WATER RELATIONS IN LETTUCE(Lactuca sativa L.Var longiflia)

Four experiments were conducted to investigate sowing rates (1, 2, 3, or 4g. seeds.mˉ2) effects on lettuce transplant performance, to improve growth and yield of lettuce by populations, plot and furrow cultivations under rainfalls and supplementary irrigation, to boost drought resistance by using either GA3 or NAA rates of 0, 50, 75 or 100 mg.lˉ1. Results of experiment 1 showed that 2 g seeds.mˉ2 was the most suitable rate for the high performance transplant productions. It gave transplant numbers of 738.3 in which large, medium and small size transplants constituted 18, 36, and 46%, respectively. Results of experiment 2 revealed that the possibility of producing lettuce under rainfalls only and supplementary irrigation is not required, furrow cultivation was superior to that of plot. It exceeded the latter by 15.4% in term of yield, high population (28 plants.mˉ2) yielded ( 109.1% ) more than that of ( 14 plnt.mˉ2),however it tended to reduce the weight of individual head. The highest yield ( 8.08 kg.mˉ2) was obtained from furrow high population interactions with either rainfalls or supplementary irrigation. Experiment 3 results confirmed that GA3 highly improved the hearting process, particularly 100mg.l-1 treatment which resulted in the highest yield(5.9 kg.mˉ2), bulk density ( 0.2 g.cmˉ3), head size(2150 cm3) and individual head weight (423.3 g). The final experiment 4 results showed that NAA rate of 50 mg.lˉ1 was the most effective treatment as it gave the highest yield ( 3.12 kg.mˉ2), and weight of single head (403.3 g). However NAA rates were inferior to these of GA3. INTRODUCTION Lettuce is a very popular vegetable crop grown in Iraq and its production facing many problems commencing with its seed germination and their inhibition by extremes temperature and light intensity ( Gray, 1977; Cantliffe et al., 1984 and Wein,1997). Lettuce heading highly influence by ambient environments and cultural practices as their negative effects display on plant stature and performance which reduce the marketable yield owing to physiological disorders and deformed heads( Wurr et al.,1987; Wurr and Fellows,1991; Quaile and Presnell,1991 and Misaghi et al.,1992). Irrigation is one of the most effective cultural practice in vegetable production especially in lettuce owing to their shallow effective root system (30 cm ) depth ( Jackson, 1995; Schwarz et al., 1995 and Gallardo et al., 1996). Mosul city is located on 360o, 42′′ latitude and Altitude of 230m , it receives an actual rainfalls (381 mm.yearˉ1), however, there are uneven yearly rain distribution and also among raining months in the season which is usually commence on November and ends on May(Guest,1966). Mosul suffer from water shortages owing to the topography which makes water conduction be very difficult and cost much besides its scarcity , Received 23/8/2005 Accepted 4/1/2006 Mesopotamia J. of Agric (ISSN 1815-316 X) Vol. ( 33 ) NO ( 4 ) 2005 and therefore four experiments were included to investigate lettuce productions through attempts to produce transplants and grow lettuce under rainfalls and complementary irrigation to find if there are any possibilities to produce this crop under rainfalls by manipulating some cultural practices. MATERIALS AND METHODS Four experiments was carried out in Mosul City which is located on north latitude of 36o,42" and Altitude of 230m to investigate the possibility of producing lettuce under the available rainfalls during fall growing seasons of 2001-2002 and 2003-2004 through simulating cultural practices and use of some growth regulators, and thus we have conducted the following experiments . Experiment 1: Production of lettuce ( Lactuca sativa var longifolia Local cv.) transplants under rainfalls: This experiment was carried out during 2001-2002 fall growing season at Yarimja vegetable growing farms, Yarimja, Mosul city, IRAQ. Largest headed plants of good stature were selected from the previous fall season (200-2001) and cut off 5 cm above soil surface at harvesting , while other undesired