Abstract: A field study was conducted to evaluate the effects of timing [single spray at pre-anthesis (T1) and at anthesis (T2) and double spray at pre- + at anthesis (T3)] and concentration (0, 10-6, 10-5, 10-4 M) of exogenous application of GA3 on growth, dry matter production, nutrient uptake and yield attributes of Indian mustard (Brassica juncea L. Czern and Coss) cv. Varuna. Depression of flowering and sink strength is associated with lower endogenous level of gibberellic acid. Therefore, it was hypothesized that foliar application of GA3 will enhance the flower number and create a balance between source and sink. The impact of GA3 application at T1 was most conspicuous and resulted in a higher growth, efficient translocation and utilization of nutrients although; T3 was equally effective but is not preferable as it requires the spray at two time intervals. Among different concentrations of GA3, 10-5 M registered the maximum values for all the parameters studied. GA3 increased partitioning of biomass to the leaves at the expense of appropriation to the stem at 20 DAF. In this way, an appreciation of the timing of foliar application of GA3 can be used to manage the resources for maximum production of Indian mustard.
INTRODUCTION
Remarkable accomplishments of Plant Growth Regulators (PGR) such as manipulating plant developments, enhancing yield and quality have been actualized in recent years using new emerging and efficient plant growth regulators. It has long been ascertained that plant hormones including auxins, gibberellic acid, cytokinin, abscisic acid and ethylene are involved in controlling developmental events such as cell division, cell elongation and protein synthesis. PGRs have been implicated in efficient utilization of nutrients and translocation of photo-assimilates in established sink-source system (Khan et al., 1996; Patrick and Steains, 1987; Pereto and Beltran, 1987) Several factors either endogenous or environmental contribute to sink strength but sink activity can mainly be enhanced by gibberellins (Kuiper, 1993). Extensive studies have demonstrated that Gibberellic Acid (GA3) has potential to enhance growth, flowering, photosynthesis, nutrient transport and yielding ability of mustard (Hayat et al., 2001; Khan et al., 1996, 2005). Precise timing of plant activities and the adaptive morphological modulations can only be realized if plants perceive signals of its direct environments. In comparison to the large number of studies on the foliar application of PGRs, much less effort has been applied to understanding how this exogenous application of PGR (GA3) may elicit change in the allocation pattern with the leaf age. For the Brassica juncea, we suggest that ontogenetic differences in leaf age will differently mediate the reception of gibberellic acid signals (a mediator of sink-strength). To test this hypothesis, we applied single foliar spray of 0, 10-6, 10-5, 10-4M GA3 at 40 (pre-anthesis) and 60 (at anthesis) DAS, or double spray in equal doses of 0, 10-12, 10-10, 10-8M GA3 both at 40 and 60 Days after Sowing (DAS) and studied the pattern of growth, dry matter partitioning, growth ratios, nutrient uptake and yield attributes of Indian mustard (Brassica juncea L.) cv. Varuna.
MATERIALS AND METHODS
Experiments were conducted at the Experimental Farm of Aligarh Muslim University; Aligarh, India (situated at 27°52' N latitude, 78°51' E longitude and 187.45 m altitude above sea level). It has a semiarid and subtropical weather with severest hot dry summers and intense cold winters. The mean annual rainfall is about 847.3 mm. More than 85% of the total downpour is delivered during a short span of four months from June to September.
Soil characteristics: Arbitrary samples of soil were collected from various chosen spots, upto depth of 15 cm, spread over the entire experimental field before sowing of the experimental crops and analyzed for physico-chemical characteristics of the soil. Data obtained on chemical characteristics and physical constant for soil are presented in Table 1.
