Caobilla (
Currently, the interest in establishing
Current practices in many forest nurseries involve container seedlings planted in local forest soil and little or no fertilizer application. Regardless of the production system, seedling germplasm quality along with cultural practices serve a major role in promoting successful morphological and physiological characteristics, resulting in satisfactory plant performance at the plantation site (
Drought and high temperatures in the dry tropics can limit nutrient availability, which often severely affects reforestation (
Top-pruning seedlings in the nursery is an uncommon practice; nevertheless, Cetina et al. (
Seedling fertilization is a common nursery practice, which serves to enhance growth and photosynthetic capacity. Reserves can be accumulated through fertilization when nutrient availability exceeds seedling requirements (
Top-pruning and exponential fertilization effects in the nursery have not been documented in
The study was conducted under greenhouse conditions in the Nursery of the Forestry Graduate School of
Seeds were collected from healthy
The two factors tested were top-pruning (TP), with three levels (no-pruning - NP, TP 25%, and TP 50% intensity), and fertilization, with two levels (traditional fertilization - TF, and exponential fertilization - EXF). The six treatment combinations were labeled as T1 (NP + TF), T2 (TP 25% + TF), T3 (TP 50% + TF), T4 (NP + EXF), T5 (TP 25% + EXF), and T6 (TP 50% + EXF). Depending on the effects, NP and TF levels, and their combinations (NP + TF), were considered controls.
Treatments were initiated on 22 October 2012, when nutrient reserves in the seedling endosperm were largely exhausted. Top-pruning (TP) was conducted once, when fertilization was first applied, by removing the stem portion between the second node and the apex (including foliage). Fertilization was further supplied from 22 October to 28 December 2012. The fertilizer amount supplied was designed for a 10-week period, and based on nitrogen (N) concentration estimates in seedling foliage.
TF treatment followed the PRONARE-CONAFOR fertilization program for tropical species using Peters® fertilizers (
The EXF treatment was designed to supply nutrients exponentially, increasing the fertilizer addition levels through time. Calculations were derived from an exponential function described by Timmer (
where
The
where
Fertilizer solution pH and electrical conductivity were adjusted following
Three weeks after the final fertilization (21 January 2013), a random sample of 16 seedlings per treatment (four from each replicate) was selected to examine shoot height, root-collar diameter, net photosynthetic rate, and chlorophyll content. Twelve of the above 16 seedlings per treatment were further analyzed for leaf area, shoot and root dry weight, and nitrogen (N), phosphorus (P), and potassium (K) concentrations.
Shoot height (cm) and root-collar diameter (mm) were respectively measured with a graduated ruler and a Mitutoyo digital caliper. The sturdiness index (
Shoot and root dry weight were obtained by oven-drying seedlings at 70 °C for 72 h. Dry weight (g) of aboveground components (shoots + leaves), and roots with shoot-heights and root-collar diameters were combined to estimate the Dickson quality index (
Finally, N, P, and K concentrations were determined from tissue samples (three per treatment) integrated over the entire seedling by the micro-Kjeldahl method, using photocolorimetry by reduction with Molibdo-Vanadate and flame emission spectrometry, respectively. Total dry biomass and nutrient concentrations were used to calculate N, P, and K content.
Seedling quality,
The quality test trial ended in May 2013. During the four month period, seedlings were periodically watered with distilled water, which was used to prevent any source of external nutrient supply. Seedling height (cm) and stem diameter (mm) were measured at the beginning and end of the test, and monthly relative growth rates were estimated based on the following equation (
where
Comparisons of the factor effect and its interactions on morphological and physiological quality indices were performed through analysis of variance (ANOVA, α = 0.05). Significant differences among treatment means were analyzed using the
Statistical analyses were carried out using the package INFOSTAT© (
Additionally, monthly relative growth rates were analyzed by treatment and not factors, since the quality test was specifically designed to assess seedling performance as a function of nutritional reserves under low soil fertility conditions. Therefore, treatments better reflected the influence of plant nutrient status on seedling growth.