plants were pulled with their roots out of the soil. Shoots were sprouted at nodes on the remained stem cuts and new small plants were formed, they were fertilized by NPK (27, 27,0) and weeds were manually eradicated .Finally seeds were collected and then kept at dry place to use them in the next season. A randomized Complete Block Design (RCBD) was used to include four sowing rates( 1, 2, 3 or 4 g. seeds. mˉ2) replicated four times, each replicate was represented by a plot of 1m2. The given seeds of each plot were equally distributed on 9 lines 10cm apart , and therefore this experiments had 16 plots. A laboratory seed germination at 20Co showed a germination percentage of (95%). Soil was dissected in to 16 plots . A sheep manure (0.1 m.mˉ2) and nitrogen fertilizer urea ( 25g.mˉ2) were applied before sowing on November,1 ,2000 .Seedlings were harvested on December, 20 , 2001. The harvested transplants were put into three size categories (large, medium and small) then for each category seedling numbers, seedling lengths, root length, stem lengths, leaf numbers , leaf fresh weights, root fresh weights, stem fresh weighs and weight of fresh transplants were measured . Leaves , roots and stems were oven dried at 60 Co for 72 h and the weighed to calculate their dry matter percentages. Soil analysis (table 1, 2) was carried out in agricultural college, soil department. Were as meteorological data (figure 1, 2) was obtained from Al-Rashidia station, Mosul, Iraq. Table (1): Physical analysis for loam soil of Yarimja field. Soil field Soil bulk density Particle sizes (%) Soil separation capacity (%) (g.cmˉ3) (g.Kgˉ1 0.21 1.58 159.8 Clay


INTRODUCTION
Lettuce is a very popular vegetable crop grown in Iraq and its production facing many problems commencing with its seed germination and their inhibition by extremes temperature and light intensity ( Gray, 1977;Cantliffe et al., 1984 andWein,1997). Lettuce heading highly influence by ambient environments and cultural practices as their negative effects display on plant stature and performance which reduce the marketable yield owing to physiological disorders and deformed heads ( Wurr et al.,1987;Wurr and Fellows,1991;Quaile andPresnell,1991 andMisaghi et al.,1992).
Irrigation is one of the most effective cultural practice in vegetable production especially in lettuce owing to their shallow effective root system (30 cm ) depth ( Jackson, 1995;Schwarz et al., 1995 andGallardo et al., 1996). Mosul city is located on 360º, 42″ latitude and Altitude of 230m , it receives an actual rainfalls (381 mm.yearˉ1), however, there are uneven yearly rain distribution and also among raining months in the season which is usually commence on November and ends on May (Guest,1966). Mosul suffer from longifolia Local cv). Seedlings obtained from Experiment 1 were used in this trail at the same season and location, and therefore a split-split plot with in Factorial Randomized Complete Block Design (split-split F-RCBD) was used to include supplementary irrigation and rainfalls as main plots , Furrow and plot cultivations as sub-plots and two plant populations as sub sub-plots. Each of the 8 treatments was replicated 4 times and each replicate was represented either by a plot of (3 m²) or a furrow of (0.75x4 m). Furrows of population 1 of (14 plant.mˉ²) had one line at the upper third on both sides, while population 11 of (28 plants. Mˉ²) had two lines one at the lower and the other at the upper thirds of each furrow side. Furrow lines were planted with (20 cm) apart, while lines in plots were planted with plant intra spaces of (20 cm) or (40 cm) for populations 1 and 11, respectively .
Weeds were eradicated manually on 29 th ., January and on 2 nd ., March. NPK (27,27,0) was mixed with urea at rate of ( 20 g.m-2 of each) broadcasted immediately after the first weed eradication and a mixture rate of (30g.mˉ² urea + 20 g.mˉ² NPK) were applied after the second manual weed control. Supplemental watering was applied whenever a depletion of 25% of available water capacity at a soil depth of 30cm, determined by gypsum blocks.
