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Original Article • Rev. bras. eng. agríc. ambient. 30 (8) • 2026 • https://doi.org/10.1590/1807-1929/agriambi.v30n8e300556 linkcopy

Open-access Responses of cassava varieties to harvest timing under rainfed conditions in the Agreste region of Alagoas, Brazil1

Respostas de variedades de mandioca a épocas de colheita em condições de sequeiro no Agreste de Alagoas, Brasil

AuthorshipSCIMAGO INSTITUTIONS RANKINGS

ABSTRACT

This study evaluated the growth, ecophysiological performance, and yield of cassava varieties as affected by harvest timing under rainfed conditions in the Agreste region of Alagoas, Brazil. The experiment was conducted from April 2023 to December 2024, using a 2 × 5 factorial scheme in a randomized block design with four replicates. Two cassava varieties (Caravela and Pretinha) were assessed at five harvest times (12, 14, 16, 18, and 20 months after planting), with agrometeorological, growth, physiological, and yield variables monitored throughout the cycle. Although the regional climate, characterized by adequate annual rainfall and mean air temperature, was generally favorable for cassava cultivation, summer droughts limited vegetative growth. The Pretinha variety showed greater shoot dry biomass accumulation, reaching 51.4 t ha-1 at 20 months after planting (MAP). By contrast, Caravela exhibited superior root performance, achieving higher fresh root yield (61.5 t ha-1), dry root yield (34.6 t ha-1), starch yield (18.7 t ha-1), and harvest index (0.7). Physiological evaluations revealed significant cassava plasticity in response to seasonal water deficit, with coordinated adjustments in photosynthesis, stomatal conductance, and transpiration across harvest times. Overall, Caravela is more suitable for industrial purposes, whereas Pretinha is better adapted to production systems that require high shoot biomass.

Key words:
Manihot esculenta Crantz; rainfed agriculture; yield components; physiological plasticity; water deficit

HIGHLIGHTS:

Caravela achieved higher fresh root yield at 17 months after planting (MAP) and higher dry matter and starch yields at 20 MAP.

Pretinha showed greater vegetative vigor and shoot dry biomass accumulation, reaching 51.4 t ha⁻1 at 20 MAP.

Seasonal water deficit induced coordinated gas exchange adjustments, demonstrating high physiological plasticity.

RESUMO

Este estudo teve como objetivo avaliar o crescimento, o desempenho ecofisiológico e a produtividade de variedades de mandioca em função das épocas de colheita na região do Agreste de Alagoas, Brasil. O experimento envolveu duas variedades de mandioca (Caravela e Pretinha) e cinco épocas de colheita (12, 14, 16, 18 e 20 meses após o plantio), dispostas em delineamento em blocos casualizados, com quatro repetições, em esquema fatorial 2 × 5. O período experimental estendeu-se de abril de 2023 a dezembro de 2024. Foram avaliadas variáveis agrometeorológicas, de crescimento, fisiológicas e de produtividade. As condições climáticas do Agreste de Alagoas, caracterizadas por precipitação anual e temperatura média do ar adequadas, foram geralmente favoráveis ao cultivo da mandioca; entretanto, a seca sazonal durante o verão limita o crescimento vegetativo. A variedade Pretinha apresentou maior acúmulo de biomassa seca da parte aérea, alcançando 51,4 t ha-1 aos 20 meses após o plantio. Em contraste, a variedade Caravela destacou-se na produção de raízes, apresentando maiores produtividades de massa fresca (61,5 t ha-1), massa seca (34,6 t ha-1), produtividade de amido (18,7 t ha-1) e índice de colheita (0,7). As avaliações fisiológicas revelaram elevada plasticidade da mandioca em resposta à sazonalidade hídrica, com ajustes na fotossíntese, na condutância estomática e na transpiração ao longo do ciclo da cultura. De modo geral, a variedade Caravela é mais indicada para uso industrial, enquanto a Pretinha é mais adaptada a sistemas de produção que demandam elevada biomassa aérea.

