 months of culture in both experiments. Key Results Rhizomes elongated without enlargement when the seeds were sown in April and June. Sowing the seeds in August and October resulted in rhizome enlargement from the tenth and fifth internodes, respectively. Rhizomes enlarged in the November-sowing but elongated in the May-sowing irrespective of temperature treatments under natural daylength in the phytotron rooms. The seedlings cultivated from May at 25–30 C for 2 months had more leaves, and more rhizome branches and nodes than those cultivated from November. Conclusions Short days led to induced dormancy in lotus. Key words: Dormancy, leaf production, Nelumbo nucifera, photoperiod, rhizome branching, rhizome enlargement, temperature.
INTRODUCTION Lotus (genus Nelumbo), an aquatic plant, consists of two species, N. nucifera (Indian lotus) and N. pentapetara (American lotus). The former originated in the eastern part of Asia and the northern part of Australia, whereas the latter originated in the eastern part of North America and the northern part of South America (Sakamoto, 1977; Toyoda, 1981). These species show considerable variations in flower colour and shape, and have been cultivated as ornamental plants as a result. The rhizome and seeds of N. pentapetara were commonly eaten by Native Americans, and those of N. nucifera are still a part of the Oriental diet. Commonly called the edible lotus, the rhizomes of N. nucifera are grown, cooked and used very much like potatoes. The crop was introduced from China and has been cultivated for more than 1000 years in Japan (Komatsu et al., 1975; Sakamoto, 1977; Toyoda, 1981). The enlarged rhizome sprouts in early spring and the rhizome elongates with emerging floating and upright leaves until late summer. Three to four distal rhizome internodes begin to enlarge in late summer and enlarged rhizomes are harvested from late summer to the following spring. Among geophytes, underground organs modified from parts of the stem are classified into corms, tubers and rhizomes. It is recognized that these organs, being used for asexual propagation, are key to the survival strategy of * For correspondence. E-mail
[email protected]
plants during unfavourable periods for their growth. The formation of tubers has been studied with regard to photoperiod and temperature in Solanum tuberosum (potato), Helianthus tuberosus and Begonia evansiana (Hamner and Long, 1939; Esashi and Nagao, 1958; Esashi et al., 1964; Snyder and Ewing, 1989). There are also some studies on the environmental factors affecting formation of corms and cormels (Tsukamoto and Inaba, 1961; Asahira et al., 1968; Imanishi et al., 1970). However, there are few reports on whether rhizome enlargement is affected by environmental factors, e.g. temperature and/or photoperiod. An enlarged rhizome is also found in N. nucifera, acting as a dormant organ to aid survival of the plant under unfavourable circumstances. Although N. nucifera is an important vegetable crop in Japan, the dynamics of its rhizome enlargement are relatively poorly known. Rhizomes enlarged in the previous year are usually used for commercial cultivation. The length of the main stem reaches to about 11 m when enlarged rhizomes are planted (Fig. 1), and each plant requires a large area (more than 2 m2 per plant) for cultivation. Detailed investigation of belowground rhizomes is laborious and time-consuming, explaining why research into the physiology of these organs in N. nucifera has been limited. Small rhizomes with compact plants can be obtained when seedlings are cultivated (Fig. 1). The effect of temperature and photoperiod on induction of dormancy was investigated by using seedlings to provide an efficient physiological approach.
Published by Oxford University Press on behalf of the Annals of Botany Company 2005
Masuda et al. — Environmental Factors and Dormancy in Lotus
40
From enlarged rhizome
Upright leaf 1m
Floating leaf Water level Soil level
Enlarged rhizome From seed
Upright leaf
10 cm
Floating leaf Water level Soil level Enlarged rhizome Seed Rhizome
F I G . 1. Schematic drawing illustrating growth in lotus.
MATERIALS AND METHODS Plant materials
Open-pollinated seeds of Nelumbo nucifera Gaertn. ‘Chugoku’ were used in all experiments. The seeds were prepared for germination by soaking in concentrated H2SO4 for 3 h and then rinsing with distilled water. They were then soaked in distilled water for 1 d at 25 C. After removing softened seed coats, the seeds were incubated in distilled water at 25 C under continuous fluorescent light (391 mmol m 2 s 1) until germination (6 days). Effects of sowing date on induction of dormancy
Seeds were prepared for germination from 6 April to 6 October at 2-month intervals. After germination, four seedlings were transplanted to a plastic container (28 · 40 · 15 cm) containing sandy soil with 30 g slow-release fertilizer (N : P : K=16 : 5 : 10 %). Each container was filled with water, which was replaced each week. Twelve seedlings were used for each treatment and the containers were placed in line. The seedlings were grown under natural daylength conditions in an unheated greenhouse. The average ambient temperature in Fukuoka gradually increased from April (15 C) to June (23 C) to August (30 C), whereas it decreased from August (30 C) to October (20 C) to December (10 C). The latitude in Fukuoka is 33.35 N, and the maximum and minimum daylengths were, respectively, 14 h 20 min and 12 h 44 min from
6 April to 6 June, 14 h 26 min and 13 h 43 min from 6 June to 6 August, 13 h 43 min and 11 h 43 min from 6 August to 6 October, and 11 h 43 min and 10 h 4 min from 6 October to 6 December; i.e. daylength gradually increased from 6 April to 21 June, gradually decreasing from 21 June to 6 August thereafter. The rhizome elongates with emerging leaves of two types: floating and upright. Rhizome enlargement proceeds by enlarging of 3–4 distal internodes with a break in leaf production and in rhizome elongation and branching. The numbers of leaves and rhizome branches on the main stem were counted, and growth of rhizomes in the main stem was calculated using a rhizome enlargement index (=maximum internode diameter/internode length) after 2 months of culture. Effects of temperature and photoperiod on induction of dormancy
Seeds were prepared for germination on 16 November, 2000 and 16 May, 2001. Four seedlings were transplanted to one plastic container as in the previous experiment, and one container was used for each treatment. The seedlings were grown at 25 or 30 C under natural daylength in a phytotron glass room of the Biotron Institute, Kyushu University. After 1 month of cultivation, the seedlings were cultivated at 20, 25 and 30 C as follows: (A) 30 C for 2 months; (B) 25 C for 1 month after 30 C for 1 month; (C) 20 C for 1 month after 30 C for 1 month; (D) 30 C for 1 month after 25 C for 1 month; (E) 25 C for 2 months; and (F) 20 C for
41
Masuda et al. — Environmental Factors and Dormancy in Lotus 10
15 Number of rhizome branches
Number of leaves
Upright leaf
d c
5 b
c
10
b b 5
a a
0
Number of leaves
Floating leaf
b
0
a
April
April
August June Sowing time
October
F I G . 3. Effect of time of sowing on the number of rhizome branches of plants grown in an unheated greenhouse for 2 months. Vertical bars represent 6s.e. Statistically significant differences at P
