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Cotton lint yield response to accumulated heat units and soil water supply
Field Crops Research, 19 (1989) 253-262 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
253
Cotton Lint Yield R e s p o n s e to A c c u m u l a t e d Heat U n i t s and Soil Water S u p p l y S. PENG, D.R. KRIEG and S.K. HICKS
Plant and Soil Science Department, Texas Tech University, Lubbock, TX 79409 (U.S.A.) (Accepted 16 May 1988)
ABSTRACT Peng, S., Krieg, D.R. and Hicks, S.K., 1989. Cotton lint yield response to accumulated heat units and soil water supply. Field Crops Res., 19: 253-262. Crop growth and development are directly associated with the thermal regime (heat-unit concept), when other environmental factors are not limiting. The growing-season length on the Texas High Plains is less than optimal for cotton production, based upon total available heat units. In addition, water stress, due to inadequate rainfall, coupled with the high evaporative demand, represents the major environmental constraint to cotton yield. A field experiment was conducted to evaluate the relationship between cotton lint yield and available heat units in this climate. When water supply was not the major limitation to productivity, lint yield was highly correlated with the available heat units during the growing season (r2= 0.90). When water stress represented the major growth constraint (seasonal water supply less than 400 mm), no significant relationship existed between lint yield and seasonal heat-unit accumulation. Evaluation of the response of specific growth stages revealed that the accumulation of heat units during the early fruiting period explained as much of the yield variability as did the seasonal heat-unit accumulation.
INTRODUCTION
The southern High Plains of Texas is one of the largest contiguous cotton production areas in the United States. Over 1.5 million ha of cotton are planted annually within a 200-km radius of Lubbock, Texas. Production in this area is limited by three major environmental constraints. Lack of adequate rainfall to meet the evaporative demand throughout the growing season is the first major limitation to production. Supplemental irrigation is practiced on about 50% of the cotton production area, although the applied water supply is usually less than required to match the potential evapotranspiration, therefore water stress is still prevalent during the growing season. Growing-season length, measured in terms of available thermal energy rather than in number of days, is the second major yield-limiting environmental factor for the area, and becomes 0378-4290/89/$03.50
© 1989 Elsevier Science Publishers B.V.
254 the primary limitation to yield when ample rainfall occurs or irrigation water is provided. Due to the relatively high elevation ( > 1000 m) of the area, cool nights are prevalent, especially during seedling establishment (May) and again during fruit maturation (September and October). The third major environmental constraint is the inherent supply of nitrogen and phosphorus in the soil, due largely to a cotton monoculture system being practiced and very little organic matter being returned to the soil system. Both water and thermal energy are wasted when the nutrient supply represents the primary constraint to growth and yield. The heat-unit concept, which is a measure of the available thermal energy above some threshold level, has been used to describe development of a large number of plant species. Numerous experiments have been conducted with cotton which have resulted in growth-and-development models using a base temperature of 15°C (Krieg, 1985; Kerby et al., 1985; Wanjura et al., 1985). However, the correlation between lint yield and total seasonal heat-unit accumulation is quite variable across many of the cotton-production areas of the southern United States. Malm and Kerby (1981) reported that only 37% of the year-to-year variability in lint yield in New Mexico could be accounted for by seasonal heat-unit accumulation from 1 April to 31 October. Mitchell and Bourland (1981), however, reported a very high correlation (r 2 = 0.90) between lint yield and seasonal heat-unit accumulation in Mississippi. Kerby (1986) indicated that lint yield per boll was highly correlated with heat-unit accumulation during the last half of the development period (beginning at flowering). The purposes of this report are: (1) to evaluate the correlation between cotton lint yield and seasonal heat-unit accumulation under both irrigated and rainfed conditions on the southern High Plains of Texas; (2) to determine the responsiveness of various developmental periods to heat-unit availability in the life of the cotton plant; and (3) to evaluate genetically controlled growth habit (degrees of indeterminancy) influences on the relationship between cotton lint yield and available heat-units. MATERIALSAND METHODS The field study was conducted at the Texas Tech Agricultural Research Center, north central Lubbock County, Texas, during the 1980 and 1981 growing seasons. The soil type was a Pullman clay loam (fine mixed thermic Torrertic Paleustalfs) approximately 1 m deep underlain by a calcic horizon. A fertilizer mixture consisting of 50 kg N and 20 kg P per ha was band-applied preplant. The experimental design was a split-split plot where water supply represented the main plots. All plots received a preplant irrigation to fill the soil profile in late April and were subsequently divided into irrigated and rainfed plots. The supplemental water-supply treatment was based upon replacement
255
of 100-mm evapotranspiration. The total seasonal rainfall (May-15 October) was 200 m m in 1980 and 400 m m in 1981 (Fig. 1). Supplemental irrigation during the growing season was 200 m m each year applied in two applications (Table 1 ). Sub-plots were planting dates arranged in a randomized, completeblock design with three replications. Mid-May, early June, and mid-June planting dates were used both years. Different planting dates were utilized to provide both time and heat-unit accumulation differences as well as a range of temperature regimes at various growth stages. The last split was cultivars. The determinate cultivar, Paymaster Dwarf, the moderately determinate cv. Tam35 j [ ] [] 3o [ ]
1
UJ n-
et'
LONG-TERMAVE. lg8o 1~1
25
UJ
a.
