Crop Coefficient Estimation and Effect of Abscisic Acid on Red Cabbage Plants (Brassica oleracea var. Capitata) under Water-Stress Conditions

Abstract

Understanding the anticipated impact of climate change on agriculture, as well as water conservation, is critical to achieving food security. Therefore, during this critical time and due to reduced water resources and increased food demand, it is important to study the impact of water-stress conditions on crops. Two successive seasons were carried out through the 2021 and 2022 seasons to estimate the crop coefficient (K_c) and study the effect of abscisic acid on red cabbage plants (Brassica oleracea var. capitata) under water-stress conditions at a private farm in the Bilbeis region, Sharqia Governorate, Egypt. The aim was to estimate the crop coefficient (K_c) and effect of different concentrations of abscisic acid (ABA) (0, 25, 50, and 75 ppm) under various irrigation levels (100, 80, and 60% of field capacity “FC”) on the growth process and yield parameters of red cabbage plants. The results revealed that the average estimated crop coefficient (K_c) for red cabbage crops under standard conditions, 100% of FC, was 0.75, 1.07, 1.2 and 0.88 and 0.77, 1.2, 1.25 and 0.82 for Initial, Development, Mid, and End stages during the 2021 and 2022 growing seasons, respectively. Data collected during both seasons clearly showed that all treatments significantly increased both the plant’s growth process and yield parameters when compared to the control. However, abscisic acid (ABA, 75 ppm) application with irrigation requirements (80% of FC) was statistically the most effective treatment in this study. Hence, this means a water savings of 20% can be achieved without significantly compromising the yield.

1. Introduction

Climate change will have a negative impact on the agricultural sector and the elements of agricultural development, especially in developing countries. Crop production will be impacted by climate change, which will exert significant stress on their growth and development, and any change in climatic conditions is likely to threaten global food production and security [1,2]. The anticipated increase in temperature and the altered seasonal rainfall pattern will have a negative impact on some crops’ production. Understanding the predicted effects of climate change on the production of different crops is crucial for studying future global food and water security in order to put in place all possible measures to counter those effects [3,4,5].

Cabbage is a good source of vitamins A and C, fiber, and iron. It contains natural chemical compounds that may help prevent specific types of cancer and can boost the body’s resistance to disease. Red or purple cabbage’s color changes according to the pH of the soil because the plant contains an anthocyanin-related pigment (flavins). The leaves are colored from dark red to purple in varying degrees, as anthocyanins accumulate in different plant tissues and under the influence of different conditions [6]. During the 2019–2020 season, Egypt’s total cabbage production area was 44,474 fed (18,531 ha), which produced 533,700 tons and an average of 12 tons fed⁻¹ (29 tons ha⁻¹) of cabbage plants [7].

Drought, a form of abiotic stress that has become more prevalent in arid and semi-arid places such as Egypt, directly inhibits crop growth by reducing cell elongation, cell turgor, or cell volume due to the covering of xylem and phloem vessels, which prevents any translocations [8].

Therefore, it is very important, when facing future changes in water availability, to develop a sustainable water management plan. Abscisic acid (ABA) plays a very important role as a plant growth regulator and is adaptive to biotic and abiotic stresses. [9,10]. Abscisic acid plays a crucial role in the processes of plant resistance and adaptability to a variety of abiotic stress situations. Additionally, ABA plays an important role in many physiological processes, including seed formation, dormancy, germination, and reproduction. Moreover, ABA regulates reactions to environmental stresses, including heat, cold, salt, drought, and excessive levels of radiation [11].

For these reasons mentioned above, estimating the amount of irrigation is crucial for managing irrigation. In addition, irrigation management includes regulating irrigation frequency and duration to maximize plant stress reduction and assure irrigation efficacy [12]. Therefore, it is vital to understand the relationship between field capacity (FC) and root zone depletion, where root zone depletion (D_r) represents the water shortage relative to field capacity (FC), and reflects soil moisture content [13].

On the other hand, to estimate crop evapotranspiration (ET_c) and the ideal crop water requirement, it is important to use the actual crop coefficient (K_c). This coefficient results from the relationship between crop evapotranspiration (ET_c) and reference evapotranspiration (ET_o) [14,15]. Moreover, climate change will affect evapotranspiration rates and may even lead to drought, thereby impacting soil moisture, crop water use, and the crop coefficient [1].

Therefore, this study aimed to estimate the crop coefficient (K_c) and the effect of different concentrations of abscisic acid (ABA) (0, 25, 50, and 75 ppm) under various irrigation levels (100, 80, and 60% of field capacity “FC”) on the growth process and yield parameters of red cabbage plants in sandy loam soil conditions.

