For most African countries, agriculture remains one of the main sources of economic income, and its development has had a considerable impact on food security and poverty reduction [1] . However, in recent decades, it has been observed that the increase in agricultural production is rather due to an expansion of agricultural land and very little to a change in production techniques and an improvement in yield. Currently, one in four people suffer from malnutrition; and this situation is likely to deteriorate over the years because it is estimated that there will be a 20% reduction in growing periods by 2050 in West and South Africa, which will lead to a drop in the yield of certain crops [1] . In addition, it should not be forgotten that the world population is projected to increase by 2.4 billion between 2013 and 2050, half of which will occur in sub-Saharan Africa [2] . Agriculture in African countries must, therefore, feed an ever-growing population while in recent years, we have observed a strong urbanization in these regions leading to a decrease in arable land [3] ; thus, new practices have emerged to increase the efficiency of agricultural production, among which we can cite the intense use of pesticides. However, this practice, although supposed to prevent pests, is often criticized for its harmful effect on health and the environment [4] . In addition, Africa is suffering from the adverse effects of climate change, which threatens the stability and yield of agricultural production. Indeed, we observe an irregularity of growing seasons, formerly well-known and controlled; and projections of extreme climatic conditions such as heavy rains, downpours, often long and intense periods of sunshine, soil erosion causing nutrient drainage [5] . In Côte d’Ivoire, there is a shortening and shift in growing seasons [6] : over the period 1951-2000, the growing season was reduced by 30 days [7] . Thus, it is necessary to make a significant transformation in agriculture in Africa so that it can meet the challenges related to the implementation of sustainable solutions in response to climate change and food insecurity [8] . To do this, new cultivation techniques could be integrated in which the climatic environment of the plant is controlled to avoid extreme external conditions. In this sense, important factors such as irrigation, nutrient supply, temperature, humidity and the plant’s light environment must be controlled to ensure optimal production. Light is an important and essential factor for plant development; through different processes such as photosynthesis, photomorphogenesis, photoperiodism and phototropism, light acts on the development, growth, secondary metabolisms of plants; and thus on the quality and quantity of agricultural production [9] . To assess the light environment in agricultural production, global radiation is very often used as a performance indicator; this indicator is part of the parameters measured and evaluated in agri-meteorology bulletins (intended for agricultural production) regularly presented by national meteorological agencies as is the case in Côte d’Ivoire [10] . However, the spectral distribution of global solar radiation measured at the earth’s surface has a broad band ranging from 280 nm to 3000 nm. And only a portion of the global radiation between 400 and 700 nm, defined as photosynthetically active radiation by the International Commission on Illumination, is actually used by plants for their growth, development, physiology and metabolism; and this is done through plant photoreceptors such as chlorophylls, phytochromes, cryptochromes, phototropins that use the captured energy from light to mediate various important biological processes [11] [12] . The DLI constitutes the daily integral of this photosynthetically active radiation; in other words, the DLI describes the daily number of photosynthetically active photons delivered to a given area during a day [13] . The concept of DLI was introduced in the 1980s when Armitage et al., on an experiment conducted on Geranium seeds, demonstrated in their work that minimum DLIs of 1.94 mol∙m⁻²∙d⁻¹ and 3.25 mol∙m⁻²∙d⁻¹ are necessary to initiate flowering and flower development respectively [14] . In 2002, a DLI map of 48 states of the United States was published by Korczynski et al., then updated in 2018 by Faust and Logan [13] [15] ; The DLI concept initially little used by producers in the United States has gradually integrated the jargon of horticulturists thanks to strong awareness and training programs dedicated to professionals; and this has ultimately contributed to improving the management of the light environment in commercial agricultural production of a wide variety of crops [13] . In 2021, a DLI mapping of the Nordic and Baltic countries was carried out by Hernandez [16] ; then in 2024, András Jung et al., proposed in a first publication a DLI mapping of Hungary, and in a second publication that of Spain [17] [18] . Knowing the DLI for a given area, growers can optimize their crop management strategy, such as selecting the most suitable crops at the DLI level, determining optimal growing periods, and selecting the best location for their crops [17] . DLI has also proven to be a very useful and reliable tool for greenhouse cultivation, allowing growers to assess their light requirements and determine the need for supplemental lighting or shading to reach a certain DLI threshold [16] . In addition, several studies have shown the influence of DLI on plants. An experiment conducted on cucumber showed that a DLI of 30 mol∙m⁻²∙d⁻¹ reduced the growth period compared to 10 mol∙m⁻²∙d⁻¹ and 5.5 mol∙m⁻²∙d⁻¹ [19] . Gavhane et al. have also demonstrated, in iceberg lettuce culture, that DLI affects physiological (photosynthesis rate, transpiration rate, stomatal conductance), morphological (leaf width, root size of the plant) and nutritional (antioxidant, total phenol, vitamin C) parameters [20] . In addition to this, research on tomato has shown that DLI influences fresh mass; there is an increase in fresh mass when going from a DLI of 10.4 mol∙m⁻²∙d⁻¹ to a DLI of 18.4 mol∙m⁻²∙d⁻¹ [21] . Experiments carried out on leafy vegetables such as lettuce, chicory, basil and arugula confirm the influence of DLI on fresh biomass [22] . Visual appearance is also influenced by DLI; It was found in a study conducted on lettuce that an increase in DLI deteriorates the visual appearance of lettuce and reduces the photosynthetic quantum yield of PSII as well as the light use efficiency [23] . Thus, global radiation, although proportionally linked to DLI, has a high proportion not used by plants; it therefore reflects less of the production activity of plants compared to DLI which turns out to be a more effective performance indicator for plants. DLI should be monitored, regularly evaluated, and integrated into agricultural management systems as an essential decision-making tool. In this context, our objective is to propose a mapping of DLI in the main regions of Côte d’Ivoire with the aim of making it a decision-making tool in the management of agricultural production; this tool, once integrated, could enable farmers to optimize their production.

