Growing season characterization:
Climate in Bandarban is characterized by humid sub-tropical with distinct wet and dry seasons. Usually the wet season starts in the month of July that continues up to September. Compared with rainy season is referred to as atmospheric drought. After the termination of rainy season, however, post-monsoon moist period during October-November prevails. In the valleys residual soil moisture can usually support crop growth during this period. From December onward a prolonged dry period prevails when evaporative demand supersedes natural precipitation.
Prevailing weather during growing seasons:
Temperatures varied a little between the two growing seasons; intra-seasonal variation in temperatures was, however, substantial (Fig. 1). The difference between the highest and the lowest temperatures was the minimum at the beginning of the growing season. But as the season advanced, air temperatures started dropping; the diurnal variation in temperatures began widening. In both the years, diurnal variation in temperatures was the highest during mid-January through mid-February.
In 2018-19 season, night temperatures dropped to 9oC on several occasions; but in the subsequent year night temperatures remained higher than 11oC throughout the season. In contrast, day temperatures surged higher past March 15 in 2018-19 season, occasionally exceeding 38oC although the average temperatures remained relatively higher in2019-20 season. Optimal temperature for the growth and development of cotton is 25oC with lower and upper threshold temperatures ranging between 12oC and 32oC (Reddy et al. 1999; Bange and Milroy 2004). However, the range of temperatures required for the growth and development of cotton plants varies depending on phenological events. For instance, optimal temperature for germination and seedling development ranges between 28 and 30oC while base temperature is about 12oC for seed germination and 15.5oC for seedling growth (Azhar et al. 2020). Conversely, high temperature stress during reproductive and post-reproductive phases hastens maturity shortening the length of growing season (Wheeler et al. 2000) and reducing yields (Pettigrew 2008; Reddy et al. 2017) of annual crops. Schlenkera and Roberts (2009) determined the critical threshold temperature for cotton as 32oC, raising temperature further causes yield decline. Earlier, Reddy et al. (1992) suggested 30/22oC as optimal temperature optimum for stem elongation, leaf area expansion, and biomass accumulation. Temperature regime prevailing in 2018-19 season was thus sub-optimal during early growth stages while it was supra-optimal past February in both the seasons.
Trend of monthly rainfall during the growing seasons are presented in Fig. 2. Rainfalls during October through May were 757 mm and 579 mm, respectively in 2018-19 and 2019-20 seasons. Rainfall pattern remained similar in both the growing seasons. There was scanty rainfall in October followed by a long dry spell until April. In 2019-20, however, the amount of rainfall in May far exceeded the amount received in the earlier season. From winter rainfall pattern it becomes apparent that irrigation is necessary for growing crops during dry season. Frequency of incidence and amount of rainfall could be an added advantage in reducing irrigation requirements provided the incidence is in vegetative or reproductive stage. Rainfall beyond April synchronized with boll opening could result in rain damage of cotton.
Seasonal variation in plant growth characters:
Data on the seasonal variation in growth characteristics of cotton plants are given in Table 1. There was a great deal of differences in plant height between two growing seasons. Plants were generally taller in 2018-19 than in succeeding season. In 2018-19, plant height ranged between 61.3 cm and 116.62 cm giving an average of 84.4 cm. However, a high value of SD (± 22.06) indicates a high degree of variability in plant height within the population. In the subsequent season, plant height varied between 51 cm and 75 cm. An average plant of 2019-20 growing season was 26% shorter than that of an average plant of 2018-19 season. In 2018-19, seeds were planted more than two weeks earlier that helped early germination and seedling establishment prior to experiencing suboptimal temperatures. But in 2019-20 season, seeds were planted on December 1 and such that took longer time to germinate in cooler temperatures. Late planting of seeds in 2019-20 season synchronizing the onset of winter might have delayed germination and poor early growth (Patterson and Flint 1979; Hake et al. 1990; Oosterhuis and Jernstedt 1999; Reddy et al. 2005).The seedlings of both the growing seasons, however, encountered cool temperatures; but seedlings of November 15 planted cotton of 2018-19 season could have been established prior to encountering sub-optimal temperatures (Bange and Milroy 2004). Earlier studies have shown that air temperatures < 16 °C essentially halt cotton seedling growth (Esparza et al. 2007). In both the seasons, cotton plants suffered from sub-optimal temperatures during early vegetative stages.