2. Material and Methods

2.1. The Experimental Site and Crop

Two successive seasons carried out through the 2021 and 2022 seasons to estimate the crop coefficient and investigate the abscisic acid effect on red cabbage plants (Brassica oleracea L. var. capitata “Primero”) under water stress. The goal of this study was enhanced red cabbage plant growth process and yield parameters by using different concentrations of abscisic acid (ABA) of 25, 50, and 75 ppm as well as the control (0 ppm) under different levels of irrigation (100, 80, and 60% of field capacity “FC”) in sandy loam soil conditions at a private farm in the Bilbeis region, Sharqia Governorate, Egypt (latitude 30°22′04.9″ N, longitude 31°37′38.2″ E, and mean altitude 21 m above sea level), Figure 1. The main physical properties analyses were determined in the site and in the laboratory at the beginning of the trial and are reported in

Table 1. Some physical properties of the soil for Bilbeis experimental site.

Depth (cm)	Particle Size Distribution (%)				Moisture Content (%)
	Coarse Sand	Fine Sand	Silt	Clay	Saturation Point (S.P.)	FC	AW	PWP
0–30	43.2	19.5	24	13.3	28.6	14.3	7.15	7.15
30–60	44.1	18.5	24.2	13.2	28.6	14.5	7.4	7.1

FC: Field Capacity, PWP: Permanent Wilting Point, AW: Available Water.

.

2.2. Irrigation System and Experimental Description

An automatic drip irrigation system was set up according to the treatments, and hydraulically tested prior to use in the pilot site. The system consisted of a pump, pressure gauges, a filter, an injection unit, center control unit, solenoid valves, soil moisture sensors, and a measurement unit. The emitters were built-in with an average discharge of 4.0 L h⁻¹ at 1.0 bar pressure and a 0.3 m emitter spacing, and 0.7 m lateral spacing. Scheduled irrigation was applied using soil moisture sensors connected to a solenoid valve and center control unit, to automatically irrigate at 45% depletion of field capacity (FC) and stop irrigation when the soil moisture content reached 100, 80, and 60% of the field capacity, under a drip irrigation system. The sensors were installed at depths from 0 cm to 15 cm and from 15 cm to 30 cm.

2.3. Moisture Deficit and Depletion % of Irrigation

To estimate the moisture content depletion root zone (between 0.2 and 0.4 m from planting to harvest at different growth stages) at the end of the day is used to obtain the daily root zone water balance by the following equation [13,16]:

$$D_{ri}=D_{ri-1}\left(P_i-R{o}_{i }\right)-I_i-C{R}_i+E{T}_{ci}+D{P}_i$$

(1)

where: D_ri: the root zone depletion per day i (mm), D_ri−1: the moisture content in the root zone at the previous day (mm), P_i: the precipitation on day i (mm), Ro_i: the surface soil runoff on day i (mm), I_i: the irrigation depth per day i which infiltrates the soil (mm), CR_i: the capillary rise from the groundwater table per day i (mm), ETc_i: the crop evapotranspiration per day i (mm), and DP_i: the lost water of the root zone on day i (mm). Moisture deficit is the amount of water (mm) below field capacity before irrigation.

2.4. Estimation of the Crop Coefficient (K_c) for Red Cabbage

The crop coefficient (K_c) under standard conditions and soil water-stress conditions was calculated according to [13]: Equations (2)–(10). The average meteorological parameters needed for crop evapotranspiration (ET_c) calculation were recorded using a computer model and applying the Penman–Monteith equation, and to estimate the crop coefficient (K_c) values.

$$I{R}_g=\frac{\left(E{T}_c-R\right)K_r }{E_i}$$

(2)

$$E{T}_c=E{T}_o\times K_c$$

(3)

$$K_c=\frac{E{T}_c}{E{T}_o}$$

(4)

$$K_c=K_{Cb}+K_e$$

(5)

$$E{T}_{c adj}=(K_{s }{K}_{Cb}+K_e) E{T}_o$$

(6)

From Equation (5):

$$E{T}_{c adj}=K_{C } K_s\times E{T}_o$$

(7)

where:

$$K_{C adjust }=K_c K_s$$

(8)

Then:

$$K_{C adjust }=\frac{E{T}_{c adj}}{E{T}_o}$$

(9)

$$K_s=\frac{K_{C adjust }}{K_{C }}$$

(10)

where: IR_g = gross irrigation requirements, mm day⁻¹, ET_C = crop evapotranspiration, mm day⁻¹. R = water received by plant from sources other than irrigation, mm (for example, rainfall), K_r = ground cover reduction factor, correction factor for limited wetting according to the 80% red cabbage canopy coverage, K_r = 0.90 [17], E_i = irrigation efficiency, %, ET_o = reference evapotranspiration mm day⁻¹, K_c = calculated crop coefficient under standard conditions, K_cb = basal crop coefficient, K_e = soil water evaporation coefficient, ET_{C adjust} = crop evapotranspiration under soil water-stress conditions, and K_S = water stress coefficient, for soil water-limiting conditions, K_s < 1. Where there is no soil water stress, K_s = 1.

