Effects of Water Stress on Leaf Photosynthesis and Yield of Melon and Tomato Crops Grown under Mediterranean Conditions of the Northeast of Morocco ^†

Abstract

Drought is the most important limiting factor affecting plant yield and photosynthesis and has impacts on reducing yields. The objective of this work is to study the impacts of drought regimes of irrigation on Chlorophyll a fluorescence, stress status and yield of melon and tomato crops. Experiments were conducted under the Mediterranean climate conditions. Two cultivars of each crop and three irrigation strategies (100%, 75% and 50%) were used. Many physiological stress indices based on chlorophyll a fluorescence parameters were measured. Results showed a change in crop light phase of photosynthesis and a decrease in yields according to treatments and cultivars.

1. Introduction

The United Nations Climate Change Conference (COP26) held recently in Glasgow, Scotland, adopted an agreement to accelerate the process in order to face global warming [1]. Morocco is among the African countries affected by the irregularity and the decrease in rainfall during the last decade [2]. The focus of research on water management is an important step in order to decrease the impact of drought on field production.

Many researchers studied the impact of water scarcity on physiological aspects such as plant growth, photosynthesis, metabolite translocation, plant yield and quality, etc. Yildirim et al. (2009) [3] reported that plants should be irrigated during the whole period of the production. Fabeiro et al. (2002) [4] found that water stress applied during the flowering period negatively influenced the fruit yield and quality of muskmelon. Moreover, low stress levels were found to result in maximum yields and high stress resulted in the lowest yields [5]. Birhanu and Zeleke (2010) [6] also conducted a study to determine the effect of 0%, 25%, 50% and 75% crop evapotranspiration deficits on the fruit yield and quality on drip-irrigated tomatoes. They showed that growth, yield and quality were directly related to water shortage levels.

Moreover, it has been shown that under high temperature and radiation levels, drought stress enhances the inhibition of electron transport [7]. Chlorophyll a fluorescence is a non-destructive measurement for studying the equilibrium between metabolic and energy evolving processes that may be affected by temperature and drought stresses [8]. Chlorophyll a fluorescence indicates the transfer of electrons during the light phase of photosynthesis from the excitation of chlorophyll by light energy to the transfer of electrons for the dark phase [9]. Drought stress reduces variable fluorescence (F_v), initiative fluorescence (F₀) and quantum yield (F_v, F_m) [10]. Chlorophyll a fluorescence was considered as a useful tool for screening and breeding under dry conditions and for high temperature resistance. The ratio F_v/F_m varies between 0.75 and 0.85 in non-stressed plants [11], and it is a good indicator for stress level status. Other parameters of the Chl a fluorescence (Vi, Vj, N, S, ABS, TR, ETR, DI, PI, etc.) also present good indicators for plant stress status.

The objective of this research work is to study, under Mediterranean climate conditions and at a commercial production level, the impacts of different drought strategies of irrigation on chlorophyll a fluorescence, stress status and yield of melon and tomato, which represent two of the main crop productions in the country.

2. Material and Methods

This study was conducted in two sites spaced out at 70 km. The first site is located in the Driouch area, Morocco (34°59′30.3″ N 3°22′30.6″ W), and was intended for melon experiments. However, the second site was located in the ARID area, Nador, Morocco (35°08′52.6″ N 2°57′40.3″ W), and was intended for tomato experiments. Both sites were realized at the commercial farm production scale. For the melon trial, two commercial cultivars were used: “cv. Missoura” and “cv. Wifaq”. An area of 0.5 ha was used for each cultivar, at a plant density of 8333 plants per ha. Rows were spaced by 2 m, and the distance between two plants of the same row was 60 cm. Date of plantation was 23 May 2021. The trial ended on 10 September 2021. For tomato trials, two commercial cultivars were used: “cv. Karima” and “cv. Jad”. An area of 1200 m was used for each cultivar, at plant density of 14285 plants per ha. Each row was spaced at 1.20 m and the distance between two plants of the same row was 70 cm. Date of plantation was 28 August 2021, and the trial was finished on 25 January 2022. Both crops were managed according to commercial production practices.

Three irrigation treatments were applied: 100%, 75% and 50% water. An irrigation tape (200 micron with dripper of 1.5 L/h, Rolland tape, Roll-Drip, France) was used. The distances of 20 cm, 15 cm and 10 cm between two drippers were used for 100%, 75% and 50% water, respectively. In the case of melon experiments, the length of each irrigation lateral was 100 m and split on 2 semi-lines of 50 m. However, for tomato experiments, the length of each irrigation lateral was 50 m long.

For the melon trial, six repetitions of each treatment were used. Each treatment replicate contains 6 blocks of 6 plants. For tomato experiments, 8 replicates of each treatment were used containing 6 blocks of 3 plants.

The chlorophyll a fluorescence measurements were recorded on the fifth young expanded leaf, during a hot and sunny day. The HPEA device (Handey PEA, Hansatech, King’s Lynn, UK) was used. Both cultivars of melon and tomato leaves were adapted to the dark for 30 min using a clip. Then, a light flash of 3000 µmol/m/s (650 nm) was applied for 1 s (gain = ×1) on the leaf adapted to darkness for 30 min. The measurements were taken on 6 plants for each treatment, from 12:00 to 14:00. F_v/F_m and DI0/CS0 were measured (F_v = variable fluorescence; F_m = maximum fluorescence; DI/CS₀ = Dissipated heat per cross section).

