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Bubenheim, D.L., C.A. Mitchell, and S.S. Nielsen. 1990. Utility of cowpea foliage in a crop production system for space. p. 535-538. In: J. Janick and J.E. Simon (eds.), Advances in new crops. Timber Press, Portland, OR.

Utility of Cowpea Foliage in a Crop Production System for Space

David L. Bubenheim, Cary A. Mitchell, and Suzanne S. Nielsen


  1. INTRODUCTION
  2. EXPERIMENTAL METHODS
  3. RESULTS
    1. Dry Weight Accumulation
    2. Biomass Partitioning
    3. Seed and Pod Yield
    4. Yield Efficiency
    5. Nutritional Composition
  4. CONCLUSION
  5. REFERENCES
  6. Table 1
  7. Table 2
  8. Table 3
  9. Table 4

INTRODUCTION

Controlled Ecological Life Support Systems (CELSS) will utilize green plants to supply food, oxygen, and purified water for inhabitants of future spacecraft and planetary bases. Candidate crop plants are selected for yield optimization research based on potential contribution to a well-balanced vegetarian diet, low-volume canopy characteristics, and short life-cycle duration.

Cowpea (Vigna unguiculata) is a candidate crop for CELSS because of the low-fat, high-complex carbohydrate, moderate protein characteristics of the edible portion. The seeds ("black-eyed peas") are considered the edible form of the plant in the United States, but the leaves serve as a major staple in the diet throughout Africa and Asia (Bittenbender et al. 1984). Young leaves or shoot tips are harvested and consumed as cooked greens or dried for storage. Harvest strategies practiced in the field to utilize foliage include harvest of the entire vegetative plant prior to flowering or partial defoliation and later seed harvest from the same plants.

Utilization of cowpea as a leafy vegetable and grain crop may provide nutritional and harvest versatility not available with purely vegetative crops like lettuce or monocarpic crops like wheat (Bubenheim and Mitchell 1987, 1988). A purely vegetative system of cowpea might be desirable in a CELSS. Only one phase of the life cycle need be satisfied in a vegetative system, that is, peculiar environmental requirements during any single developmental phase would not complicate the system. Vegetative culture of cowpea, a short-day or day-neutral plant for flowering, could be accommodated with the long photoperiod and high photosynthetic photon flux optimizing environments required for some long-day candidate crops. The nutrient composition of cowpea foliage is more desirable than that of many vegetative crops and cowpea could provide dietary versatility by utilization of either foliage or seeds from the same crop species.

The ultimate efficiency and feasibility of a CELSS will be evaluated on the basis of productivity of the system per unit inputs necessary to operate the system. The inputs of the final CELSS will be volume, mass, time, and energy In the early stages of yield optimization with cowpea, we considered time and area inputs and evaluated yield efficiency on a daily basis per unit area. Since both cowpea foliage and seed are edible, and since both plant parts are utilized in field agriculture, productivity and yield efficiency were determined for cowpea following three different harvest strategies. Harvest of the whole, vegetative plant prior to flowering, traditional seed harvest, and a combination of removal of young leaves and eventual seed harvest from the same plants were evaluated. Nutritional composition of the edible plant parts also was determined.

EXPERIMENTAL METHODS

Two cowpea cultivars 'Bainkey-21' and 'IT84E-124' were grown using soilless culture in a greenhouse at Purdue University. All plants were exposed to a 10-hour photoperiod by enclosing the plants in a light exclusive curtain during each dark period. Temperature was maintained at 21 ± 5°C.

In the vegetative harvest treatment, whole plants (except roots) were harvested at 15-day intervals beginning 10 days after germination. At each harvest, 4 plants of each cultivar were separated into leaves, petioles, and stems of individual branches. All leaves present on a plant at each vegetative harvest were considered edible. For the traditional seed-harvest treatment, only dried seed was considered the edible portion of the plant. The combination treatment of vegetative and seed harvest from the same plants consisted of removal of 2 to 4 recently formed trifoliolate leaves (not fully expanded) from each branch on a plant 25 and 40 days after germination, and then seed was harvested from those same plants. Carbohydrate, protein, fat, and ash content were determined for leaves and seeds of harvested plants.

RESULTS

Dry Weight Accumulation

A similar pattern of dry weight accumulation was exhibited by both cowpea cultivars, although each exhibited a different growth habit (Chaturvedi 1980). 'Bainkey-21' developed vigorous vining branches, whereas 'IT84E-124' exhibited a determinate growth habit and developed a compact, low-volume canopy As typical for cowpea, leaf dry weight decreased in support of pod development for both cultivars (Ezedinma 1973, Huxley and Summerfield 1976, Stewart et al. 1978). Biomass production was greatest and a larger proportion of biomass was partitioned to foliage by 'IT84E-124' compared with 'Bainkey'. Data presented are for 'IT84E-124' only.

Biomass Partitioning

Removal of young expanding leaves during the vegetative phase just prior to flowering as part of the vegetative/seed-harvest strategy suppressed total plant biomass and altered partitioning compared with plants in a traditional seed-harvest strategy (Table 1). Periodic, partial defoliation stimulated leaf production; 68% of cumulative biomass was in the form of leaves compared with 57% for vegetative plants. Twice as much cumulative leaf dry weight was produced by plants in the vegetative/seed-harvest strategy than by plants in the traditional seed-harvest strategy.