The coefficient (K_s) will vary under the irrigation treatments of 60 and 80% of the field capacity.

A gross irrigation requirement was converted from m³/ha/day to mm/ha/day [18]. The daily meteorological data and reference evapotranspiration for the experimental site of Bilbeis region over the two seasons are shown in Figure 2.

2.5. Experimental Design and Layout

The experimental layout was a split-plot system in a complete randomized block design that included 12 treatments with five replicates. The main plot (first factor) comprised three irrigation levels (100, 80, and 60% of FC) and the sub-plot (second factor) was an abscisic acid application (control, 25, 50, and 75 ppm). Additionally, the experimental unit area was 52.5 m². It contained three drip lines 25 m in length and 0.7 m in width. Healthy seedlings with uniform size (35 days from seed sowing) of red cabbage (Brassica oleracea L. var. capitata (Primero) were selected from a commercial nursery and transplanted on the 10^th of September in both seasons. Then, they were transplanted on both sides of the drip line at a distance of 0.3 m. Irrigation treatments (100, 80, and 60% of field capacity) were applied after planting. These treatments reflect conditions labeled severe water stress, moderate, and optimum level of water supply, respectively. The plants in every treatment were sprayed with abscisic acid and applied at 20 and 40 days from planting. In addition, the tested irrigation levels based on different rates of irrigation water, i.e., 2285, 1855, and 1518 m³ ha⁻¹ in the 2021 season and 2219, 1801, and 1485 m³ ha⁻¹ in the 2022 season.

2.6. Crop Characteristics

2.6.1. Vegetative Growth Characteristics

Red cabbage heads were harvested when they reached horticultural maturity on the first of December (about 82 days after transplanting), and the samples were taken randomly in both seasons on the first of December to record the vegetative growth characteristics: plant height (cm), head diameter (cm), head volume (cm³ by water displacement), plant fresh weight (g), head fresh weight (g), root fresh weight (g), number of green leaves per plant, number of red leaves per plant, and number of total leaves per plant.

2.6.2. Leaf Total Chlorophyll

Leaves were collected from the mid-section of plant to reduce age effects and the leaf total chlorophyll was recorded in fresh leaves per each plant using a portable chlorophyll meter SPAD 502, according to [19].

2.6.3. Leaf TSS Content

Total soluble solids percentage (TSS) was determined using a hand refractometer from the leaves of the plant mid-section.

2.6.4. Leaf Cell Sap Osmotic Pressure (atm)

Leaves’ cell sap concentration and osmotic pressure (atm) were determined according to [20] from the leaves of the plant mid-section.

2.7. Total Yield of Red Cabbage

At harvest time of red cabbage, the total weight of plants in each treatment was recorded by harvesting red cabbage as Kg per 1 m² calculated, and then the total yield as ton ha⁻¹ was calculated.

2.8. Statistical Analysis

The experimental design was a split-plot system in a complete randomized block design with five replicates. The data obtained were statistically analyzed using the analysis of variance method; the values of least significant differences (L.S.D. at 5% level) were calculated to compare the means of the different treatments, as reported by [21]. Additionally, the differences between means were differentiated by using Duncan’s range test [22].

3. Results and Discussion

3.1. Moisture Deficit and Depletion % of Irrigation

The estimation of irrigation amount is an important factor for irrigation management. Moreover, irrigation management is about controlling the amount and time to control the rate and intervals of irrigation to be effective, without stressing the plants [12].

Figure 3 shows the depletion %, moisture deficit, and irrigation (mm) under irrigation rates (100, 80, and 60% of field capacity “FC”) for the 2021 and 2022 seasons. The total irrigation amounts for red cabbage were 2285, 1855, and 1518 m³ ha⁻¹ and 2219, 1801, and 1485 m³ ha⁻¹ under 100, 80, and 60% of FC for the 2021 and 2022 seasons, respectively. On the other hand, the deficit rates (mm) increased with high depletion %; in contrast, the number of irrigation intervals decreased with the very close values between them, for 60% of FC, compared with 80 and 100% of FC for the 2021 and 2022 growing seasons.

However, there were moderate deficit rates (mm) with moderate depletion (%) for 80% of FC compared with 60% of FC. It was observed that the irrigation intervals were very close, with increasing depletion (%) and deficit rates (mm) in the red cabbage initial stage. This is due to the increase in evaporation from the soil surface as a result of the low density of vegetation cover with an increase in evapotranspiration during the initial stage of plant growth [23,24].

