Effects of Gibberellic Acid (GA₃) on Physiological Contributing Characters of Mungbean (Vigna radiata L.)

MATERIALS AND METHODS

The experiment was conducted at the Field Laboratory of the Department of Crop Botany, Bangladesh Agricultural University, Mymensingh during the period from October, 2000 to February, 2001. Two varieties of mungbean namely BARI moog-4 (V₁) and BARI moog-5 (V₂) collected from Bangladesh Agricultural Research Institute, Joydebpur, Gazipur were used in study.

The land of the experimental site was first opened with a power tiller. After ploughing and laddering all the stubbles and uprooted weeds were removed to make the land ready for sowing and the basal dose of fertilizers was incorporated thoroughly before planting (BARC, 1997). The two factors experiment was laid out in Randomized Complete Block Design (RCBD) with 3 replications. The whole area was divided into 3 blocks and each block into 16 unit plots. The size of the unit plot was 2m² and the distance between plots was 0.5m. The plots were raised up to 15cm from the soil surface. The seeds of mungbean were sown in rows made by hand plough on October 24, 2000. The distances between rows and seeds were 30cm and 15cm, respectively and 2 seeds were placed in each point at 2-3cm depth. Seedlings were transferred to fill up the gaps where seeds failed to germinate. Irrigation was done as per necessity. The crop field was weeded twice: First weeding followed by thinning was done at 15 days after sowing (DAS) and second weeding was done at 45 DAS. Dimecrone 50 EC was sprayed @ 1 L/ha to prevent pod borer infestation as and when required. GA₃ solution was prepared two times first for seed treatment and second for spray according to the procedure of Roy et al. (1991).

The first crop sampling was done on 15 days after sowing (DAS) and it was continued at an interval of 10 days till on 55 DAS. At the time of each harvest, five plants were selected randomly from each plot. The sampling was done until maturity. Physiological characters like- leaf area index, total dry matter, crop growth rate, relative growth rate and net assimilation rate were recorded. The data collected on different parameters under the experiment were statistically analyzed to obtain the level of significance using the MSTAT-computer package programme developed by Russel (1986). The differences between pairs of means were compared by Duncan’s Multiple Range Test (DMRT).

RESULTS AND DISCUSSION

The leaf area index (LAI) was recorded at 10 days interval starting from 15 days after sowing (DAS) to 55 DAS. It revealed that both seed treatment and foliar spray of GA₃ had stimulatory effect on leaf area index (Table 1). The lowest concentration (50 ppm), however, finally became superior to 100, 200 ppm and the control (Table 1). Moreover, 50 ppm GA₃ became the best treatment with the highest LAI at the later stages, although the same treatments were more effective at early stages. Seed treated plants of BARI-4 had the highest LAI at all the growth stages except 15 DAS. While foliar application on the seedlings of BARI-5 had higher LAI than BARI-4 throughout the entire growth stages except 15 DAS (Table 2).

Table 1:	Physiological characters of mungbean as influenced by pre-sowing seed treatment and foliar spray with GA3
Figures followed by different letters within a column differ significantly by DMRT  NS = Not significant
T1 = Control     T2 = 50 ppm of GA3    T3 = 100 ppm of GA3    T4 = 200 ppm of GA3

Table 2:	Physiological characters of mungbean cultivars grown after seed treatment and foliar spray with GA3
Figures followed by different letters within a column differ significantly by DMRT
NS = Not Significant       V1 = BARI– 4       V2 = BARI –5

Table 3:	Physiological characters of mungbean as influenced by pre-sowing seed treatment and foliar spay by GA3
Figures followed by different letters within a column differ significantly by DMRT
NS = Not Significant      T1 = Control     T2 = 50 ppm of GA3      T3 = 100 ppm of GA3    T4 = 200 ppm of GA3

Table 4:	Variation in physiological character in different cultivars of mungbean as a result of seed treatment and foliar spray with GA3.
Figures followed by different letters within a column differ significantly by DMRT
NS = Not Significant      V1 = BARI– 4     V2 = BARI –5

This difference indicates a differential responsiveness of the cultivars at different stages of growth to GA₃ and its mode of application. GA₃ was reported to increase leaf area in seed treatment (Deotale et al., 1998) as well as in foliar application (Naidu and Swamy, 1995).

