Changes in Photosynthetic Rate, Specific Leaf Weight and Sugar Contents in Mango (Mangifera indica L.)

To evaluate the cultivar variation in photosynthetic rate (Pn) of mango, observations were taken in 25 cultivars representing different agro-climatic region of the country at two stages i.e. full bloom (March) and developed fruit stage (May). Seven cultivars, Papatio, Lucknow Safeda, Neelam, Mulgoa, Khas-ul-khas, Himsagar and Nisar Pasand showed the higher Pn rate at both the stages, while nine genotypes, Kishan Bhog, Langra, Chausa, Kesar, Fazri, Vanraj, Fernandin, Dashehari and Mallika showed medium Pn rate at both the stages. Specific leaf weight (SLW), total sugar and reducing sugar contents in leaves were also estimated in five commercially grown mango varieties, viz., Dashehari, Langra, Chausa, Amrapali and Mallika during the day at three times, 7:30 a.m., 12:30 p.m. and 5:30 p.m. at flower bud differentiation (November) and full bloom (March) stage to evaluate the cultivar variation in photosynthate production and their translocation efficiency. Total sugar and reducing sugar contents were generally low during November, while SLW was observed at lower level in March in all the cultivars, indicated better photosynthate movement from leaves in March and SLW level increased likewise sugar content from morning to noon and decreased gradually till evening. Higher photosynthates production and greater translocation was observed in Chausa and Langra and its maximum level was recorded during full bloom stage (March). The study brings out the importance of SLW for screening the genetic stocks of mango cultivars with regard to photosynthetic rate and translocation of photosynthates from leaves to other plant parts. INTRODUCTION Photosynthesis process seems to be quite important as it contributes a lot to growth, development and fruit yield in plants but perennial trees like mango have very low orchard efficiency [1] in terms of commercial fruit production and many commercial cultivars have a tendency for biennial or alternate bearing although it bears luxuriant growth. The exhaustion of the carbohydrate supply by fruiting has been implicated as one of the factors responsible for biennial bearing in mango [1]. Vigorously growing mango cultivars have low starch contents [2, 3] and diversion of assimilates from shoot apices to floral primordia results floral inhibition. Low rate of photosynthate production and lower partitioning may be one of the reasons for its low productivity. Plant growth and yield is the product of photosynthesis and accumulated photosynthates which regulate critical growth events under stress situations but at flowering and fruiting, the increased demand is to be met by current photosynthesis from the existing source, the leaves. Photosynthates produced by leaves are either used immediately by the nearest sink, the flowers and fruits [4], or in other situations stored throughout the tree, if the sink is limited. Hence, photosynthates production plays an important role in the plant productivity however, it varies from location to location and genotype to genotype. Chacko et al. [5] also reported significant differences in photosynthesis rate in mango cultivars. Keeping this in view, mango cultivars were evaluated for leaf photosynthesis at two developmental stages, full flowering and fully developed *Address correspondence to this author at the Central Institute for Subtropical Horticulture, Rehmankhera, Lucknow 227 107, India; E-mail: [email protected] fruit stage. Photosynthates translocation also plays an important role in meeting the urgent requirement of sink, fruit and leaves. Therefore, changes in SLW, total and reducing sugar contents were worked out to know the behavior of photosynthate movement / translocation from the leaves. The photosynthetic potential of leaves is reported to be quantitatively inherited [6] may be more useful if it is coupled with faster translocation efficiency which results in higher yield. However, the information on the variability in leaf photosynthates translocation efficiency in mango is very meager. Screening large number mango genotypes for photosynthates translocation efficiency is very difficult, as it is time consuming hence, the development of selection criterion would be desirable. Specific leaf weight (SLW), defined as the leaf dry weight per unit area, has been reported to have a strong association with total sugar and photosynthates production [7, 8]. Therefore, the present study was undertaken with the view to assess the variability in the photosynthetic rate and their translocation efficiency in terms of leaf total and reducing sugar contents and their impact on SLW. MATERIALS AND METHODS Twenty five cultivars of 25 years of age representing different agro-climatic regions at the Central Institute for Subtropical Horticulture, Lucknow, located at latitude 26.55°N and longitude 85.59°E in the subtropical zone of India, were evaluated for leaf photosynthetic rate at two stages, i.e. full bloom (March) and full grown stage of fruit (May). The photosynthetic rate (Pn) was measured in the previously tagged leaves by portable photosynthesis system Changes in Photosynthetic Rate, Specific Leaf Weight The Open Horticulture Journal, 2009, Volume 2 41 (LI 6200) at 11:00 a.m. at both the stages. The fully matured middle leaves as were standardized for mango [9] facing towards east-south direction was used for measuring photosynthetic rate. Ten leaves per tree were