Experimental layout, preparatory tillage and cultural operations: The
field experiments were laid out in randomized block design with three replications
for each treatment. The individual plot size was 10 m2 (2x5 m). Prior
to each trial, diligent ploughing of fields was done to turn the soil for maximum
aeration and weed eradication. The plots were made with proper bunds along with
necessary irrigation channels and then irrigated lightly before sowing to maintain
proper moisture in the sub-surface of the soil. The seeds were sown by the usual
behind the plough method at the rate of 10 kg ha-1. Rows were separated
by a distance of 30 cm while the plants in the row were 15 cm apart. After the
establishment of the crop i.e., after 12 days of sowing seedlings were thinned
to maintain the uniformity (approximately 22 plants m-2) of the plant
population. Crop was irrigated prior to sowing and subsequently, when ever found
essential.
| Table 1: | Physico-chemical characteristics of the soil |
| Data are the means of 5 samples | |
There were two irrigations during the entire growth period of the crop.
Homogenous broadcasting of 80 kg N, 30 kg P and 30 kg K ha-1 was applied to the soil at the time of leveling of the individual plot. The sources of nutrients were urea, Single Super Phosphate (SSP) and Muriate of Potash (MOP). Exogenous application of gibberellic acid was made at pre-anthesis (40 days after sowing) and at post-anthesis (60 days after sowing). The scheme of the treatments is given in Table 2. GA3 was sprayed at the rate of 600 L ha-1 together with 0.5% teepol (a surfactant). In control set, equal amount of de-ionized water with 0.5% teepol was sprayed simultaneously with the treatment.
Determination of growth variables and nutrient uptake: Plants were harvested by cutting at the ground level and were allowed for sun drying. After sun drying, threshing was done. Seeds were cleared and collected for seed yield. The observations were carried out at 20 days interval from 80 DAS till harvest (120 DAS). There were total three samplings, i.e., at 80, 100 and 120 DAS. Five plants from each plot were cut at the soil level at various sampling stages for analysis of different growth parameters using suitable covenant and nutrient concentrations and their accumulation in plants. At harvest (120 DAS), twenty five plants (equivalent to 1 m2 land area) were removed to record the yield attributes.
Sampled plants were divided into different parts, like leaf, stem and pod corresponding
to different sampling stages and were dried in hot air oven at 80°C for
two days. The dried materials were weighed on physical balance and weight was
recorded as dry weight. Leaf area was ascertained by gravimetric method. For
analysis of growth ratio, crop growth rate (Watson, 1952), relative growth rate
(Radford, 1967) and net assimilation rate (Milthorpe and Moorby, 1979) was calculated.
Specific Leaf Area (SLA) and Specific Leaf Weight (SLW) was calculated according
to Hunt (1978). Content of nitrogen (Lindner, 1944) and phosphorus (Fiske and
Subba Row, 1925) was determined by the Kjeldahl method while content of potassium
was estimated with the help of flame photometer. Uptake of nitrogen, phosphorus
and potassium was calculated as the product of N, P and K content and their
respective dry matter at these stages.
| Table 2: | Scheme of treatments |
| +: Present, -: Absent | |
RESULTS AND DISCUSSION
Growth parameters: Table 3 shows that timing and concentration of GA3 spray significantly affected all the growth parameters at 20 days after flowering. Dry weight of a plant represents a successful manifestation of a complex processes ranging from interception of solar radiation, availability of nutrients and water and active photosynthesizing area to hormonal status of the crop. Maximum above Ground Dry Matter (AGDM) was attained in 10-5 M GA3 in pre-anthesis spray treatment (T1) which was equal in effect of the double foliar spray (T3) i.e., 40+60 DAS. The spray of this concentration at pre-anthesis (T1) produced 22.5% more AGDM than the plants sprayed at anthesis (T2). Significant differences were noted in the GA3 concentration effect in T1 and T3 treatments, but in T2 all the concentrations were equal in effect. The spray concentration of 10-5 M GA3 recorded 34.3, 13.2 and 47.2% more AGDM than control in treatments T1, T2 and T3, respectively. Exogenous application of GA3 produced considerable increase in dry weight of rice (Singh, 1996) and mustard (Khan et al., 1996). Plant height was significantly lower in treatment T3 than T1 and T2 which were equal in effect. The concentration effect of GA3 at each spray application showed a similar trend of increase over control. The increase in plant height was 19.3, 29.0 and 34.0% in T1, 14.2, 29.6 and 26.5% in T2 and 19.1, 26.1 and 33.8% in T3 in 10-6 M, 10-5 M and 10-4 M GA3 in comparison to control, respectively. Treatment with GA3 causes microtubule reorientation favoring axial elongation (Shibaoka, 1994). The enzyme Xyloglucan Endotransglycosylase (XET) catalyzes the breaking and reforming of bonds between xyloglucan residues, thus permitting transient increase in wall extensibility. Increase in XET activity is correlated with GA enhanced elongation in number of plant species (Potter and Fry, 1994). Possibly all these factors contributed for the increase in the plant height due to GA3 treatment. Several investigators have reported GA3 induce increase in plant height in rice varieties (Singh, 1996), Steria anceps (Carrer et al., 1997) and flax (El- Shourbagy et al., 1994).