UJ I--
20
>. .J
15
¢3
10 MAY
J U N E JULY
AUG.
SEP.
OCT.
175 150 E E Z
[] [] []
b
LONG-TERMAVE. lgS0 1gel
125 100
a. I.g n>. .J
.T-
75 50 25
0 MAY
JUNE JULY
AUG.
SEP.
OCT
Fig. 1. Mean daily temperature for each month (a) and total monthly precipitation (b) in 1980 and 1981 compared to the long-term averages for Lubbock, Texas.
256 TABLE 1 Treatment characterization for each year of the experiment Variable Planting date Mid-May Early June Mid-June Growing-season length* (days and heat units) Mid-May Early June Mid-June Irrigation applied** Preplant First Second
1980
1981
May 20 June 2 June 16
May 21 June 9 June 18
149 136 122
1531 1429 1280
April 15 July 2 July 22
148 129 120
1239 1104 1009
April 20 June 29 July 24
*Growing-season length was defined from planting to Oct. 15. **Allplots received irrigation at the same time (100 mm per application). cot 788, and the indeterminate cv. Coker 5110 provided a wide range of potential growth-habit responses to the prevailing environment. Individual plots were 4 rows wide (1 m between rows) and 10 m long. Hand-harvest of 5-m 2 plot area was conducted after first frost. Plant density, fruit number, and total fruit weight were determined. Lint and seed yield and the components of yield were expressed on a ground-area basis. Daily weather data were collected by an automated weather station located at the experimental site. Daily heat-units were calculated from the average of maximum and minimum temperatures minus 15 ° C, and were accumulated from planting until October 15 when they approached 0. Statistical analyses of the data included least significant difference test and regression. RESULTS AND DISCUSSION The temperature and rainfall data during the growing season for each year as compared with long-term mean data for the Lubbock area are depicted in Figs. la and lb. In 1980, the mean daily temperatures from June through September were higher than the long-term average. The mean daily temperatures for 1981 were comparable to the long-term average. Rainfall in 1980 was below the long-term average in June, July and August and above normal in May and September. July was extremely hot and dry. The 1981 growing season was characterized by below-normal rainfall in May, June and September, but abovenormal rainfall in July, August and October. Lint yield (LY) and components of yield as affected by planting date and
257
water supplies are listed in Table 2. Lint yield was significantly greater in 1980 than in 1981. Irrigated yields were 20% greater than rainfed yields, with the impact of supplemental irrigation more pronounced in 1980 than in 1981. In 1980, planting date significantly affected lint yield of the irrigated plots but not the rainfed plots. Regression of yield on yield components across all treatments reveals that 78% of the yield variability was explained by the fruit number (FN), and only 19% by the lint weight per fruit (LWF) [ regression equation: L¥ = -- 372.4 + 1.228 FN -k 308.2 LWF ]. Total accumulated heat-units were summed from each planting date to 15 October within each year. Figure 2a reveals that lint yields of the irrigated plots across years, planting dates, and genotypes were linearly correlated with total heat-units (r e = 0.90). The linear regression equation indicated that approximately 830 (i.e. intercept on the X axis) heat-units were required to support any lint yield with an increase of 1.12 kg/ha per heat-unit thereafter. Since the different planting dates provided not only different total heatunits but also different lengths of time for development, lint yield response to planting date must also be defined as a function of growing-season length. We defined the growing-season length as the number of days after planting (DAP) to 15 October when daily heat-unit accumulation became, essentially, 0. Cotton lint yield was positively correlated with growing-season length in days (r 2= 0.40 ), and the correlation coefficient between accumulated heat-units and growing-season length was 0.44. Since growing-season length measured in both chronological time and metabolic time were correlated, it is possible to elimiTABLE 2 Yield and yield components as affected by planting date and water supply treatment within each year of the experiment Year
1980
1981
Planting date
Lintyield (kg/ha)
Fruit number (×103/ha)
Lint weight perfruit (g)
Irrigated
Rainfed
Irrigated
Rainfed
Irrigated
Rainfed
Mid-May Early June Mid-June
747 a* 666 ab 557 c
444 b 456 b 449 b
498 a 444 ab 378 bc
292 b 332 b 278 b
1.48 a 1.49 a 1.51 a
1.53 ab 1.39 a 1.61 a
Mean
657
450
440
301
1.49
1.51
Mid-May Early June Mid-June
508 cd 269 e 179 f
593 a 319 c 129 d
491 a 313 c 229 d
490 a 284 b 149 c
1.05 b 0.91 b 0.81 b
1.16 bc 1.27 abc 0.89 c
Mean
319
347
344
308
0.92
1.11
*Within columns, means followed by the same letter are not significantly different at the 5% level according to least significant difference test.