Results obtained, as illustrated in Table 1 and Table 2 , give DLIs that vary between 28.73 ± 0.62 to 52.47 ± 0.53 mol∙m⁻²∙d⁻¹ across the country depending on the region and the time of year. The lowest DLIs are recorded around the month of August and the peaks are observed during the months of March-April. The north of the country, precisely the regions of Korhogo and Odienné give the highest DLI values and the regions of Adiaké and Tabou in the south give the lowest values. In terms of spatio-temporal variation, the highest DLIs are observed in the north of the country in the regions of Korhogo and Odienné in the month of March (respectively 52.47 ± 0.53 and 50.18 ± 0.83 mol∙m⁻²∙d⁻¹); and the lowest DLIs appear in the southern regions of Tabou and Adiaké in August (respectively 28.92 ± 0.57 and 28.73 ± 0.62 mol∙m⁻²∙d⁻¹).

In the United States of America, DLI ranges from 5 - 10 mol∙m⁻²∙d⁻¹ in the north of the country in December to 55 - 60 mol∙m⁻²∙d⁻¹ in the southwest of the country in May-July [15] ; this map has been updated with a new peak of 60 - 65 mol∙m⁻²∙d⁻¹ appearing in the southwestern United States only in June, and a minimum of 0 - 5 mol∙m⁻²∙d⁻¹ in the north of the country due to the inclusion of Alaska on the map [13] . In Hungary, in Central Europe, DLI ranges from 4-5 mol∙m−2∙d−1 in winter to 46 - 47 mol∙m⁻²∙d⁻¹ in summer [17] . The DLI in Spain, a southwestern European country, ranges from 5 mol∙m⁻²∙d⁻¹ in winter to 55 mol∙m⁻²∙d⁻¹ in summer [18] . The DLI results obtained for Côte d’Ivoire, ranging from 28.73 ± 0.62 to 52.47 ± 0.53 mol∙m⁻²∙d⁻¹, show a much smaller variation gap and, therefore, a certain relative stability compared to the regions mentioned above.

Furthermore, several studies have shown that DLI has effects on plant growth, and recommendations have been made on appropriate DLI values for the cultivation of certain species (as shown in Table 3 ). For greenhouse cultivation of lettuce, tomato, sweet pepper, and cucumber, the minimum required DLIs are 12, 12 - 14, 20 - 30, and 30 mol∙m⁻²∙d⁻¹, respectively [19] [33] - [35] . An experiment conducted on cucumber seedlings showed that an increase in DLI by 1% using supplemental artificial lighting resulted in an increase in cucumber shoot dry mass by 1.2% - 1.8%; the growth and morphology of these cucumber plants were significantly improved under a solar DLI of 16.2 ± 5.3 mol∙m⁻²∙d⁻¹ compared to a DLI of 5.2 ± 1.2 mol∙m⁻²∙d⁻¹ [36] . Similarly, according to Dorais, one way to estimate the effects of light on production is to use the 1% rule, which states that a 1% reduction in light will also result in a 1% reduction in production [19] . For hydroponic spinach cultivation, a DLI of 17.3 mol∙m⁻²∙d⁻¹ is recommended, which yields the highest net photosynthesis rates [37] . For vertical farming of leafy vegetables such as lettuce, basil, chicory, and arugula, a DLI of 14.4 mol∙m⁻²∙d⁻¹ is recommended [22] .

