Evaluation of the Effects of Recent Weather Variations on Winter-Wheat Agronomic Characteristics, and Their Correlations in Jinju, Republic of Korea

Abstract

Wheat grain productivity is different from year to year because growing environments are highly seasonally variable as a result of climate change. This study analyzed the variation in the weather conditions in the 2010–2023 growing seasons and evaluated the crop developmental phase, yield-related components, and the correlations of the variables in the southern plain of South Korea, measuring agronomic traits, including the above-ground dry weight, young-panicle length, spike number per m², number of grains per spike, thousand-grain weight, and grain yield. The number of days in the heading and ripening phase showed less differences than the other growth phases. The thousand-grain weight showed low variations over the fourteen years observed, unlike the number of grains per spike, the marketable grain yield, and the straw yield, with comparatively high variations. The grain yield was negatively correlated with the average air temperature during the winter dormancy phase (R = −0.687, p = 0.007) and precipitation (R = −0.726, p = 0.003), but showed positive associations with the number of days in the winter dormancy phase (R = 0.597, p = 0.024) and the number of grains per spike (R = 0.809, p = 0.000). In conclusion, longer winter dormancy and a longer tillering phase delay young-panicle development but increase the number of spikes and the number of grains per spike, resulting in a higher wheat grain yield in Southern Korean weather conditions.

1. Introduction

In 2022, wheat (Triticum aestivum L.) ranked second among cereal crops globally, totaling 946 million mega grams (Mg) in 243 million hectares [1], 4.65 million Mg of which were consumed in the Republic of Korea, with 2.54 and 2.08 million Mg for human and animal consumption, respectively [2]. The annual wheat demand has averaged 4.00 million Mg since the mid-1980s. However, the country produced only 35 thousand Mg in 2022, responsible for 0.7% of the total demand. Wheat production in the country has rapidly decreased since the 1970s, mainly due to mass import as credit assistance from the United States [3]. In recent years, food self-sufficiency, sovereignty, and security have become important issues in the country. In addition, because of a decrease in rice consumption and overproduction, the government currently demands farmers to alternate other crops, such as soybean, forage crop, and wheat, to rice in paddy soil, with strategic direct payments.

Wheat or barley is typically produced in the double-cropping system, followed by rice, in the Southern Republic of Korea. Rice is transplanted from May to June and harvested in October, while winter wheat is sown from October to November and harvested in June. Farmers willingly choose early-maturing rice cultivars to plant overwintering field vegetables, such as garlic or bulb onions, but prefer late-maturing rice cultivars before planting wheat due to income differences. Thus, late-wheat-seed sowing often results in the poor establishment, especially in poor environmental conditions, such as frequent rainfall or cold temperatures in early winter. In addition, wheat breeding has focused on early maturity and high yields [4].

Climate change in South Korea and East Asia is characterized by an increase in the annual average temperature [5,6] and in the frequencies of extreme temperatures or rainfall events [7,8]. There are several climate change-related physiological disorders in winter wheat, affected by high temperatures or drought during the anthesis and ripening phases [9,10,11,12], winter or early spring warming [13,14,15], temporary frost or cold in the spring [16,17], and frequent rains during grain filling [18,19].

Heat stress and high temperatures during the reproductive or flowering phase are among the most concerning issues in temperate regions, as they can reduce wheat grain number and size, seriously influencing the yield [20,21,22,23]. Heat stress during anthesis increases floret abortion and causes pollen sterility, tissue dehydration, and lower assimilation [24]. In addition, elevated temperatures shorten assimilate translocation and grain filling duration [25,26,27].

During the last several decades, low temperatures have caused serious wheat production losses in Australia, the United States, Europe, and China [17,28,29,30]. Post-head-emergence frost frequently occurs in Australia, contributing to yield reduction [28,31], while frosts often occur before head emergence in China [17,32]. A previous study [33] hypothesized that mild winters and warm, early springs may induce premature plant development, resulting in the exposure of vulnerable plant tissues and organs to subsequent late-season frosts. In Korea, winter cereal crop cold or frost injuries in the early spring frequently occur, but are rarely reported [16].