Palavras-chave:
Manihot esculenta Crantz; agricultura de sequeiro; componentes de produtividade; plasticidade fisiológica; déficit hídrico

INTRODUCTION

Cassava is widely recognized for its adaptability to different edaphoclimatic conditions. This plasticity enables cultivation in low-fertility soils and under significant water stress (Rocha et al., 2020; Tabaglio, 2023). Cassava derivatives are essential for human and animal nutrition, especially in South America and Africa, making the crop a strategic resource in efforts to combat hunger (Byju & Suja, 2020). According to the FAO (2025), global cassava root production reached 333 million tons in 2023, with a mean yield of 10 t ha -1. In Brazil production totaled 18.5 million tons (15 t ha -1), and the state of Alagoas contributed 518,000 tons to this total (15 t ha-1) (IBGE, 2025). The relatively low national yield is primarily attributed to inadequate management practices, such as non -optimized planting and harvesting schedules and the use of low-yielding varieties (Sousa et al., 2020).

Agriculture is particularly vulnerable to climatic variability, particularly in rainfed cassava production systems in the Agreste region of Alagoas, where yields depend heavily on rainfall. Extreme weather events can compromise production, disturb the supply-demand balance, and lead to market price fluctuations (Souza et al., 2023; Onyeneke et al., 2024). Notably, cassava pricing is a complex phenomenon, driven not only by availability but also by root quality (Oliveira et al., 2023a). In Brazil, the industrial sector requires roots with high starch and dry matter contents. Thus, cultivating varieties with different phenological cycles, directly associated with harvest time, is essential to mitigate climatic uncertainty and ensure a year-round supply of high -quality raw material (Silva et al., 2023). Furthermore, selecting varieties adapted to regional edaphoclimatic conditions should prioritize physiological traits associated with drought tolerance, such as high photosynthetic capacity, efficient stomatal regulation, and controlled transpiration (Rangel et al., 2021; Silva et al., 2021a).

Existing research indicates that physiological traits (photosynthetic efficiency and photoassimilate allocation) and productive parameters (root yield and shoot biomass) vary with variety and harvest timing and are influenced by environmental conditions and the plant developmental stage (Silva et al., 2021a). However, data remain scarce for rainfed cassava systems in the Agreste region of Alagoas (Brazil), particularly regarding the performance of local varieties across multiple harvest times. Studies in Alagoas have shown that cassava root yields under rainfed conditions are markedly lower than those achieved under improved water availability, where yields can reach 90 t ha-1 and generate up to 167% higher gross revenue. This contrast underscores the severe constraints imposed by water deficit and suggests that adopting more tolerant varieties could substantially increase producer profitability (Silva et al., 2023). Additionally, variability in cassava raw material quality affects the concentration of fermentable compounds in processing by-products, which in turn influences the efficiency of downstream valorization processes, such as biosurfactant production (Oliveira et al., 2023b).

In light of these factors, this study aimed to evaluate the growth, ecophysiological performance, and yield of cassava varieties as affected by harvest timing under rainfed conditions in the Agreste region of Alagoas, Brazil.

MATERIAL AND METHODS

The experiment was conducted from April 2023 to December 2024 in themunicipality of Arapiraca, Alagoasstate, Northeastern Brazil (9° 45’ 58” S, 35° 38’ 58” W, 324 m altitude). According to the Köppen classification, the local climate is classified as Tropical As, with a dry season from spring to summer and a rainy season from autumn to winter. Historical, mean annual rainfall is approximately 800 mm (Barros et al., 2012). The soil in the cultivated area is classified as Argissolo Vermelho-Amarelo (Santos et al., 2018), corresponding to Ultisols and some Kandic Oxisols (Soil Survey Staff, 2022), with a sandy loam texture. Soil chemical, physical and hydric attributes were determined in the 0.0-0.2 m layer before cultivation, following the procedures described by Teixeira et al. (2017). Table 1 summarizes the soil properties in the experimental area.