ANOVA results indicated the main factors,
Under the NP treatment, the most divergent morphological and physiological quality index response occurred for shoot and root dry weight, leaf area, and net photosynthetic rate (
Results of the fertilization effect showed seedlings under the EXF treatment regime produced 17% more biomass and had 50% more leaf area compared with TF treatment seedlings. Similarly, physiological variables, such as chlorophyll content (CC) and net photosynthetic rate (NPR) were significantly affected by EXF, evidenced by a respective 20% and 60% higher CC and NPR in seedlings compared with those grown under the TF regime (
Differences in plant nutrient status were detected in response to treatments. Compared with the control treatment (T1), seedlings receiving treatments T4, T5, and T6 exhibited increased N concentration and content, respectively (126% and 157% for T4, 75% and 67% for T5, and 364% and 260% for T6). P concentration in T4 and T6 was respectively 53% and 74% higher than in T1. Similarly, P content in T4 and T6 treatments were respectively 57% and 21% higher than in T1. Finally, only T6 revealed higher K concentration (> 55%) and content (> 20%) than T1 (
Seedling growth in the quality test trial revealed significant effects for height (F[5, 54] = 3.4, P < 0.01) and diameter (F[5, 54] = 4.91, P < 0.001), with T4 seedlings showing the highest relative growth rates (
Our results showed TP at 25% or 50% did not improve the morphological or physiological quality of
Previous reports demonstrated that after defoliation, plants accumulated nonstructural carbohydrates for regrowth to compensate tissue loss (
Cetina et al. (
Exponential nutrient loading applied in this study improved the morphological and physiological attributes of
Enhanced seedling growth,
In the present study, nutrient reserve amounts (N, P, and K) were higher in plants fertilized with the EXF treatment regime (
Results from the quality test in Tezontle (
Varied interpretations of species differential responses to exponential fertilization have been proposed.
Based on our short-term trial results, the doses applied (17 to 412 mg N l-1) using a complete formula of soluble fertilizer (20 N: 20 P2O5: 20 K2O) resulted in beneficial effects on accumulation reserves to support growth, when no external nutrient supply was available. However, we cannot exclude toxic effects due to exponential fertilization, particularly when the 50% TP + EXF treatment was administered. Indeed, exponential fertilization assumes a relationship between fertilizer dose and plant size (
We are grateful to Celi G. Calixto Valencia, Noé Bahena Tamayo† and Itzel Dinoran Galindez for their technical support.
Concentration of N per liter of nutrient solution provided in each application of the exponential fertilization regime.
Nomograms for response of nitrogen (a), phosphorus (b), and potassium (c), to the application of different top pruning intensities and different fertilization regimes in
ANOVA results for top pruning and fertilization regime effects on morphological and physiological variables related to seedling quality in