Wheat grain productivity is different from year to year because growing environments are highly seasonally variable in terms of temperature and rainfall. Indeed, grain yield is the result of the growth and development of yield components during the growing season, influenced by environmental factors, genetic and physiological controls, and evolutionary constraints [34,35]. The role of grain number per unit area, comprising spike number per unit area and grain number per spike, and single-grain weight as the main components for determining cereal grain yield is somewhat controversial. While genetic progress in grain yield has been mainly achieved via increases in the grain number per unit area [36,37,38] and grain weight [39,40], the former is highly responsive to favorable growth conditions and dominates the latter, representing the main component determining grain yield [41]. This is attributed to the plasticity of seed number in relation to resource availability compared to seed size [42].

This study aimed to analyze the characteristics of variations in the daily average, maximum and minimum air temperatures, precipitation, and sunshine hours during the winter-wheat-crop growing seasons from 2010 to 2023, in Jinju, in the southern province of Gyeongsang. We evaluated the changes in the period of each developmental phase and yield-related components, and analyzed the between-variable correlations, providing insights into managing winter-wheat cropping in climates and cropping regions similar to the southern plain in South Korea.

2. Materials and Methods

2.1. Description of the Experimental Field Site

The experiment was conducted at Gyeongsangnam-do Agricultural Research and Extension Services, located in Chojeon-dong 874, Jinju, Gyeongsangnam-do, Republic of Korea (35°20′56″ N, 128°11′74″ E), with a warm temperate continental monsoon climate, in the 2010–2023 growing seasons. The experimental sites have been under a continuous rice–wheat rotation system for approximately forty years. The soil type was silt loam in the surface soil and silty clay loam in the subsoil. The soil was of the inceptisol order and moderately well drained, with 2021 pre-plant chemical properties of 7.0 pH, 29.0 g kg⁻¹ organic matter, 1.57 g kg⁻¹ nitrogen, 83.7 mg kg⁻¹ available phosphorus, and 0.23 cmol_c kg⁻¹ exchangeable potassium.

2.2. Description of the Field Experiment

This long-term field trial was established in 2010 to analyze the effects of weather conditions on winter wheat’s agronomic traits under rainfed conditions. A randomized, complete, block experimental design was used, with two wheat cultivars and three replications. Wheat seeds cvs. Geumgang and Jokyong were sown by hand in late October to early November, followed by molding with a ridging machine, resulting in beds of 1.50 m from center to center between adjacent ones with a bed width of 1.10 m, a height of 0.20 m, and a plant density of 16.0 g per m². The cvs. Geumgang (cv. Geuru sibling x Gwandong75) and Jokyong (Seri82 x cv. Geumgang) were registered new cultivars on 1 May 2000 and 18 April 2007, respectively, and supplied to growers as seeds certificated by the government.

The nitrogen rate adopted in the experiment was 76 kg ha⁻¹, with 40% and 60% for basal and additional applications, respectively. Phosphate and potassium at 73 kg ha⁻¹ P₂O₅ and 31 kg ha⁻¹ K₂O rates, respectively, were applied once before sowing. Butachlor was applied to control weeds one day after sowing, at 30.0 kg ha⁻¹. Thifensulfuron-methyl was used at 70 g ha⁻¹ (58.5 g 1000 kg⁻¹ liquid) in early spring to control the grassy weeds that germinated in winter. Pesticides were not applied due to no serious pathogen or insect problems.

2.3. Crop Phenology and Wheat Agronomic Traits

Crop development is the progression of a plant’s lifecycle, independent of the biomass accumulation-related growth. Our measurements of growth and development referred to the methods by Feekes [43], Hay and Kirby [44], Hyles et al. [45], Wang et al. [46], and Zadoks et al. [47], recording the timing of six wheat growth stages each year: the sowing date, winter dormancy onset (the first day of an average daily air temperature of zero or under zero degrees for five consecutive days), green-up (the start of regrowth in the spring, after dormancy, when new leaves and roots measure more than 1.0 cm and 0.2 cm, respectively), maximum tillering, heading (40% heading), and maturity (physiological maturity, when the flag leaf and spikes turn yellow). The crop growth period was, therefore, divided into five phases—germination and seedling growth (sowing to winter dormancy onset), winter dormancy (winter dormancy onset to green-up), tillering (green-up to maximum tillering), stem elongation (maximum tillering or jointing to heading), and heading and ripening (heading to maturity)—referencing the classification of Choi et al. [48] and He et al. [49].

