Banana is one of the important fruit crops of the world, especially in the tropics. The world produces 40 million tonnes of banana each year, but most of them are consumed locally. In India, to meet the increasing food demand of the ever increasing population, stepping up of fruit production is part of the strategy to solve the food crisis. And, banana has a dominant role to play among the fruit crops. India produces about 30 million tonnes of bananas from an area of 0.83 million ha. Among horticultural crops, contribution of banana to Agricultural Gross Domestic Product (AGDP) is the highest (Turner, 1990).

There is a need to focus on standardization of improved production technologies suitable for different systems of cultivation to realize potential yields in many commercial cultivars for targeted banana production. Selection of high-yielding varieties, planting of healthy, disease-free planting material, choosing the right planting density, need-based and timely application of inputs, viz., irrigation water and nutrients, maintenance of weed-free conditions, etc., are important to bridge the gap between actual yield and potential yield per unit area (David, 2002).

With a view to manipulate vegetative growth, source size and its activity by varying nitrogen levels, and to convert the increased biomass to yield advantage by improving the assimilate translocation to developing sink by using various growth regulating chemicals, the present study has been taken up.

Results 
Stomatal diffusive resistance
Stomatal resistance in banana clearly established showed an increasing trend upto shooting stage. There existed significant differences in main plot and subplot treatments at all the stages. The interaction effects were also found to be significant at all the growth stages. Among the doses of nitrogen, M₄ was found to be poorer in stomatal resistance than M₃, M₂ or M₁ by recording 12.7%, 29.1% and 5.7% lower values than control at 5 MAP, 7 MAP and at harvest stages.

Among the plant growth regulators, salicylic acid spray (S₅) recorded values of 1.46, 5.18, 6.55 and 1.24 at 3 MAP, 5 MAP, 7 MAP and at harvest which were 19.7%, 12.4%, 14.3% and 49.4% increase over control respectively. Benzyl adenine (S₇) and 2, 4-D performed poorly with low values in most of the stages. The results indicated that salicylic acid and growth retardant chemicals tended to increase stomatal diffusive resistance, while nitrobenzene, benzyl adenine and 2,4-D appeared to reduce the resistance (Table 1).

Table 1 Effect of different levels of nitrogen and plant growth regulators on stomatal resistance (s/cm) at different growth stages of banana cv. Ney Poovan

M₁S₁ and M₂S₅ recorded maximum values at 7 MAP (7.98) and at harvest (1.37), while M₄S₅ recording high value at 3 MAP (1.59) and M₃S₄ at 5 MAP (5.49). Very low values were mostly seen in M₄S₇ and M₄S₈ which recorded the lowest mean values of 2.66 by pooling observations of all the stages.

Transpiration rate (μg H₂O cm^-2 s^-1)
Data on transpiration rate revealed there were significant impacts of various plant growth regulators on transpiration rate at all growth stages. However, nitrogen levels failed to elicit any significant differences among themselves.

Among the subplots treatments, high transpiration rate was observed in control plants in all the chosen stages except at 3 MAP, when 2, 4-D and benzyl adenine treatments registered enhanced rates over control. Almost all the growth regulator treatments distinctly recorded lower transpiration rate than control. Among them salicylic acid spray at 100 ppm recorded lowest rate at 3 and 7 MAP (21.8% and 21.4% lower than control respectively), mepiquat chloride and CCC registered lower transpiration rate at 5 MAP and at harvest by 19.5% and 27.2% respectively below the values of control. Among the growth regulators, nitrobenzene, benzyl adenine and 2,4-D showed high transpiration rate almost nearer to that of untreated control plants (Figure 1; Table 2)

Figure 1 Effect of different growth regulators and chemicals on transpiration at different growth stages

Table 2 Effect of different levels of nitrogen and plant growth regulators on leaf temperature (oC) different growth stages of banana cv. Ney Poovan (Main effect)

Significant interaction effects were observed in the stages of observation M₂S₈ showed maximum transpiration rate at 3 MAP (8.44) which were comparable with that of M₃S₈. At 5 MAP and at harvest, M₁S₇ revealed lower rates at 5 and 7 MAP; control recorded the highest rate (8.50), on the contrary, M₄S₅ recorded lower rates at 5 and 7 MAP (6.91 and 6.14) respectively. Considering the mean values of all the stages, M₃S₈ recorded the maximum value of 6.97, while M₁S₅ recorded the lowest mean value of  5.10.

Table 3 Effect of different levels of nitrogen and plant growth regulators on transpiration (µg H2O cm-2s-1) different growth stages of banana cv. Ney Poovan.