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Table 1
Chemical and physical-hydric properties of the soil in the experimental area before planting (0.0-0.2 m layer)

The experiment followed a randomized complete block design with four replicates, in a factorial arrangement (two varieties × five harvest times). Treatments consisted of two industrial cassava varieties (C - Caravela and P - Pretinha) evaluated at five harvest times 12 (E1), 14 (E2), 16 (E3), 18 (E4), and 20 months after planting (E 5), totaling 10 treatments. Planting was carried out on April 28, 2023. Both varieties are widely cultivated in Alagoas state and are recognized for high yield; Pretinha is also reported to produce greater shoot biomass (Santiago et al., 2018). Plots measured 5 × 5 m (25 m2) and contained 50 plants, with a total experimental area of 1,000 m2. Planting spacing was 1.0 × 0.5 m, corresponding to a density of 20,000 ha-1.

Basal fertilization consisted of 40 kg ha-1 of P2O5. Topdressing fertilization, applied w45 days after planting, supplied 40 kg ha-1 of N and K2O, following the recommendations of Ribeiro et al. (1999). Single superphosphate (18% P2O5), urea (45% N), and potassium chloride (58% K2O) were used as sources of phosphorus, nitrogen, and potassium, respectively. Weed control was performed manually throughout the experimental period. Daily mean air temperature and rainfall were obtained from an agrometeorological station (Datalogger - CR 3000, Campbell Scientific, Logan, Utah) installed 100 m from the experimental area. Figure 1 presents the agrometeorological data for the experimental period.

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Figure 1
Rainfall (mm) and mean air temperature (TM - °C) during the experimental period (April 2023 to December 2024) and the timing of cassava harvests for the five treatments in the Agreste region of Alagoas, Brazil

Evaluations began 12 months after planting (MAP), which correspondws to the first commercially relevant harvest under rainfed conditions. At each harvest time, six plants were collected from the net plot area (9.0 m2) of each plot. The following growth and yield component variables were measured: plant height (PH - m), root length (RL - cm), root diameter (RD - cm), number of roots (NR), and stem diameter (SD - cm). Fresh yield (t ha-1) was determined for roots (YR), stump (YS), stem (YST), leaves (YL), shoot (stem + leaves - YAP), and total biomass (root + stump + stem + leaves - TB). Dry mass variables included root dry mass (DMR), stem dry mass (DMST), leaf dry mass (DML), shoot dry mass (DMAP), and total dry mass (TDM). Harvest index (HI) and starch yield (SY) were also calculated.

Root length and diameter were measured using a tape measure and digital caliper, respectively. Fresh mass of each plant fraction (roots, stump, stem, and leaves) and total fresh mass were determined by weighing six sampled plants per plot using a digital scale (Mila, China) with 0.01 kg precision. Mean fresh mass per plant was calculated by dividing the total fresh mass of the six plants by six. Yield (t ha⁻1) was estimated by multiplying the mean fresh mass per plant by the plant population density (20,000 plants ha-1). Dry mass determination began at 14 MAP because an oven malfunction prevented drying at 12 MAP (when other variables were first assessed). Dry mass was obtained by drying samples in a forced-air oven at 65 °C for 72 hours, according to Sagrilo et al. (2008). Harvest index was calculated as the ratio of root mass to total plant mass (Silva et al., 2021b). Root starch content was estimated according to Grossmann & Freitas (1950).