selected for its measurement and average data were presented. In an another experiment five commercially grown mango varieties, Dashehari, Langra, Chausa, Amrapali and Mallika of the same age were selected for the purpose of determination of leaf area, leaf fresh weight, total sugar and reducing sugar contents thrice a day i.e. 7:30 a.m. morning, 12:30 p.m. noon and 5:30 p.m. evening at flower bud differentiation and full bloom stages. The leaf samples were collected in the morning, noon and evening brought in polythene bags to the laboratory with minimum loss of time. Leaf area was measured by leaf area meter (LI 3000) and fresh weight was recorded for each sample each time. The samples were then kept in preheated oven at 100°C and temperature maintained at 80°C till the constant weight of the dry leaves was recorded. SLW was calculated using dry weight of leaves and leaf area. Total and reducing sugar contents were analysed in the leaf samples following the method of Morris [10] using Spectrophotometer Chemito 2010. All measurements were performed in triplicate and repeated thrice. The critical difference of the data were calculated at 5% (p = 0.05) as described by Panse and Sukhatme [11]. RESULTS AND DISCUSSION The results presented in Table 1 revealed that the value of photosynthetic rate (Pn) was in general, comparatively low at full bloom stage (March) as compared to fully develTable 1. Leaf Photosynthetic Rate ( mol m -2 s -1 ) in Different Mango Cultivars at Full Bloom and Fully Developed Fruit Stage Photosynthetic Rate S. No. Genotypes Full Bloom Stage Fully Developed Fruit Stage 1 Papatio 7.518 8.900 2 Lucknow Safeda 7.479 8.610 3 Neelam 7.384 8.134 4 Mulgoa 7.194 7.547 5 Khas-ul-Khas 7.156 7.468 6 Himsagar 7.155 7.144 7 Nisar Pasand 7.144 7.140 8 Kishan Bhog 6.040 6.080 9 Langra 6.006 6.061 10 Chausa 6.004 6.060 11 Kesar 5.896 6.047 12 Fazari 5.808 6.020 13 Vanraj 5.588 6.012 14 Fernandin 5.562 5.990 15 Dashehari 5.469 5.965 16 Mallika 5.300 5.593 17 Amrapali 4.660 4.724 18 Rataul 4.090 4.106 19 Alphonso 4.000 4.091 20 Banglora 3.195 3.902 21 Swernerkha 3.100 3.190 22 Benganpalli 2.993 3.100 23 Bombay green 2.990 3.092 24 Zardalu 2.982 2.990 25 Mankurad 1.998 2.580 CD at 5% (p = 0.05) 0.412 0.802 42 The Open Horticulture Journal, 2009, Volume 2 Singh and Rajan oped fruit stage (May) in all the genotypes but the trend of genotypic Pn was almost similar at both the stages. Seven genotypes Papatio, Lucknow Safeda, Neelam, Mulgoa, Khas ul Khas, Himsagar and Nisar Pasand showed the higher photosynthetic rate at both the stages while nine genotypes, Kishan Bhog, Langra, Chausa, Kesar, Fazri, Vanraj, Fernandin, Dashehari and Mallika showed medium Pn in the same environmental conditions. Rest of the genotypes was poor in Pn rate at both the stages. Significant variation in the Pn in mango cultivars suggested that there is a possibility of improvement in their photosynthetic efficiency through breeding manipulations. Results presented in Table 2 suggest that specific leaf weight (SLW) differed significantly and was comparatively low during full bloom stage (March) in all the mango cultivars. The increase in SLW was maximum from morning to noon as was recorded in Langra (36.0 and 31.2%) and Chausa (33.8 and 28.5%) during flower bud differentiation (November) and full bloom stage (March), respectively. This indicated the higher photosynthate production and accumulation in leaves due to higher photosynthetic rate from morning to noon. The SLW decreased at evening and the maximum reduction was recorded in Langra (15.3 and 14.6%) and in Chausa (15.5 and 15.8%) in both the stage of observation i.e. flower bud and full bloom stages, respectively which indicates the higher rate of photosynthate translocation from source (leaves) to sink (other plant parts). Saini and Joshi [7] also observed similar trend while working on ragi leaves. Total sugar contents was also varied significantly in different mango cultivars during both the stages of flowering (Table 2) and total sugar markedly increased in leaves up to noon and maximum increase was recorded in Chausa (303.0 and 304.1%) and in Langra (249.0 and 298.0%) during flowering bud differentiation stage (November) and full bloom stage (March), respectively. However its level was decreased from noon to evening and greater reduction was observed in Langra (47.9 and 57.5%) and in Chausa (41.4 and 51.6%) in the corresponding stages of flowering, which suggests the superiority of these cultivars in Pn and photosynthate translocation efficiency. Similar informations are available showing marked variation in total sugar content during day time in other crops [7]. Reducing sugar contents has also showed significant variation among the cultivars (Table 2). In general reducing sugar was higher at full bloom stage in all the cultivars. Increase in reducing sugar content from morning to noon was noticed in both the stages but was more marked at full bloom stage in both Chausa (94.2 and 105.2%) and Langra (119.1 and 137.1%) cultivars. The reducing sugar contents decreased at evening and decline was observed maximum in Chausa (72.5 to 73.5%) and Langra (70.0 to 74.4%) in corresponding stage of flowering as compared to rest of the cultivars. However, the rate of reduction of reducing sugar was comparatively more than total sugar content. The present study clearly showed the similar pattern of diurnal changes in SLW, total and reducing sugar in fully Table 2. Changes in SLW, Total Sugar and Reducing