On 5-7 th , April plants were harvested and data on plant heights ,leaf numbers, weight of marketable head and yields were recorded, finally leaf and stem samples were oven-dried at 80Cº for 72 h. to calculate their dry matter percentages. Four GA3 rate treatments (0, 50, 75 or 100 mg.lˉ¹) were replicated five times and they were included in Randomized Complete Block Design (RCBD) , each replicate was represented by a furrow of (4X0.75m).
Plants were harvested on April, 4 th , 2004 and data on plant heights, head sizes , number of unfolded leaves, number of folded leaves , stem lengths, weight of single folded head, weight of single unfolded plant, head bulk density, unmarketable yields and marketable yields were recorded. Leaf samples were weighed and then oven-dried at 80 Cº for 72 h, to calculate the leaf dry matter percentages. Experiment 4: The possibility of improving lettuce (Lactuca sativa var lonigfolia , Paris Island Cv.) yield under rainfalls by the application of naphthalene acetic acid (NAA) in Mosul City: The materials and methods of this experiment were similar to these of experiment 3 except in that of altering naphthalene acetic acid (NAA) in stead of GA3 in which NAA rates (0, 50,75,or 100 mg.lˉ¹) were sprayed two times started on March, 1 st , 2004 and repeated after two weeks RESULTS AND DISCUSSIONS Experiment 1:Transplant production lasted for 50 days owing to the gradual decreases in temperature from November, 1 st to December, 20 th (table1). Low temperature adversely influenced seed germination , seedlings development and final transplant performances. Similar results were found by OH et al. (1987) and El-Sabary ( 2005). Low temperature had indirect effects on enzymes activities which participate in the hydrophobic and hydrophilic balance of cellular membrane since it alter the Sol-Gel transition state of membrane, and subsequently unequivocal discrepancies in plant responses to temperature were observed ( Levitt, 1980 andTurner andKramer, 1980). Total number of transplants obtained from sowing rates 1, 2, 3 and 4 g.m-2 were 324,738.3, 1062.5 and 110.9 transplants, respectively. The percentage of large size transplants were 46, 18, 9 and 8% which represent transplant numbers of 148.5, 129.5, 99.5 and 86.5 , respectively. Transplants had length (22.6 cm), root length (6.1 cm), stem length (2.2 cm), leaf numbers (8.8), leaf fresh weight (5g), root fresh weight 0.4g), stem fresh weight (0.19), leaves dry weight (0.39g), root dry weight (0.035g), stem dry weight (0.029g) and transplant fresh weight (5.8g) means were categorized as a large lettuce transplant (table 5 ). Large sized lettuce transplants of fresh weight (4-6g) were recommended for greenhouses and early spring season production of lettuce to shorten growth duration and consequently reduce cost, avoid bolting, extend marketing duration before critical day length for bolting are reached and ensure high competition ability of lettuce to weeds ( Guzman, 1987;Mangal et al., 1988;Wein, 1997 andEl-Sabary, 2005). However, using transplant of large size in fall season to produce lettuce in the early following spring are not preferred owing to their low duration requirement of verbalization which resulted in high unfolded head percentages, more vulnerable to freezing and frost injuries of the ensuing winter and earlier bolting in relation to smaller and younger transplants of high biologically active cells (Xing andRajashekar,2001 andDavid andHenson, 1998). These ambient environments disable plants to perform hearting process perfectly rendering the plant vegetative apical meristems expose to sunlight , altering them to flowering meristems synchronized with assimilate diversion to the reproductive ( 4 )  2005 sink on the account of high rate of leaf differentiations to ensure marketable yields ( Kinet et al., 1985, Wein, 1997and Ryder ,1999.