258 1000 CI 80O
y = -929.9 + 1.12X
r2_-0.90
800
400
.~
200 4-
q "
IRRIGATED
0
80O
600 1980
÷
400'
200 RAINFED 0 ,. 1200 . 1000 . . . 1100 . ' ' 1300 14 'o0 15 ; ,0 1600
TOTAL HEAT UNITS (C)
Fig. 2. Relationship between cotton lint yield and total heat-units during the growing season in the (a) irrigated and (b) rainfed conditions. At discrete total heat-unit accumulations, each point represents one of the three genotypes. Each genotype point represents the mean of three replications.
nate the effect of the growing-season length in order to accurately assess the relationship between seasonal accumulated heat-units and lint yield. The partial correlation coefficient expresses the relationship between a dependent and an independent variable with the influence of one or more other independent variables eliminated. The partial correlation coefficient between cotton lint yield and the accumulation of seasonal heat units was 0.88 with the effects of growing-season length in days removed. The partial correlation coefficient decreased by only 2 % compared with the simple correlation coefficient (r 2= 0.90 ), which means growing-season length in days had little effect on cotton production in this study.
259
The total water supply from rainfall was 200 mm in 1980 and 400 mm in 1981. Water supply played a major role in the difference in lint yield between 1980 and 1981. Figure 2b depicts the relationship between lint yield and total heat-units under rainfed conditions in 1980 and 1981 separately. In 1981, the seasonal water supply was not a limitation to lint production and yield was linearly correlated with total heat-units, similar to the response under irrigated conditions. In 1980, however, water supply was the first limitation to yield and no positive relationship with total heat-units existed. A multiple-regression model of LY based on seasonal heat-units (THU) and water supply (W) was developed from data of this 2-year experiment. The regression equation was: LY ------ 5.482.9 + 7.805 (THU) -- 0.00268 (THU) 2 nu 2.068 W - 0 . 0 0 2 1 6 W 2, which accounted for 93% of the variability of lint yield. Total heat-units and water supply were related to lint yield in a curvilinear manner
7{E
A
O _3 w ). jZ _3
60O
4@@
o
2@@
1@@
U~VlT8 (C)
~o@@
Fig. 3. Lint yield as a function of total seasonal h e a t - u n i t s and water supply (including precipitation a n d irrigation ).
260
similar to most production functions (Fig. 3 ). Maximum lint yield was achieved in this experiment from a combination of total seasonal heat-units of 1450 ° C and a water supply of 550 mm. In order to determine whether any specific developmental period was more responsive to available heat-units than other periods under irrigated conditions, lint yields were regressed on heat-units accumulated during each 30-day period during the growing season (Table 3 and Fig. 4). Irrigated lint yield was highly correlated (r 2 =0.92) with heat-units accumulated during the period 61-90 DAB. Irrigated lint yields increased by 3.86 kg/ha for each heat-unit accumulated above 166 during this period. The fruit number was highly correlated with the heat-units accumulated during this period (r2= 0.83 ). This peTABLE 3 Regression analyses of lint yield and total heat units during the different growing periods under irrigated conditions (data from both 1980 and 1981 ) Growing period
Regression equation
r~
(DAP) 1- 30 31-60 61-90 91-120
Lint yield-490.5 - 0.009 Lint yield---1150.8+4.362 Lint yield-- -640.1+3.864 Lintyield= -56.5+2.801
( HU) (HU) (HU) (HU)
n.s. 0.67** 0.92** 0.67**
* and ** indicate significance at the 5% and 1% levels respectively. HU= heat units accumulated during the growing period. lOOO y : -640.1 + 3.86x
r2=0.92
#
8oo
J=
6o0
9 U~ 4o0 -1
2oo +
IRRIGATED
do
I
s2o
3eo
HEAT UNIT8 {$1-~ DAP)
Fig. 4. Relationship between cotton lint yield and heat-units during 61-90 days after planting under irrigated conditions. At discrete heat-unit accumulations, each point represents one of the three genotypes. Each genotype point represents the mean of three replications.
