Effects of Water Stress on Vegetative and Reproductive Traits of Three Commercial Cotton Varieties in Field Conditions in Central Iran

Background water scarcity is one of the most important factors that restricts crop production specially, cotton which must planted in areas without cold temperature limitation. Most of such area in Iran encounters drought events, hot temperatures and high atmospheric evaporative demand. So, understanding of stress severity and cultivar responses will help to better management of crop in stress conditions. Our previous study showed that cultivar responses in view of some physiological and morphological aspects were highly different in water stress condition. In this study we focused on yield formatting traits. Results Three cotton commercial varieties; Khorshid, Khordad and Varamin studied in sever, mild and without water stress. In normal condition zero type cultivar, the khorshid, produced the highest seed cotton yield. Varamin cultivar had more and longer sympodial branches which could raise it’s yield. Also, Varamin cultivar’s seed cotton yield was higher than the others (3617 kg -1 ha compared with 2477 and 3060 for khordad and khorshid, respectively). Khorshid was superior to the others at sever water stress. Seed cotton yield showed high correlation whit boll number and boll weight and vegetative aspects such as plant height, node number and sympodial branches number. Management for developing more sympodial branches results in higher bud and ower and will increase the yield. Totally, we recommend Khorshid and Varamin cultivars for normal condition and Khorshid for sever stress conditions. caused to attenuate most of tolerance mechanisms and shedding of vegetative organs for plant survival. Conrming this Ul-Allah et highlighted the negative effects of drought stress on assimilate accumulation and portioning in reproductive tissues of cotton which nally converts into the ber. Competition of bolls for assimilates will reduce boll weight and expected to this correlation be low or negative. Probably, lower number of bolls, especially in stress conditions leads to sucient loading of each boll and correlated positively. Despite our results, other researches indicated negative correlation (Balkcom et al., 2010; Liu et al., 2013).

signi cant, but response of cultivars to water stress in view of plant height, monopodial and sympodial branches number and length, were signi cantly different and this differences affected by conditions of the year of experiment (table 2).

Vegetative traits
The highest number of node observed in Khorshid cultivar. The heihighest ratio of height to node, monopodial branches and length of monopodial branches belong to varamin and Khordad cultivars. Varamin Also, had the highest number of sympodial branches. In conditions of unlimited water availability (S0) Khorshid had the most number of boll and seed cotton yield (table 8), but as soil water content reduced to (45% of Fc. or mild stress), the Varamin cv.
Performed better than the others in view of boll number and seed cotton yield (table 2).
Water stress affected monopodial branch number, signi cantly, but length of this type of branches did not show signi cant response to water stress levels (table 1). In low level of water availability, the number of monopodial were low but relatively long, which appeared in insigni cant differences between stress levels (table 3). Varamin had more (5.5) and longer (39.3 cm) monopodial branches than Khorshid and Khordad (table 10). In the other study in research conditions, Varamin monopodial branch number and length reported as 4 and 30 cm, respectively (Naderi Are et al., 2015). Khorshid had the lowest and shortest sympodial branches than two other cultivars (table 2).
Water stress levels in uenced on number and length of sympodial branches, signi cantly (p<0.01). The cultivars differences were signi cant, too (table 1). In 85% Fc, Khordad had the longest (27.5) and most number of sympodial branches and was superior to the others. Increasing of water stress led to similar response of two cultivars in view of length (24.5 cm). The lowest number of sympodia branches observed in Khordad at 25% Fc, and the lowest length of sympodial belong to Khorshid at 25% Fc (3.6 cm, table 3). Similar differences in reduction of sympodial of different genotypes by water stress reported by sahito et al., (2015) and ehsan et al., (2008).