Winter wheat’s phenological date is different to that of spring wheat due to the former’s overwintering processes. Tillers can be formed at multiple nodes on the main stem, and secondary and tertiary tillers can form from nodes on the tillers themselves [50,51]. Because tillers develop from the three-leaf stage to jointing (Zadoks13–30) [50], tillers in winter wheat can form prior to winter dormancy onset, during dormancy, or after green-up, which is controlled by the environment during tiller development [52]. We defined the tillering phase as the period ranging from green-up to the maximum tiller stage, according to Choi et al. [48].

The above-ground dry weight per m² was recorded after drying the whole plant above the surface at 70 °C for at least 72 h. The young-panicle length was measured with a magnifier, and the yield-related components were measured according to Kwon et al.’s standard method [53]. The number of spikes per 0.10 m × 1.50 m unit area was counted during the ripening phase, and the number of grains per spike was counted, using twenty representative spikes, on the same date. The wheat crop was harvested at full maturity using a sickle in 1.50 m × 2.20 m plots, put in a mesh bag, and dried in a plastic greenhouse for approximately one month. The wheat straw was removed using a thresher, while grain hull, awns, and other particles were removed with a screener. Immature grains were sorted using a 2 mm mesh sieve. The grain yield was adjusted to a 14% moisture content.

2.4. Meteorological Data

Meteorological data, such as the average, maximum, and minimum daily air temperature, rainfall, and sunshine hours during wheat production, were obtained from annual reports from regional weather stations in Jinju (35°66′42″ N, 128°04′02″ E, 29 m altitude), operated by the Korea Meteorological Administration [54]. The distance between the experimental site and the weather station was 4.75 km in a straight line. The meteorological data were processed at 10 d intervals, and the average, maximum, and minimum daily air temperatures, precipitation, and sunshine hours per day in each growth period, over five different developmental phases, were calculated from the average value for the investigated fourteen years.

2.5. Statistical Analysis

Statistical analyses were performed using XLSTAT Pro 2013.1.01 (Addinsoft, New York, NY, USA). A correlation analysis was performed to determine the relationship between each growth stage, the growth characteristics, the yield-related traits, and the yield of winter wheat. A principal component analysis (PCA) was conducted to investigate the difference in the relationships among the properties, as affected by 14 variables over 14 years.

3. Results and Discussion

3.1. Trends of Weather Conditions

The average, maximum, and minimum daily air temperatures of 10 d intervals ranging from mid-November to late May in the 2010–2014, 2015–2019, and 2020–2023 growing seasons are shown in Figure 1. The average daily temperature during the whole growth cycle was 1.2 °C, 0.4 °C higher in 2020–2023 than in 2010–2014 and 2015–2019. The average temperature rose more during the stem elongation phase (from late March to mid-April) than during the other growth phases, measuring 1.9 °C higher in 2020–2023 than in 2010–2014. The maximum daily air temperature increased more during the tillering (mid-February to mid-March) and stem elongation phases than during the other phases, while the minimum temperature increased the most during the winter dormancy phase (late December to early February).

The precipitation amount during the whole growth cycle was 450.8 mm in 2020–2023, which was 121.8 mm and 72.1 mm lower than in 2010–2014 and 2015–2019, respectively (Figure 2). There were increases in precipitation during the winter dormancy (33.8 mm) and stem elongation phases (27.6 mm) in 2020–2023 compared to 2010–2014, while it decreased during the other phases. There was a significant decrease in precipitation during the heading and grain filling phases (late April to late May), recently reaching extreme amounts of 70.0 mm and 395.9 mm in 2022 and 2023, respectively. The average sunshine hours per day increased during all growth phases, except for winter dormancy, in the 2020–2023 growing season compared to 2010–2014.

