*2.2. Effects on Early Vegetative Growth Stages*

In addition to germination, uniform and rapid emergence of seedlings from soil is necessary to achieve vigorous growth of plants and high yield [22]. High temperature stress during the early stages of plant growth affects the emergence of seedlings and results in the development of poor seedlings. In a previous experiment, researchers grew seedlings of cotton under various temperature regimes, i.e., 38 ◦C and 32 ◦C for 8 days. They recorded 152 mm shoot and 173 mm root growth at 32 ◦C temperature, while at 38 ◦C only 50 mm shoot and 86 mm root growths were observed [17]. The emergences and growths of cotton seedlings were also assessed for four different temperatures, i.e., 20, 30, 40, and 50 ◦C. The maximum emergence of seedling with vigorous growth was observed at 30 ◦C. The genotypes developed for hot tropical environments with higher seed weight showed vigorous seedlings even at 40 ◦C than compared to genotypes with smaller seed size and weight. Cotton seedlings did not emerge at 50 ◦C [23].

Along with the above ground parts of plants, roots are also severely affected by high temperature stress. The damaging of roots minimizes their uptake of nutrients and water from soil that can disturb the entire physiological mechanism of the plant and limit its productivity. The cotton plant has a taproot system. Studies showed that cotton roots develop poorly under high soil temperature and result in a poor crop stand [24]. A study was conducted on 10 upland cotton genotypes in order to determine the heat stress ability of roots by measuring related parameters. After 120 min of seed germinations, the seedlings were subjected to seven different temperature regimes between 25 to 45 ◦C. The results revealed that all the genotypes showed normal root growth up to 35 ◦C. The roots' growth declined when temperature exceeded from 35 ◦C, and irreversible damage to roots of all the cultivars occurred at 45 ◦C [25].