261
riod corresponds with the period of rapid floral-bud production and early fruit development. The heat-units accumulated during this particular period explained as much of the yield variability as did the total seasonal accumulation, reflecting the importance of fruit number to yield. During the period of seedling development (0-30 DAP),heat-unit accumulation had no significant effect on lint yield. Lint yield was positively correlated with heat-units accumulated during the period 31-60 DAP (floral site initiation) and the period 91-120 DAP (fruit maturation ) (r 2= 0.67 for each period). The relationship between lint yield and heat-unit accumulation was also affected by cultivar. The irrigated lint yields of the indeterminate cultivar Coker 5110 and the moderately determinate Tamcot 788 were linearly correlated to total seasonal heat-unit accumulation. However, a curvilinear relationship ex1000
8~
y • -1190,7 ÷ 1.32x
r2~0.@4
y • -1000.4 ÷ 1.10x
r2~O.06
o .~
1
600
400
//I + o •J lU
0
IRRIGATED
e
!
i
i
e
e
b + ";
COKER 5110 TAMCOT 788
• 'I
8oo,
PM DWARF
600,
400.
200 DRYLAND ! !
"
|
1000
|
•
1100
e
1200
•
|
1300
•
!
1400
•
!
1500
1600
TOTAL HEAT UNITS (C)
Fig. 5. Comparison of the relationship between lint yield and seasonal accumulation of heat-units among three cultivars under (a) irrigated and (b) rainfed conditions.
262
isted between irrigated lint yield and total heat-units for the determinate cultivar Paymaster Dwarf (Fig. 5a). The determinate cultivar produced more lint than the indeterminate cultivar when the seasonal heat-unit accumulation was less than 1300. The cultivar response was observed under both irrigated and rainfed conditions. Differences in the lint yield of the cultivars were not significant under hot and dry conditions in 1980, since lint production was determined by water-stress conditions (Fig. 5b). Therefore, the more determinate cultivar performed better under cool environments than the indeterminate cultivar, while the inverse was true under warmer environments. Wang (1960) and Lowry (1969) pointed out that the heat-unit concept is limited in application and reliability because it does not take into account other factors which influence plant growth and development, such as irradiance, water, photoperiod, etc. Our data indicate that cotton lint yield is highly correlated with the accumulation of heat-units when water-stress is minimized; however, no significant relationship exists between lint yield and the accumulation of heat-units when water-stress is the major limitation. More importantly, the significance of the early reproductive development period to total lint yield was elucidated. This period is most critical in determining the total number of fruiting sites produced and in largely determining lint yield due to high dependence of yield on fruit number. Heat-units can be used to accurately predict cotton growth, development, and lint yield when water supply is not a major limitation to productivity. Both water supply and growth habit will modify the relationship between cotton lint yield and available heat-units. REFERENCES Kerby, T.A., 1986. Boll growth and development. Cotton Comments, vol. 21. Cooperative Extension, Univ. of California, Berkeley, pp. 1-4. Kerby, T.A., Wilson, L.T. and Johnson, S., 1985. Upper threshold required for heat unit calculations for cotton growth in the far west. In: Proc. Beltwide Cotton Producers Conf. National Cotton Council, Memphis, TN, pp. 366-368. Krieg, D.R., 1985. Developmental and physiological responses of short-season cotton to temperature. In: Proc. Beltwide Cotton Producers' Conf. National Cotton Council, Memphis, TN, p. 366. Lowry, W.P., 1969. Weather and Life. Academic, New York, pp. 193-197. Malm, N.R. and Kerby, T.A., 1981. Association between cotton yields and heat units in New Mexico over a 25-year period. In: J.M. Brown (Editor), Proc. Beltwide Cotton Producers' Conf., 4-8 January 1981, New Orleans. National Cotton Council, Memphis, Tenn., pp. 55-57. Mitchell, J.F. and Bourland, F.M., 1981. Heat unit accumulation and phenological development of four cotton cultivars in Mississippi. In: Proc. Beltwide Cotton Producers' Conf. National Cotton Council, Memphis, TN, p. 113. Wang, J.Y., 1960. A critique of the heat unit approach to plant response studies. Ecology, 41: 785790. Wanjura, D.F. and Supak, J.R., 1985. Temperature methods for monitoring cotton development. In: J.M. Brown (Editor), Proc. Beltwide Cotton Producers' Conf., 4-8 January 1981, New Orleans. National Cotton Council, Memphis, Tenn., pp. 369-370.
