Source-sink relations in cotton: Genetic and environmental affectors
Cotton (Gossypium hirsutum) production represents the largest crop enterprise in Texas with over 60% of the state’s cotton acreage on the High Plains around Lubbock. Large year-to-year and field-to-field variation exists in lint yield across this vast area under both irrigated and dryland conditions. Both environment and genetics contribute to yield variation with the environmental complex being the primary yield affector. Lack of an adequate water supply throughout the growing season represents the single greatest limitation to cotton productivity in this semi-arid region. Approximately 70% of the regions annual rain (450 mm) does occur during the cotton growing season but potential evaporation exceeds precipitation by a factor of over 3 times. Approximately 50% of the cotton acreage in this area has supplemental irrigation capability; however, the application volumes are highly variable from system to system and rarely adequate to provide all the water the cotton crop needs each year. Irrigated cotton yields on the Southern High Plains are highly correlated with water supply; however, dryland yields are more closely related to when the summer rains occur with July being most important. Lint yield can be assessed in terms of the respective yield components: boll number and boll size. Regression analyses indicate that boll number accounts for over 80% of the yield variation and is largely influenced by the growing environment, including management. Boll number is determined by the production of fruiting sites and retention and final size of fruit. Water stress has a major impact on the production of mainstem nodes and thus the number of fruiting branches and fruiting sites. Fruit retention is strongly related to the supply of reduced carbon and nitrogen from the subtending leaf associated with each fruit form. Many factors influence average boll size, seed weight and lint turnout, with genetics being the major factor and water and nutrient supplies being secondary.
The total water available, or degree of irrigation (percent of ETp replaced), has a direct effect on yield and yield components Considerable evidence exists to say that as water supply increases, the number of fruiting sites and the number of harvestable fruit produced per plant increases, but the retention of fruit per plant decreased. Guinn and Mauney (1984) determined that cotton yields are proportional to the number of bolls produced. Grimes et. al.(1969) found final yield to be most highly correlated with the number of fruiting sites produced, and determined that boll retention decreased as irrigation rates increased, but that the total number of fruiting sites per plant actually increased, and offset the decreased retention effects resulting in more bolls per plant. Previous research shows that the final number of fruiting sites is most affected by water supply (Morrow and Krieg, 1987).
On the Texas Southern High Plains, water supply (or the lack of) is the single most limiting factor to the cotton producer. Water supply is also the single most important factor determining the production of fruiting sites. The water supply from the 6 leaf stage through the first week of flowering produces all the fruiting sites that have a chance to produce a mature fruit on the Texas High Plains. Average irrigated yields, are approximately two times dryland yields across the Southern High Plains; however, with adequate water the physical environment can support yields of greater than 5 times the dryland yields.
The cotton plant normally produces several times more fruiting sites than it retains and matures fruit. Numerous hypotheses and observations have been proposed for fruit abortion of the cotton crop. The "Nutritional" and "Hormonal" hypotheses have the most credibility. The Nutritional Hypothesis states that fruit abortion during the first 10 days of the embryo life results from inadequate supplies of reduced carbon and nitrogen products arriving from the leaf factory. The Hormonal Hypothesis states that fruit abortion during the early life of the embryo results from an increase in abscissic acid and a reduction in auxin flow to the developing fruit. Both hypotheses have convincing evidence for support. Fruit shedding is increased by high temperatures common to the cotton belt in the summer months. Sarvella (1966) observed pollen sterility at high day and night temperatures (above 28/20 degrees C, respectively) in the field, concluding that this might be a factor in poor boll set. Mauney et. al.(1978) and Guinn and Mauney (1984) showed that at high temperatures, nutritional and water shortages were responsible for fruit loss. Ehlig and LeMert (1973) conducted a trial in which daily temperatures remained above 40 C throughout the flowering period under different boll load scenarios, concluding that abscission rates are determined by boll loads, even at high temperatures. Baker (1994) concludes that even though boll shedding at high temperatures is not well understood, source-sink imbalances account for the "vast majority of fruit loss in cotton". Overall, boll shedding at or above 40 C can be attributed to source-sink imbalances. Baker et. al.(1983) also showed that temperature also plays a role in fruit development. He determined that when the day/night temperatures are 30/20 C (respectively), maximum boll growth rates were obtained. He concludes that although data strongly supports there being a temperature optimum, that it cannot be used as a basis for calculating sink strengths. He also notes that there is very little information available that establishes the effects of temperature on sink strength in whole bolls, and practically no information that breaks this down into boll components.