3.2. Wheat Phenological Characteristics

The number of days from the sowing date to the maximum tillering, heading, and maturity dates tended to decrease from 2010 to 2023 (Figure 3), with the tillering phase significantly shortening over the fourteen years (Figure 4) due to the higher temperatures. The rising temperatures during the seedling and winter dormancy phases accelerated tiller formation, although the onset of winter dormancy and green-up did not change. The average temperatures in early and mid-December (winter dormancy onset) and early February (start of green-up) remained similar over the fourteen years, but the average temperatures of ten-day intervals from late December to late January rose above zero degrees starting from 2015. We deduced that tillering might have occurred prior to or during winter dormancy depending on the years, although the number of tillers was not counted during our study period.

The number of days from sowing to green-up, heading, and maturity did not differ between the years compared to other growth stages, with coefficient of variations (CVs) of 0.05, 0.04, and 0.03, respectively. The number of days in the heading and ripening phase remained the most similar throughout the investigated years, possibly due to the lesser impact of the average temperature on this phase.

Another study reported that the period from green-up to anthesis and from anthesis to maturity shortened and lengthened, respectively, in most of the investigated stations in 1981–2009 [55], concluding that, although climate warming favors early anthesis and maturity, the duration of the grain filling phase is less susceptible to high temperatures. Similarly, Wang et al. [46] showed that the period from sowing to flowering or maturity shortened, in agreement with changes in the temperature, with no changes in the period from maximum tillering to flowering and from flowering to maturity. In our study, although the temperature during the period from maximum tillering to heading (stem elongation phase) increased over the years, the number of days from sowing to heading decreased less than from sowing to maximum tillering. The heading date is very close to anthesis (flowering) and corresponds to the developmental conversion point from vegetative to reproductive growth [56,57]. Photoperiod and temperature are major environmental factors determining the time to flower initiation and appearance in plants, and wheat flowering is more sensitive to the photoperiod [58,59]. Hammes and Marshall [60] previously reported that there is a tendency for the photoperiod to have a more pronounced effect on the development stage than temperature, especially for anthesis. Additionally, Lv et al. [61] reported that changes in the grain filling period are relatively stable compared to other growth periods due to fewer temperature increases in the former.

3.3. Wheat Growth Characteristics

The maximum number of tillers (CV = 0.21) showed greater variation than the number of spikes (CV= 0.17) or culm length (CV = 0.07) over the fourteen-year study period (Figure 5), with a slightly positive correlation with the average air temperature (R = 0.346, p = 0.225) during the tillering phase, precipitation (R = 0.396, p = 0.161) during winter dormancy, and sunshine hours (R = 0.439, p = 0.116) during the seedling phase, without statistical significance (

Table 1. Correlation coefficients of linear regression between the weather conditions during each growth phase and the growth characteristics or yield-related traits of winter wheat in Jinju, South Korea, from 2010 to 2023.