253
Cotton Lint Yield R e s p o n s e to A c c u m u l a t e d Heat U n i t s and Soil Water S u p p l y S. PENG, D.R. KRIEG and S.K. HICKS
Plant and Soil Science Department, Texas Tech University, Lubbock, TX 79409 (U.S.A.) (Accepted 16 May 1988)
ABSTRACT Peng, S., Krieg, D.R. and Hicks, S.K., 1989. Cotton lint yield response to accumulated heat units and soil water supply. Field Crops Res., 19: 253-262. Crop growth and development are directly associated with the thermal regime (heat-unit concept), when other environmental factors are not limiting. The growing-season length on the Texas High Plains is less than optimal for cotton production, based upon total available heat units. In addition, water stress, due to inadequate rainfall, coupled with the high evaporative demand, represents the major environmental constraint to cotton yield. A field experiment was conducted to evaluate the relationship between cotton lint yield and available heat units in this climate. When water supply was not the major limitation to productivity, lint yield was highly correlated with the available heat units during the growing season (r2= 0.90). When water stress represented the major growth constraint (seasonal water supply less than 400 mm), no significant relationship existed between lint yield and seasonal heat-unit accumulation. Evaluation of the response of specific growth stages revealed that the accumulation of heat units during the early fruiting period explained as much of the yield variability as did the seasonal heat-unit accumulation.
INTRODUCTION
The southern High Plains of Texas is one of the largest contiguous cotton production areas in the United States. Over 1.5 million ha of cotton are planted annually within a 200-km radius of Lubbock, Texas. Production in this area is limited by three major environmental constraints. Lack of adequate rainfall to meet the evaporative demand throughout the growing season is the first major limitation to production. Supplemental irrigation is practiced on about 50% of the cotton production area, although the applied water supply is usually less than required to match the potential evapotranspiration, therefore water stress is still prevalent during the growing season. Growing-season length, measured in terms of available thermal energy rather than in number of days, is the second major yield-limiting environmental factor for the area, and becomes 0378-4290/89/$03.50
© 1989 Elsevier Science Publishers B.V.
254 the primary limitation to yield when ample rainfall occurs or irrigation water is provided. Due to the relatively high elevation ( > 1000 m) of the area, cool nights are prevalent, especially during seedling establishment (May) and again during fruit maturation (September and October). The third major environmental constraint is the inherent supply of nitrogen and phosphorus in the soil, due largely to a cotton monoculture system being practiced and very little organic matter being returned to the soil system. Both water and thermal energy are wasted when the nutrient supply represents the primary constraint to growth and yield. The heat-unit concept, which is a measure of the available thermal energy above some threshold level, has been used to describe development of a large number of plant species. Numerous experiments have been conducted with cotton which have resulted in growth-and-development models using a base temperature of 15°C (Krieg, 1985; Kerby et al., 1985; Wanjura et al., 1985). However, the correlation between lint yield and total seasonal heat-unit accumulation is quite variable across many of the cotton-production areas of the southern United States. Malm and Kerby (1981) reported that only 37% of the year-to-year variability in lint yield in New Mexico could be accounted for by seasonal heat-unit accumulation from 1 April to 31 October. Mitchell and Bourland (1981), however, reported a very high correlation (r 2 = 0.90) between lint yield and seasonal heat-unit accumulation in Mississippi. Kerby (1986) indicated that lint yield per boll was highly correlated with heat-unit accumulation during the last half of the development period (beginning at flowering). The purposes of this report are: (1) to evaluate the correlation between cotton lint yield and seasonal heat-unit accumulation under both irrigated and rainfed conditions on the southern High Plains of Texas; (2) to determine the responsiveness of various developmental periods to heat-unit availability in the life of the cotton plant; and (3) to evaluate genetically controlled growth habit (degrees of indeterminancy) influences on the relationship between cotton lint yield and available heat-units. MATERIALSAND METHODS The field study was conducted at the Texas Tech Agricultural Research Center, north central Lubbock County, Texas, during the 1980 and 1981 growing seasons. The soil type was a Pullman clay loam (fine mixed thermic Torrertic Paleustalfs) approximately 1 m deep underlain by a calcic horizon. A fertilizer mixture consisting of 50 kg N and 20 kg P per ha was band-applied preplant. The experimental design was a split-split plot where water supply represented the main plots. All plots received a preplant irrigation to fill the soil profile in late April and were subsequently divided into irrigated and rainfed plots. The supplemental water-supply treatment was based upon replacement
255
of 100-mm evapotranspiration. The total seasonal rainfall (May-15 October) was 200 m m in 1980 and 400 m m in 1981 (Fig. 1). Supplemental irrigation during the growing season was 200 m m each year applied in two applications (Table 1 ). Sub-plots were planting dates arranged in a randomized, completeblock design with three replications. Mid-May, early June, and mid-June planting dates were used both years. Different planting dates were utilized to provide both time and heat-unit accumulation differences as well as a range of temperature regimes at various growth stages. The last split was cultivars. The determinate cultivar, Paymaster Dwarf, the moderately determinate cv. Tam35 j [ ] [] 3o [ ]
1
UJ n-
et'
LONG-TERMAVE. lg8o 1~1
25
UJ
a.