Physiological measurements were performed starting at 12 MAP and included net photosynthetic rate (A), transpiration rate (E), leaf temperature (LT), and stomatal conductance (gs). These variables were measured using an infrared gas analyzer (IRGA, LCi model, ADC BioScientific, Hoddesdon, UK) equipped with a 2,000 μmol m-2 s-1 light source for all treatments, following Silva et al. (2021a). Measurements were taken between 08:00 and 10:00 h on two representative plants per plot, using the fifth fully expanded leaf, counted downward from the apex of the main branch. Thus, 16 measurements were obtained per harvest time (2 varieties × 2 plants × 4 blocks). Leaf chlorophyll content (SPAD index) was estimated indirectly using a SPAD-502 chlorophyll meter (Soil Plant Analysis Development Section, Minolta Camera Co., Osaka, Japan). Ten readings were taken on the central lobe of each of five leaves from the upper third of the plant, and the mean value was used for analysis.

Data were subjected to analysis of variance (ANOVA). When the F-test was significant, means for the variety factor were compared using Tukey’s test (p ≤ 0.05), and the harvest time factor was analyzed by regression. Regression coefficients were tested using the t-test (p ≤ 0.05) (Ferreira, 2018).

RESULTS AND DISCUSSION

Mean air temperature during the experimental period was 24.8 °C, with the highest value recorded in January 2024 (27.2 °C) and the lowest in July 2023 (22.8 °C) (Figure 1). Under similar conditions in Alagoas, Silva et al. (2021a) reported normal cassava growth at a mean temperature of 25 °C when comparing irrigated and rainfed cultivation. Similarly, Silva et al. (2025) observed normal growth at a mean temperature of 24 °C assessing cassava planting dates in the Agreste region of Alagoas. Alves (2002) reported that mean temperatures between 25 and 29 °C are optimal for cassava growth and development. Consequently, temperature conditions in the Agreste region of Alagoas were not limiting for cassava cultivation.

Throughout the 20-month experimental period, cumulative rainfall reached 1,532 mm. From April to December 2023, rainfall was 827 mm, while in 2024 (January to December), it totaled 768 mm. The highest monthly rainfall occurred in June 2023 (231 mm) and May 2024 (191 mm). These data indicate adequate water availability during the most critical phenological stages, specifically up to the 5th month after planting (MAP), as recommended by Conceição (1981), for April planting. Notably, November recorded the lowest rainfall in both years, with totals of 4.6 and 2.0 mm, respectively (Figure 1).

Mean annual rainfall in the Agreste region of Alagoas typically ranges from 750 to 1,000 mm (Barros et al., 2012). In the same region, Silva et al. (2025) reported annual rainfall exceeding 1,000 mm in years affected by strong La Niña events. Alves (2002) noted that major cassava-producing regions generally receive around 800 mm of rainfall annually, and areas exceeding this threshold are considered suitable for cultivation in terms of water supply.

Overall, rainfall in the Agreste region of Alagoas was compatible with cassava water requirements. However, strong seasonality (Figure 1) frequently leads to water deficit during spring and summer, triggering the physiological phenomenon known as vegetative dormancy. Tironi et al. (2019) described this adaptive mechanism as a marked reduction in leaf area to mitigate the effects of water stress.

For shoot-related traits, Pretinha generally showed more robust growth. Pretinha plants had a larger stem diameter (2.4 cm) than the Caravela variety (2.0 cm) (Figure 2A), and plant height was also greater in Pretinha (3.1 m) than in Caravela (2.5 m) (Figure 2C). Pretinha also produced higher fresh yields of total shoot biomass (42.2 t ha-1), stems (37.2 t ha-1), and stumps (5.60 t ha-1) (Figures 2H, I, and J). Similarly, Pretinha accumulated more shoot dry matter, particularly in stems (13.7 t ha-1) and total shoot biomass (15.1 t ha-1), whereas Caravela reached 8.70 and 9.80 t ha-1, respectively (Figures 2D and F). Despite its lower vegetative growth, Caravela showed superior root-related performance, including a higher average number of roots per plant (7.7 vs. 6.5; Figure 2B) and greater root dry mass (26.2 t ha-1 vs. 21.5 t ha-1; Figure 2E). These traits contributed to higher starch yield (18.7 t ha-1 vs. 15.8 t ha-1) and harvest index in Caravela (0.7 vs 0.5) (Figures 2K and L). Root length was not significantly affected by variety, harvest time, or their interaction (p > 0.05), with an overall mean of 21.6 cm (C.V. (%) = 7.3).

