A DOUBLE SHELL PLANT GROWTH CABINET BY L H. RORISON Botany Department, The University, Sheffield {Received z Fehruary 1964) SUMMARY The construction and performance of a plant growth cabinet which stands in an insulated room are described. Its performance does not match that of a closed circuit system, but the uniformity of environmental conditions maintained, despite frequent access, and the ease of access, recommend it to those who need frequent contact with their experimental material. The cost is modest by present-day standards. INTRODUCTION Recently, emphasis has been placed upon the development of closed circuit systems which allow a fine degree of environmental control within a given growing space and which are excellent so long as plants may grow undisturbed for the duration of each experimental period. When experiments involve the need for frequent access to plants the problem arises of whether (a) to open the closed circuit into what may be very different ambient conditions, or (b) to create an identical surrounding climate so that on opening the closed circuit a minimum of (climatic) disturbance results. To maintain such a buffer zone could add considerably to construction and running costs, but without it the refinements of the original closed circuits controls may be superfluous. A compromise unit is outlined below in which one set of equipment is used to maintain a constant environment in both a cabinet and in the room surrounding it. It allows easy access, and the environment provided is sufficiently constant to fall within the limits of error imposed by the rest of the experimental system employed. THE GROWTH CABINET Structure A cabinet of 4.5 X4.5 ft base stands on legs in the centre of an 8 ft cubed room (Fig. IA). The walls and ceiling of the room are insulated with a 2 in. thickness of expanded polystyrene and a standard door (i) opens outwards into a broad corridor. There is a 1.5 ft wide access round three sides of the cabinet, while the fourth side is sealed to an adjacent room wall and houses the constant running fan of the refrigeration unit (4). The cabinet walls (2) in line with the fan are of pegboard, while the two walls at right angles (3) comprise framed hardboard shutters. The compressor motor of the refrigeration unit (5) is housed in a small hutch externally. All control gear is fixed on to the opposite external wall adjacent to the door (6). A vertical section taken through the line X-Y (Fig. IB) shows the position of the rest of the equipment and also the direction of air flow. On the floor of the room immediately beneath the cooler outlet is a 2 kw fan heater (7) 358
A plant growth cabinet 359 and beneath the far end of the cabinet base is a constant level humidifier (8). On the outer peg board wall of the cabinet hangs a make and break thermostat (10) and a break on rise humidistat (9). B A 3 -- EOlO 5. r -2 : 2 15 --Y 1 1ft 1 Scale E : 3 1 L Fig. I. Scale diagram of the growth cabmet. A: Ground plan; B: Elevation through lme X-Y on the ground plan. The effective working base of the cabinet measures 4.5 /4-25 ft and is covexed by a fibre-glass tray with 2 in. sides and a drain tap at one corner. Head room is 3.5 ft and all vertical walls are lined with highly reflecting aluminium foil. The ceiling is of quarter plate glass. N.P. G
360 I. H. RORISON Above the growth cabinet is the light compartment in which thirty, 5 ft, 80 W warm white fluorescent tubes (11) are suspended at 1.75 in. centres, 5 in. above the glass. Over the tubes a moveable reflecting surface (12) is suspended. A 9.5 in. fan (13) wired through the light switch draws air from the corridor, through three 10 in. square air filters (14) and across the light bank. This air flow removes excess heat from both the light bank and from the tube starters (6) in the corridor. Both sides of the light bank have shutters which allow access for maintenance. Outside the room (6) are arranged: (a) the lamp starters and chokes, (b) a 24-hour time switch, (c) a night temperature depression unit, (d) a Transmatic electronic control panel (15) which controls both cooling and heating cycles in sequence from the one sensing element, (e) miniature hour meters to record light usage, and individual switching to allow selection of }, f, or full light intensity. Performance The following are controllable: day length; light intensity; 'day' and 'night' temperatures; relative humidity (above ambient). Temperature is controlled to within ±0.5 C between 5 and 30"" C. Maximum light intensity midway on the plateau of tube life (Phillip's Warm White) is approximately (a) 2200 ft-candles (0.165 cal/cm^/min) immediately beneath the screen, (b) 1800 ft-candles (0.135 cal/cm^/min) at bench height, and (c) 1500 ft-candles (o.iio cal/cm2/min) at 4 in. above bench height, with plants in position. There is a decrease in light intensity of less than 10% from the centre to the far corners of the cabinet. With both inspection shutters down, maximum