UJ I--
20
>. .J
15
¢3
10 MAY
J U N E JULY
AUG.
SEP.
OCT.
175 150 E E Z
[] [] []
b
LONG-TERMAVE. lgS0 1gel
125 100
a. I.g n>. .J
.T-
75 50 25
0 MAY
JUNE JULY
AUG.
SEP.
OCT
Fig. 1. Mean daily temperature for each month (a) and total monthly precipitation (b) in 1980 and 1981 compared to the long-term averages for Lubbock, Texas.
256 TABLE 1 Treatment characterization for each year of the experiment Variable Planting date Mid-May Early June Mid-June Growing-season length* (days and heat units) Mid-May Early June Mid-June Irrigation applied** Preplant First Second
1980
1981
May 20 June 2 June 16
May 21 June 9 June 18
149 136 122
1531 1429 1280
April 15 July 2 July 22
148 129 120
1239 1104 1009
April 20 June 29 July 24
*Growing-season length was defined from planting to Oct. 15. **Allplots received irrigation at the same time (100 mm per application). cot 788, and the indeterminate cv. Coker 5110 provided a wide range of potential growth-habit responses to the prevailing environment. Individual plots were 4 rows wide (1 m between rows) and 10 m long. Hand-harvest of 5-m 2 plot area was conducted after first frost. Plant density, fruit number, and total fruit weight were determined. Lint and seed yield and the components of yield were expressed on a ground-area basis. Daily weather data were collected by an automated weather station located at the experimental site. Daily heat-units were calculated from the average of maximum and minimum temperatures minus 15 ° C, and were accumulated from planting until October 15 when they approached 0. Statistical analyses of the data included least significant difference test and regression. RESULTS AND DISCUSSION The temperature and rainfall data during the growing season for each year as compared with long-term mean data for the Lubbock area are depicted in Figs. la and lb. In 1980, the mean daily temperatures from June through September were higher than the long-term average. The mean daily temperatures for 1981 were comparable to the long-term average. Rainfall in 1980 was below the long-term average in June, July and August and above normal in May and September. July was extremely hot and dry. The 1981 growing season was characterized by below-normal rainfall in May, June and September, but abovenormal rainfall in July, August and October. Lint yield (LY) and components of yield as affected by planting date and
257
water supplies are listed in Table 2. Lint yield was significantly greater in 1980 than in 1981. Irrigated yields were 20% greater than rainfed yields, with the impact of supplemental irrigation more pronounced in 1980 than in 1981. In 1980, planting date significantly affected lint yield of the irrigated plots but not the rainfed plots. Regression of yield on yield components across all treatments reveals that 78% of the yield variability was explained by the fruit number (FN), and only 19% by the lint weight per fruit (LWF) [ regression equation: L¥ = -- 372.4 + 1.228 FN -k 308.2 LWF ]. Total accumulated heat-units were summed from each planting date to 15 October within each year. Figure 2a reveals that lint yields of the irrigated plots across years, planting dates, and genotypes were linearly correlated with total heat-units (r e = 0.90). The linear regression equation indicated that approximately 830 (i.e. intercept on the X axis) heat-units were required to support any lint yield with an increase of 1.12 kg/ha per heat-unit thereafter. Since the different planting dates provided not only different total heatunits but also different lengths of time for development, lint yield response to planting date must also be defined as a function of growing-season length. We defined the growing-season length as the number of days after planting (DAP) to 15 October when daily heat-unit accumulation became, essentially, 0. Cotton lint yield was positively correlated with growing-season length in days (r 2= 0.40 ), and the correlation coefficient between accumulated heat-units and growing-season length was 0.44. Since growing-season length measured in both chronological time and metabolic time were correlated, it is possible to elimiTABLE 2 Yield and yield components as affected by planting date and water supply treatment within each year of the experiment Year
1980
1981
Planting date
Lintyield (kg/ha)
Fruit number (×103/ha)
Lint weight perfruit (g)
Irrigated
Rainfed
Irrigated
Rainfed
Irrigated
Rainfed
Mid-May Early June Mid-June
747 a* 666 ab 557 c
444 b 456 b 449 b
498 a 444 ab 378 bc
292 b 332 b 278 b
1.48 a 1.49 a 1.51 a
1.53 ab 1.39 a 1.61 a
Mean
657
450
440
301
1.49
1.51
Mid-May Early June Mid-June
508 cd 269 e 179 f
593 a 319 c 129 d
491 a 313 c 229 d
490 a 284 b 149 c
1.05 b 0.91 b 0.81 b
1.16 bc 1.27 abc 0.89 c
Mean
319
347
344
308
0.92
1.11
*Within columns, means followed by the same letter are not significantly different at the 5% level according to least significant difference test.