Total dry matter accumulation was significantly enhanced by the application of GA₃ at all the growth stages (Table 1). However, no single dose was found to be clearly superior to the others. The data revealed that the highest total dry matter (3.17) was recorded at 55 DAS with 100 ppm of GA₃ and the lowest one at 15 DAS with the same concentration of GA₃ in case of seed treated plants (Table 1). Highest total dry matter (3.52) was found at 55 DAS with 100 ppm of GA₃ while the lowest was resulted in control at 15 DAS when foliar application was made. The two varieties of mungbean differed significantly in their ability to accumulate dry matter at all growth stages (Table 2) except 15 DAS in pre-sowing. The difference was lower at early growth stages but remarkably higher at later stages. The data revealed that V₁ had superiority over V₂ in accumulating dry matter throughout the entire growing season in seed treated plants (Table 2); while in foliar spray, V₂ was superior to V₁ at all growth stages except for 35 DAS (Table 2). Application of 100 ppm of GA₃ was reported to increase the total dry matter in french bean (Gabal et al., 1990) and soybean (Deotale et al., 1998). The present study also shows that GA₃ efficiently influenced dry matter accumulation in mungbean.

The various concentrations of GA₃ significantly regulated relative growth rate (RGR) at different growth stages (Table 3). The data revealed that RGR was maximum at 25 DAS and decreased with the advancement of crop growth in all treatments (Table 3). The treated plants had higher RGR over their respective control. No treatment could be identified having the highest RGR value at all growth stages. However, 50 ppm had higher RGR at 35 & 45 DAS, 200 ppm had higher at 45 and 55 DAS; while 100 ppm had the highest RGR (0.156) at 35 DAS (Table 3). In foliar spray, 200 ppm GA₃ had the highest RGR except only the final stage (55 DAS) when 100 ppm superceeded it (Table 3). Difference of mungbean varieties on relative growth rate was found significant at all the growth stages (Table 4) except at 35 DAS in foliar spray. Maximum RGR in pre-sowing treatment was found in V₁ at all stages except 45 DAS. V₂ had higher RGR than V₁ at 25 and 55 DAS in foliar spray; while V₁ had higher RGR at 35 and 45 DAS. Crop growth rate (CGR) was estimated at 10 days interval starting from 25 DAS to 55 DAS. In all treatments, CGR increased gradually with the advancement of crop growth, maximized at 45 DAS and thereafter decreased slightly (Table 3). Crop growth rate increased due to the influence of GA₃ applied as seed treatment and foliar spray. However, 50 ppm was superior among the seed treatments; while 100 ppm was the most effective dose as foliar spray .It was interesting to note that BARI-4 responded better than BARI-5 in seed treatment, while BARI-5 had higher CGR values than BARI-4 due to the influence of foliar spray of GA₃ (Table 4). Similar increase of CGR was reported at all the treatments over control (Katiyar, 1980). The final decrease of CGR at 55 DAS might be due to the approach of crop maturity.

The effect of GA₃ was significant as was observed on net assimilation rate (NAR) at all growth stages (Table 3). The data revealed in seed treatment that the highest NAR in pre-sowing was obtained by 200, 50, 100 and 200 ppm at 25, 35, 45 and 55 DAS, respectively. In the foliar spray, the highest NAR was found with 200 ppm and control at 25 and 35 DAS, respectively and with 100ppm at 45 and 55 DAS. Difference of varieties on net assimilation rate of mungbean was found significant at all growth stages (Table 4). The data showed that V₁ had higher NAR values than V₂ at all growth stages except 45 DAS in seed treatment. In foliar spray V₂ had higher NAR at 25 and 55 DAS and those of BARI-4 had higher NAR at 35 and 45 DAS. Maximum NAR (230mg g^-1 day^-1) in green gram was reported with 150 ppm GA₃ (Suma Bai et al., 1987). The present study is in agreement with the above report.

The results of the present study indicate that physiological parameters of mungbean can be favourably influenced by the application of GA₃ with a consequent yield increase. This result shows potential application of GA₃ for the increase of mungbean yield in Bangladesh.