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En el estudio se evaluó el rendimiento productivo de los cultivares de yuca CM 3306-4 y MCOL 2215, cultivados en un suelo arcillo-limoso, bajo diferentes condiciones hídricas: (1) H1 = en secano desde la siembra hasta la cosecha (9 meses) con una precipitación de 841 mm; (2) H2 = riego complementario desde la siembra hasta la cosecha y una precipitación de 1247 mm; y (3) H3 = riego a partir del cuarto mes y una precipitación de 998 mm. El diseño utilizado fue parcelas subdivididas con tres repeticiones. En el tratamiento H2 el contenido de almidón en campo (27.45%), el rendimiento de almidón (8.23 t/ha), la altura de planta (249 cm) y el rendimiento de yuca fresca (30 t/ha) fueron más altos que en los tratamientos H1 y H3. El cultivar CM 3306-4 presentó los mayores incrementos promedio en rendimiento (26.4 t/ha), altura (249 cm) y contenido de almidón (26.64%) en los diferentes tratamientos (P < 0.05) en relación con el cultivar MCOL 2215. El riego incrementó el rendimiento del cultivo de yuca, presentando diferencia significativa entre la condición hídrica y los parámetros evaluados; igualmente se encontraron diferencias en contenido de almidón entre variedades.

The causes of species rarity are of critical concern because of the high extinction risk associated with rarity. Studies examining individual rare species have limited generality, whereas trait-based approaches offer a means to identify functional causes of rarity that can be applied to communities with disparate species pools. Differences in functional traits between rare and common species may be indicative of the functional causes of species rarity and may therefore be useful in crafting species conservation strategies. However, there is a conspicuous lack of studies comparing the functional traits of rare species and co-occurring common species. We measured 18 important functional traits for 19 rare and 134 common understory plant species from North Carolina's Sandhills region and compared their trait distributions to determine whether there are significant functional differences that may explain species rarity. Flowering, fire, and tissue-chemistry traits differed significantly between rare and common, co-occurring species. Differences in specific traits suggest that fire suppression has driven rarity in this system and that changes to the timing and severity of prescribed fire may improve conservation success. Our method provides a useful tool to prioritize conservation efforts in other systems based on the likelihood that rare species are functionally capable of persisting. Las causas de la rareza de las especies son de mucho interés debido al alto riesgo de extinción asociado con la rareza. Los estudios que examinan a las especies raras una por una tienen una generalidad limitada, mientras que las estrategias basadas en las características ofrecen los medios para identificar las causas funcionales de la rareza que pueden aplicarse a las comunidades con pools dispares de especies. Las diferencias de las características funcionales entre las especies comunes y las raras pueden ser indicadoras de las causas funcionales de la rareza de las especies y por lo tanto puede ser útil para elaborar estrategias de conservación de especies. Sin embargo, existe una carencia conspicua de estudios que comparen las características funcionales de las especies raras con las de las especies comunes co-presentes. Medimos 18 características funcionales importantes de 19 especies raras y 134 especies comunes de plantas de sotobosque de la región de las Dunas de Carolina del Norte y comparamos sus características de distribución para determinar si hay diferencias funcionales significativas que pueden explicar la rareza de especies. Las características de florecimiento, fuego y química de los tejidos difirieron significativamente entre las especies raras y comunes que ocurren en el mismo sitio. Las diferencias en las características específicas sugieren que la supresión del fuego ha propiciado la rareza en este sistema y que los cambios en la sincronización y la gravedad del fuego prescrito pueden aumentar el éxito de la conservación. Nuestro método proporciona una herramienta útil para priorizar los esfuerzos de conservación en otros sistemas basados en la probabilidad de que las especies raras son funcionalmente capaces de persistir.

Cleft grafting methods were applied in order to evaluate the effect of grafting on tomato fruit yield and quality. The tomato cultivars used as scions were Yeni Talya, Swanson and Beri. The cultivars Beaufort and Arnold were used as rootstocks. The following characteristics of grafted and nongrafted plants were recorded: fruit index, number of fruits/truss, fruit weight, fruit yield, dry matter, pH, concentration of soluble solids, titratable acidity, concentrations of total sugar, lycopene and vitamin C. The results showed that fruit yield and fruit index, number of fruits/truss and fruit weights were improved by grafting. Fruit quality, measured in terms of concentrations of dry matter, soluble solids, total sugar, and vitamin C, was lower in grafted plants than in nongrafted ones. No significant difference in lycopene level and pH was found. Titratable acidity was improved by grafting. A positive effect of grafting was recorded when Beaufort was used as rootstock. These results showed that grafting could be an advantageous alternative in tomato production.

The concept of critical nitrogen concentration (% Nc) has been proposed as the minimum % N in shoots required to produce the maximum aerial biomass at a given time. Several authors have shown that % Nc declines as a function of aerial biomass accumulation (W) and the % Nc - W relationship has been proposed as a diagnostic tool of N status in different crops, excluding maize. From data obtained in five nitrogen fertilisation experiments in irrigated maize crops, 26 critical data-points were selected with a precise statistical procedure. An allometric relation was fitted and a critical % N - W relationship model is proposed in maize as: If W < 1 t ha⁻¹ % Nc = 3.40 If 1 t h a⁻¹ < W < 22 t ha⁻¹ % Nc = 3.40 (W)-0.37 The model is applicable to maize crop development between emergence and silking + 25 days. The model was tested and validated with data obtained in a network of 17 N fertilisation experiments conducted in France under contrasting pedoclimatic conditions. In only nine out of 280 data-points (3.2%), the plant N status was mispredicted when ± 5% error around % Nc was allowed. A critical N uptake model (Nuc, kg N ha⁻¹) is proposed as Nuc = 34 (W)0.63 A comparison between Nuc and N uptake observed in N treatments giving the maximal grain yields has shown that maize crops assimilate at least 30 kg N ha⁻¹ in a storage N pool at the silking stage. The significance of the critical % N — W and Nu—W relationships is discussed in relation to theoretical models proposed in whole plant ecophysiology. Different relationships calculated between leaf area index and aerial biomass accumulation, and between N uptake and leaf area were consistent with previous results for other crops. This strengthens the interest of the critical % N — W relationship for use as diagnostic tool of nitrogen status in maize crops.