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Fluid Journal : Fluid Journal 1996-1998
4 Fluid Journal Winter 1998 with yield, especially boll size. Increasing water supplies during late season causes the plant to divert more of its daily photosynthate into root growth and storage for overwintering, reflecting the woody, perennial nature of the cotton plant. Across all water supplies, years, and cultural treatments a strong linear relationship existed between water supply/plant from square initiation to peak flower and fruit/plant (Figure 2). Distribution of the fruit load on the plant was strongly influenced by water supply per plant. First position fruit increased rapidly as water supply per plant increased, reaching a maximum of 6-7 bolls per plant at 10 gallons per plant. Lint per boll was correlated with water supply from square initiation to first flower. However, magnitude of the change was relatively small and considerable influence due to genetics and temperature during the later part of the fruiting period was more important than water supply. Total plant productivity (lint per plant) was strongly correlated with water supply from square initiation to peak bloom with an r2 of 0.70. Lint per plant strongly correlated with lint yield/A with an F of 0.76 (Figure 3). These results strongly suggest that plant population should be based on water supply per plant. Take-home message To maximize lint production per plant and thus lint yield/A, an individual plant requires approximately 10 gallons of water per plant. An acre- inch of water contains 27,500 gallons so that each inch of available water will support 2,500-2,700 plants/A. If one considers stored water at planting and the potential rainfall and/or irrigation from square initiation to peak bloom, a decision as to appropriate plant density can be made and planting rates adjusted accordingly. For instance, on the Texas southern High plains, stored water at planting is normally 3 to 4 inches. Approximately 6 to 7 inches of precipitation occurs between square initiation and peak bloom, providing a total water supply of 9 to 11 inches up to peak bloom. If an inch supports 2,500 plants, then dryland systems should target 25,000 to 30,000 plants/A as optimum. For each inch of irrigation water that can be provided during square initiation to peak bloom (approximately 6 to 7 weeks), an additional 2,500 plants/A should be produced. Typical irrigation water supplies range from 2 to 4 gallons per minute per acre (gpma). Each gpma of irrigation water will support an additional 5,500 to 6,000 plants/A. The cotton plant has a tremendous compensatory capacity. If one is going to hedge the risk of water stress, it would be better to be on the low rather than the high side from a plant density perspective. Dr. Krieg is professor of Crop Physiology, Texas Tech University.
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