Table 1
Seasonal variations in plant characters of cotton grown in 2018-19 and 2019-20 seasons
Growing season
|
Plant characters
|
Min
|
Max
|
Average
|
SD (±)
|
2018-19
(15 Nov sowing)
|
Plant ht, cm
|
61.3
|
111.62
|
84.4
|
22.06
|
Nodes on main stem (number)
|
23.12
|
33.06
|
25.07
|
3.81
|
Number of branches per plant
|
9.8
|
17.5
|
11.3
|
2.04
|
Node of first sympodial branch
|
6
|
9.38
|
7.3
|
1.06
|
2019-20
(01 Dec sowing)
|
Plant ht, cm
|
51
|
75
|
62.8
|
7.02
|
Nodes on main stem (number)
|
20
|
25
|
22.6
|
1.71
|
Number of branches per plant
|
7
|
11
|
8.4
|
1.17
|
Node of first sympodial branch
|
7
|
10
|
8.8
|
1.03
|
The two highly correlated and important criteria (Ray and Richmond 1966) for determining the length of growth duration- the length of the pre-fruiting period and the nodal position of the first sympodium are influenced by temperature (Reddy et al. 1991. Reddy et al. (2017) demonstrated that regardless of cultivar differences, node numbers and root tips increased linearly with increase in temperature. Length and number of internodes determine the final height of the plant. The number of nodes on main stem varied between 23.2 and 33.06 making an average of 25.07 (± 3.81) in 2018-19 season. The corresponding numbers of nodes in the following season were 20 and 25 respectively with an average of 22.6 (± 1.71). Internode lengths were short in both the seasons (3.82 cm and 2.91 cm, respectively) and there was no sign of rank growth. Rather, internode length was too short in the second season making up the plants of short stature. In 2019-20, the number of nodes on the main stem was less variable than in 2018-19.
Each node on the main stem of cotton plant is capable of bearing a branch; but all branches do not bear fruits. Normally the branches developed on the lower 4–7 nodes on the main stem do not bear bolls (fruits) and are called monopodial branches. Fruiting branches are called sympodial. The number of branches per plant varied between 9.8 and 17.5 with an average of 11.3 (± 2.04) branches in2018-19 season. The number of branches was unusually low compared with the number of nodes developed on the main stem. Fewer branches were produced in the second year which ranged between 7 and 11 with a mean of 8.4 (± 1.17). The first sympodial branch was on node 7.3 in 2018-19 and on node 8.8 in the subsequent season. Development of the first sympodial branch on node 8.4 was rather unusual suggesting a prolonged vegetative growth prior to shifting to reproductive growth. This is also indicative of the plants undergoing stress environment prior to switching to reproductive stage. Vegetative growth has bearing on the initiation and the rate of formation of sympodial branches (Mauney 1986), and again the rate of vegetative growth prior to onset of flowering is temperature-dependent (Hesketh et al. 1972). Low temperatures for a longer period during seedling and vegetative stages or prior to squiring in both the seasons could have increased the number of nodes and branches (El-Shahawy et al. 1994). But in the present study, such a prolonged vegetative phase encountering cool temperatures might have slowed the rate of node and branch formation. Our observations compare favorably with that of Khan (2003) who reported that plants growing in cooler temperatures produce a greater number of nodes to first sympodia than those growing in warmer temperatures.