258 1000 CI 80O
y = -929.9 + 1.12X
r2_-0.90
800
400
.~
200 4-
q "
IRRIGATED
0
80O
600 1980
÷
400'
200 RAINFED 0 ,. 1200 . 1000 . . . 1100 . ' ' 1300 14 'o0 15 ; ,0 1600
TOTAL HEAT UNITS (C)
Fig. 2. Relationship between cotton lint yield and total heat-units during the growing season in the (a) irrigated and (b) rainfed conditions. At discrete total heat-unit accumulations, each point represents one of the three genotypes. Each genotype point represents the mean of three replications.
nate the effect of the growing-season length in order to accurately assess the relationship between seasonal accumulated heat-units and lint yield. The partial correlation coefficient expresses the relationship between a dependent and an independent variable with the influence of one or more other independent variables eliminated. The partial correlation coefficient between cotton lint yield and the accumulation of seasonal heat units was 0.88 with the effects of growing-season length in days removed. The partial correlation coefficient decreased by only 2 % compared with the simple correlation coefficient (r 2= 0.90 ), which means growing-season length in days had little effect on cotton production in this study.
259
The total water supply from rainfall was 200 mm in 1980 and 400 mm in 1981. Water supply played a major role in the difference in lint yield between 1980 and 1981. Figure 2b depicts the relationship between lint yield and total heat-units under rainfed conditions in 1980 and 1981 separately. In 1981, the seasonal water supply was not a limitation to lint production and yield was linearly correlated with total heat-units, similar to the response under irrigated conditions. In 1980, however, water supply was the first limitation to yield and no positive relationship with total heat-units existed. A multiple-regression model of LY based on seasonal heat-units (THU) and water supply (W) was developed from data of this 2-year experiment. The regression equation was: LY ------ 5.482.9 + 7.805 (THU) -- 0.00268 (THU) 2 nu 2.068 W - 0 . 0 0 2 1 6 W 2, which accounted for 93% of the variability of lint yield. Total heat-units and water supply were related to lint yield in a curvilinear manner
7{E
A
O _3 w ). jZ _3
60O
4@@
o
2@@
1@@
U~VlT8 (C)
~o@@
Fig. 3. Lint yield as a function of total seasonal h e a t - u n i t s and water supply (including precipitation a n d irrigation ).
260
similar to most production functions (Fig. 3 ). Maximum lint yield was achieved in this experiment from a combination of total seasonal heat-units of 1450 ° C and a water supply of 550 mm. In order to determine whether any specific developmental period was more responsive to available heat-units than other periods under irrigated conditions, lint yields were regressed on heat-units accumulated during each 30-day period during the growing season (Table 3 and Fig. 4). Irrigated lint yield was highly correlated (r 2 =0.92) with heat-units accumulated during the period 61-90 DAB. Irrigated lint yields increased by 3.86 kg/ha for each heat-unit accumulated above 166 during this period. The fruit number was highly correlated with the heat-units accumulated during this period (r2= 0.83 ). This peTABLE 3 Regression analyses of lint yield and total heat units during the different growing periods under irrigated conditions (data from both 1980 and 1981 ) Growing period
Regression equation
r~
(DAP) 1- 30 31-60 61-90 91-120
Lint yield-490.5 - 0.009 Lint yield---1150.8+4.362 Lint yield-- -640.1+3.864 Lintyield= -56.5+2.801
( HU) (HU) (HU) (HU)
n.s. 0.67** 0.92** 0.67**
* and ** indicate significance at the 5% and 1% levels respectively. HU= heat units accumulated during the growing period. lOOO y : -640.1 + 3.86x
r2=0.92
#
8oo
J=
6o0
9 U~ 4o0 -1
2oo +
IRRIGATED
do
I
s2o
3eo
HEAT UNIT8 {$1-~ DAP)
Fig. 4. Relationship between cotton lint yield and heat-units during 61-90 days after planting under irrigated conditions. At discrete heat-unit accumulations, each point represents one of the three genotypes. Each genotype point represents the mean of three replications.