The numbers of medium transplant size of 18.5 cm length, 6 cm root length,1.3 cm stem length, 6.6 leaf numbers, 1.8g leaf fresh weight,0.2g root fresh weight, 0.1g stem fresh weight, 2.1g fresh weight and 0.29g dry weight were 111.5, 266.8, 343 and 438 transplants obtained from 1, 2, 3 and 4g.mˉ² seed rates , respectively. Medium size transplants (2-4g fresh weight) are perfect for lettuce production in moderate zone areas above north latitude 40º where long spring season durations and summer day temperature of 25Cº. These transplants are somehow have an ability of cold stress tolerance owing to their biological activity of cells. Medium size transplants also recommended for fall growing season in the middle east rejoin where moderate short freeze duration occurs in some years ( El-Shal et al., 1986, El-Sabary, 2005. Transplants which posses the lowest values in term of length(14.2 cm), root length (5.5 cm), stem length (0.8 cm), leaf numbers (4.7), leaf fresh weight(0.5g), root fresh weight(0.04g), stem fresh weight(0.06g) and mean of total transplant fresh weight( 0.6g) were classified as small transplant size. Vast numbers of these were confined the two higher sowing rates and gradual decreases were obtained as sowing rate decreased. Transplant numbers of small size were 529 at rate of 3g.mˉ² and 583.8 at rate of 4g.mˉ² which constitute 50 and 53% of the total produced transplants, respectively. These transplant size may be suitable for late production of lettuce in fall, spring and summer seasons where the growing season is so long that enable these transplants to ensure well establishment in the permanent field during their early stages of growth, particularly when the prevail ambient environments are in favor of perfect growth and heading. Small transplant size possesses small cells of highly active condensed protoplasts which enable them to display high cold stress avoidance and cold acclimation to the ensuing winter ( Abdel, 1997). The environmental factors, such as day length, temperature, nutrition, water availability, light intensity and physiological maturity of plant or plant part are known to play a role in cold acclimation , generally, plant growth ceases before cold acclimation begins, decreasing day length provide the primary stimulus or trigger for cold acclimation in many plants (Dewayne et al., 2001).
Negative correlation were found among sowing rates in term of transplant length(r=-0.1), root length(r=-0.49), leaf numbers(r=-0.26), leaf fresh weight(r=-0.81),root fresh weight(r=-0.083), root dry matter percentage(r=-0.064), stem dry matter percentage(r=-0.07), large transplant fresh weight(r=-0.08),small transplant fresh weight(r=-0.09) and large transplant dry weight (r=-0.089), however, positive correlations were found in stem length (r=0.088), stem fresh weight(r=0.049), transplant numbers(r=0.55), medium transplant fresh weight(r=0.092) medium transplant dry weight(r=0.0031and small transplant dry weight(r=0.024). From the obtained negative and positive correlations of most detected traits we may inferred that the higher the sowing rate was applied the weaker the transplant was produced. This weakness explains the degree of competition among germinating seeds, seedlings or transplants on nutrients and lights. It is well established that high plant populations create ambient condition of low light intensities in which far-red light is Mesopotamia J. of Agric (ISSN 1815-316 X) Vol. ( 33 ) NO ( 4 )  2005 prevailed over red light, such conditions force the plant to direct their assimilate for the favor of internodes elongation to enable them to intercept more photosynthesis active radiation(PAR) on the account of other competed plants ( Levitt, 1980;Kinet et al., 1985). Competition on light reinforces the apical dominance and increases endogenous IAA which causes the release of H+ ion from plasma lemma membrane, directed them to cell walls to weaken the glycosides and hydrogen bonds occur among cellulose microfibrils and other cell wall components through pH reductions to facilitate cell wall expansion by tergour pressure ( Goodwin and Mercer, 1985). Low light intensity induce expansion in the cells of the intercalary meristems located at the internodes and therefore elongation of internodes are occurred ( Wareing andPhillips, 1978 andBurgess, 1985).  (Dewayne et al., 2001). In Iraq, daily temperature fluctuations are greatest when clear skies exist, since solar radiation warms the earth surface during the day and air temperature of 10 to 15ºC are common even though the temperature at night may be less than -5ºC. This phenomenon highly affects lettuce growth, since the apical meristem of lettuce plants being very close to the soil and thus these Mesopotamia J. of Agric (ISSN 1815-316 X) Vol. ( 33 ) NO ( 4 )  2005 meristems are more influenced by soil temperature than air temperature. Lettuce plants are more related to heat lost from soil surface by radiation cooling moves away from earth surface, as the soil continues to lose heat until it is colder than the air just above it (Wurr e. al. ,1981). Kimball et al. (1967) determined that lettuce production in areas that had at least 2 months temperature of 17 to 28º C day and 3 to 12º C night, at temperature higher than these ranges, the cultivar grown in these areas develop a high incidence of tipburn, bolting and formation of loose puffy heads. Air temperature considered optimum for growth of lettuce in Florida from December to April ranges from maximum 26ºC to minimum 13ºC (Quaile and Presnell, 1991). Rainfall data illustrate that rainfalls significantly exceeded water losses by evapotranspiration estimated by Blay-Cridle equation. Therefore supplemental watering was not required during the first three months of the growing season , however complementary irrigation of 10.07 cm was applied once late in March to bring up the soil to field capacity to a depth of 25cm. However, the obtained results confirmed the ability of producing lettuce under rain fed in Mosul and supplementary irrigation mostly is not required this conclusion is inferred from our results and 30 years data of rainfalls ( Guest, 1966). Evapotranspiration (ETo) for irrigated lettuce between thinning and harvesting was 146 mm and maximum crop coefficient of 0.81 to 1.02 were obtained at maturity 55-53 days after planting and lettuce Etc was higher in Cos cultivars than in two crisp head cultivars, furthermore lettuce root length density and soil water extraction were greatest in the top 0 to 45cm and decreased rapidly below 45cm depths, however, soil water extraction by root increased at low depths when irrigation was reduced (Gallardo et al., 1996), they also stated that soil water tension at 0.3m depth should be maintained at 6 to 7 kpa for optimum yield of trickle irrigated lettuce.
Furrow cultivation method significantly exceeded that of plot method in term of leaf numbers per plant (34.8%), single head weight (19.3%) and yield (15.4%). This superiority might be referred to the ability of furrow to sustain better soil physical properties at lettuce root system such as low soil bulk density which facilitate optimal gas exchange between air and root zone in relation to plot cultivation. Plot cultivation usually manifests gradual bulk density increases during the growing season. These increases are obviously resulted from the continuous rainfalls and labor movement to achieve demanded cultural practices. In furrow cultivation, labor movements are usually demonstrated at the furrow bottom rendering other parts loose and aerated. Root establish a critical link in the soil-plant-air continuum and this link has to be maintained in the most adverse environment or physiological conditions ( Jackson et al., 1994 andSteudle, 2001 ). Anoxia results in profound physiological disturbances including an increase in CO 2 content in the soil solution and a decrease in O 2 supply anaerobiosis ( Dell Amico et al., 2001). Root absorption of mineral and water as well depend on energy generated by root respiration which in term depend on soil aeration. Kamaluddin and Zwiazek, (2001) applied 0.5 mM NaN3 sodium azide, a respiration inhibitor on red-osier roots they found a 35% decreases in both O 2 uptake and root hydraulic conductivity after 2 hours. Contrary results were reported by El-Sabary (2005) he obtained higher lettuce yield in plot cultivation than that of furrow cultivation. This Mesopotamia J. of Agric (ISSN 1815-316 X) Vol. ( 33 ) NO ( 4 )  2005 disagreement may be attributed to soil texture which reflects on aeration and gas exchange and therefore soils of prevail clay minerals may differ apparently from soils of prevail sand.
