In the Cerrado, the sequential chaining of phenological events during the dry season is a pattern observed in many plant species. In this season, many plants completely lose their leaves, and soon after deciduous, there is an expressive production of leaf buds. In this study, we investigated the effect of irrigation and early defoliation on the triggering of leaf budding of the deciduous species Peixotoa tomentosa A.Juss. in the dry season of a seasonal environment with water restrictions. Therefore, we set up an experiment with three groups of plants: control (n = 15), irrigation treatment (n = 15), and removal treatment (n = 15), and after the complete deciduousness of the plants, we carried out phenological monitoring of the development of leaf buds in these plants. From July to August 2022, the leaf budding phenology of the 45 individuals was evaluated twice a week. To test whether there is a difference in the number of leaf buds between treatments, we built generalized linear mixed models (GLMMs). Plants in the removal treatment had a statistically higher number of leaf buds produced than the plants in the irrigation and control groups (P < 0.05). However, the control group and the irrigation treatment did not differ from each other (P > 0.05). We showed that early defoliation influenced the triggering of leaf buds in P. tomentosa, increasing the production of young leaves in their individuals in a seasonal environment with water restrictions. Irrigation was not able to break the dormancy of leaf buds. Our findings contribute to a better understanding of the triggering of vegetative phenophases in deciduous Cerrado plants, showing that leaf fall may play a more important role than rain in the production of leaf buds in the dry season.

Cerrado, leaf buds, leaf fall, Malpighiaceae, phenology, savanna, seasonality

In the Cerrado, the sequential chaining of phenological events during the dry season is a pattern observed in many plant species (Pereira et al. 2022). In this season, many plants completely lose their leaves, and soon after deciduous, there is an expressive production of leaf buds (Araújo and Haridasan 2007; Pereira et al. 2022).

Water stress is an important physiological trigger for leaf abscission (Kikuzawa 1991). The decrease in the amount of water in the more superficial layers of the soil, at the beginning of the dry season, has been associated with leaf fall due to the decline in water potential, which would induce this phenophase (Santos et al. 2021). With reduced precipitation and humidity, leaf fall can help conserve water, preventing the plant’s vital activities from being compromised in the dry season (Goldstein et al. 2008). Thus, as a result of leaf fall, rehydration promotes adjustments in plant water demand and supply, which are regulated by reducing leaf area, transpiration, increasing root system depth, and internal water reservoir (Scholz et al. 2002), which guarantees the subsequent opening of leaf buds and flower buds (Pereira et al. 2022).

Leaf bud production is linked to a wide range of adaptive responses to seasonal stresses (Pereira et al. 2022). The development of young leaves in the dry season minimizes photosynthetic losses in this period of low light, showing higher efficiency at the beginning of the rainy season of high light, when mature leaves will have their photosynthetic apparatus developed, providing the energy needed for the subsequent peak flowering and fruiting (Goldstein et al. 2008). In addition, predation on young leaves is lower due to adverse conditions for insect populations (Murali and Sukumar 1993), which may increase the chances of leaf development due to the lower abundance of chewing insects at this time (Silva et al. 2020).

Knowledge of seasonal plant variations has been considered essential for the study of the ecology and evolution of the Cerrado and for understanding the spatio-temporal organization of available resources (Pereira et al. 2022). In the tropics, climate change has been causing variations in the intensity of abiotic factors (for example, changes in the amount of rainfall, Mendoza et al. 2017), changing the seasonal rhythms of species by affecting the sequential triggering of phenological events and, consequently, the plant development (Vilela et al. 2017) and community dynamics (Fourcade et al. 2021). Thus, the investigation of the vegetative phenology of Cerrado deciduous plants helps us to better understand how these abiotic events interfere with the development of typical plants in tropical communities subjected to seasonality.

In this study, we investigated the effect of irrigation and early defoliation on the triggering of leaf budding of the deciduous species Peixotoa tomentosa A.Juss. in the dry season amidst a seasonal environment with water restrictions. We tested the following hypotheses: (i) The irrigation of plants in the water deficit period increases the production of leaf buds; and (ii) The early removal of senescent leaves accelerates the production of leaf buds. In light of these hypotheses, we made the following predictions: (i) Irrigation reduces water stress and serves as a stimulus to break the dormancy of leaf buds; (ii) Early leaf removal promotes the rehydration of branches due to reduced transpiration and a reallocation of nutrients that are transferred to the growth of young leaves. To test the hypotheses above, we set up an experiment with three groups of plants: control, irrigation treatment, and removal treatment, and after the complete deciduity of the plants, we carried out the phenological monitoring of the development of leaf buds in these plants.

There were no rainfall records during the evaluation period (0 mm of rainfall between July 1^st and August 25^th).

