Introduction Directory UMM :Data Elmu:jurnal:E:Environmental and Experimental Botany:Vol43.Issue2.Apr2000:

Environmental and Experimental Botany 43 2000 1 – 9 Factors inducing cavity formation in the vascular cylinders of pea roots Pisum sati6um L., cv. Alaska Daniel K. Gladish a, , Teruo Niki b a Department of Botany, Miami Uni6ersity, Hamilton, OH 45011 , USA b Department of Cell Technology, Faculty of Engineering, Takushoku Uni6ersity, Hachioji, Tokyo 193 , Japan Received 24 February 1999; received in revised form 27 May 1999; accepted 31 May 1999 Abstract When grown at relatively high temperatures pea primary roots Pisum sati6um L.cv. Alaska often form long lysigenous cavities in the centers of their vascular cylinders. Factors other than temperature may be involved, however. Pea seedlings were grown at 10 and 25°C in vermiculite in a water-availability series 750 – 2200 ml water2 l vermiculite and hydroponically at various levels of aeration 0, 400, 800 ml airmin. Pea seedlings were also grown at 25°C in vermiculite moistened with 750 ml water2 l along an oxygen gradient 2 – 21 O 2 . Growth was much slower and vascular cavities never formed at 10°C. At 25°C in vermiculite the rate of cavity formation 22 – 100 was positively correlated with water-availability, so water-availability was a significant factor. In the hydroponic system, aeration had little effect on growth at 10°C but increased growth at 25°C. All primary roots grown hydroponically at 25°C contained cavities. As ambient oxygen level was increased so did growth rate, but the reverse was true for cavity formation. Growth rate, therefore, was not a factor in cavity formation but oxygen availability was. Primary roots that did not develop cavities at 25°C had intercellular spaces in parenchymatous tissues of their vascular cylinders, whereas those with cavities did not. These results support the hypothesis that at high temperature elevated respiratory demand exceeds the rate of oxygen diffusion to the center of the mature portions of pea roots that form cavities, and that this situation is aggravated by wet conditions. Therefore, vascular cavities may be functioning as a type of aerenchyma. © 2000 Elsevier Science B.V. All rights reserved. Keywords : Root cavity formation; Root growth; Water-availability; Flooding response; Aerenchyma www.elsevier.comlocateenvexpbot

1. Introduction

The observations of Popham 1955 regarding the formation of lysigenous cavities in the centers of vascular cylinders of primary roots of pea were largely disregarded until Rost et al. 1988 re- ported vascular cavities beginning near the tips of Alaska pea primary roots grown under standard conditions in vermiculite. Lu et al. 1991 showed good evidence of routine cavity formation in pri- mary roots cavities do not form in lateral roots extending continuously from ca. 2 cm from the Corresponding author. Tel.: + 1-513-7853257; fax: + 1- 513-7853145. E-mail address : gladisdkmuohio.edu D.K. Gladish S0098-847200 - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 9 8 - 8 4 7 2 9 9 0 0 0 3 8 - 6 tips to within 2 – 3 cm of the bases. They described the phenomenon as temperature-dependent; i.e. vascular cavities form in pea primary roots at temperatures above 15°C when grown in a wide range of media. Cavities formed in up to 30 cm of the root axis. They also described specialized par- enchyma cells SP-cells, which later acropetally fill the cavities. In an examination of the vascular anatomies of the primary roots of 20 legume species, this phenomenon was shown to be general in cool season legumes Rost et al., 1991. Niki et al. 1995 and Niki et al. 1998 have since de- scribed the fine details of the process of cavity formation by lysigeny and the later intrusion of SP-cells into the cavities in vascular cylinders of pea primary roots using thin-sectioning methods for light microscopy, transmission electron mi- croscopy and scanning electron microscopy. In a population of pea seedlings growing at an inductive temperature the frequency of occurrence of root cavities was usually not 100 Lu et al., 1991. Even with seeds of the same lot, germi- nated and grown together under the same grow- ing conditions, usually a portion of these previously described populations did not produce root cavities at inductive temperatures. This sug- gests that other factors in addition to temperature may be involved in inducing vascular cavity for- mation. In the present study we manipulated wa- ter conditions and the gaseous environment around developing seedlings as modulators of root development in order to examine the rela- tionship of water status and oxygen availability with root growth and cavity formation. We hy- pothesized that at elevated temperature high wa- ter availability would be associated with hypoxia in the central vascular tissue and that this would contribute to cavity formation.

2. Materials and methods