261
riod corresponds with the period of rapid floral-bud production and early fruit development. The heat-units accumulated during this particular period explained as much of the yield variability as did the total seasonal accumulation, reflecting the importance of fruit number to yield. During the period of seedling development (0-30 DAP),heat-unit accumulation had no significant effect on lint yield. Lint yield was positively correlated with heat-units accumulated during the period 31-60 DAP (floral site initiation) and the period 91-120 DAP (fruit maturation ) (r 2= 0.67 for each period). The relationship between lint yield and heat-unit accumulation was also affected by cultivar. The irrigated lint yields of the indeterminate cultivar Coker 5110 and the moderately determinate Tamcot 788 were linearly correlated to total seasonal heat-unit accumulation. However, a curvilinear relationship ex1000
8~
y • -1190,7 ÷ 1.32x
r2~0.@4
y • -1000.4 ÷ 1.10x
r2~O.06
o .~
1
600
400
//I + o •J lU
0
IRRIGATED
e
!
i
i
e
e
b + ";
COKER 5110 TAMCOT 788
• 'I
8oo,
PM DWARF
600,
400.
200 DRYLAND ! !
"
|
1000
|
•
1100
e
1200
•
|
1300
•
!
1400
•
!
1500
1600
TOTAL HEAT UNITS (C)
Fig. 5. Comparison of the relationship between lint yield and seasonal accumulation of heat-units among three cultivars under (a) irrigated and (b) rainfed conditions.
262
isted between irrigated lint yield and total heat-units for the determinate cultivar Paymaster Dwarf (Fig. 5a). The determinate cultivar produced more lint than the indeterminate cultivar when the seasonal heat-unit accumulation was less than 1300. The cultivar response was observed under both irrigated and rainfed conditions. Differences in the lint yield of the cultivars were not significant under hot and dry conditions in 1980, since lint production was determined by water-stress conditions (Fig. 5b). Therefore, the more determinate cultivar performed better under cool environments than the indeterminate cultivar, while the inverse was true under warmer environments. Wang (1960) and Lowry (1969) pointed out that the heat-unit concept is limited in application and reliability because it does not take into account other factors which influence plant growth and development, such as irradiance, water, photoperiod, etc. Our data indicate that cotton lint yield is highly correlated with the accumulation of heat-units when water-stress is minimized; however, no significant relationship exists between lint yield and the accumulation of heat-units when water-stress is the major limitation. More importantly, the significance of the early reproductive development period to total lint yield was elucidated. This period is most critical in determining the total number of fruiting sites produced and in largely determining lint yield due to high dependence of yield on fruit number. Heat-units can be used to accurately predict cotton growth, development, and lint yield when water supply is not a major limitation to productivity. Both water supply and growth habit will modify the relationship between cotton lint yield and available heat-units. REFERENCES Kerby, T.A., 1986. Boll growth and development. Cotton Comments, vol. 21. Cooperative Extension, Univ. of California, Berkeley, pp. 1-4. Kerby, T.A., Wilson, L.T. and Johnson, S., 1985. Upper threshold required for heat unit calculations for cotton growth in the far west. In: Proc. Beltwide Cotton Producers Conf. National Cotton Council, Memphis, TN, pp. 366-368. Krieg, D.R., 1985. Developmental and physiological responses of short-season cotton to temperature. In: Proc. Beltwide Cotton Producers' Conf. National Cotton Council, Memphis, TN, p. 366. Lowry, W.P., 1969. Weather and Life. Academic, New York, pp. 193-197. Malm, N.R. and Kerby, T.A., 1981. Association between cotton yields and heat units in New Mexico over a 25-year period. In: J.M. Brown (Editor), Proc. Beltwide Cotton Producers' Conf., 4-8 January 1981, New Orleans. National Cotton Council, Memphis, Tenn., pp. 55-57. Mitchell, J.F. and Bourland, F.M., 1981. Heat unit accumulation and phenological development of four cotton cultivars in Mississippi. In: Proc. Beltwide Cotton Producers' Conf. National Cotton Council, Memphis, TN, p. 113. Wang, J.Y., 1960. A critique of the heat unit approach to plant response studies. Ecology, 41: 785790. Wanjura, D.F. and Supak, J.R., 1985. Temperature methods for monitoring cotton development. In: J.M. Brown (Editor), Proc. Beltwide Cotton Producers' Conf., 4-8 January 1981, New Orleans. National Cotton Council, Memphis, Tenn., pp. 369-370.