 

Figure 2 Effect of different growth regulators and chemicals on transpiration at different growth stages

Discussion
By increasing nitrogen dose or by giving urea spray, stomatal resistance was found to decrease (Table 1). Among the growth regulators, salicylic acid was found more effective in increasing stomatal resistance, while benzyl adenine performed poorly with low values. Apart from benzyl adenine, nitrobenzene and 2,4-D also showed low stomatal resistance. As stomatal resistance is an useful parameter in identifying drought resistant character, salicylic acid appears to be an useful tool for inducting resistance to drought. Eris (1983) observed significant increase in stomatal resistance in pepper with salicylic acid treatment. Anitha (2003) reported promotary effect of cycocel treatment on stomatal resistance in banana leaves, which is in conformity with the present findings as all growth retarding chemicals including CCC (cycocel) maintained positive influence on stomatal resistance.

Nitrogen produced only neglible effect on leaf temperature changes in banana. Salicylic acid, mepiquat chloride and cycocel were effective in reducing leaf temperature below that of control. On the other hand, nitrobenzene, benzyl adenine and 2,4-D had higher leaf temperature values. The reduced leaf temperature values in certain growth regulator treatments might be attributed to the maintenance of better water status in leaf tissues (Robinson and Bower, 1988)

Conclusion
The findings of this research also showed that the leaf temperature, transpiration rate and stomatal diffusive resistance were greatly influenced by different nitrogen levels and plant growth regulators. Net photosynthesis and stomatal conductance revealed very high positive relationship with bunch yield, which were showing high rates at shooting stage in plants receiving nitrogen 200 g plant^-1 with urea two per cent foliar spray and salicylic acid 100 ppm as combined spray. Salicylic acid treated leaves showed high stomatal resistance with low transpiration.

Materials and Methods
A field experiment in banana cv. Ney Poovan was conducted with various levels of nitrogen and plant growth regulators. The main plots are, M₁ (Control-150 g N plant^-1 (45 g+75 g+30 g N plant^-1 at 3 MAP, 5 MAP and 7 MAP, respectively), M₂ (M₁+Urea 2% foliar spray), M₃ (200 g N plant^-1 (60 g+100 g+40 g N plant^-1 at 3 MAP, 5 MAP and 7 MAP respectively), M₄ (M₃+Urea 2% foliar spray). The sub plot treatments are, S₁ (Control (water spray), S₂ (Mepiquat chloride (MC) 500 ppm), S₃ (Chlormequat chloride (CCC) 100 ppm), S₄ (Ethrel 500 ppm), S₅ (Salicylic acid (SA) 100 ppm), S₆ (Nitrobenzene 100 ppm), S₇ (Benzyl adenine (BA) 20 ppm), S₈ (2,4- Dichlorophenoxy acetic acid (2,4-D) 25 ppm). The leaf temperature, transpiration rate and stomatal diffusive resistance were estimated during 3rd, 5^th and 7th month after planting and at harvest stages of the crop with measuring procedure was given below:

Leaf temperature
Leaf temperature was recorded between 10.00 A.M to 12.00 noon at 3^rd, 5^th and 7^th month after planting and at harvest stage using Steady State Porometer (LICOR 1600, Licor Inc, Nebraska, USA) and expressed as °C.

Stomatal resistance
Stomatal resistance was measured at 3, 5^th and 7^th month after planting and at harvest by using Steady State Porometer between 10.00 AM to 12.00 noon (LICOR 1600, Licor Inc, Nebraska, USA) and expressed as s cm^-1.

Transpiration rate
Transpiration rate was measured by using Steady State Porometer (LICOR-1600, Licor Inc, Nebraska, USA) at 3^rd,5^th,7^th month after planting  and at harvest and expressed as µg H₂O cm^-2 s^-1.

Reference
Anitha R., 2003, Physiological investigations in banana cv. Grand Nain as influenced by certain plant growth regulators and chemicals. M.Sc.(Ag.) Thesis, Tamil Nadu Agricultural University, Coimbatore.

David W., 2002, Limitation to photosynthesis in water stressed leaves: stomata vs. metabolism and the role of ATP, Annals of Botany, 89: 871-885

Eris A., 1983, Effect of salicylic acid and some growth regulators on stomatal resistance of pepper seedling leaves, Acta Hort., 137:189-192.

Robinson J.C., and Bower J.P., 1988, Transpiration from banana leaves in the subtropics in response to diurnal and seasonal factors and high evaporative demand, Scientia Horticulturae, (37): 129-143

Saavedra A.L., 1978. The antitranspirant effect of acetylsalicylic acid on Phaseolus vulgaris. Physiol. Plant., 43: 126-128

Turner D.W., 1990, Modelling demand for nitrogen in the banana, Acta Horticulturae, 275: 497-503