Leaf area was significantly higher for T1 (20.8%) and T3
(25.8%) than T2. Increase in the plant height, in the treatment T1
and T3, at early stage enhanced the opportunity for the formation
of more leaf initials which may later developed into leaf increasing the total
leaf area. Spray concentration of 10-4 M GA3 at T3
gave maximum leaf area which was equal in effect to that of 10-5
M and 10-4 M GA3 at spray time T2. The increase
in the leaf area of GA3 applied plants could be attributed to the
increase in the phyllochron. In Aegilops caudata and Aegilops tauschii,
addition of GA3 increased the phyllochron as observed by Bultynck
and Lambers (2004). The partitioning of dry matter to leaf area is an important
determinant of plant growth rate during early phases of development (Nelson,
1988). GA3 treatment at T1 and T3 had lower
specific leaf area (leaf area/ leaf weight) than T2. SLA was 11.2,
15.5 and 15.1% in T1 and 11.9, 15.4 and 15.0% in T3, less
in 10-6 M, 10-5 M and 10-4 M GA3
than water spray control. Dijkstra et al. (1990) recorded an increase
in SLA of a slow-growing inbred line of Plantago major supplied with
GA3. Specific leaf weight (leaf weight/leaf area) was unaffected
by the spray time T3 in spite of large response in AGDM suggesting
that leaf thickness was not altered. Allocation pattern of dry weight in the
leaves (SLW) was similar for the plants received treatments at spray time T1
and T3.
| Table 3: | Growth parameters of Indian mustard (Brassica juncea L.) cv. Varuna as influenced by the timing and concentration of exogenous GA3 application at 20 Days After Flowering (DAF) |
| Table 4: | Distribution of dry matter in to leaf, stem and pod (%) as influenced by the timing and concentration of exogenous GA3 application at 20 (pod-initiation) and 60 (pod-fill) Days After Flowering (DAF) in Indian mustard (Brassica juncea L.) cv. Varuna |
Percent distribution of dry weight, number of flower and pod: Table 4 shows the effects of GA3 concentrations and timing of exogenous application on the pattern of biomass allocation at pod-initiation (20 DAF) and pod-maturity (60 DAF). The pattern of distribution of dry weight among different plant parts was strongly influenced by timing of GA3 treatment while concentration was non significant. The contribution of leaf dry weight to the total plant dry weight was 33.7, 31.9 and 35.8% at spray time T1, T2 and T3, respectively at pod-initiation phase. This was declined to 8.0-11.0% during pod-maturation phase of the crop. Interestingly, the period of decline in the specific leaf weight coincides with an increase in the pod dry weight. This was evident from the contribution of pod dry weight to the total dry weight which showed a steady increase from meager 9.0-10.0% at 20 DAF to 30.0-33.0% at maturity (60 DAF). It was found that GA3 has changed the pattern of assimilate distribution and more assimilates were translocated to reproductive parts of the GA3 treated plants from the source (leaves) organs. GA3 directed mass movement of photosynthetic materials was also evident from the distribution of dry weight towards stem. In this case, per cent stem dry weight of water sprayed treatments were found to be much higher than GA3 treated plants. This was due to the reduced translocation of photo-assimilates towards leaves and pods in water sprayed plants.
