Abstract
Water plays a vital role in various structural, physical and metabolic processes in plants. The study of plant water relations drew the attention of plant research sci-entists by the start of this century. To quantify the plant water relations various mea-surable physical parameters were designed such as water potential along with its components, Le, turgor and osmotic pressure. Different techniques/equipments were fabricated for the purpose, however, most of those were having some theoreti-cal and technical limitations, Therefore, pressure probe was developed by Iiiisken et al. (1978), that could measure cell turgor pressure directly in individual cells. Various other parameters could also be measured using the same equipment. This equipment can provide better understanding of plant mechanisms under control and stressed conditions. Therefore, it is urgently needed in broadening up the research horizon in Pakistan, It will also introduce us to new and challenging aspects of plant water relations in various crops, fruits, vcgetables,.halophytes etc.
INTRODUCTION
The importance of water relations had been realized since the start of this century and scientists had started describing various aspects (Thoday, 1918). In the beginning these were discussed and studied at whole plant and/or crop levels. Later, the studies were concentrated at tissue and organ levels. Water potential was considered to be a suit-able parameter, that could represent the plant water relations properly. To measure this parameter various techniques have been designed. Among these the pressure cham-ber or bomb (Scholander et al., 1965; Tyree and Hammel, 1972) was considered to be the most suitable. However, it is accompa-nied with some potential errors, such as that in its theory the cell wall solutes have been ignored (Tomos, 1988) which can influence the cell water relations directly. Another er-ror (Murphy and Smith, 1989) is that it does
not measure the water potential directly in-stead it measures the hydrostatic pressure in xylem. This argument has further been sup-ported by the findings of Zimmerman et al. (Personal communication) who measured the negative hydrostatic pressure in the xylem, using the same equipment. Since 50's, emphasis has also been laid on the importance of water relations of indi-vidual plant cells (Thoday, 1950; Kamiya and Tazawa, 1956). An excellent review by Dainty (1963) has set the foundations of the modern plain cell water relations. During early studies sonic techniques were designed in this context such as plasmonietry, Iran-scellular osmosis, perfusion techniques, shrinking and swelling technique (Tomos, 1988). These techniques were aimed at mea-suring the hydrostatic component of water potential that could never be measured di-rectly before, rather, it was calculated by the difference of water potential and osmotic
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