Comparative ecology of selected herbs and shrubs in relation to soil moisture and soil types

Abstract

Eco-physiology and nature of adaptation of Cassia tora, C. occidentalis and C. sophera the pioneer of dry derelict land were investigated in relation to environmental conditions. Comparative study with these three Cassia spp. was carried out with reference to germination eco-physiology and seedling growth in relation to seed storage, salinity and pH; growth in relation to wet (W) and dry-moist (DM) conditions; sun (100% light) and shade (15% light) treatments and tissue water relation in their ecological significance. The objectives of the present investigation were to know the reasons of abundance of C. tora and low population or causes of the absence of Cassia occidentalis and Cassia sophera in the natural habitat. Seed morphology of these species showed variation in colour, size and shape and also showed similarities and dissimilarities. Freshly harvested, one week , one month and three months old seeds showed very little germination in Cassia tora ; in contrast freshly harvested seeds of Cassia occidentalis and Cassia sophera showed better germination than Cassia tora. However, the scarified seeds of Cassia occidentalis and Cassia sophera showed high percentage of germination. The effect of salinity on germination of three Cassia spp. was investigated to know their degree of tolerance at different concentrations of NaCl solution and showed a strong effect on the percentage of seed germination. Germination percentage was decreased in all the three species with the increase of the NaCl concentrations and showed lowest germination at 0.1M NaCl. Lack of germination of normal seeds of C. occidentalis and C. sophera at 0.2M and 0.5 M NaCl concentrations indicated their sensitivity to salinity. The seedling length of C. tora at 0.05M NaCl was significantly (p=0.05) decreased to 5.71cm and further increase of salinity to 0.1M, the seedling length was decreased to less than one third (2.15 cm) from that of control (6.92cm). Seeds of all the species failed to germinate at 0.5M NaCl solution. Different pH levels did not show any significant (p=0.05) difference in seedling growth except Cassia sophera which at acidic pH significantly (p=0.05) decreased seedling length and dry weight. As far as germination and early establishment are concerned it was established that low germination percentage in C. occidentalis and C. sophera, and sensitivity to salinity are the factors most likely to exclude the growth, and adaptation in the natural habitat. The three species grown in wet (W) and dry-moist (DM) conditions and at two light intensities (sun-100% light and shade 15% light) in potted soil to explain the nature of adaptation. Vegetative growth was vigorous in all water regimes in the early stages of growth of three Cassia spp. The roots of wet plants were on the top 1-2 cm of the soil surface, stunted and dark brown to black in colour. In contrast, the roots were penetrated deep into the base of the pot in DM treatment. The individual leaves of C. tora, C. occidentalis and C. sophera at different stages of development were found to be progressively larger with taller petiole (approximately 70-135 mm) as soil water was decreased i.e. in DM treatment. There was a progressive increase in leaf size, leaf area with the decrease of soil moisture in all three Cassia spp. At dry moist treatment C. tora started flowering 3 weeks earlier than wet treatment; under DM condition C. tora showed flowering 12 weeks earlier than C. occidentalis and 17 weeks earlier than C. sophera. In wet treatment there is no flower and fruit production at all in C. occidentalis and C. sophera. However, C. tora shows very poor flowers and fruit production under wet condition. Root growth was poor in wet condition. In the dry-moist treatment the largest component of the plant is leaf material in all three Cassia spp. Vegetative growth under sun (100% light) treatment was more vigorous than shade (100% light) condition. At sun (100% light) treatment C. tora showed earlier flowering than C. sophera and C. occidentalis . The relation between leaf water content and stomatal closure, the rate of transpiration and the relative turgidity under two different soil moisture (wet and dry-moist) and two light (sun and shade) treatments were investigated. The plant showed contrasting behavior with respect to transpiration rate, stomatal behavior and the development of water deficits. Highest total transpiration rate was observed in C. occidentalis followed by C. sophera and C. tora. In all the species, there was a relation between the total water and stomatal water loss. The value of RT was very similar between sun and shade conditions in Cassia spp. except C. sophera where the value of sun condition is significantly lower (p=0.05) than shade condition.