Variables	Weather	GSP	WDP	TP	SEP	HRP
MNT	Average temp.	−0.110	−0.072	0.346	-	-
	Maximum temp.	0.192	−0.077	0.319	-	-
	Minimum temp.	−0.125	−0.027	0.270	-	-
	Precipitation	−0.040	0.396	−0.194	-	-
	Sunshine hours	0.439	−0.014	−0.048	-	-
AGDW	Average temp.	0.165	0.549 *	0.831 **	0.373	-
	Maximum temp.	0.348	0.601 *	0.619 *	0.332	-
	Minimum temp.	0.117	0.529	0.820 **	0.240	-
	Precipitation	−0.139	0.409	0.006	0.132	-
	Sunshine hours	0.232	−0.278	−0.239	−0.060	-
YPL	Average temp.	0.239	0.600 *	0.843 **	0.670 **	-
	Maximum temp.	0.487	0.603 *	0.810 **	0.661 **	-
	Minimum temp.	0.080	0.595 *	0.627 *	0.422	-
	Precipitation	−0.268	0.366	−0.175	0.258	-
	Sunshine hours	0.268	−0.246	0.176	0.236	-
CL	Average temp.	−0.227	−0.179	0.473	0.015	−0.504
	Maximum temp.	−0.239	−0.227	0.269	0.000	−0.593 *
	Minimum temp.	−0.134	−0.103	0.587 *	0.093	−0.184
	Precipitation	−0.143	0.197	0.458	0.363	0.538 *
	Sunshine hours	0.211	−0.239	0.515	−0.350	−0.550
NS	Average temp.	−0.376	−0.448	−0.242	−0.693 **	−0.693 **
	Maximum temp.	−0.504	−0.396	−0.444	−0.639 *	−0.598 *
	Minimum temp.	−0.159	−0.474	0.119	−0.447	−0.430
	Precipitation	0.033	−0.276	0.416	−0.117	0.533 *
	Sunshine hours	−0.043	0.045	−0.640 *	−0.433	−0.349
NGS	Average temp.	−0.201	−0.650 *	−0.516	−0.467	−0.061
	Maximum temp.	−0.579 *	−0.622 *	−0.474	−0.454	0.034
	Minimum temp.	0.013	−0.642 *	−0.363	−0.356	−0.132
	Precipitation	0.355	−0.607 *	0.050	−0.115	0.148
	Sunshine hours	−0.302	0.423	−0.147	−0.012	−0.009
TGW	Average temp.	0.381	0.492	0.265	0.406	0.157
	Maximum temp.	0.591*	0.438	0.179	0.375	0.024
	Minimum temp.	0.186	0.519	0.200	0.269	0.304
	Precipitation	−0.053	0.451	−0.255	0.126	−0.298
	Sunshine hours	0.374	−0.322	0.132	0.108	0.069
GY	Average temp.	−0.455	−0.687 **	−0.386	−0.166	0.064
	Maximum temp.	−0.641 *	−0.552 *	−0.308	−0.066	0.127
	Minimum temp.	−0.319	−0.742 **	−0.318	−0.187	0.038
	Precipitation	0.036	−0.726 **	0.113	−0.099	0.049
	Sunshine hours	0.035	0.555 *	−0.124	0.088	−0.013
SY	Average temp.	0.227	−0.088	0.124	−0.128	−0.362
	Maximum temp.	−0.006	−0.009	−0.253	−0.242	−0.419
	Minimum temp.	0.328	−0.113	0.566 *	0.075	0.052
	Precipitation	0.203	−0.249	0.528	0.442	0.436
	Sunshine hours	−0.209	−0.029	−0.793 **	−0.452	−0.435

** and * indicate significant values at 1% and 5% levels of significance, respectively. GSP = germination and seedling phase; WDP = winter dormancy phase; TP = tillering phase; SEP = stem elongation phase; HRP = heading and ripening phase; AGDW = above-ground dry weight per m² on 10 April (g m⁻²); YPL = young-panicle length on 10 April (mm); CL = culm length (mm); MNT = maximum number of tillers per m²; NS = number of spikes per m²; NGS = number of grains per spike; TGW = thousand-grain weight (g); GY = grain yield (Mg ha⁻¹); and SY = straw yield (Mg ha⁻¹).

). The maximum number of tillers was positively correlated with the culm length (R = 0.553, p = 0.040) and the above-ground dry weight (R = 0.450, p = 0.107), but negatively correlated with the number of grains per spike (R = 0.403, p = 0.145) (

Table 2. Pearson’s correlation coefficient between each growth period, growth characteristics, yield-related traits, and winter wheat yield in Jinju, South Korea, from 2010 to 2023.