3.2. Estimation of the Crop Coefficient (K_c) for Red Cabbage

During this study, the crop coefficient was estimated using the ratio between the adjusted or actual crop evapotranspiration (ET_{c adjust}) and potential (ET_o) from the equation of Penman–Monteith [13]. Figure 4 shows the crop coefficient (K_c) estimation for red cabbage crops in the Bilbeis region and the current experiment conditions with irrigation at 100, 80, and 60% of (FC). It is clear that the average crop coefficient (K_c) for red cabbage crops was 0.75, 1.07, 1.2, and 0.88 and 0.77, 1.2, 1.25, and 0.82 under standard conditions, 100% of FC for Initial, Development, Mid, and End stages during the 2021 and 2022 growing seasons, respectively.

Management is well done when actual standard conditions generally prevail. Then, the reference evapotranspiration is calculated under standard conditions. Otherwise, when conditions in the field differ from the standard conditions, a lack of soil water may reduce crop evapotranspiration and soil water uptake. Then, the reference evapotranspiration is recalculated and, in turn, the crop coefficient (K_c) differs [13,14,15,25]. This is demonstrated in Figure 4; the estimation of the red cabbage crop coefficient differed depending on irrigation amount (60 and 80% FC). According to Figure 4, the average crop coefficient (K_c) for red cabbage crops was 0.61, 0.88, 0.97, and 0.71 and 0.62, 0.99, 1.01, and 0.67 under standard conditions, 80% of FC for Initial, Development, Mid, and End stages during the 2021 and 2022 growing seasons, respectively. The average crop coefficient (K_c) for red cabbage crops was 0.5, 0.72, 0.8, and 0.59 and (0.52, 0.82, 0.84, and 0.55 under non-standard conditions, 60% of FC for Initial, Development, Mid, and End stages during the 2021 and 2022 growing seasons, respectively.

The crop coefficient depends on the variation in time and growth stage, as well as the change in the weather conditions and in soil moisture by evaporation, always expressed as a function of the crop coefficient. The results indicated that the optimal values of crop coefficient (K_c) were achieved at 100% FC, followed by 80% FC, when compared with the results reported in FAO Paper 56. The red cabbage crop coefficient was 0.7, 1.05, and 0.95 for Initial, Mid, and End stages [13].

The different methods available, focusing on the FAO 56 framework, commonly used for estimation of the actual crop coefficient (K_c), have shown that the estimation of K_c is affected by changes in soil water stress [26]. Therefore, to adjust the crop coefficient under water stress, the water stress coefficient (K_s) is used [27]. The water stress coefficient (K_s), which is the relationship between the K_c of non-standard conditions and standard conditions, was calculated from Equation (10) [13]. Figure 5 shows the average calculated K_s under water-stress conditions for red cabbage crops was 0.81 and 0.67 for 80 and 60% of FC, respectively, for all growth stages and both growing seasons.

Climate change may have an impact on estimates of crop evapotranspiration (ET_c). Obtaining the actual values of the K_c coefficient considering the new scenarios can fill the current gap on this topic at this time. For this reason, studies of yield coefficient (K_c) estimation are needed for new cultivars, different growing seasons, and different regions under different conditions [28].

3.3. Crop Characteristics

The data presented in

Table 2. Effect of water stress and abscisic acid on plant height, head diameter, and head volume of red cabbage plants (2021–2022 seasons).

Treatments	Plant Height (cm)				Head Diameter (cm)				Head Volume (cm3)
	First Season		Second Season		First Season		Second Season		First Season		Second Season
Field Capacity
100% of FC	50.97	A	47.20	A	31.18	A	34.18	A	354.92	A	385.45	A
80% of FC	44.20	B	41.18	B	27.93	B	31.02	B	315.77	B	335.71	B
60% of FC	36.07	C	33.62	C	24.89	C	27.15	C	283.97	C	297.81	C
LSD at 5%	0.224		0.439		0.194		0.910		2.312		2.846
Abscisic Acid
ABA 00 ppm	46.32	A	43.19	A	26.83	D	29.44	D	300.70	D	326.94	D
ABA 25 ppm	44.47	B	41.30	B	27.61	C	30.29	C	313.35	C	333.44	C
ABA 50 ppm	42.90	C	39.74	C	28.34	B	31.25	B	325.85	B	343.71	B
ABA 75 ppm	41.29	D	38.44	D	29.23	A	32.14	A	332.96	A	354.52	A
LSD at 5%	0.372		0.373		0.217		0.177		3.281		1.996
Interaction
100% of FC × ABA 00 ppm	52.73	a	48.96	a	30.21	d	32.98	d	329.51	d	372.79	d
100% of FC × ABA 25 ppm	51.60	b	47.40	b	30.87	c	33.72	c	349.33	c	378.02	c
100% of FC × ABA 50 ppm	50.53	c	46.51	c	31.51	b	34.55	b	367.50	b	391.54	b
100% of FC × ABA 75 ppm	49.01	d	45.95	d	32.16	a	35.45	a	373.32	a	399.43	a
80% of FC × ABA 00 ppm	46.26	e	44.38	e	26.35	h	30.00	h	304.17	h	320.63	h
80% of FC × ABA 25 ppm	45.34	f	42.23	f	27.50	g	30.66	g	310.27	g	328.52	g
80% of FC × ABA 50 ppm	43.61	g	40.04	g	28.37	f	31.22	f	321.31	f	339.02	f
80% of FC × ABA 75 ppm	41.58	h	38.05	h	29.49	e	32.20	e	327.32	e	354.67	e
60% of FC × ABA 00 ppm	39.97	i	36.22	i	23.93	l	25.33	l	268.42	l	287.40	l
60% of FC × ABA 25 ppm	36.47	j	34.26	j	24.45	k	26.50	k	280.44	k	293.79	k
60% of FC × ABA 50 ppm	34.56	k	32.67	k	25.16	j	27.98	j	288.75	j	300.57	j
60% of FC × ABA 75 ppm	33.29	l	31.32	l	26.03	i	28.78	i	298.25	i	309.46	i
LSD at 5%	0.593		0.641		0.358		0.279		5.283		3.580