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Figure 2
Mean comparison test of growth variables: stem diameter - SD (A), number of roots - NR (B), and plant height - PH (C); and yield components: shoot dry mass - DMAP (D), root dry mass - DMR (E), stem dry mass - DMST (F), leaf dry mass - DML (G), shoot yield - YAP (H), stem yield - YST (I), stump yield - YS (J), harvest index - HI (K) and starch yield - SY (L) of cassava varieties evaluated from April 2023 to December 2024, in the Agreste region of Alagoas, Brazil

The results revealed marked differences in vegetative growth and productive performance between the cassava varieties Pretinha and Caravela. Pretinha’s greater capacity for shoot biomass production is likely associated with a higher leaf area index, directly linked to increased leaf yield (discussed below). A larger canopy can enhance light-use efficiency and promote greater temporary reserve accumulation in the stem (Figures 2F and I). El-Sharkawy (2004) highlighted these traits as key drivers of high shoot biomass production in cassava.

Alves (2002) noted that excessive vegetative growth in certain cassava varieties may compete with photoassimilate allocation to roots, especially in water-limited (rainfed) conditions, as observed in the present study. This trade-off helps explain the pattern observed for Pretinha, which produced high shoot biomass but comparatively lower root performance. Conversely, despite producing less shoot biomass, Caravela exhibited greater efficiency in allocating photoassimilates to storage roots, even under rainfed conditions, a desirable trait for starch and flour production. This was supported by the harvest index, a key indicator of physiological efficiency in photoassimilate partitioning. The higher harvest index observed in Caravela suggests a metabolic prioritization of storage root growth over shoot development. Cock et al. (1985) attributed this behavior to hormonal regulation, particularly auxin and cytokinin levels, which influence apical dominance and photoassimilate translocation.

With respect to harvest timing, vegetative growth was greatest at 20 MAP. At this harvest time, plants reached a mean height of 3.3 m and root diameter of 5.8 cm, representing increases of 39 and 33%, respectively, compared with 12 MAP (2.3 m height and 4.3 cm root diameter; Figures 3A and B). Harvesting at 20 MAP also resulted in the highest yields of shoot components, including stem yield (38.7 t ha⁻1), total biomass (102.2 t ha-1), stem dry mass (14.4 t ha-1), and total shoot dry mass (15.2 t ha-1). By contrast, the lowest stem yield and total biomass occurred at 12 MAP (23.4 and 79.8 t ha-1, respectively). Thus, delaying harvest to 20 MAP increased stem yield and total biomass by 65 and 28%, respectively (Figures 3C and D). For dry mass variables, the lowest values were recorded at 14 MAP, with stem and shoot dry mass yields of 7.9 and 9.6 t ha-1, respectively. By comparison, harvesting at 20 MAP increased stem and shoot dry mass by 81 and 58%, respectively (Figures 3E and G).

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Figure 3
Plant height - PH (A), root diameter - RD (B), stem yield - YST (C), total biomass - TB (D), stem dry mass yield - DMST (E), starch yield - SY (F), and shoot dry mass yield - DMAP (G) of cassava as affected by harvest time in the Agreste region of Alagoas, Brazil

Starch yield was best described by a second-degree polynomial model (Figure 3F), peaking at 19.2 t ha-1 around 17 MAP. The lowest starch yield (14.3 t ha-1) occurred at the earliest harvest (12 MAP), whereas the latest harvest (20 MAP) resulted in a starch yield of 17.4 t ha-1. Thus, harvesting at 17 MAP increased starch yield by 34 and 10% relative to 12 and 20 MAP, respectively.