Yleld and yield components
The highest mean of boll number (28.2 bolls) produced by Khorshid in appropriate condition (water availability of 85% Fc). Lowering water content to 45% Fc reduced boll number of this cultivar. Varamin performed better in this level of water stress with 18.2 boll per plant (table 2). More reduction of water availability to 25% Fc led to insigni cant difference between cultivars, while Khorshid was relatively better, but laid in same statistical group (table 2).
Khorshid as a zero type cultivar, possess very short sympodial branches and lower leaf area which probably led to thrifty use of water and retention of bolls on this points and so, performed better than the others in severe stress (25% Fc, table 2). In varamin and khorshid varieties with higher potential of vegetative development and size, water stress reduced the number and length of sympodial branches which re ected in reduced boll number.
In high level of water availability, cultivars did not show signi cant differences in boll weight and Varamin was superior to others with boll weight of 4.8 gr (table 2).
In condition of water availability (85% Fc) seed cotton yield of Khorshid and Varaimin cultivars was higher than Khordad, but in mild stress (45% Fc) potential of Varamin in retention of more and heavier bolls, increased the yield of this cultivar to 3617 kg.ha -1 , while the yield of Khorshid and Khordad were 3060 and 2477 kg.ha -1 , respectively (table 2). In severe stress (25% Fc) Khorshid as a zero type cultivar performed better than the others.

Correlations
Correlation in the broadest sense is a measure of an association between variables. The Pearson correlation coe cient which used for better understanding of treats relations is typically used for jointly normally distributed data. This correlation coe cient scaled such that it range from -1 to +1, where 0 indicates that there is no linear or monotonic association, and the relationship gets stronger and ultimately approaches a straight as the coe cient approaches an absolute Correlation of seed cotton yield with boll number was high and positive (0.95**, table 3). Seed cotton yield correlation with boll weight was positive but lower than that of boll number (0.64**, table 3). Correlation of boll number and boll weight was positive (0.42*). Seed cotton correlation with plant height and node number was positive and signi cant (0.73 and 0.57, respectively). Correlation of yield with height to node ration and number of sympodial branches was signi cant, too (table 3). Sympodial branch number highly correlated with yield components imply that the higher number of this branches, increases probability of higher seed cotton yield. Because of their effects on development of sympodial branches and reproductive organs, the number of monopodial branches is amongst the important traits of cotton plants (Sahito et al., 2015). The number of monopodial branches of Varamin cultivar highly reduced in response to water stress, but this trait changes in Khorsid was not considerable. This response is in accordance with mentioned growth habit of them.

Discussion
Node number in uences by cultivar potential in exploitation of resources and growth of internodes. Length of internodes mostly affected by availability of soil water (Arab Salmani and Baniani, 2015), while node number usually affected by nitrogen availability (khan et al., 2019). Because of light induced effects on growth, dense canopy of cultivar can be effective in main stem height, node number and nally, height to node ratio. Thus, because of potential for dense planting of Khorshid cultivar, its early and dense canopy could exploit inputs, especially water, e ciently and can better response in stress conditions as well as showed in this study.
Height to node ratio is a mean for monitoring cotton plant growth and development rate which must be determined for different regions (Stewart et al., 2010). In this study, combination of plant height and node number were such that the height to node ratio ranged between 3.3 to 4.4 and the highest value observed in Varamin cultivar. Kerby et al., (1998) evaluated the information of 104 eld (1982)(1983)(1984)(1985)(1986)(1987)(1988)(1989)(1990)(1991) and reported that the appropriate level of the ratio is 4.54. Naderi are and Hamidi (2013) reported similar results at the conditions of Garmsar in central Iran, too which is similar to our result in normal condition. Reduction of soil water leads to shorter internodes and reduction of height to node ratio. In Oklahoma, ratios of less than 1.5 observed in cotton planted after wheat (bowman et al., 2013). In condition of this study, the ratio was higher even at stress treatments. At the early of season, temperature is not high but, because of higher relative humidity, atmospheric demand is low and this condition helps plant to have more vegetative growth, more nodes and internode formation. This Drought promoted carbon allocation in older bolls (Zhao et al., 2019). In Khorshid cultivar owering points (specially, rst and second) are near to main stem and may directly related to main stem vascular bundle and receive water, minerals and assimilates directly, from stem and subtending leaves, so that, in condition of sever water shortage, produced 1955 kg.ha -1 seed cotton. Chen et al., (2021) innovated a boll-leaf system (BLS) study, which includes the mainstem leaf, sympodial leaf, and non-leaf organs, as the basic unit of the cotton source-sink relationship and yield formation. Based on their research, there was a better linear correlation between the net CO 2 assimilation rate, respiration rate of BLSs and boll biomass. This correlation can be a cue of higher boll retention ability of Khorshid which its short branches leaves shedding is lower in stress condition than more vegetative cultivars with higher leaf damage.
Furthermore, Pilon et al., (2019) reported that soluble carbohydrates and starch concentration in leaves were more affected by drought than those of oral tissues, with corresponding reduction in dry matter, suggesting that owers are more buffered from water-de cit conditions than the adjacent leaves. So, it seems that, cultivars like Khorshid which produces more reproductive organs relative to total leaf per plant, performs better in severe stress condition.