FC = field capacity, ABA = abscisic acid. Mean followed by the same letter\s within each column are not significantly different from each other at 0.5% level.

showed a great positive effect of abscisic acid concentrations under different rates of field capacity and their interaction with some growth parameters of red cabbage plants in both seasons.

As for the water stress factor, the highest significant value for plant height was 50.97 cm with 100% of FC compared to 60% of FC, which recorded 36.07 cm in the first season. Additionally, the head diameter and head volume took the same direction to plant height; this was true in both seasons. For the abscisic acid concentration factor, the highest significant value for plant height was 46.32 cm with the control compared to abscisic acid (75 ppm), for which the record was 41.29 cm in the first season. On the other hand, the head diameter and head volume took the contrary direction to plant height. They achieved the highest significant values with abscisic acid (75 ppm), while the lowest values were under the control treatment. For interaction, the highest significant value for plant height was 52.73 cm with 100% of FC, combined with abscisic acid (0 ppm) in the first season. On the other hand, the head diameter and head volume achieved the highest significant values with 100% of FC combined with abscisic acid (75 ppm).

Generally, the crop parameters were increased by increasing the % of FC, which may be due to increased soil moisture availability, moderate evaporation from the soil surface, and N, P, and K values [29,30]. Results regarding conserving water supply levels were in agreement with those mentioned by [31,32]. On the other hand, particularly at moderate and high-water potentials, plants treated with abscisic acid showed higher relative water content than untreated plants. These effects are due to decreased transpiration, which influences stomata control and may increase water flux into roots [33]. The results are congruent with those obtained by [11,34].

Table 3. Effect of water stress and abscisic acid on plant fresh weight, head fresh weight, and root fresh weight of red cabbage plants (2021–2022 seasons).

Treatments	Plant Fresh Weight (g)				Head Fresh Weight (g)				Root Fresh Weight (g)
	First Season		Second Season		First Season		Second Season		First Season		Second Season
Field Capacity
100% of FC	572.69	A	622.13	A	507.97	A	542.12	A	29.71	A	34.05	A
80% of FC	443.32	B	467.09	B	390.15	B	403.71	B	25.81	B	26.54	B
60% of FC	283.18	C	352.82	C	244.79	C	306.08	C	18.66	C	21.03	C
LSD at 5%	12.081		12.566		11.644		12.207		0.680		0.587
Abscisic Acid
ABA 00 ppm	391.14	D	437.23	D	333.83	D	367.37	D	26.87	A	30.10	A
ABA 25 ppm	416.54	C	462.95	C	361.69	C	396.92	C	25.94	B	28.55	B
ABA 50 ppm	448.49	B	494.23	B	398.54	B	432.46	B	24.07	C	26.25	C
ABA 75 ppm	476.08	A	528.31	A	429.83	A	472.45	A	22.04	D	23.93	D
LSD at 5%	9.951		6.521		10.136		6.358		0.515		0.525
Interaction
100% of FC × ABA 00 ppm	535.06	d	558.58	d	465.44	d	471.40	d	30.96	a	37.99	a
100% of FC × ABA 25 ppm	545.91	c	595.01	c	479.53	c	512.31	c	30.11	b	35.50	b
100% of FC × ABA 50 ppm	582.83	b	640.41	b	520.86	b	562.69	b	29.04	c	32.67	c
100% of FC × ABA 75 ppm	626.98	a	694.50	a	566.04	a	622.06	a	28.73	d	30.04	d
80% of FC × ABA 00 ppm	385.98	h	441.34	h	326.42	h	371.25	h	28.39	e	28.78	e
80% of FC × ABA 25 ppm	419.94	g	457.37	g	363.13	g	391.61	g	27.54	f	27.71	f
80% of FC. × ABA 50 ppm	467.85	f	467.62	f	416.98	f	406.27	f	25.08	g	25.55	g
80% of FC × ABA 75 ppm	499.50	e	502.06	e	454.07	e	445.72	e	22.24	h	24.13	h
60% of FC × ABA 00 ppm	252.37	l	311.78	l	209.61	l	259.46	l	21.27	i	23.55	i
60% of FC × ABA 25 ppm	283.78	k	336.48	k	242.41	k	286.85	k	20.16	j	22.45	j
60% of FC × ABA 50 ppm	294.79	j	374.65	j	257.78	j	328.42	j	18.09	k	20.52	k
60% of FC × ABA 75 ppm	301.77	i	388.36	i	269.36	i	349.57	i	15.13	l	17.61	l
LSD at 5%	17.205		12.912		17.341		12.536		0.907		0.894

