Plant Stress Ecology

Reforestation projects in semiarid lands often yield poor
results. We evaluated a variety of low-cost planting techniques in order to increase water availability to plants. Treatments included various combinations of traditional planting holes; waterharvesting microcatchments; stone or plastic mulches; small waterproof sheets to increase water harvesting; dry wells; buried clay pots; and deep irrigation. Treatment effects on the survival and growth of Olea europaea seedlings varied between experimental sites.

Australian rainforests extend from tropical climates in the north to temperate climates in the south, thus providing an opportunity to investigate the physiological responses to temperature of temperate and tropical species within the same forest type.  Eight rainforest canopy tree species were selected to cover the 33o latitudinal range of rainforests in eastern Australia.  Temperature tolerance was measured from six-year-old plants grown under a common environment, by exposing leaves to a series of hot temperatures during late summer and a series of freezing temperatures during midwinter.  Damage was evaluated based on chlorophyll fluorescence measurements 2 hr after exposure and by visual assessment a week after exposure.  Leaves of the tropical species had a higher heat tolerance and lower frost tolerance than leaves of the temperate species, which is consistent with their climate distributions.  In contrast, the temperature tolerance of the photosynthetic apparatus was not correlated with the native climates of species.  However, the tropical species had significant photoinhibition during winter.  All species maintained the integrity of the photosynthetic apparatus and avoided tissue damage over a similar span of temperatures (ca. 60 oC), which reflects the similar annual temperature range of their climates.  Chlorophyll fluorescence and visual assessment provided different estimates of the absolute and relative temperature tolerance of the species, thus stressing the importance of visual assessments of injury to the determination of the relative temperature tolerance of species.

The recently developed homesteads in small tank villages were evolved from traditional shifting cultivation areas due to the population increase.  At present, the income generating from home garden is very low due to poor management.  Anatomy of the home garden is a result of the interaction between man and environment taken place in its evolution.  Thus, any improvement on the system for obtaining high income should also be based on proper understanding of this interacting phenomena worked in the development process.  Therefore, a floristic composition survey was planned to identify the effects of man environment interaction on the anatomy of home gardens.

A survey was carried out in 5 AGA divisions in the Anuradhapura District during 1992 using 27 home gardens.  About 4000 trees were surveyed therein 140 plant species were found.  The survey included various measurements and observations on the farm family, tree species, farmland and the surrounding.  Data were analyzed in various ways to elucidate the mechanism worked in this eco-system to create the present anatomy of the tank village home garden.

The results indicate that there are 11 prominent plant species in the home garden.  They are Jak (Artocarpus heterophyllum), Drumstick (Moringa oleiferam), Coconut (Cocos nucifera), Mango (Mangifera indica), Papaya (Carica papaya),  Margoza (Azadirachta indica), Lime (Citrus), Guava (Psidium guajava), Orange (Citrus), Banana (Musa sapientum) and Tamarind (Tamerindus indica).  Some other 9 species are more frequently found, and another 20 species are also found in more than 50 % of the homesteads.  Major purposes served by these trees can be ranked in descending order as firewood, fruits, timber, medicine, vegetable and spice.  Of tree species found in the home garden , more than 75 % are multipurpose.  About 70 % of the trees serve as the source of firewood and timber.

The plant density is affected by number of factors such as the farm size, canopy size, availability of well, origin of trees etc.  The plant density is adjusted according to the availability of water by changing proportion of large to medium and small canopy trees.  This is possible because 70 % of the large tree population is of natural origin and hence adopted to the climatic vagaries.  Therefore, it is more vital to consider this balance between different canopy sizes according to the availability of water and the farmer preference to the purposes served by them in planning future home-garden models.

Abstract: Osmotic adjustment represents one of the most important mechanisms of plant stress responses to drought and some other stresses. Accumulation of soluble compounds in plant cells enables turgor maintenance, improved soil water extraction and water saving; moreover, organic compounds (eg proline) provide stabilization and protection of cell membrane system. In our experiments with varieties of winter wheat we observed the changes in osmotic potential of leaf tissues during progressing drought stress.