Harvest time strongly affected cassava agronomic performance, and harvesting at 20 MAP promoted significant gains in vegetative growth and shoot biomass production. These increases reflect greater reserve accumulation and root system expansion, which are essential for both flour and starch production but also favor vegetative propagation via stem cuttings (Oliveira et al., 2014; Ezui et al., 2016). However, under rainfed conditions, water availability limits management options. Cassava planting in the region ideally occurs between April 10 and May 5, coinciding with rain onset (Silva et al., 2025); consequently, longer cycles require biennial planting. Higher yields at later harvests are expected because a longer crop cycle allows progressive biomass accumulation, root thickening, and photoassimilate allocation to storage organs. This pattern was reflected in the significant increases observed at 20 MAP for plant height, stem yield, total biomass, and dry matter (Figure 3). However, extending the crop cycle under rainfed conditions also increases exposure to seasonal water deficits. Therefore, annual cycles with intermediate yields may optimize land use efficiency and production stability while accounting for regional climatic variability.

The low dry mass yields at 14 MAP may indicate metabolic transition or photoassimulate redistribution, as reported by El-Sharkawy (2012) for vegetative growth versus dry matter accumulation in cassava. By contrast, starch yield followed a distinct pattern, likely associated with the mobilization of starch reserves in later stages to support vegetative maintenance, flowering, or lignification (Ceballos et al., 2016). Rainfall during the harvest period can also influence starch yield by increasing root moisture and reducing starch concentration (Alves, 2002). In the present study, the high rainfall recorded between May and July (429 mm; Figure 1) likely increased root moisture and reduced starch yield at the earlier harvest (12 MAP).

In addition to the main effects of variety and harvest time, the interaction between these factors was analyzed to better characterize varietal responses to changes in harvest timing under rainfed conditions. This approach allows the identification of differential growth and yield patterns across the crop cycle. In the interaction analysis (Figure 4), Caravela exhibited root yields of 41.5 and 55.4 t ha-1 at 12 and 20 MAP, respectively, peaking at 61.5 t ha-1 (17 MAP). Pretinha, on the other hand, yielded 41.1 and 50.0 t ha -1 at 12 and 20 MAP, respectively, showing a more gradual increase with no distinct peak. Caravela accumulated more root mass, outperforming Pretinha by 5.4 t ha-1 at 20 MAP and throughout the cycle (Figure 4A). Santiago et al. (2022) evaluated the productive performance of cassava varieties in the Agreste region of Alagoas, reporting an average root yield of 35 t ha-1 for Pretinha, while Santiago et al. (2016) observed root yields of 55.5 t ha-1 for Caravela. Combined with our data, these findings suggest that Caravela has a more vigorous root system and more efficient photoassimilate translocation to roots, which may explain its earlier peak yield at 17 MAP.

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Figure 4
Root yield - YR (A), root dry mass - DMR (B), leave yield - YL (C), and total dry mass - TDM (D) of cassava varieties as affected by harvest time in the Agreste region of Alagoas, Brazil

At 20 MAP, Caravela reached a root dry mass yield of 34.6 t ha-1, while Pretinha achieved 31.8 t ha-1 (Figure 4B). This 8.8% advantage suggests that Caravela is more efficient at converting assimilates into root dry matter, which tends to generate higher industrial processing yields. However, Pretinha produced greater total dry mass (roots + shoot biomass), reaching 51.4 t ha-1 at 20 MAP, while Caravela achieved 40.6 t ha-1. This difference reflects Pretinha’s greater shoot biomass accumulation, even at later stages of the crop cycle, resulting on a 26% higher total dry mass yield (Figure 4D).

Leaf biomass yield varied significantly over time, indicating physiological changes related to plant age and climate interactions. In Caravela, leaf yield decreased from 4.4 t ha-1 at 12 MAP to 2.4 t ha-1 at 20 MAP, corresponding to a 45% reduction. Pretinha maintained higher shoot biomass accumulation during the same period, with yields of 7.1 and 3.0 t ha -1 at 12 and 20 MAP, respectively, representing a 57.7 % reduction (Figure 4C).