FC = field capacity, ABA = abscisic acid. Mean followed by the same letter\s within each column are not significantly different from each other at 0.5% level.

shows the significant statistical impact of abscisic acid concentrations at different rates of field capacity and their interaction on some yield parameter indicators (plant, head, and root fresh weight) of red cabbage plants in both seasons.

As for the field capacity factor, the highest significant value for plant fresh weight was 572.69 g with 100% of FC, compared to 60% of FC, which recorded 283.18 g in the first season. Additionally, the head fresh weight and root fresh weight took the same direction to plant fresh weight; this was true in both seasons. For the abscisic acid concentration factor, the highest significant value for plant fresh weight was 476.08 g with abscisic acid (75 ppm) compared to the control, which recorded 391.14 g in the first season. Additionally, head fresh weight took the same direction to plant fresh weight in both seasons, while the root fresh weight took the contrary direction to plant fresh weight. Under interaction, the highest significant value for plant fresh weight was 626.98 g with 100% of FC combined with abscisic acid (75 ppm) in the first season. In addition, head fresh weight took the same direction as plant fresh weight in both seasons. On the other hand, the root fresh weight took the contrary direction to plant fresh weight.

The results of the current study showed that water supply levels had a significant effect on yield characteristics; the present investigation agrees with [35,36,37]. The increase in crop yield is due to the facilitation of plant water uptake under moderate water levels without exposure to water stress [30,38]. Additionally, this might be due to the effect of water on some metabolic processes in the plant cell, and the availability of N, K, and P uptake in the plant root zone as well as enhanced photosynthetic processes and carbohydrate production [39,40,41].

Treatments with abscisic acid had higher yield parameter indicators than untreated plants, especially under moderate and high-water contents. Such effects are due to reduced transpiration via its effects on stomatal regulation and are possible by increasing water flux into roots [33].

The existing data in

Table 4. Effect of water stress and abscisic acid on numbers of green leaves, numbers of red leaves, and total numbers of leaves per plant of red cabbage (2021–2022 seasons).

Treatments	No. of Green Leaves Per Plant				No. of Red Leaves Per Plant				No. of Total Leaves Per Plant
	First Season		Second Season		First Season		Second Season		First Season		Second Season
Field Capacity
100% of FC	6.69	A	10.45	A	17.32	A	20.46	A	24.01	A	30.91	A
80% of FC	6.19	B	9.33	B	15.99	B	17.36	B	22.18	B	26.70	B
60% of FC	5.53	C	8.42	C	13.67	C	15.17	C	19.21	C	23.59	C
LSD at 5%	0.026		0.045		0.149		0.194		0.162		0.156
Abscisic Acid
ABA 00 ppm	6.37	A	9.75	A	16.45	A	18.59	A	22.82	A	28.34	A
ABA 25 ppm	6.23	B	9.50	B	15.81	B	18.00	B	22.04	B	27.50	B
ABA 50 ppm	6.07	C	9.25	C	15.42	C	17.42	C	21.49	C	26.67	C
ABA 75 ppm	5.88	D	9.11	D	14.96	D	16.65	D	20.84	D	25.76	D
LSD at 5%	0.515		0.062		0.119		0.170		0.157		0.166
Interaction
100% of FC × ABA 00 ppm	6.94	a	10.81	A	17.80	a	21.82	a	24.75	a	32.62	a
100% of FC × ABA 25 ppm	6.78	b	10.48	B	17.41	b	20.93	b	24.20	b	31.41	b
100% of FC × ABA 50 ppm	6.59	c	10.33	C	17.16	c	20.07	c	23.75	c	30.40	c
100% of FC × ABA 75 ppm	6.44	d	10.19	D	16.91	d	19.04	d	23.34	d	29.23	d
80% of FC × ABA 00 ppm	6.34	e	9.66	E	16.60	e	18.25	e	22.94	e	27.90	e
80% of FC × ABA 25 ppm	6.22	f	9.51	F	16.15	f	17.60	f	22.38	f	27.10	f
80% of FC × ABA 50 ppm	6.15	g	9.14	G	15.76	g	17.11	g	21.91	g	26.25	g
80% of FC × ABA 75 ppm	6.05	h	9.03	H	15.43	h	16.49	h	21.49	h	25.52	h
60% of FC × ABA 00 ppm	5.83	i	8.78	I	14.93	i	15.72	i	20.76	i	24.50	i
60% of FC × ABA 25 ppm	5.69	j	8.51	J	13.87	j	15.47	j	19.55	j	23.98	j
60% of FC × ABA 50 ppm	5.47	k	8.28	K	13.35	k	15.08	k	18.82	k	23.37	k
60% of FC × ABA 75 ppm	5.16	l	8.10	L	12.54	l	14.42	l	17.70	l	22.52	l
LSD at 5%	0.081		0.099		0.208		0.291		0.264		0.276