Leaf budding

The mean leaf buds ranged from 9.67 (± 0.80 SE) in the removal treatment on July 21^th to zero in all groups from August 25^th onwards. The evaluated individuals of P. tomentosa presented leaf buds from July 18^th. Subsequently, as these leaf buds developed into young leaves, the number of leaf buds dropped throughout August (Fig. 3, Table 1). Thus, from the last fortnight of August, the leaves began to mature, with great speed and synchrony: the evaluated population changed from young leaves to mature, hairy and green leaves.

Figure 3. 

Time series analysis showing the mean of leaf buds (points) and the standard error (vertical limits) of the 15 individuals of Peixotoa tomentosa A.Juss. in each of the 12 observation dates between July and August 2022. Control group ( A), irrigation treatment (B), and removal treatment (C).

Plants in the removal treatment (4.98 ± 0.29 SE) had a statistically higher number of leaf buds produced than the plants in the control group (3.62 ± 0.16 SE) and in the irrigation treatment (3.85 ± 0.11 SE) (F_2.41 = 14.995, P = 0.0001). The control group and the irrigation treatment did not differ from each other (P > 0.05) (Fig. 4, Table 4).

Table 4.

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Pairwise comparisons (P < 0.05; GLMM/Tuckey’s post hoc test. α = 0.05) showing the differences between the control group, irrigation, and removal treatments. * = Significant difference between treatments.

Treatments	Control	Irrigation	Removal
Control	–	0.671	0.0001*
Irrigation	0.671	–	0.0001*
Removal	0.0001*	0.0001*	–

Figure 4. 

Mean leaf buds in individuals of Peixotoa tomentosa A.Juss. from the control, irrigation treatment, and removal treatment. Error bars represent ± SE. Means followed by the same letters do not differ statistically from each other (P < 0.05; GLMM/Tuckey’s post hoc test. α = 0.05).

Our findings showed that early defoliation influenced the triggering of leaf buds on P. tomentosa, increasing the production of young leaves in its individuals in a seasonal tropical environment with water restrictions. However, irrigation was not able to break the dormancy of leaf buds.

Many Cerrado woody plants have an extensive root system that, in many cases, penetrates deeper into the soil, reaching the water table. These roots, by remaining humid throughout the year, ensure a stable water source for plants, minimizing the effects of seasonal water deficit (Nardoto et al. 1998; Scholz et al. 2002). Thus, we suggest that despite the irrigation treatment having provided a more humid soil for the plants, the depth, and extension of the root system of P. tomentosa allow these plants to capture enough water to minimize the effects of the water deficit, not interfering with the production of leaves in the dry season (Scholz et al. 2002).

Although irrigation was not related to leaf budding, the removal of senescent leaves proved to be an important factor in triggering young leaves. The reduction in transpiration appears to be a key factor in triggering the leaf buds of P. tomentosa (Pereira et al. 2022). Studies have shown that the leaf fall can avoid water stress in tropical deciduous species, as it reduces the transpiration surface of the foliage and, therefore, reduces water demand and the risks of xylem embolism and plant desiccation (Goldstein et al. 2008; Lima et al. 2021; Santos et al. 2021). Thus, leaf fall is closely related to leaf senescence in such a way that it can be used as a proxy for leaf budding, as this would reduce water loss by the plant, leading to rehydration of leafless branches and leaf production in P. tomentosa, even under severe water stress (John et al. 2018). Furthermore, we suggest that the defoliation of senescent leaves did not prevent the translocation of nutrients, and, since these plants had all their leaves removed at the same time, there was a drastic reduction in the transpiration of these individuals, which produced new leaves in higher numbers than found in the other groups.

Evidence is accumulating that the timing and intensity of the vegetative phenophases of deciduous plants are changing as a result of climate change (Gordo and Sanz 2009; Gunderson et al. 2012; Estiarte and Peñuelas 2015; Liu et al. 2022). On average, climate warming will delay, and drought will advance, leaf senescence, but to varying degrees depending on the species (Gunderson et al. 2012). Warming and drought, therefore, have opposite effects on the phenology of leaf senescence, and the impact of climate change will depend on the relative importance of each factor in specific regions (Estiarte and Peñuelas 2015). Furthermore, the overall effects of climate change on nutrient reabsorption will depend on the contrasting effects of warming and drought, because the construction of new leaves depends almost exclusively on nutrients reabsorbed from the leaves during the previous leaf fall (Estiarte and Peñuelas 2015). Thus, changes in leaf falling and leaf budding phenology will impact carbon uptake, but also nutrient cycling in the ecosystem (Jia et al. 2022).

Our findings contribute to a better understanding of the triggering of vegetative phenophases in Cerrado deciduous plants, showing that leaf fall may play a more important role than rainfall in the production of leaf buds in the dry season.

The authors would like to thank CSEC, PPG-ECMVS and CAPES for continuous support. This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Finance Code 001.