Variables | Factors | df | F-value | P-value |
---|---|---|---|---|
Root-collar diameter | Top pruning | 2, 90 | 9.01 | 0.0003 |
Fertilization regime | 1, 90 | 1.32 | 0.2542 | |
Interaction | 2, 90 | 1.47 | 0.2362 | |
Sturdiness index | Top pruning | 2, 90 | 3.01 | 0.0542 |
Fertilization regime | 1, 90 | 5.25 | 0.0243 | |
Interaction | 1, 90 | 1.89 | 0.1572 | |
Shoot dry weight | Top pruning | 2, 66 | 5.93 | 0.0001 |
Fertilization regime | 1, 66 | 10.31 | 0.0176 | |
Interaction | 2, 66 | 0.01 | 0.99 | |
Root dry weight | Top pruning | 2, 66 | 10.01 | 0.0002 |
Fertilization regime | 1, 66 | 1.28 | 0.2614 | |
Interaction | 2, 66 | 0.02 | 0.9754 | |
Dickson”s quality index | Top pruning | 2, 66 | 6.75 | 0.0022 |
Fertilization regime | 1, 66 | 0.56 | 0.4572 | |
Interaction | 2, 66 | 0.06 | 0.9387 | |
Leaf area | Top pruning | 2, 66 | 7.71 | 0.001 |
Fertilization regime | 1, 66 | 33.78 | <0.0001 | |
Interaction | 2, 66 | 0.7 | 0.5025 | |
Chlorophyll content | Top pruning | 2, 90 | 0.24 | 0.791 |
Fertilization regime | 1, 90 | 26.34 | <0.0001 | |
Interaction | 2, 90 | 0.81 | 0.4493 | |
Net photosynthesis rate | Top pruning | 2, 90 | 40.88 | <0.0001 |
Fertilization regime | 2, 90 | 175.3 | <0.0001 | |
Interaction | 2, 90 | 0.59 | 0.5543 |
Average values (± SE) of morphological and physiological quality indexes assessed in
Variables | Pruning | Fertilization | |||
---|---|---|---|---|---|
NP | TP 25% | TP 50% | TF | EXF | |
Diameter (cm) | 4.97 ± 0.11a | 4.88 ± 0.11a | 4.37 ± 0.11b | - | - |
SI (cm mm-1) | - | - | - | 2.20 ± 0.06a | 2.39 ± 0.06b |
Shoot dry weight (g) | 1.16 ± 0.06a | 0.99 ± 0.06b | 0.79 ± 0.06c | 0.90 ± 0.05a | 1.06 ± 0.05b |
Root dry weight (g) | 0.51 ± 0.03a | 0.41 ± 0.03b | 0.34 ± 0.03b | - | - |
DQI | 0.36 ± 0.02a | 0.31 ± 0.02a | 0.25 ± 0.02b | - | - |
Leaf area (cm2 ) | 73.56 ± 4.07a | 52.21 ± 4.07b | 54.07 ± 4.07b | 46.82 ± 3.33a | 73.28 ± 3.33b |
NPR (µmol m-2 s-1) | 3.54 ± 0.09a | 2.37 ± 0.09b | 2.69 ± 0.09c | 2.18 ± 0.08a | 3.55 ± 0.08b |
CC (SPAD units) | - | - | - | 31.83 ± 0.89a | 38.28 ± 0.89b |
Concentration and content of N, P and K in response to pruning and fertilization treatments in
Treatmentcombination | Concentration (mg g-1) | Content (mg plant-1) | ||||
---|---|---|---|---|---|---|
N | P | K | N | P | K | |
NP + TF (T1) | 8.1 | 3 | 12.2 | 12.7 | 4.7 | 19.13 |
TP 25% + TF (T2) | 8.1 | 2.4 | 11.4 | 10.56 | 3.13 | 14.87 |
TP 50% + TF (T3) | 9.1 | 2.7 | 11.6 | 9.49 | 2.81 | 12.1 |
NP + EXF (T4) | 18.3 | 4.6 | 11.3 | 32.6 | 8.19 | 20.13 |
TP 25% + EXF(T5) | 14.2 | 3.2 | 11.5 | 21.16 | 4.77 | 17.14 |
TP 50% + EXF (T6) | 37.6 | 4.7 | 18.9 | 45.68 | 5.71 | 22.96 |
Average relative growth rates in
Treatmentcombination | Relative growth rates | |
---|---|---|
Height(cm cm-1month-1) | Diameter(mm mm-1month-1) | |
NP + TF (T1) | 0.10 ± 0.01a | 0.05 ± 0.01ab |
TP 25% + TF (T2) | 0.09 ± 0.01ª | 0.04 ± 0.01ª |
TP 50% + TF (T3) | 0.12 ± 0.01ab | 0.08 ± 0.01bc |
NP + EXF (T4) | 0.15 ± 0.01b | 0.11 ± 0.01d |
TP 25% + EXF (T5) | 0.11 ± 0.01ª | 0.05 ± 0.01abc |
TP 50% + EXF (T6) | 0.14 ± 0.01b | 0.09 ± 0.01cd |