FC = field capacity, ABA = abscisic acid. Mean followed by the same letter\s within each column are not significantly different from each other at 0.5% level.

clearly show the statistical impact of abscisic acid concentrations under different rates of field capacity and their interaction on some growth parameters of red cabbage plants in both seasons.

As for the water stress factor, the highest significant value for the total number of leaves per plant was 24.01 with 100% of FC compared to 60% of FC, which recorded 19.21 in the first season. Additionally, the numbers of green and red leaves took the same direction as the total number of leaves; this was true in both seasons. For abscisic acid concentration, the highest significant value for the total number of leaves per plant was 22.82 with the control compared to abscisic acid (75 ppm), which recorded 20.84 in the first season. Additionally, the numbers of green and red leaves took the same direction as the total number of leaves; this was true in both seasons. For interaction, the highest significant value for the total number of leaves per plant was 24.75 with 100% of FC combined with (0 ppm) of abscisic acid in the first season. In addition, the numbers of green and red leaves showed a nearly similar trend as the total number of leaves per plant in both seasons. Overall, the results of water stress are consistent with those reported by [9,10,42,43]. The effect of abscisic acid is that it enhances the drought tolerance of coffee seedlings by delaying the onset of wilting point [11]. These results are in agreement with those obtained by [34,43].

Table 5. Effect of water stress and abscisic acid on total chlorophyll, leaf cell sap TSS, and leaf cell sap osmotic pressure of red cabbage plants (2021–2022 seasons).

Treatments	Total Chlorophyll SPAD				Leaf Cell Sap TSS				Leaf Cell Sap Osmotic Pressure (atm)
	First Season		Second Season		First Season		Second Season		First Season		Second Season
Field Capacity
100% of FC	89.29	A	85.67	A	4.63	C	4.77	C	3.58	C	3.69	C
80% of FC	75.81	B	80.68	B	5.54	B	5.63	B	4.28	B	4.35	B
60% of FC	66.59	C	71.75	C	6.50	A	6.39	A	5.04	A	4.95	A
LSD at 5%	3.096		0.789		0.189		0.094		0.151		0.067
Abscisic Acid
ABA 00 ppm	82.63	A	82.73	A	5.00	D	5.20	D	3.86	D	4.02	D
ABA 25 ppm	78.24	B	80.50	B	5.28	C	5.50	C	4.08	C	4.25	C
ABA 50 ppm	75.69	C	78.30	C	5.94	B	5.74	B	4.59	B	4.44	B
ABA 75 ppm	72.36	D	75.94	D	6.00	A	5.94	A	4.64	A	4.59	A
LSD at 5%	3.200		0.581		0.185		0.048		0.147		0.068
Interaction
100% of FC × ABA 00 ppm	98.15	a	88.35	A	4.00	g	4.00	k	3.12	g	3.12	k
100% of FC × ABA 25 ppm	89.37	b	86.13	B	4.50	f	4.60	j	3.49	f	3.56	j
100% of FC × ABA 50 ppm	87.33	c	84.63	C	5.00	e	5.13	i	3.86	e	3.97	i
100% of FC × ABA 75 ppm	82.30	d	83.57	D	5.00	e	5.33	h	3.86	e	4.12	h
80% of FC × ABA 00 ppm	80.00	e	82.87	E	5.00	e	5.50	g	3.86	e	4.25	g
80% of FC × ABA 25 ppm	77.60	f	81.47	F	5.33	d	5.50	g	4.12	d	4.25	g
80% of FC × ABA 50 ppm	74.67	g	80.37	G	5.83	c	5.60	f	4.51	c	4.32	f
80% of FC × ABA 75 ppm	70.97	h	78.03	H	6.00	b	5.93	e	4.64	b	4.59	e
60% of FC × ABA 00 ppm	69.73	i	76.97	I	6.00	b	6.10	d	4.64	b	4.72	d
60% of FC × ABA 25 ppm	67.77	j	73.90	J	6.00	b	6.40	c	4.64	b	4.96	c
60% of FC × ABA 50 ppm	65.07	k	69.90	K	7.00	a	6.50	b	5.45	a	5.04	b
60% of FC × ABA 75 ppm	63.80	l	66.23	L	7.00	a	6.57	a	5.45	a	5.09	a
LSD at 5%	5.327		1.029		0.309		0.143		0.247		0.116