Seed cotton yield and yield components:
Table 2 summarizes data on seed cotton yield and yield components as influenced by growing season. The number of plants per unit area, harvestable open bolls per plant and individual open boll weight make up the seed cotton yield per unit area. In the present study, we harvested open bolls from 15 m2 pre-demarcated sample area of each unit plot to record seed cotton yield. Separately sampled 15 plants were harvested from each unit plot cutting at the base and yield attributes determined. As expected, the number of bolls, boll weight and seed cotton yield differed appreciably between the growing seasons. In 2018-19, the number of open bolls per plant ranged between 13.07 and 35.7 with an average of 20.39. A substantial percentage (25%) of bolls did not open and could not be harvested. Cotton was harvested for a single picking in 2019-20 growing season and the crop damage due to rainstorm following cyclone Amphan did not allow second picking. The number of bolls per plant decreased appreciably in 2019-20 season recording 9 to 13 harvestable bolls per plant. More than 31% bolls remained immature and not harvested. Regardless of growing seasons, the number of bolls was appreciably low. The crop encountered low temperature stress following planting that endured throughout seedling and vegetative stages while high temperature stress during reproductive stage enhanced maturity reducing boll development phase. In both the seasons, there was long spell of low temperatures with night temperatures very often lower than 15oC (Fig. 1). Cotton plants exposed to temperatures ≤ 20oC at any phenological stage have a slowing down effect on the growth (Burke et al. 1988; Reddy et al., 1991. Since cotton seeds were planted 15 November (2018-19 season) or 1 December (2019-20 season), cotton plants had to undergo low temperature stress with occasional ‘cold shock’ (Bange and Milroy 2004) during seedling stage. Again, as day temperatures exceeded 32oC past mid-February (Fig. 1), plants encountered high temperature stress during reproductive and boll development stages.
Table 2
Seasonal variation in yield and yield attributes of cotton
Growing season
|
Yield attributes
|
Minimum
|
Maximum
|
Average
|
SD (±)
|
2018-19
(15 Nov sowing)
|
No. open bolls per plant
|
13.07
|
35.7
|
20.39
|
3.44
|
No. bolls not opened
|
2.1
|
8.7
|
6.8
|
2.83
|
% open boll
|
58.19
|
80.66
|
74.99
|
4.17
|
Open boll wt per plant
|
61.08
|
94.41
|
91.55
|
10.03
|
Boll weight (g boll− 1)
|
3.22
|
6.03
|
4.63
|
0.71
|
Seed cotton yield (kg ha− 1)
|
961
|
2,743
|
2,047
|
127
|
2019-20
(01 Dec sowing)
|
No. open bolls per plant
|
9
|
13
|
10.6
|
1.17
|
No. bolls not opened
|
4
|
7
|
4.8
|
1.03
|
% open boll
|
65
|
69.23
|
68.83
|
53.2
|
Open boll wt per plant
|
28
|
39
|
31.9
|
3.67
|
Boll weight (g boll− 1)
|
2.69
|
3.4
|
3.01
|
0.26
|
Seed cotton yield (kg ha− 1)
|
738
|
2,106
|
1,186
|
114.4
|
A high degree of heterogeneity in seed cotton yield was observed (Table 2) in both the growing seasons. In 2018-19, average yield was 2,047 (± 127) kg ha− 1, which was reduced to 1186 (± 114) kg ha− 1 in 2019-20. The yields were rather low in both the growing seasons. The low yield was associated with fewer numbers of bolls per plant and low boll weight. Low temperatures at the early growth stages might have reduced boll formation while high temperatures reduced the size and number of bolls retained. Prolonged supra-optimal temperatures (> 35oC) past vegetative phase might have caused shedding of squares and bolls (Reddy et al., 1992). Costa (2015) indicated that 38/30oC (day/night) temperature conditions presented a reduced boll size and a substantial decline in non-structural carbohydrate content.
Temperatures influence cotton growth and development and growing season is regulated by the onset and termination of the growing season depends on temperature regime. Bolls per plant and boll weight are the two yield components playing major role in the variation in seed cotton yield. In our study, seed cotton yield varied widely between the seasons. Variation in seed cotton yield was associated with the seasonal variation in both the components. Differences in yield attributes might be attributed to the differences in temperature regimes prevailing during the growing seasons. Apart from the differences in planting dates between the two growing seasons, adverse effects of supra- and sub-optimal temperatures, rain damage of open bolls also reduced seed cotton yield in 2019-20 season.