Dense plant population (p2) highly exceeded normal population (p1) in term of plant height (15.4%) and yield (109.1%), however, p1 significantly increased leaf number per plant, weight of single head and stem dry matter percentage as compared to p2. Our results are in agreement with Mangal et al. (1988), they found that spaces ranges 15 to 20 cm gave the highest yield. Plant population in lettuce is a very important factor in production. Subsequently growers have to pay intensive care to the optimal population in order to improve yield quantity and quality. Low populations resulted in high yield reductions because of the low number of plants per unit area , whereas high populations resulted in weak etiolated plants of puffy, lose leaves, elongated internodes and low weight of single head. Several morphological conditions must be met for head formation to succeed, these are large individual leaves, slow rate of stem elongation short petioles and a high rate of leaf production ( Dullforce, 1962 ). Jonson( 1983 ) described the importance of leaf differentiation rate in the lettuce heading process, he reported that marketable head formation resulted from the accumulation of young leaves under the layer of leaves covering the growing point. High population usually suffers low light intensity which in term divert the assimilates for the benefit of stem elongation on the account of other part developments ( Wareing and Phillips, 1978 ).
Results of irrigation and cultivation methods interactions showed that lettuce plant cultivated on furrows under irrigated or rainfalls significantly exceeded these of plots in leaf numbers per plant, weight of single head and yield. These results explain the occurrences of adequate rainfalls through out the growing season and therefore cultivation methods had the dominated effects on watering. Irrigation and populations interaction results showed prevalence of populations over both rainfalls and irrigation. Population 1 under either rainfalls or supplementary irrigation treatments significantly increased leaf numbers per plant, single head weight and stem dry matter percentages , whereas population 2 resulted in apparent yield and plant height increases under rainfalls and watering treatments. Plants of population 2 grown on furrow was the most effective treatment . It gave the highest yield (8.08 kg.mˉ²) and single head weight (317.4g), however, it resulted in significant leaf numbers per plant reductions. The obvious domination of population and furrow cultivation under either rainfalls and supplemental watering explained the moderate degree of inter and intra competition among lettuce plants grown at the rows of each furrow . Population 2 degree of competitions were ameliorated and withdrawn below their critical point by furrow cultivation. High light incidence in Mosul may be another interpretation for completion being under the critical point which highly exceeded that of lettuce requirement. Glenn (1984) measured continued growth increases up to 500cal.cmˉ².dayˉ¹ incident radiation in his green house trail. Whereas, Wein (1997) found that lettuce dry matter production was maximum at 150cal.cmˉ².dayˉ¹ in field plantings in Italy and United Kingdom (UK). Ryder (1999) reported lettuce light requirement of 15klux. In comparison to Mosul City located on 36º 42" N latitude and 230m altitude has a radiation incidence ranges from 520 to 610 klux during lettuce growing season (Wein, 1997). 5.6a 5.65a 6.03a 5.22b 3.64b 7.61a 6.05a 5.16b 6.01a 5.28b Leaf dry matter (%) 9.95a 10.15a 10.7a 9.7a 10.6a 9.8b 10.5a 9.4a 10.3a 10a Stem dry matter (%) 5.65a 5.85a 5.7a 5.8a 6.25a 5.25b 10.5a 9.4b 10.3a 10.1ab (P1,P2= P0pulations);(P=plot=furrow);(R=rainfall's=supplemental irrigation). Figure of shared characters are not significant at 0.05 level/Duncan tests. ( 7 ) results showed that gradual increases in the applied rates of GA3 were concomitant by gradual growth and yield improvement of lettuce, and therefore apparent increases were observed in plant height , stem length numbers of folded leaves, single head weight, head size bulk density of head and marketable yield. 100 ppm GA3 treatments appeared to be the most effective. It highly increased plant height(24.6%), stem length(75.5%), numbers of folded leaves(22.1%), weight of single head(51.2%), head bulk density(25%) and marketable yield(114.5%) in comparison to check. Furthermore it gave the lowest values in the number of unfolded leaves and non-marketable yield. However it slightly reduced dry matter percentage of leaves. Growth and yield increases gained from GA3 application explained the huge dry matter production in relation to control ,and thus we could ignore dry mater percentage reduction.