Number of flowers and pods were significantly responsive towards timing of
spray application (Fig. 1 A and B). The
setting of flower to pods were remarkably increased in the GA3 treated
plants at spray time T1, but spray time T2 recorded the
lowest pod number even at higher concentration of GA3. The spray
at T1 (pre-anthesis) and at T3 (pre- + post anthesis)
checked the abortion of flower which finally enhanced the chance for the plants
to increase its yield potential by increasing the number of pods.
| Fig. 1: | Effect of timing and concentrations of GA3 on number of flowers (A) at anthesis (20 DAF) and number of pods (B) at maturity (60 DAF) of Indian mustard (Brassica juncea L.) cv. Varuna. Vertical bars indicate ±SE (n = 5) |
Several investigators have positively correlated the initiation of flower with GA3 treatment and endogenous level (Chandler and Dean, 1994; Takahashi and Kaufmann, 1992).
Growth analysis: According to Lambers (1987), growth analysis is an important first step in an analysis of morphological, physiological, or biochemical factors determining relative growth rate. The results show that all the growth variables were strongly affected by the timing of spray application (Fig. 2A-F). Between 0-20 DAF, crop growth rate (CGR) was found to be highest in T1 while T2 and T3 recorded maximum between 20-40 DAF and 40-60
DAF, respectively. GA3 spray gave significantly lower CGR than water
sprayed control between 20-40 DAF this was a period which coincided with pod-filling
stage and GA3 sprayed plant has invested all its resources to pods
(Fig. 2A and B). Relative Growth Rate (RGR),
was significantly higher at T1 while others i.e., T2 and
T3 were equal in response between 0-20 DAF. Rate of increase in dry
mass per unit starting mass and time (RGR) was enhanced under the influence
GA3 in comparison to the water sprayed control when applied at T1
and T3 between 0-20, 20-40 and 40-60 DAF. RGR in water sprayed control
did not vary between any sampling stages but between 20-40 and 40-60 DAF it
decreased significantly (Fig. 2C and D).
In the present experiment a reduction in SLA was consistently associated with
a decrease in RGR and the two parameters were positively correlated underlining
the significance of leaf expansion for dry matter accumulation. A positive relationship
between SLA and RGR has also been reported by others (Lambers and Poorter, 1992;
Poorter and Remkes, 1990; Reich et al., 1998).
| Fig. 2: | Crop growth rate (A-B), relative growth rate (C-D) and net assimilation rate of Indian mustard (Brassica juncea L.) cv. Varuna, as influenced by timing of exogenous application of GA3 at 20 days after flowering (20 DAF). Points are means of five replicates. Vertical bars indicate ±SE |
Net Assimilation Rate (NAR) is basically a function of photosynthesis particularly during the initial stage of growth. In GA3 sprayed plants the rate of dry matter accumulation in a unit (NAR) was always higher in comparison to the water sprayed plants. This is consistent with the positive effect of GA on photosynthetic characteristics (Khan et al., 1996, 2005). Significant differences were observed for the effect of timing of spray and T3 was superior in effect between 0-20 DAF and 20-40 DAF while T2 recorded maximum between 40-60 DAF (Fig. 2E and F). Contrary to our findings an increase in NAR was observed in low-GA mutant (Nagel et al., 2001).