This behavior aligns with senescence dynamics and increased photoassimilate allocation to storage organs, common in cassava phenological stages after 12 MAP. Furthermore, climatic conditions during specific stages of the crop cycle directly influence leaf area. In the Agreste region of Alagoas, water deficits are common during spring and summer (Figure 1), coinciding with harvests at 18 and 20 MAP. These conditions triggered vegetative dormancy, an adaptive mechanism characterized by a reduced leaf area that limits water loss under stress (Tironi et al., 2019). This explains the inverse relationship between root dry mass and leaf biomass. During this phase, cassava undergoes senescence and partial leaf abscission while continuing to allocate photoassimilates to the storage roots. Consequently, leaf biomass decreases concomitantly with the increase in root dry mass at later harvest times. These findings align with Santiago et al. (2022), who evaluated Pretinha and Caravela in the same region observing that Pretinha maintained higher leaf biomass, while Caravela showed a sharper decline in shoot structure. These results indicate that Pretinha has a greater capacity to maintain an active leaf area, as observed in this study.

Net photosynthetic rate varied significantly across the crop cycle. The maximum rate was recorded at 15.6 MAP, reaching 13.4 μmol CO 2 m-2 s-1. After this point, photosynthetic activity declined sharply, with the lowest value recorded at 20 MAP, when the rate dropped to 7.2 μmol CO 2 m-2 s-1, a 46% reduction (Figure 5A).

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Figure 5
Net photosynthetic rate - A (A), SPAD index (B), stomatal conductance - gs (C), and transpiration rate - E (D) of cassava varieties as affected by harvest time under rainfed conditions in the Agreste region of Alagoas, Brazil

The high car bon assimilation observed around 16 MAP may be associated with intrinsic water-use efficiency (A/gs), a characteristic typical of hardy plants such as cassava. Even under moderate or low stomatal conductance, cassava can maintain high photosynthetic rates. Moreover, efficient biochemical mechanisms, such as higher Rubisco activity and rapid RuBP regeneration, may enhance CO2 assimilation under restricted stomatal opening. High irradiance during this stage may also contribute to sustaining photosynthetic performance, particularly in environments with intense solar radiation (El-Sharkawy, 2007), such as the Agreste region of Alagoas, Northeast Brazil.

The subsequent decline in net photosynthesis at 18 and 20 MAP is likely attributable to non-stomatal limitations, including enzymatic inhibition, free radical accumulation, and photochemical damage induced by water stress combined with high radiation. These processes compromise chloroplast integrity and limit the use of NADPH and ATP (Chaves et al., 2009).

Changes in source-sink relationships may contribute to the observed reductions in photosynthesis during the later stages of the cassava cycle. This can lead to sugar accumulation in leaves and negative feedback on photosynthesis, reducing the activity of enzymes involved in carbon fixation (Paul & Foyer, 2001).

The SPAD index, an indirect indicator of leaf chlorophyll concentration, reached its lowest value (41.1) at 16.7 MAP and then increased moderately up to 20 MAP (45.7), although it remained below the value recorded at 12 MAP (50.4) (Figure 5B). This variation throughout the cassava cycle likely resulted from the plant’s response to environmental conditions and phenological development. The higher SPAD value at 12 MAP, which coincided with the onset of the rainy season, may be associated with intense vegetative growth supported by higher soil moisture and nutrient availability, particularly nitrogen, which is directly involved in chlorophyll synthesis (Taiz et al., 2017).

However, as the rainy season progressed (12 and 16.7 MAP), SPAD values decreased significantly. This decline may be explained by two main factors: (i) nutrient dilution in leaves due to higher vegetative growth, and (ii) shading of basal and intermediate leaves, reducing the need for high chlorophyll concentrations in these tissues (Fukuda et al., 2010). Around 17 MAP, with the end of the rainy season and increasing solar radiation until the summer peak (20 MAP), SPAD values increased again. This rise may reflect enhanced chlorophyll synthesis under higher light availability, particularly in hardy plants such as cassava. Additionally, water restriction during the dry season tends to reduce vegetative growth, which can concentrate nutrients in the most physiologically active leaves, contributing to higher SPAD values (Lichtenthaler & Buschmann, 2001).