Plant height increases as a result of GA3 application were attributed to the ability of GA3 in enhancing higher rate of node differentiations. This enhancement ability proved by the production of a huge number of folded and unfolded leaves , otherwise plant height increase should be caused by internodes elongation and hence significant reduction would be observed in head firmness , numbers of folded leaves and yield. Lettuce stem elongation usually occurs after exposing plants to cold temperature and critical long day periods. Bolting is the real problem facing lettuce growers in Mosul.
Lettuce maturation are usually synchronized with the commence of critical bolting photoperiods and thus growers have to harvest their crops with in few days and sell it very cheap. GA-stimulated stem elongation is due to cell elongation rather than increased cell division after exposure to the cold temperatures of the ensuing winter they flower in the second year (Goodwin and Mercer, 1985), they also found that the level of extractable GA-like materials in rosette plant is equal or greater than that in photo periodically induce bolting plant, the possible explanation of negative correlation between GA-like materials and stem elongation is that under short days GA is compartmentalized in the cell and no available for the stimulation of stem elongation until long days initiate it release.
GA3 application resulted in high differentiation rate of leaves which is reflected on head bulk density (firmness), head size head weight, hearting process, plant height, yield and on entire head performance. These improvements might be attributed to the influence of GA3 on cell metabolism through its ability to increase activities of hydrolytic enzymes which provide crude materials to facilitate cell wall expansions. Mapelli, et al., (1984) demonstrated increase in activities of amylase (15.7%0, A phosphotase (46.2%), peroxidase (34.4%), lipase(37.7%), pectinase (37.7%), esterase(2.1%), Pytase(45.9%), invertase8%) glucosidase 11.7%) and nuclease(11.2%). Treatment with GA1 tended to induce the production of ethylene with begin 10h after application, showed a peak at about 24h and the declined , the yield of ethylene was proportional to the amount of GA1, ethylene production has a boosting effect on cell wall expansion (Garcia-Martinez, et al., 1984 ;Goodwin and Mercer, 1985). Experiment 4: Application of NAA rates (Table 8), resulted in positive correlations in plant height(r=0.021), weight of single head(r=0.1), unmarketable yield (r=0.33) and marketable yield(r=0.7). However, they also showed negative correlations in stem length(r=-0.36), numbers of unfolded leaves(r=-0.7), weight of unfolded plant(r=-0.09), bulk density of head(r=-0.04) and leaf dry matter percentage (r=-0.13). 50 ppm NAA appeared to be the most effective treatment It highly increased the individual head weight(19.8%), head size(21.1%), unmarketable yield (19.5%) and marketable yield (44.6%), followed by 100 ppm NAA treatment which gave the highest plant height (30.7cm) and marketable Mesopotamia J. of Agric (ISSN 1815-316 X) Vol. ( 33 ) NO ( 4 )  2005 yield(3.12kg.mˉ²) when it compared to untreated control. In general, NAA application tended to reduce stem length(9.8%) and numbers of unfolded leaves (43.5) in relation to check treatment. Lettuce growth and yield increases confirmed the positive effects of NAA applications on lettuce at heading stage. Similar results were obtained by Gallardo et al. (1996) Ryder (1999 and El-Sabary (2005), They confirmed that 60 to 80% of lettuce growth occurs during the last month of the growing season, and therefore hearting stage should be paid an intensive care in all cultural practices such as watering, fertilizing and pest eradications . Studies reported that the primary effect of auxin is to cause the cell to lower the pH of the aqueous phase of the cell wall, possibly by stimulating a membranes-bond H+ ion pump, increase in H+ ion concentration would then weaken hydrogen bonding between the cellulose microfibrils and xyloglucans and allow them to slide past each other under the pull of turgor pressure to keep cell wall expansion ( Goodwin and Mercer, 1985). The indirect effects of auxin are fulfilled through ethylene actions which is its generation boosts by auxins . Pretreatment of leaves with IAA elicited a rapid ethylene production and in endogenous ACC 1-aminocyclopropane-1carboxylic acid ( Mackeon et al., 1982 andGarcia-Martinez et al., 1984).