Nutrient uptake: Improving the use efficiency of applied nutrients to Brassica requires an understanding of the pattern of nutrient uptake by the crop during growth. The yield response of Brassica to applied nutrient will depend, in part, on the capacity of the crop to mobilize the nutrients from senescing vegetative organs and to translocate this to developing seeds. To achieve this, the crop should be manipulated in such a way as to utilize the maximum possible available resources. Present results on the pattern of uptake of nutrients showed that it was much more affected by the timing of spray at 20 DAF than at 60 DAF (Table 5). Pronounced effect of GA3 was noted for the uptake of nitrogen, phosphorus and potassium in treatment T1 and T3 at 20 DAF. But the effect of spray stage T3 on nutrient uptake was negligible. The difference in the nutrient uptake in response to spray timing remained as such until 60 DAF (pod-maturation) where T3 recorded significantly lowest value. The observations in the present experiment clearly manifest the importance of GA3 application at pre-anthesis stage for enhanced uptake and better utilization of nutrients. Plant growth regulators control most of the characteristics of root systems, including primary root growth and the formation of lateral roots and root hairs. Many plant species respond to the exogenous application of auxins by producing large numbers of lateral roots and to auxins and ethylene by increasing the density and length of root hairs. In case of gibberellic acid, it is assumed that exogenous application of GA3 at T1 sends a signal to the roots for enhanced uptake of nutrients. Similar enhancement of nutrient uptake in response to GA3 application was also noted in Plantago major (Kuiper and Saal, 1987) and Brassica (Khan et al., 2005). However, application of GA3 and IAA to flax (Linum usitatissimum L.) increased P, K and Ca accumulation in all plant organs (El- Shourbagy et al., 1994).
Yield characteristics: Yield is the culmination of several comprehensive
phases which starts at germination and end at harvest, encompassing through
shoot growth, leaf development, photosynthesis, flowering, pollination and seed
set. Better vegetative growth of a crop is largely responsible for higher seed
yield because number of photosynthesizing sites i.e., number of vegetative branches
is affected by initial growth stages. Two sequential steps are necessary for
a mustard plant to produce pods; a sink of pollinated pods capable of further
development must be created and this must be supplied with photosynthates over
subsequent period of development. The consequences for yield attributes are
clearly evident (Fig. 3A-C). Again there
was consistent and significant reductions (17.8%) in the seed yield due to spray
time T2 than T1. This reinforces our view that application
at the phase that coincides with intense vegetative growth had the tissues which
was most responsive and created higher demands with efficient translocation
of nutrients that finally culminated in to higher seed yield.
| Table 5: | Effect of timing of foliar application of GA3 on nitrogen, phosphorus and potassium uptake of Indian mustard (Brassica juncea L.) cv. Varuna at 20 Days After Flowering (DAF) |
| Values are means (±SE) of five plants. Different letters in the same column indicate significant differences (p<0.05) | |
| Fig. 3: | Seed yield (A), biological yield (B) and harvest index (C) of Indian mustard (Brassica juncea L.) cv. Varuna, as influenced by timing and concentrations of GA3 at harvest (60DAF). Vertical bars indicate ±SE (n = 5) |
As for concentration effect on the seed yield, the spray application of 10-5 M GA3 produced 21.6% more seeds than water sprayed control. Similar increases in seed yield due to exogenous application of GA3 have been observed by several workers (Harkess et al., 1994; Metzger, 1987; Zeevart, 1983).
The increase in biological yield was 7.9 and 11.5% more in spray time T1 and T3 than T2. The spray application of 10-5 M GA3 increased the biological yield by 22.5% compared to water sprayed control. It was shown that photoassimilates are remobilized to support both seed development and vegetative growth when plant source-sink relations were artificially modified (Griffith, 1992). Factors that control sink strength also control the partitioning in the crop. Harvest index of GA3 applied plants were increased by 15.8% compared to water sprayed control. The increase in biological yield and harvest index owe much to the production of enough leaves due to GA, with improved radiation interception efficiency which ultimately resulted in higher translocation of photoassimilates towards seeds.
In conclusion, most suitable stage of growth for the exogenous application of GA3 would appear to be at pre-anthesis for Indian mustard (Brassica juncea L.). The findings are relevant when considering the impact of nutrient utilization and related promotion of partitioning of photoassimilates under the spray influence of GA. Taking all together, GA 3 spray had a significant stimulatory effect on dry matter production and nutrient uptake. The findings reported here also have implications with respect to the use of plant growth regulators (GA) that lower the incidence of flower abortion and promote the formation of pods.