Variables	GSP	WDP	TP	SEP	HRP	MNT	AGDW	YPL	CL	NS	NGS	TGW	GY
WDP	−0.883 **
TP	−0.256	0.388
SEP	0.094	−0.317	−0.681 *
HRP	−0.039	−0.153	−0.490	0.363
MNT	−0.045	−0.037	−0.313	0.478	−0.136
AGDW	0.137	−0.341	−0.756 **	0.704 **	0.521	0.45
YPL	0.233	−0.451	−0.837 **	0.466	0.567 *	0.245	0.796 **
CL	−0.321	0.196	−0.187	0.228	−0.070	0.553 *	0.451	0.261
NS	−0.462	0.434	0.495	0.047	−0.259	0.143	−0.123	−0.600 *	0.411
NGS	−0.596 *	0.655 *	0.509	−0.373	−0.261	−0.403	−0.640 *	−0.685 **	−0.085	0.499
TGW	0.633 *	−0.647 *	−0.636 *	0.522	−0.115	0.4	0.501	0.536 *	0.042	−0.428	−0.707 **
GY	−0.478	0.597 *	0.419	−0.495	−0.328	−0.285	−0.522	−0.473	0.111	0.419	0.809 **	−0.557 *
SY	0.048	−0.094	−0.212	0.4	−0.120	0.112	0.349	0.014	0.575 *	0.545 *	0.128	0.127	0.22

** and * indicate significant values at the 1% and 5% levels of significance, respectively. GSP = germination and seedling phase (days); WDP = winter dormancy phase (days); TP = tillering phase (days); SEP = stem elongation phase (days); HRP = heading and ripening phase (days); AGDW = above-ground dry weight per m² on 10 April (g m⁻²); YPL = young-panicle length on 10 April (mm); CL = culm length (mm); MNT = maximum number of tillers per m²; NS = number of spikes per m²; NGS = number of grains per spike; TGW = thousand-grain weight (g); GY = grain yield (Mg ha⁻¹); and SY = straw yield (Mg ha⁻¹).

).

Over the years, the number of tillers tended to increase during winter dormancy rather than after green-up (Figure 6), measuring less than 1000 m⁻² on 20 February from 2010 to 2013, increasing thereafter by more than 1000 on the same date. Although the maximum number of tillers was higher in 2020 and 2023 than in the other years, it was not significantly different right after heading between the years. In addition, the number of spikes tended to decrease over the fourteen-year period. The number of tillers produced per plant is controlled by the environment during tiller development and by tiller mortality in the period from jointing to anthesis [50,62]. The timing of tiller initiation can influence tiller mortality [63]. Tilley et al. [52] confirmed that tillers created before 1 March were responsible for the majority of plants producing spikes and contributed the most to grain yield, while those created in March contributed little to the latter. However, there were some differences in the effects of earlier tillers depending on the sowing rate, environmental conditions, and nitrogen application timing, among others. Wheat plants are more vulnerable to cold temperature during the jointing and booting growth phases, decreasing the numbers of spikes per plant and grains per spike [64]. In our study, the number of tillers in 2023 increased from 1280 m⁻² on 10 December to 2342 m⁻² on 20 March, then decreased to 762 m⁻² on 30 April. Although tiller development can be accelerated by favorable environmental conditions, the final number of spikes per m² was consistent, irrespective of the growing years, and more dependent on the genetic character than on the growth environmental conditions.

In most years, the above-ground dry weight started to increase drastically from 20 March, but, more recently, this has been anticipated for 20 February or even earlier (Figure 7). The variations in the young-panicle length were similar to those in the above-ground dry weight (Figure 8), with the latter’s per unit area value on 10 April highly positively correlated with the former (Table 2). Greater above-ground dry weight or young-panicle-length values were attributed to the environmental conditions in the previous phases. Both the above-ground dry weight and the young-panicle length were positively correlated with the average air temperature during winter dormancy (R = 0.549 and p = 0.042, and R = 0.600 and p = 0.023, respectively) and tillering phases (R = 0.831 and p = 0.000, and R = 0.843 and p = 0.000, respectively) (Table 1). However, the increase in both values on 10 April did not have a positive effect on the number of spikes per unit area or grains per spike and grain yield (Table 2). Brasier et al. [65] reported that, although the association between the above-ground biomass and the grain yield was least significant at anthesis, it was not significant during the booting stage. Considering the negative or lack of effects on increases in the above-ground dry weight and young-panicle length right before the heading stage on the number of grains per spike, the number of spikes, and the grain yield in our study, the vegetative growth rate should not decide winter wheat’s grain yield.

