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Lulo (Solanum quitoense) is a promising Andean fruit tree that is highly valued in international markets because of its nutritional characteristics, flavor, aroma, and high content of vitamins A and C with antioxidant properties. However, certain sanitary problems, such as Fusarium oxysporum and soil nematodes, pose challenges to the competitiveness of lulo crops. Cultivating lulo plants grafted onto resistant patterns, such as S. hirtum, is a plausible solution to these sanitary issues; so, determining the behavior of grafted plants is relevant. In this study, the phenology and growth dynamics of grafted lulo plants were evaluated with functional analyses based on thermal time (degree days-DD). Completion of the vegetative phase of the lulo required 561 DD from planting to the onset of flower bud sprouting, 715 DD to reach anthesis, 801 DD for fruit set, and a cumulative total of 2,464 DD to reach fruit ripening. The functional analyses showed that Richards’s model fit the dynamics of the growth variables (height, leaf area index, crop growth index, total dry weight, leaf dry weight, and stem dry weight). Of the total plant dry biomass, 39.7% corresponded to the stems, 33.8% was from the fruits, 15.54% was from the leaves, 10.56% was from the roots, and 0.4% was from the flowers. The yield was not affected by the grafting process of lulo S. quitoense onto S. hirtum stocks; on the contrary, the crop population and productivity were maintained for a longer time. Additional key words: plant phenology; functional analysis; day-degrees; allometry; partition. 1 Universidad de Nariño, Faculty of Agriculture Sciences, Grupo de Investigación en Producción de Frutales Andinos (GPFA). San Juan de Pasto (Colombia). ORCID Criollo-Escobar, H.: 0000-0002-8690-4486; ORCID Lagos-Burbano, T.C.: 0000-0001-9222-4674 2 Universidad de Nariño; Servicio Nacional de Aprendizaje (SENA), Cartago (Colombia). ORCID Moncayo-Palacios, M.-F.: 0000-0001-7200-309X 3 Corresponding author. tclagosb@udenar.edu.co In Colombia, the total crop area for lulo (Solanum quitoense Lam.) in the productive stage in 2018 was 8,821.3 ha with a production of 89,050 t of fruits and an average yield of 10.09 t ha-1. In the same year, the Department of Huila was the highest producer, with 14,340 t, followed by Valle del Cauca, Antioquia, Boyaca, and Cauca (Agronet, 2020). Lulo is one of the most promising fruit crops for the Andean region, thanks to its good organoleptic characteristics and unique aptitude for the agro-industry (Gallo et al., 2018). Lulo is susceptible to sanitary issues that limit crop production and cause high losses for producers. Therefore, creating disease-tolerant plants is necessary for ambitious exportation projects (Criollo et al., 2014). Also, the lulo production system has severe limitations associated with technological, phytosanitary, and commercial factors that threaten sustainability (Muñoz et al., 2013). According to Muñoz (2010), the main problems in lulo crops are Meloidogyne incognita, which affects water and nutrient uptake, Neoleucinodes elegantalis, which perforates fruits and induces premature fruit drop, Colletotrichum gloesporioides, which causes fruit rotting, Sclerotinia sclerotiorum or cottony soft rot, Phytophthora sp., which leads to dark lesions in the leaves and stems, defoliation, and plant death, and Fusarium oxysporum, which affects roots and causes plant death (Noboa and Viera, 2020; Benítez et al., 2020). Grafting fruit tree species, such as lulo, is a promising technique used for the vegetative propagation of outstanding plants, in which the rootstock is diseasetolerant or resistant (Rojas et al., 2004). Considering the resistance of several wild Solananceae species to Fusarium oxysporum and Meilodigine incognita (Arizala et al., 2011; Clements et al., 2016; Navarrete et al., 2018; Vargas et al., 2018). There are studies on lulo plants grafted onto resistant rootstocks, such as Solanum hirtum; however, the behavior of these grafts and their implications on crop productivity and fruit quality are unknown. An important aspect when evaluating grafted lulo plants is the rhythm of the appearance of vegetative and reproductive structures, which depends on the interaction of biotic and abiotic factors (Browning et al., 2019). According to Casiano and Paz (2018), anthropic and climatic effects can cause variations in growth patterns and the appearance of plant structures. Thus, site growth conditions, such as climate, soil and plant management, can affect the duration of phenological stages (Fischer and Melgarejo, 2020). RESUMEN El lulo (Solanum quitoense) es un prometedor árbol frutal andino muy apreciado en los mercados internacionales por sus características nutricionales, su sabor y aroma, y su alto contenido de vitaminas A y C con propiedades antioxidantes. Sin embargo, ciertos problemas sanitarios, como Fusarium oxysporum y nematodos del suelo, plantean problemas para la competitividad del cultivo del lulo. Al respecto, el