Therefore, SPAD oscillations across the cassava cycle can be attributed to the combined effects of plant phenological stage, nitrogen availability, and environmental variations in light intensity and soil moisture. These results reinforce the importance of management practices that support adequate plant nutrition throughout the cycle, particularly during transitions between rainy and dry periods.

Stomatal conductance exhibited a decreasing trend during the intermediate phase of the cycle, reaching the lowest value at 16.2 MAP (0.05 mmol H2O m-2 s-1). A partial recovery was observed by 20 MAP (0.18 mmol H2O m-2 s-1), although values remained below those measured at 12 MAP (0.21 mmol H2O m-2 s-1) (Figure 5C). The significant drop in stomatal conductance at 16 MAP likely reflects declining soil moisture, since the dry season began abruptly in August, when only 27 mm of irregularly distributed rainfall was recorded. This response is consistent with stomatal closure as an adaptive mechanism under water deficit. The partial recovery of this parameter by 20 MAP (0.18 mmol H2O m-2 s-1) suggests compensatory strategies, such as water extraction from deeper soil layers via a robust root system, as previously described by other researchers (Alves & Setter, 2004; Daryanto et al., 2016).

Leaf temperature did not differ significantly among varieties or harvest times, and no interaction effects were detected (p > 0.05). Mean leaf temperature remained stable around 29.1 °C (C.V. (%) = 1.8) throughout the crop cycle, suggesting effective thermal regulation and limited sensitivity of this trait to seasonal climatic variations under rainfed conditions in the Agreste region. Transpiration followed a pattern similar to that of stomatal conductance, reaching a minimum of 0.87 mmol H2O m-2 s-1 at 15.9 MAP, a marked decline from 3.7 mmol H2O m-2 s-1 at 12 MAP. By 20 MAP, transpiration increased to 3.8 mmol H2O m-2 s-1, indicating reactivation of transpirational activity (Figure 5D). This recovery suggests that cassava can partially restore water loss through transpiration even during prolonged dry periods, likely supported by access to deeper soil water and high root plasticity (Punyasu et al., 2025). However, the recovery in transpiration was not accompanied by a proportional increase in photosynthesis, reinforcing the interpretation that internal metabolic limitations become more important than CO2 diffusion constraints under these conditions (Taiz et al., 2017).

CONCLUSIONS

  • 1. Pretinha showed greater shoot biomass accumulation, whereas Caravela exhibited higher root yield, starch yield, and harvest index, indicating greater suitability for industrial purposes.

  • 2. Harvesting at 20 MAP increases shoot biomass and dry mass, supporting multiple uses of the crop, whereas maximum starch yield occurred at approximately 17 MAP. Under rainfed conditions, gas exchange varied across the crop cycle in response to seasonal water availability, with pronounced changes in net photosynthetic rate, stomatal conductance, and transpiration, particularly during periods of low rainfall.

  • 3. Climatic conditions in the Agreste region of Alagoas are generally favorable for cassava cultivation; however, seasonal water deficits affect growth dynamics and should be considered when defining the optimal harvest time.

  • 1
    Research developed at Universidade Federal de Alagoas, Arapiraca, AL, Brazil
  • Ref. 300556
  • Financing statement:
    The research received no external funding.

Data Availability Statement:

The authors declare that there are no data underlying this text.

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Edited by

  • Editors:
    Ítalo Herbet Lucena Cavalcante & Walter Esfrain Pereira

Publication Dates

  • Publication in this collection
    21 Aug 2026
  • Date of issue
    2026

History

  • Received
    04 Sept 2025
  • Accepted
    12 Apr 2026
  • Published
    31 July 2026
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