3.4. Wheat Grain Characteristics and Yield

The thousand-grain weight showed a low variation (CV = 0.07) over the fourteen years, while the number of grains per spike, the marketable grain yield, and the straw yield showed comparatively high variations, with CVs = 0.21, 0.19, and 0.32, respectively (Figure 9). The grain yield in cereals depends on three yield components—the number of spikes per unit area, the number of grains per spike, and the grain weight—with strong mutual compensation, despite the fact that one component’s limitation cannot be completely compensated by the others [66]. In our study, the thousand-grain weight was negatively correlated with the number of grains per spike (R = −0.707, p = 0.005) and the number of spikes per m² (R = −0.428, p = 0.127), while the number of grains per spike was slightly positively correlated with the number of spikes per m² (R = 0.499, p = 0.069) (Table 2). The wheat grain yield was positively correlated with the number of grains per spike (R = 0.809, p = 0.000), while it was negatively correlated with the thousand-grain weight (R = −0.557, p = 0.039). Mandea et al. [67] reported that most correlations between the number of spikes per unit area or the number of grains per spike and the thousand-grain weight were negative in trials involving 26 different cultivars and 10 locations, although the statistical value varied depending on the cultivars.

The thousand-grain weight was negatively correlated with the winter dormancy (R = −0.647, p = 0.012) or tillering periods (R = −0.636, p = 0.015), but positively correlated with the young-panicle length (R = 0.536, p = 0.048) or the above-ground dry weight on 10 April (R = 0.501, p = 0.068) (Table 2). Indeed, the thousand-grain weight might have increased when the above-ground dry weight and young-panicle development were promoted by the high air temperature during winter dormancy and tillering. The number of spikes per m² showed a significantly negative correlation with the young-panicle length (R = −0.600, p = 0.023) and slightly positive correlations with the winter dormancy (R = 0.434, p = 0.121) and tillering periods (R = 0.495, p = 0.072). The correlation of the number of grains per spike was similar to that of the number of spikes per m², with negative correlations with the above-ground weight (R = −0.640, p = 0.014) and the young-panicle length (R = −0.685, p = 0.007), but positive correlations with the winter dormancy (R = 0.655, p = 0.011) and the tillering periods (R = 0.509, p = 0.063). The number of spikes per m² was not significantly correlated with the maximum number of tillers per m².

The number of spikes had a significantly negative correlation with the average (R = −0.693, p = 0.006) and maximum air temperatures (R = −0.639, p = 0.014) during the stem elongation phase (Table 1), as well as during all other growth phases, either significantly or insignificantly. The number of grains per spike had a significantly negative correlation with the average (R = −0.650, p = 0.012) and maximum air temperatures (R = −0.622, p = 0.017) during the winter dormancy phase (Table 1), as well as during the tillering or stem elongation phases, with statistical insignificance. On the contrary, the thousand-grain weight was slightly positively correlated with the average or maximum air temperature during the winter dormancy or stem elongation phase. Precipitation only had a positive correlation with the number of spikes (R = 0.533, p = 0.050) during grain filling and ripening, while sunshine hour only had a negative correlation with this component (R = −0.639, p = 0.013) during the tillering phase.

Considering the negative correlation between the thousand-grain weight and the number of spikes or grains per spike, the positive effect of the latter components, and the negative effect of the thousand-grain weight on the grain yield in our study, the number of grains per spike and the number of spikes should be the most important traits for determining wheat grain yield. Wheat grain yield is more closely related to the grain number per unit area, which integrates two main components, the number of spikes per unit area and grains per spike, than grain size or weight [34,41,68,69]. The dominant role of grain number in determining yield stems from evolutionary constraints, and the environmental modulation of plant reproductive output largely depends on seed number adjustments, whereas a stable seed size is adaptive [34,42,70,71]. In addition, the grain number per unit area is more flexible than the grain weight as a yield-determining component; hence, the grain number responds to favorable growing conditions more effectively than the grain weight [41]. However, under unfavorable conditions, such as temperatures as high as 30 °C during the grain filling phase, wheat yield reduction was attributed mostly to a lower grain weight and only slightly to a lower grain number [72,73].