Conclusions
This study highlights the effects of water de cit stress on vegetative and reproductive aspects of cotton plant. It also highlights the differences of cotton cultivars in response to drought stress. So that, zero type cultivar of Khorshid responses was more stable than the others and produced the most seed cotton yield in severe stress condition. In normal condition, all cultivars produced high yield, but Khorshid was superior to Varamin and Khordad. Superiority of Khorshid was due to possibility of dense planting and higher number of boll per plant and so, unit area. In mild stress Varamin cultivar with more and longer sympodial branches, produced more boll and had the highest seed cotton yield. Yield highly correlated with number and weight of boll. Also, yield correlation with plant height, node number and number of sympodial branches was signi cant. Thus, input management must be so that the potential of plant for producing of this trait exploited highly, and the most amount of resources could be allocated to bolls.

Methods
The experiment conducted in led located at the Garmsar agricultural research station of Semnan province in central Iran. The experimental design was split plot in RCBD with three replications. Water stress treatments arranged in main plots and three cotton commercial genotypes including Varamin, Khorshid and Khordad planted as subplots. Planting date in 2017 season was May 10 and in 2018 was May 13. For land preparation, the eld ploughed in autumn of past years. Conventional operations including disk and land leveling done before planting. Starter fertilizers including nitrogen, phosphorus and potassium broadcasted and disked then. For pre-emergence control of weeds Tri uralin 48% EC consumed and in season control of weeds done by hand.
Each sub plot included of 5 row with 8 meter long and plant spaced on rows by 10 centimeters. Row spacing was 75 centimeter (thinned in 2 true leaf stage to 8 plant per m 2 ). Seeds of three commercial cultivars (Varamin and Khordad) planted by hand in depth of 5 centimeters. Khorshid is a zero type cultivar so planted densely in 35 cm row space and 15 cm of plants on row (19 plant per m 2 ). Pest and disease monitored and controlled by recommended chemicals.
Three water stress treatments included of: S1 as control (irrigation after soil humidity of root zone reaches to 85% Fc), S2 as mild stress (irrigation after depletion of root zone humidity to 45% Fc) and S3 as severe stress (irrigation after depletion of root zone humidity to 25% Fc). Evaporation of pan used for determining time of soil samplings. in each time of samplings, after about 50, 100 and 200 mm evaporation from pan, soil samples taken and if the water level was at treatment area, irrigation accomplished and if was not in determined levels, sampled soil mass returned back to sampling hole and the hole covered by 10 centimeter of top soil. Soil water percent determined after oven drying of saturated soil according to Razkeh and Timourlou (2015) in which soil water percent calculated by following formula: Where Ww and W b are wet and dry sample weight, respectively. For stress treatments combined of plant wilting and soil water percentage of Fc were used according to table 4. So that, control plots watered before appearance of wilting symptoms and reducing soil water to less than 85% of eld capacity. Mild stress plots irrigated before slight wilt symptoms and depletion of soil water to 45% of Fc; sever stress plots irrigated before severe wilt symptoms and depletion of soil water to 25% of Fc.    Means, in each column and for each factor, followed by at least one letter in common are not signi cantly different at the 5% of probability level, using Duncan's multiple range test.