The fruit harvests were carried out on 18 July and 3 August 2021 for “cv. Missoura” and for “cv. Wifaq”, respectively. However, the last harvests were carried out on 19 September and 27 August 2021, respectively. For tomato trials, the harvest starts on 3 November and ends on 23 January 2022. Total yield was counted for both experiments.

3. Results and Discussion

During the hot period of the day and clear sky conditions (19 July 2021; 12:00–14:00 with Tmax = 35 °C; Tmin = 20 °C; Relative humidity = 58%; and wind speed = 11 km/h), the F_v/F_m ratio of the cv. Missoura melon was lowered and the DI/CS0 ratio was increased by the water shortages (75% and 50%). However, for cv. Wifaq, both ratios were not affected by the irrigation treatment (

Table 1. Fluorescence parameters (F_v/F_m and DI/CS₀) of melon and tomato plants grown under three strategies of irrigation (100%, 75% and 50% water).

			100% Water	75% Water	50% Water
Melon 19/7/2021	cv. Missoura (12:00–12:45)	Fv/Fm	0.81 ± 0.02	0.77 ± 0.03	0.77 ± 0.03
		DI/CS0	86 ± 13	114 ± 17	120 ± 36
	cv. Wifaq (13:15–14:00)	Fv/Fm	0.80 ± 0.03	0.81 ± 0.02	0.79 ± 0.01
		DI/CS0	77 ± 6	70 ± 9	78 ± 10
Tomato 29/9/2021	cv. Karima (11:40–12:10)	Fv/Fm	0.77 ± 0.04	0.73 ± 0.05	0.72 ± 0.04
		DI/CS0	123 ± 26	185 ± 64	192 ± 44
	cv. Jad (12:20–12:50)	Fv/Fm	0.73 ± 0.08	0.73 ± 0.06	0.78 ± 0.03
		DI/CS0	197 ± 77	181 ± 76	122 ± 37

F_v = variable fluorescence; F_m = maximum fluorescence; DI/CS₀ = Dissipated heat per cross section. Data are mean of 6 repetitions ± standard deviation.

). This means that the response of the cultivar was not the same for irrigation treatments and cv. Missoira was more affected by water stress compared to cv. Wifaq. It was shown by Baker and Horton (1987) [11] that the F_v/F_m ratio varies between 0.75 and 0.85 for non-stressed plants. The DI/CS₀ ratio showed high values, which indicates that plants were stressed and would have dissipated heat energy. A similar occurrence was observed for cv. Missoura, which was more stressed under the 50% and 75% treatments compared to the 100% treatment. The DI/CS₀ values were 86, 114 and 120, for the 50%, 75% and 100% irrigation treatments, respectively.

For tomato, during a day of clear sky (29 September 2021; 11:40–12:50 with T_max = 29 °C; T_min = 22 °C; Relative humidity = 62%; and wind speed = 16 km/h), the same tendencies were observed for cv. Karima than cv. Missoura. For cv. Jad, no differences were recorded for the F_v/F_m and for DI/CS₀ ratios between treatments. This could mean that cv. Karima did not have the same response to water shortage as cv. Jad. This was shown by Salinas-Vargas et al. (2021) [12], which indicates that tomato cultivars could have different responses (tolerance or resistance) for water stress.

The yield of melon was lowered by shortage in irrigation amount (

Table 2. Total yield (kg/plant) of melon and tomato crops grown under three strategies of irrigation (100%, 75% and 50% water).

		100% Water	75% Water	50% Water
Melon (kg/plant)	cv. Missoura	6.0 ± 1.5	4.9 ± 1.1	4.4 ± 1.0
	cv. Wifaq	7.3 ± 1.0	6.9 ± 1.2	5.8 ± 1.1
Tomato (kg/plant)	cv. Karima	2.9 ± 0.7	2.3 ± 0.7	2.2 ± 0.7
	cv. Jad	2.4 ± 0.9	2.2 ± 1.1	2.1 ± 0.7

Data are the mean of 36 repetitions (melon); 48 repetitions (tomato) ± standard deviation.

), for both cultivars (cv. Missoura and cv. Wifaq). This indicates that 100% treatment (adapted by the farmer) was the appropriate irrigation strategy. This is consistent with the findings of Fabeiro et al. (2002) [4], who found that water stress negatively influenced the fruit yield of melon. For tomato, the treatments did not affect the yield of both cultivars. This could be also be explained by the fact that tomato trials were carried out in fall, where the evapotranspiration demand is lower than conditions of the melon experiments in summer. As reported by Agele et al. (2011) [11], the high evapotranspiration rate has a significant effect on decreasing the plant yield for tomato crops.

4. Conclusions

This study showed that reducing the amount of irrigation water decreased the yield of both cultivars of melon (cv. Missoura and cv. Wifaq). This reduction was higher for the 50% irrigation compared to 75% and 100% irrigation. However, this shortage in water did not affect yields of both tomato cultivars (cv. Karima and cv. Jad). For chlorophyll a fluorescence, the results showed that the F_v/F_m ratio was decreased and the DI/CS0 ratio was increased for cv. Missoura (melon) and cv. Karima (tomato) with the 75% and 50% irrigation treatments compared to the control treatments, and this means that plants were more stressed. Economic studies on the amount of water saving and yield decrease will be also interesting in order to determine the best strategy of water management that could be adapted by the grower in such conditions.