cultivo de plantas de lulo injertadas en patrones resistentes, como S. hirtum, es una solución plausible a estos problemas sanitarios y, por lo tanto, determinar el comportamiento de las plantas injertadas es una cuestión relevante. En este estudio se evaluó la fenología y la dinámica de crecimiento de las plantas de lulo injertadas mediante análisis funcionales basados en el tiempo térmico (grados día-DD). La finalización de la fase vegetativa del lulo requirió 561 DD desde la plantación hasta el inicio de la brotación del botón floral, 715 DD para alcanzar la antesis, 801 DD para el cuajado de los frutos y un total acumulado de 2.464 DD para alcanzar la maduración de los frutos. Los análisis funcionales mostraron que el modelo de Richards se ajustaba a la dinámica de las variables de crecimiento (altura, índice de superficie foliar, índice de crecimiento del cultivo, peso seco total, peso seco de la hoja y peso seco del tallo). Del total de la biomasa seca de la planta, el 39,7% correspondía a los tallos, el 33,8% a los frutos, el 15,54% a las hojas, el 10,56% a las raíces y el 0,4% a las flores. El rendimiento no se vio afectado por el proceso de injerto de lulo S. quitoense en S. hirtum; por el contrario, la población de los cultivos y su productividad se mantuvieron durante más tiempo. Palabras clave adicionales: fenología vegetal; análisis funcional; grados-día; alometría; partición. Received for publication: 14-05-2020 Accepted for publication: 23-11-2020 INTRODUCTION 292 CRIOLLO-ESCOBAR / MONCAYO-PALACIOS / LAGOS-BURBANO Rev. Colomb. Cienc. Hortic. The flowering stage is the most sensitive. Luo et al. (2015) stated that the ambient temperature affects crop growth, development, and production since there are specific temperature requirements to complete each stage of the life cycle. Lulo crops show adequate development in zones with temperatures of 15-24°C, with an optimum of 20°C (Sánchez-Reinoso et al., 2019). Water and energy balance are also significantly affected by temperature (Sacks and Kucharika, 2011). To determine the effects of the environment and genetic composition of plants on biomass accumulation, techniques with plant growth analysis are needed to accurately estimate productivity variables with simple measurements, such as leaf area (LA), leaf area index (LAI), crop growth index (CGI), and biomass (Poorter and Sack, 2012). According to Di Benedetto and Tognetti (2016), evaluating crop growth in response to agronomic modifications, such as grafting, requires growth analysis techniques that can be statistically validated. The growth rate of different plant structures largely determines the partition of assimilates and plant productivity. The balance between the energetic demand of plant organs and the photosynthetic contribution of plants must be considered. The efficiency of this partition will affect the composition of the total biomass and, particularly, the distribution of photosynthates to commercially-important organs (Ochoa-Vargas et al., 2016). Crop production seeks to enhance conditions for longer periods of high productivity, with efficient partitioning, achieved with an adequate source-sink relationship (Barrientos et al., 2015). This study aimed to analyze the crop phenology, growth dynamics, and yield of lulo (Solanum quitoense var. quitoense) plants grafted onto Solanum hirtum plants. MATERIALS AND METHODS The study was conducted on the Instituto Educativo de Desarrollo Rural I.E.D.R Farm, in the municipality of La Union, Nariño, located at an elevation of 1,782 m a.s.l., at 1o35’15” N and 77o07’32” W, with an average temperature of 19.32°C, average relative humidity of 78%, and total cumulative precipitation of 1,512.6 mm. The soil of the experimental plot was composed of volcanic ash and sand with a pedological content of 70% Typic Hapludands and 30% Typic Fulvudansds, characterized by a first horizontal layer to a depth of more than one meter (González and Salamanca, 2008). The soil analysis showed adequate contents of organic matter (3.28%), potassium (0.43 cmol kg-1), and minor elements (in mg kg-1: Mn = 6.8, B = 0.6, Fe = 26.72, Zn = 2.1), with a slightly acidic pH (5.6); low contents of calcium (2.3 cmol kg-1), magnesium (1.2 mg kg-1), and phosphorus (16.43 mg kg-1); apparent density (1.09 kg dm-3), sandy loam texture, and 17% slope. The plant material was produced by grafting lulo de castilla (S. quitoense var. quitoense) onto S. hirtum rootstocks. When the S. hirtum plants reached a diameter of 8-10 mm, they were cut at a height of 10 cm and grafted with S. quitoense twigs that were tied with plastic tape and covered with a plastic bag for 1 week to prevent dehydration. After 1 month, the plants were transferred to the field. The grafted plants were sown in 0.3 × 0.3 × 0.3 m holes, arranged in five rows of 13 plants each with a distance of 3 m between rows and 2 m between plants. The planting density was 1,666 plants/ha. 80 g of dolomite lime wer

Phenology and growth of lulo (Solanum quitoense Lam) plants grafted onto Solanum hirtum Vahl.