decrease in light intensity has been of the order of 10%. This figure is low because the whole room interior is painted white and reflects light back into the cabinet. Light intensity may be reduced either by switching off a proportion of tubes or by laying mesh filters over the plate glass ceiling. Relative humidity response is within ±2.5 % of the required setting above ambient. Air changes between the outer corridor and the whole growth room occur approximately twice an hour while the rate of horizontal air flow across the cabinet is 30 ft'^/min. The total potential electrical load is approximately 5 kw. Block variation across the cabinet has been very small as may be seen from the results of the first experiment carried out in the first completed room (Table i). Cost To complete this room within an existing shell, to connect a low pressure water supply, and to fit linoleum and decorate to Isotope Laboratory grade C standard, cost ^^830 (1963). This figure included labour since the work was contracted out to a refrigeration engineer, a shop fitter, and to the local electricity board. DISCUSSION Basically, the design follows the principles advocated by Evans (1959) and Morris (1957) and later by Carpenter and Moulsley (i960). The resulting dispersal of components has led to the production of a climate inevitably less uniform than one in a closed circuit, yet surprisingly good for such a large volume. More rapid air flow^ might have increased uniformity but in order to avoid physiological complications it was decided to keep it
A plant growth cabinet 361 down to less than 30 ft/min. Horizontal air flow was chosen because of the safety of a solid bench surface w^hen using radioactive solutions in nutritional experiments. The height of the light-bank and spacing of components allows: (a) easy entry for dusting and maintenance and (b) the inclusion of more and differently shaped light sources as required. There is no built-in temperature control of air moving across the light bank but the corridor from which air is draw^n is kept between 15 C and 25 C throughout the day and this together with heat from the tube starters ensures that the Table i (a) Figures of logc mean dry weight in nigjblock I 2.708 2 2.721 3 2.720 air flow 4 2.771 2.782 6 2.781 L.S.D. 5",_, = 0.186. (b) Analysis of variance data Treatment d.f. M.S. Vr. Species Soils Harvests Blocks 2 4 5 0.63809 42.60536 65.47863 0.04105 2.9710 198.3121 304.7786 0.1911 N.S. *** *** N.S. tube envelopes remain at a temperature corresponding to the plateau for maximum light intensity. Although air is drawn along the length of the tubes, envelope temperature differs less than 15" C from end to end. A most pleasing result of the design has been the physical ease with which experiments can be carried out. Access round the cabinet is good and the centre of the bench can be reached comfortably. The cabinet sides (3) lift out and when placed upright between the cabinet legs create a tunnel which ensures that the main air flow continues to move directly across the cabinet above. In this way climatic changes on entry are minimized. The layout has proved well suited to plant growth experiments with nutritional and climatic variables which need up to several hours daily attention for maintenance of nutrient media, growth measurements, and/or harvesting. The spectral composition of the fluorescent tubes used has been adequate for apparently normal growth of a range of species, but failure of Perilla frutescens Britt. to elongate has shown the need for additional red light. It is suggested that in terms of: (a) performance flgures, (b) experimental variation experienced, and (c) cost, the unit merits consideration by both experimental ecologists and plant physiologists. At Sheffield three such rooms are in operation. They open into a corridor 24 ft long by 10.5 ft wide which provides not only a second climatic buffer but also a useful marshalling area. A standard size laboratory bench runs the length of the corridor opposite the rooms and there is still adequate space for chart recorders and infra-red gas analysers on the walls adjacent to each room. Soil preparation and chemical analysis are carried out in separate laboratories.
362 I. H. RORISON ACKNOWLEDGMENTS I should like to thank all those colleagues who have shown me their plant growth facilities and with whom I have had much valuable discussion, also Mr. R. Dobson of Refrigerator Contracts Ltd., Sheffield, who has overcome many difficulties, and the Nature Conservancy, for financing the venture, REFERENCES CARPENTER, G. A. & MOULSLEY, L. J. (i960). The artificial illumination of environmental control chambers for plant growth. J. agric. Engng. Res., 5, 283. EVANS, G. C. (1959)- Uesign of equipment for producing accurate control of artificial aerial environments at low cost. jf. agric. Sci., 53, 198. MORRIS, L. G. (1957). The design of growth rooms. In: Control of the Plant Environment, p. 139. Butterworths, London.