Clearly, the preceding paragraphs demonstrate the importance of DLI in plant growth and development. The DLI trends obtained across the country could also explain the definitions of growing seasons in the north and south of the country. Indeed, in the north of the country, during the entire growing season which begins in April-May and ends in October-November, the DLI is maintained at a relatively high level which does not fall below 40 mol∙m⁻²∙d⁻¹; and therefore, one could assume that the cultivated plants do not experience a DLI deficit during this period. While in the south of the country, there are two growing seasons separated by a period where the DLI has its lowest level, that is to say during the months of July-August with a DLI which falls below 30 mol∙m⁻²∙d⁻¹ in some regions of the south; in order to avoid a DLI deficit in crops, the interruption of growth at this time could prove necessary for a recovery at a time when the DLI rises to higher values, acceptable for the plants; Hence the definition of these two growth periods in the south, which takes on its full meaning with a justification by the DLI.

It should also be noted the increasing use of agricultural greenhouses in Africa. De Bon et al., in their expert report on market gardening in Côte d’Ivoire, recommends the use of greenhouse shelters to achieve better yields and obtain quality products available year-round [38] . Indeed, in greenhouses, the light transmission rate inside varies from 35% to 70% [13] ; and this reduction is due to the greenhouse infrastructure, the glazing materials, or the quality of the polyethylene covers used and the shade curtains. This significantly reduces the DLI for crop growth: for example, for the Adiaké region which has an average DLI of 30.8 mol∙m⁻²∙d⁻¹ in the month of September, greenhouse cultivation could lower this DLI to between 10.78 and 21.56 mol∙m⁻²∙d⁻¹; in this case, DLI mapping could be used to assess the missing DLI gap to reach the minimum DLI required by the plant to grow optimally and artificial lighting can be used as a supplement to fill this gap. In this sense, one could rely on various studies that evaluate the impact of artificial lighting on food crops such as okra [39] - [41] . In addition, it is noted that some regions of the country could face certain challenges related to excess light that could be managed by advanced techniques for reducing environmental lighting of the plant such as shading. This is the case of the northern regions of Ivory Coast, such as the Korhogo region whose average DLI reaches 52.47 mol∙m⁻²∙d⁻¹ in the month of March at the beginning of the growing season; this DLI could be saturated and inappropriate for the optimal growth of certain crops. The use of a greenhouse shelter with a transmission rate of 70% could reduce this DLI to 36.73 mol∙m⁻²∙d⁻¹. The shading technique is quite complex because, using a spectral filter, it modifies both the quantity of light but also its spectral quality; which has an impact on different microclimatic factors such as humidity, photosynthetic activity, production of secondary metabolites, enzymatic process and in the end, affect plant production [18] .

This study presented the average DLI values in the main regions of Côte d’Ivoire over a 17-year period. Monthly averages for different regions were also determined, as well as their variations throughout the year. In addition, an analysis of DLI levels during different growing seasons was performed. This analysis provides farmers with an idea of the light environment in these regions to facilitate decision-making in their agricultural management processes.

Furthermore, this study highlights several possibilities for using DLI mapping in Côte d’Ivoire: 1) as an agricultural management tool to decide on the location and appropriate time to start growing a specific crop. 2) During certain periods in locations where the DLI is high enough for cultivation, provide shading systems or agricultural greenhouses with a low transmission rate to control the DLI and reduce it to a lower level acceptable for crops. 3) For periods and regions where the DLI is low for the crops under consideration, greenhouse cultivation methods should be considered, incorporating artificial lighting in addition to natural light to increase the DLI. 4) This could be a tool in investigating the causes of low or high production during certain periods of the year or in certain regions; a link should then be made between the plant’s exposure to a certain DLI (optimal or not) and the quantity and quality of the resulting production.

Integrating DLI as an agricultural management tool requires various measures that should be included in our transformation process towards sustainable agriculture resilient to climate change, namely: 1) continuous assessment of DLIs in agricultural premises using measuring instruments. 2) Promoting scientific research to determine optimal DLIs for strategic crops for local consumption or export. 3) Awareness of the integration of the lack of control of DLI as one of the potential causes of the advent of a certain quality and quantity of production of our crops, alongside other parameters such as rainfall, soil quality.

National Polytechnic Institute Félix Houphouet-Boigny (INP-HB) of Yamoussoukro supported this research.