FC = field capacity, ABA = abscisic acid. Mean followed by the same letter\s within each column are not significantly different from each other at 0.5% level.

illustrates the impact of abscisic acid concentrations under different rates of field capacity and their interaction on total chlorophyll, leaf cell sap TSS, and leaf cell sap osmotic pressure parameters of red cabbage plants in both seasons.

During the first season, the highest significant value for total chlorophyll was 89.29 with 100% of FC compared to 60% of FC, which recorded 66.59. For the abscisic acid concentration factor, the highest significant value for total chlorophyll was 82.63 with the control compared to abscisic acid (75 ppm), which recorded 72.36 in the first season. For interaction, the highest significant value for total chlorophyll was 98.15 with 100% of FC combined with abscisic acid (0 ppm) in the first season; this was true in both seasons.

On the other hand, leaf cell sap TSS and leaf cell sap osmotic pressure parameters took a contrary direction to total chlorophyll in both seasons, which was true with water stress factor and abscisic acid concentrations as well as the interaction between them. In this respect, the results of water stress are in agreement with those obtained by other researchers [30,44].

The results show that abscisic acid has a positive effect on plants, as it protects them from drought damage by causing stomata closure to reduce water loss via transpiration and increase hydraulic conduction of water movement from roots to leaves [44,45]. The results are in agreement with those obtained by [46,47].

Generally, the use of abscisic acid (ABA) led to reduced stomatal conductance and decreased both transpiration and net photosynthetic rate [48]. This decrease in photosynthetic rate is related to the accumulation of carbohydrates. In addition, the respiration rate did not increase with higher carbohydrate levels in the presence of ABA [49].

3.4. Total Yield of Red Cabbage

Figure 6a,b showed the effect of abscisic acid concentrations under different rates of field capacity on the fresh yield per hectare of red cabbage. Overall, the highest values were 27 and 29.7 ton ha⁻¹ under 100% of FC and abscisic acid (75 ppm) for the two growing seasons, respectively. The yield under ABA (75 ppm) was higher than that of ABA (0 ppm) by 18.5% and 24% for the first and second seasons, respectively, under 100% of FC. The same pattern can be seen for 80 and 60% of FC. Irrigation with 80% FC and 75 ppm abscisic acid had the same effect on red cabbage yield as irrigation with 100% FC and 0 ppm abscisic acid for the two growing seasons; this means water savings of 20% can be achieved without significantly compromising the yield [50].

4. Conclusions

Climate change may have an impact on estimates of crop evapotranspiration (ET_c). Obtaining the actual values of the K_c coefficient considering the new scenarios can fill the current gap on this topic at this time. In this study, the crop coefficient was estimated at different irrigation levels (100, 80, and 60% of field capacity, “FC”), utilizing the ratio between the adjusted or actual crop evapotranspiration (ET_{c adjust}) and potential (ET_o) from the equation of Penman–Monteith. The results showed that the optimal values of crop coefficient (K_c) were achieved at 100% of FC, followed by 80% of FC. Using different concentrations of abscisic acid improved vegetative growth and yield at different irrigation rates of field capacity (FC) for the two growing seasons. The highest vegetative growth and yield were obtained with abscisic acid (75 ppm) and 100% of FC, followed by abscisic acid (75 ppm) and 80% of FC in both seasons. On the other hand, the use of abscisic acid (ABA) led to reduced stomatal conductance and decreased both transpiration and net photosynthetic rate. This decrease in photosynthetic rate is related to the accumulation of carbohydrates. In addition, the respiration rate did not increase with higher carbohydrate levels in the presence of ABA. Therefore, abscisic acid has a positive effect on plants, as it protects them from drought damage by causing stomata closure to reduce water loss via transpiration and increase hydraulic conduction of water movement from roots to leaves.

Overall, the use of abscisic acid (75 ppm) with 80% of (FC) results in cost-effective vegetative growth and yield of red cabbage plants. Therefore, it could be the key and most effective way to further reduce water use and increase crop productivity.