Plant height is of the most important traits that, usually affected by input and genetic structure of parental line of cultivar (Zabihi et al., 2013). Plant height of Gossypium. hirsutum and G. barbadense can be reach to more than 1.5 and 2.5 meter, respectively (Naderi Arefi et al., 2014), but in commercial variety of both species in the region plant height ranged 1-1.5 meter.
Five first node of cotton main stem supports vegetative growth and reproductive or sympodial branches commences the growth later (Ritchie et al., 2007). 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.
Node number influences 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 finally, height to node ratio. Thus, because of potential for dense planting of Khorshid cultivar, its early and dense canopy could exploit inputs, especially water, efficiently 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 field (1982-1991) and reported that the appropriate level of the ratio is 4.54. Naderi arefi 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 growth restricted highly in mead season because of sharp increment of temperature to more than 42 °C as demonstrated by reduction of all vaegetative traits in stress treatments (table 2).
The most important component of yield is boll number per unit area which it’s high correlation with yield confirmed in various studies (Smith and Hamel, 2012; Naderi Arefi and Hamidi, 2013). Also, Mehrabadi (2014) reported high correlation between seed cotton yield, boll number, biomass and harvest index in stress condition. Low humidity is one of the most important factors affecting flower and boll production and retention of these organs on cotton (Sawan et al., 2018). Management of environmental and physiological factors for build up more boll number in unit area has high effect on yield raise (Alishah and Ahmadi Khah, 2009). Most of cotton production systems, depends on first and second fruiting point for economic yield (Hake et al., 1996). So high potential of soil and leaf water is critical for development of bolls.
At sever stress condition (25% of Fc) Khorshid and Varamin were superior to Khordad in view of boll number per plant. Khorshid is a zero type cultivar with limited vegetative growth and so, lower transpiration and probably, efficient use of water. Superiority of Varamin cv. Related to its higher number of reproductive organs and it’s ability of producing more boll per plant.
Mild stress reduced boll weight of all cultivars, differently, but because of shedding of some flowers and squares, weight of remaining bolls did not reduced sharply (Papastylianou and Argyrokastritis, 2014) similar to sever stress. Also, retention of more bolls on first flowering points in stress conditions can increase mean boll weight (Petigrew, 2004). Cultivars responded evenly to severe stress (25% Fc) in view of boll weight. Similar results reported by Basal et al., (2009), Fathi Sadabadi and Navabi (2012), Najib-ullah et al., (2017). One consequence of water stress is shedding of vegetative organs. It seems that severity of stress in 25% Fc caused to attenuate most of tolerance mechanisms and shedding of vegetative organs for plant survival. Confirming this result, Ul-Allah et al., (2021) highlighted the negative effects of drought stress on assimilate accumulation and portioning in reproductive tissues of cotton which finally converts into the fiber. 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 sufficient loading of each boll and correlated positively. Despite our results, other researches indicated negative correlation (Balkcom et al., 2010; Liu et al., 2013).
Drought promoted carbon allocation in older bolls (Zhao et al., 2019). In Khorshid cultivar flowering points (specially, first 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 main-stem 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 CO2 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 floral tissues, with corresponding reduction in dry matter, suggesting that flowers are more buffered from water-deficit 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.