Aweekorsoafterseedlingestablishment,thefirsttrueleafappearsabovethecotyledons(Figure 4).Thefirst leaf shifts the plant's primary energy source from storage to photosynthesisandsignalsthemovefromemergencetovegetativegrowth.5.Soil Effects on Germination and EarlyRoot GrowthRoot growth dominates the growth of the cotton plant during germination and seedlingestablishment.Infact,thetaprootmaybeasdeep as1oinchesbythetimethecotyledonsemerge. This is a critical time for the development of the root system. Cold soils, seedlingdisease, low soil pH, water stress, hard pans and herbicide injury all inhibit root growth anddevelopment,but careful crop management can minimize most of these stresses.Therootsabsorbwater and nutrientsthatare vital tothedevelopmentof theplant,and anyhindrance ofroot development in these early stages of cotton growth may cause a disappointing productionseason.Cotton emerges the quickest from warm,moistsoil.Low temperatures(below 60degrees F)or less than adequate soilmoisture may hinder germination by slowingmetabolicprocesses(seethediscussiononheat units).Physical impedance,such ascrusting, does not slow germination, but itcan prevent the hypocotyl from emergingThis oftencausesthickening of thehypocotyland a condition referred to as “big shank" or"thick-legged"cotton, resulting in reducedseedling vigor (Figure 5)GenerallytheFigure5.Thickshankinacottonseedling.Soillonger ittakesfor emergence to occur,thecrusting,compactionorothermechanicalfactorsgreater the risk of plant death and yield loss.cancausethickshank.Arule of thumbforplantingcottoninmostregions of the U.S. Cotton Belt is that the soil temperature at 4 inches deep should be at least65degrees F for 3consecutive days, with warm temperatures in the forecast.6.RootDevelopmentAs the cottonplant grows,theradicle that originally emergedfromthe seed becomesataproot, from which lateral roots begin to form and grow.Lateral roots and the taprootcollectively make up the basal root system. Other"higher order"roots then develop from thisbasal rootsystem.Thesehigherorderrootshaveafunctional lifeof about3weeks.Theyformwhen environmentalconditionsaregood,andthendiewhennutrientsandwateraredepletedinthe area in which they developed.Astheplantmatures,theroots continueto spread andprobedeeper inthesoil profileforwater and nutrients.Therefore,the distribution of roots tends to match the most fertile soilzones.Figure6ashowsan exampleoftherootdistributionof anunstressedcottoncommunity.4
4 A week or so after seedling establishment, the first true leaf appears above the cotyledons (Figure 4). The first leaf shifts the plant’s primary energy source from storage to photosynthesis and signals the move from emergence to vegetative growth. 5. Soil Effects on Germination and Early Root Growth Root growth dominates the growth of the cotton plant during germination and seedling establishment. In fact, the taproot may be as deep as 10 inches by the time the cotyledons emerge. This is a critical time for the development of the root system. Cold soils, seedling disease, low soil pH, water stress, hard pans and herbicide injury all inhibit root growth and development, but careful crop management can minimize most of these stresses. The roots absorb water and nutrients that are vital to the development of the plant, and any hindrance o f root development in these early stages of cotton growth may cause a disappointing production season. Cotton emerges the quickest from warm, moist soil. Low temperatures (below 60 degrees F) or less than adequate soil moisture may hinder germination by slowing metabolic processes (see the discussion on heat units). Physical impedance, such as crusting, does not slow germination, but it can prevent the hypocotyl from emerging. This often causes thickening of the hypocotyl and a condition referred to as “big shank” or “thick-legged” cotton, resulting in reduced seedling vigor (Figure 5). Generally, the longer it takes for emergence to occur, the greater the risk of plant death and yield loss. A rule of thumb for planting cotton in most regions of the U.S. Cotton Belt is that the soil temperature at 4 inches deep should be at least 65 degrees F for 3 consecutive days, with warm temperatures in the forecast. 6. Root Development As the cotton plant grows, the radicle that originally emerged from the seed becomes a taproot, from which lateral roots begin to form and grow. Lateral roots and the taproot collectively make up the basal root system. Other “higher order” roots then develop from this basal root system. These higher order roots have a functional life of about 3 weeks. They form when environmental conditions are good, and then die when nutrients and water are depleted in the area in which they developed. As the plant matures, the roots continue to spread and probe deeper in the soil profile for water and nutrients. Therefore, the distribution of roots tends to match the most fertile soil zones. Figure 6a shows an example of the root distribution of an unstressed cotton community. Figure 5. Thick shank in a cotton seedling. Soil crusting, compaction or other mechanical factors can cause thick shank
Mostof theroots in this case canbe found between1and3feetdeep in the soil,but largequantitiesof rootscanstill befound morethan4feetdeep inthesoil.Theamount of rootsgenerallypeaksduringthecottonfloweringphasethendeclinesastheplantpartitionsmorecarbohydratestothedevelopingbolls(Figure6b)oeBollFillingFirstFirstFlowerSquare20364452406080101122028Depth(inches)DaysafterPlantingFigure6.(a)Comparisonofrootquantitywithsoil depthfora maturecottonplantNewrootsareconstantlyproducedinareasofthesoilprofilethathavewaterandnutrients.(b)Comparisonofrootquantity with cotton growth stage.Roots begin to declineafter flowering as the cotton plant shits itsenergyfromroottobolldevelopment.7.The MeristemsThecottonplanthasmeristems,orgrowingpoints,atthetopofthemainstemandonitsfruitingbranches.Thesemeristems allowtheplantto simultaneouslygrowupward andoutwardFigure7aisamicrographof theapicalmeristemandfirsttwofruitingbranches,whicharetoosmalltobeseenwithoutmagnificationDFigure7.(a)Lightmicrographofacottonplantapicalmeristemmagnified40x.Thetwofruitingbranchesinthismicrographaretoosmall tobe seen withthenakedeye.(b)The cells wherethrips feed on themeristemsdie,andtheresultingleavesappearcrinkledandhaveholesinthem.5
5 Most of the roots in this case can be found between 1 and 3 feet deep in the soil, but large quantities of roots can still be found more than 4 feet deep in the soil. The amount of roots generally peaks during the cotton flowering phase then declines as the plant partitions more carbohydrates to the developing bolls (Figure 6b). Figure 6. (a) Comparison of root quantity with soil depth for a mature cotton plant. New roots are constantly produced in areas of the soil profile that have water and nutrients. (b) Comparison of root quantity with cotton growth stage. Roots begin to decline after flowering as the cotton plant shifts its energy from root to boll development. 7. The Meristems The cotton plant has meristems, or growing points, at the top of the main stem and on its fruiting branches. These meristems allow the plant to simultaneously grow upward and outward. Figure 7a is a micrograph of the apical meristem and first two fruiting branches, which are too small to be seen without magnification. Figure 7. (a) Light micrograph of a cotton plant apical meristem magnified 40x. The two fruiting branches in this micrograph are too small to be seen with the naked eye. (b) The cells where thrips feed on the meristems die, and the resulting leaves appear crinkled and have holes in them