Production and Quality of White Asparagus Grown under Opaque Rowcovers

Black and white plastic rowcovers were established over field-grown ‘Jersey Giant’ asparagus (Asparagus officinalis L.). Spears were cut for 7 weeks. Season soil temperatures were lowest under white plastic and highest without plastic covering. Night air temperature under plastic covers was ≈ 1.4C higher than without plastic covering (control), but day air temperature was typically 10C higher under black plastic, with temperatures under white plastic intermediate. Marketable yield (t·ha) was improved with the use of plastics as was total yield (P = 0.05), but spear number/ ha was similar in all treatments. There was no consistent treatment effect on spear diameter. Average spear weight was higher when under plastic, whereas spear length was reduced compared with uncovered spears. There were no differences among treatments in spear fiber content, but spears grown under plastic covers were higher in soluble solids content, titratable acidity, and nitrate and lower in protein, ascorbic acid, and total phenolics than uncovered spears. Quantitative differences in these constituents were also a function of whether they were from the upper, middle, or lower spear segment. Very little chlorophyll and carotenoids were produced in the absence of light, but there was a chroma (color intensity) difference between spears grown under the two plastics. White asparagus production is common in Asia and Europe. But fresh white asparagus is rare in major U.S. markets and, when available, costs two to three times as much as green asparagus. In Germany, clear plastics have been used principally as a soil warming technique in green (Kromer, 1978) and white (Munz, 1970) asparagus production. On Cyprus, 1.5-m-high walk-in plastic tunnels in conjunction with black rowcovers provided maximum yield of white asparagus (Vakis et al., 1975). Black plastic alone did not improve earliness. When asparagus is grown under conditions that allow spears to develop in darkness, chlorophyll is not synthesized and developing spears appear white. Raw white asparagus has been reported to be similar or higher in fiber, slightly lower in ascorbic acid and protein, but higher in soluble solids content (SSC) than green asparagus (Ketsa and Uthaiburt, 1987; Lai et al., 1973). Traditionally grown white asparagus requires mounding ≈0.5 m of soil over the Received for publication 17 Apr. 1990. Mention of a proprietary product does not constitute endorsement or recommendation for its use by the U.S. Dept. of Agriculture. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact. 1Research Horticulturist. 2Associate Professor. 374 asparagus crowns. Soils subject to crusting or of high bulk density are not always suitable for white asparagus production. Harvesting is frequently done three times a day. During daylight, spears that have pierced through the soil surface are not saleable as white asparagus. Because of these and other problems (Vakis et al., 1975) associated with white asparagus production, we report on a method using opaque white and black plastic rowcovers as a simple tool for the production of white asparagus and examine the effects that cover types have on spear quality. The production site consisted of an Enders stoney silt loam (clayey, mixed, thermic Typic Hapludults) that had been planted in Apr. 1986 to ‘Jersey Giant’ seedlings. Spacing and eventual crown depth were 30 cm and 15 cm, respectively, in rows 1.4 m apart. N'(3,4-dichlorophenyl) -N, Ndimethylurea (diuron) and (2,4-dichlorophenoxy) -acetic acid (2,4-D) were applied on 10 Mar. 1989, before spear emergence, at 2 and 9 kg·ha, respectively. Ammonium nitrate was applied at 60 kg N/ha on 16 Mar. Griffolyn (Reef Ind., Houston, Texas), a nylon-reinforced 4mil (0.1 mm) plastic fabric, which is white on one side and black on the other, was then used to establish either white or black rowcovers (as viewed from the outside). Plot length was 7.3 m. Wire hoops, spaced 1.8 m apart in the row, supported the rowcovers to a height of 0.35 m. Uncovered rows were used for the control treatment to produce green asparagus. The plastic covers were removed to one side of the row each time the spears were harvested. Single-ended thermocouples were connected to a CR-10 data logger (Campbell Scientific, Logan, Utah) to measure hourly soil and air temperatures 15 cm below and above the soil surface of one replicate. Spears were cut three times a week from 27 Mar. to 12 May. Immediately after cutting, spears were placed in opaque plastic bags. Objective color determination and subsequent sample preparation were done under diffuse light. After trimming length to 15 cm, spears <9.5 mm in diameter (butt end) were counted, weighed, and culled. Marketable spears were ≥9.5 in diameter after trimming to 15 cm; those used for quality analysis weighed 10 to 20 g per 15 cm length. The spears were cut into three 5-cm segments consisting of upper (tip), middle, and lower (butt) sections, blanched for 3 min in boiling water, and frozen to –20C after each harvest. Spear segments were pooled by harvest week for each replication of the rowcover treatments. Spears were harvested after the 7th week of cutting to obtain spear length data, but only weeks 1 through 7 were included in the total season yield. On 26 May, spears 9 cm or longer from all treatments were marked 4 and 9 cm below the tip and remeasured on 29 May to determine length differences after 3 days of growth. Spear pigments from a 5-cm-long segment 5 to 10 cm below the tip were measured by reflectance and spectrophotometry. Reflectance was measured with a CR100 Chroma Meter (Minolta, Ramsey, N.J.) standardized against a white plate having values for Y, x, and y of 87.4, 0.311, and 0.318, respectively. Chlorophylls and carotenoids from lyophilized midspear segments were extracted in 80% aqueous acetone and quantified spectrometrically using published extinction values for chlorophylls and carotenoids previously determined in 80% aqueous acetone (Wellburn and Lichtenthaler, 1984). Soluble solids concentration in juice pressed from the thawed samples before fiber analysis was read with an Atago Nlt hand-held refractometer (Atago Co., Japan). Protein was determined by AOAC automated Kjeldahl method 7.021 (Assn. of Official Analytical Chemists, 1980) and nitrate by specific ion HORTSCIENCE , VOL. 26(4), APRIL 1991 Fig. 2. Rowcover × spear segment interactions for percent soluble solids, ascorbic acid and total phenols concentrations, and for pH. Mean separation for segments within each cover treatment (lowercase letters) and treatments within a given segment (uppercase letters) were performed by LSD at P = 0.05. Plots covered with white or black plastic rowcovers produced white asparagus; plots without rowcovers produced green asparagus. ZMeans of four replications separated by LSD (0.05 or 0.01). NS,*,* *Nonsignificant or significant at P = 0.05 or 0.01, respectively. Table 2. Effect of rowcovers on spear length and on 3 days of growth of a 5-cm marked spear