Seed and Pod Yield

The suppression of biomass accumulation and diversion to the vegetative portion of the plant resulting from vegetative/seed harvest was unaffected by harvest strategy (Table 2); seed was harvested from plants of both the vegetative/seed and traditional seed-harvest strategies 75 days after germination. Seed yield, seed number, and pod number per plant were, however, severely suppressed as a result of partial defoliation. While the reduction in source leaves limited reproductive sink size (seed number per plant), individual seed size was not affected. The mixed-harvest scenario of seed + leaves together increased the time to harvest by 2 days.

Yield Efficiency

Yield efficiency was greatest for the vegetative harvest strategy with whole-plant harvest 40 days after germination (Table 3). While total edible yield per plant was equal for the vegetative and traditional seed-harvest strategies, the vegetative product could be harvested 35 days earlier than the 75 days required to produce seeds. Daily yield per plant, yield per unit area, and overall yield efficiency (g m-2 day-1) were two to three times greater for the vegetative strategy than for the traditional seed-harvest strategy, reflecting the shorter time to harvest and smaller plant size. Of the three harvest strategies considered, the vegetative/seed harvest was identified as the least efficient. Total edible yield per plant and daily area, and over-all yield efficiencies were significantly less for the vegetative/seed-harvest strategy than for either the purely vegetative or traditional seed-harvest strategies.

In areas of the world where the vegetative/seed-harvest strategy is practiced, the primary goal appears to be timely availability of food rather than production of an absolute maximum amount. Final seed yield is sacrificed so that food in the form of cowpea foliage is provided throughout the season.

Harvest index, the cumulative edible biomass expressed as a percent of total plant biomass, was greatest for the vegetative/seed strategy, the least efficient strategy of those considered. While increased harvest index is a characteristic of enhanced yield it cannot be used as a sole indicator of yield potential. The potential for yield enhancement in cowpea grown under optimizing environments will be evaluated further. Both traditional seed harvest and purely vegetative harvest strategies will be used in future yield-optimization research.

Nutritional Composition

Leaf carbohydrate content increased with leaf age, but was greatest in the seed (Table 4). Protein content of older leaves was similar to that of seeds; protein content of young leaves was greatest. Fat content was greater in leaf tissue than in seed and was not affected by leaf age. Inorganic mineral (ash) content of cowpea foliage was much greater than that for seed regardless of leaf age.

CONCLUSION

Cowpea is a dynamic crop that may add versatility to the diet not provided for by other candidate crops for CELSS. Cowpea could complement a large number of other food crops by utilization of two different edible plant parts (leaves and seeds), each with different nutritional characteristics. A single cowpea crop should be grown to supply either leaves or seeds as the yield efficiency was suppressed by the combination of leaf and seed harvest. Yield efficiency was greatest for the vegetative harvest strategy, but the bioavailability of nutrients from foliage must be determined before potential use of this strategy can be adequately evaluated. Both leaves and seeds of cowpea appear to provide a low-fat high-protein food choice. By choosing leaves of various ages or seed, the proportion of carbohydrate and protein provided in the diet by cowpea could be controlled. Cowpea is a versatile legume crop that will provide high carbohydrate together with moderate protein and low fat from foliage as well as seeds for a vegetarian diet in a space-deployed bioregenerative life support system. The results of this study suggest that some plants could be grown for reproductive harvest while separate plants should be dedicated to vegetative harvest. Additional work is required to find a mixed-harvest scenario from the same plant that will be as productive.

REFERENCES


Table 1. Biomass production and partitioning for three harvest systems of cowpea cv. IT84E-124.

Yield (g/plant)
Harvest system Leaves Stem Pod Seed Total
Seed (75 days) 23.8b(c)z 28.2a 46.5a 34.6a 117.3a(a)
Vegetative (40 days) 33.8a(b) 20.4b 59.8b(c)
Vegetative + seed (75 days) 7.1c (46.5a)y 11.9c 12.7b 10.1b 33.9c (68.5b)y
zMean separation in columns by Waller-Duncan K-test (K=100), or F-test.
yIncludes leaves from vegetative harvests.


Table 2. Yield of cowpea cv. IT84E-124 as influenced by harvest system.

Seed
Harvest system (No.) (g/plant) (g/seed) Pods (no.) Days to
flowering
Seed 34.6 238.5 0.15 54.6 37
**z ** NS ** *
Vegetative + seed 10.1 66.0 0.15 15.2 39
zMean separation between rows by F-test at 1% (**), 5% (*) level or not significant (NS).


Table 3. Yield characteristics of cowpea cv. IT84E-124 as influenced by harvest system.

Harvest system
Plant part Seed Vegetative Vegetative
+ seed
Seed yield (g/plant) 34.6az 10.1b
Edible leaves (g/plant) 33.8z 14.8 b
Total edible yield (g/plant) 34.6ay 33.8az 25.0b
Harvest index (%) 30c 58b 72a
Days to harvest 75a 40b 75a
Daily yield (g/day-plant) 0.46b 0.85a 0.33c
Area yield (g/m2 canopy area) 69.2b 105.4a 45.6c
Yield efficiency (g/m2-day) 0.92b 2.64a 0.60c
zMean system within rows by Waller-Duncan K-test (K = 100) or F-test.
yAssumes all harvested leaves are edible at day 40.


Table 4. Proximate analysis of leaves and seeds of cowpea cv. IT84E-124.

Yield (% dry wt)
Plant part Carbohydrate Protein Fat Ash
Expanding leaves (7-10 days old) 31.8cz 43.0a 5.3a 14.4a
Expanded leaves (22-25 days old) 42.6b 30.5b 5.0a 14.8a
Seed 55.5a 30.9b 1.2b 3.8b
zMean separation with row by Waller-Duncan K-test (K=100).


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