The grain yield was negatively correlated with the average, maximum, and minimum air temperatures during the winter dormancy phase (R = −0.687, p = 0.007; R = −0.552, p = 0.041; R = −742, p = 0.002, respectively) and precipitation (R = −0.726, p = 0.003), while it was positively correlated with the sunshine hours during the same phase (R = 0.555, p = 0.039) (Table 1). The grain yield showed positive associations with the winter dormancy length (R = 0.597, p = 0.024) and the number of grains per spike (R = 0.809, p = 0.000), while it had a negative association with the thousand-grain weight (R = −0.557, p = 0.039).

We used principal component analysis to summarize some information relating to the length of the growth phases, growth characteristics, and yield components of winter wheat (Figure 10). PC1 explained 42.44% of the total variance in the dataset, while PC2 explained 20.51%. The highly weighted variables under PC1 included GSP (squared cosines (SCs) = 0.315), WDP (SC = 0.507), TP (SC = 0.688), SEP (SC = 0.449), AGDW (SC = 0.654), YPL (SC = 0.713), NGS (SC = 0.752), TGW (SC = 0.637), and GY (SC = 0.585), while these were CL (SC = 0.712), NS (SC = 0.435), and SY (SC = 0.475) under PC2. PC1 was characterized by the number of grains per spike, the grain yield, the winter dormancy phase, the young-panicle length, and the thousand-grain weight, while PC2 was characterized by the culm length and the straw yield. Highly weighted observations under PC1 were the years of 2013, 2020, and 2023, while these corresponded to 2010 and 2017 under PC2.

Due to global warming, heat events occur more frequently during the reproductive growth period (from anthesis to maturity) of wheat crops in many global production areas, and temperatures over 25 °C or 30 °C during the ripening phase cause a critical reduction in the wheat yield [74,75,76,77,78], as the optimum temperature for wheat growth during the reproductive growth phase ranges from 12 °C to 22 °C [79,80]. Li et al. [77] concluded, in a study performed on a cropping system similar to ours, that a maximum daily air temperature of 26 °C may be the threshold for normal wheat grain filling, beyond which wheat yield losses may occur. In our study, the average or maximum air temperature during the heading and ripening phase was negatively correlated with the number of spikes but did not impact the grain yield, probably because, over the investigated years, the maximum temperature was not higher than 25.0 °C during the reproductive stage.

Frost stress during the stem elongation (jointing) or heading stage is one of the primary meteorological disasters influencing the growth and development of winter wheat [16,17,31,81]. When the growing point switches to the production of reproductive primordia, and reproductive tissue emerges from the leaf sheath, wheat becomes the most frost sensitive [31,82]. Indeed, while wheat can tolerate temperatures as low as −10 °C before stem elongation, following this stage, a −4 °C temperature may result in severe injury to wheat forets [83]. It is also known that frost severely damages plants when the air temperature falls below −5 °C for two consecutive days [84]. Koo et al. [16] reported that a low temperature around −3.1 °C at the booting or heading stages caused spike degeneration and sterility. In our study, the minimum air temperature in April was the lowest, at 3.5 °C, in both 2013 and 2020. A low temperature below zero was recorded for 5 days in both years. The grain yield was 6.07 Mg ha⁻¹ in 2013, but 2.32 Mg ha⁻¹ in 2020, a reduction that was attributed to the decreased number of spikes per unit area and grains per spike. Because the young panicle elongated later in 2013 than in 2020, with the heading date in the former (3 May) being 12 days later than the latter’s (21 April), the wheat plants might have been less affected by frost stress in 2013 compared to 2020. In recent years, heading dates have been established earlier than usual because of global warming, meaning that frost stress in the spring might occur more frequently, consistent with the findings of Gu et al. [33].

4. Conclusions

Longer winter dormancy and tillering phases following lower air temperatures during the winter and early spring, respectively, delay the above-ground plant growth rate and young-panicle development, but increase the number of spikes and grains per spike, resulting in higher wheat grain yields in Southern Korea’s weather conditions. Global warming expedites plants’ growth rates and frequently causes heat or frost stress, resulting in an inconsistent wheat grain yield.