Krieg (1973) noted that the greatest rate of dry matter accumulation was strongly affected by night-time low temperatures, with 20 C being the optimum, with a great decrease at 15 C, coupled with very slow rates of oil and nitrogen accumulation (nitrogen starting to decrease at 25 C). As maximum daily temperature reached 40 C, he determined that, due to increased respiration, after the maximum weight was reached (40 DPA) there was an appreciable decrease in dry matter. Maximum seed size (length and volume) was achieved 20 days post anthesis, but maximum weight is not reached until just before the boll opens.
Fruit retention is highly variable and is largely dependent upon the supply of reduced C and N to the young, developing fruit. Competition for reduced C and N by the vegetative growth is one of the explanations for fruit abortion. If the plants partition less photosynthate into vegetative growth, then a higher portion of the photosynthate would be available for developing the fruit load of the plant. In this aspect, the opportunity exists to alter the source-sink relations of the plant and increase fruiting site production and retention, resulting in more bolls per acre and bolls per plant. Reducing the distance between planted rows has been evaluated for numerous years in cotton producing areas. Row spacings less than the traditional 40" (1.0m) are commonly referred to as narrow rows. According to the current literature, narrower row spacings provide substantial yield increases, due primarily to the earlier onset of fruiting (Buxton et al 1979, and Constable 1977). The literature shows a strong relationship between boll size and moisture stress, with severely water stressed cropping systems having smaller bolls with somewhat shorter fibers. Marani and Ephrath (1985) determined that with boll loading in cotton, plant height/width ratios change and a canopy which is not closed prior to boll loading will close at that time without further growth in height. This parameter has been shown to be affected by plant spacing and is manageable to some extent Hopkins (1990) showed that when comparing conventional 40 inch rows to narrow or ultra-narrow rows under the same planting density (plants per hectare) that under narrower row spacings, the plants were provided with greater spatial distribution (ground area per plant) and had greater light interception and light penetration into the canopy. The yield averages he noted were between 20% and 40% higher than the conventional 40" rows under the same environmental conditions, and the same row spacing management strategy provided 10-20% yield increases under dryland conditions. Increased yields due to narrow row spacing practices have been attributed to increased early season reproductive growth (in response to reduced competition and increased light interception) and increased fruit retention. Hopkins also concludes that the combination of shorter plant heights and narrow row systems produced less vegetative branches under narrow row conditions, suggesting that the vegetative to reproductive tissue ratios can be altered in favor of reproductive biomass being produced.
Variety selection(growth habit), plant densities and row spacings have been shown to have a direct effect on boll numbers/plant as well as boll size. The denser the populations, the fewer bolls/acre produced and the smaller the boll size, but the higher the rate of boll retention (Staggenborg, 1993). Hopkins (1990) showed that as row spacings went from 40 inches to 18 inches but plants/acre remained constant, bolls per plant almost doubled (increasing bolls per acre), and the crop had the ground covered 2 to 3 weeks earlier, which provides maximum light interception. Narrow rows have the potential to increase yield through the increased production and retention of fruiting sites as well as boll size by increasing seed per boll and fibers per seed.
Annual type plants typically go through definite growth stages. They begin with a period of vegetative growth, followed by a period of fruit growth, and then maturation and senescence, followed by death. Cotton (Gossypium hirsutum), on the other hand, is a woody perennial with an indeterminant growth habit. Cotton’s leaves and fruit behave as annuals, but it has stems and rootstock "programmed to live indefinitely" which is a perennial characteristic (Baker et. al., 1978). Upland cotton displays differences in the degree of indeterminacy exhibited within current cotton varieties. Maturity differences exist not only in the amount of heat units required to mature a given fruit load, but in vegetative and fruit development as well. Cultivar selection, based mainly on available water supply, is one of the main tools a producer has available in trying to maximize the natural resources for maximum productivity and profit on the Southern High Plains of Texas. Therefore, to optimize the efficiency of a given cotton production system, a thorough understanding of the interaction between cotton’s growth habits and the environment under which it will develop must be clear.