Root systems have an important role to play in contributing to crop performance. Improvement in yield requires efficient uptake of water and nutrients that must be captured from the soil via roots. In root length, the overall mean performance of mannitol induced drought stress treatments was recorded the highest value in control and decreased with increasing of mannitol stress (Table 3 and Figure 1). On the other hand, the highest overall mean value was recorded for the drought tolerant genotype E. Hybrid 1, while the lowest mean value was showed for the sensitive genotype Giza-177. For all studied rice genotypes the mean performance was showed linear decrease with increasing mannitol concentration, except Giza-179 which recorded the highest mean value for 125 mM mannitol treatment.The results in Table (3) and Figure (1) illustrated that, the reduction of shoot length was showed the same results as root length for all studied rice genotypes and for all treatments of mannitol induced drought stress and they in range from 0.
87 for Giza-177 cultivar to 2.81 for E. Hybrid 1 cultivar.
Germination percentage % For germination % as shown in Table (3), the high level of mannitol (500 mM) was showed the lowest percentage of germination (51.67 %) in comparison with control which showed the high percentage of germination (91.67 %). As the results recorded for root length and shoot length, the drought tolerant E. Hybrid 1 cultivar recorded the highest percentage of germination (92.50 %) and the lowest mean value (53.
33 %) was recorded for the drought sensitive cultivar Giza-177.Under mannitol induced drought stress condition, rice seedlings showed reduction in root length, shoot length and germination percentage %. This can be explained by the findings of previous researchers who reported that differences of stress tolerance efficiency among seedling cultivars are usually determined by the inhibition of growth characteristics at the whole plant level (Rampino et al., 2006 and Aboulila, 2015). The seedling of cultivars that showed lower reduction of growth characteristics could be drought-tolerant than those which showed higher reduction of growth characteristics under drought stress. This could suggest that seedlings of drought tolerant cultivars may have better adaptive responsibility such as the controlling stomatal pore and the stability of organelles within the cell (Setter and Flannigan, 2001). Drought stress can cause seedlings damage by different mechanisms related to osmotic and oxidative damages at cellular levels.
In drought stressed plants, several kinds of stress inducible genes are differently expressed (Seki et al., 2002). The gene expressions depend on the genetic content inside each of the rice cultivars and effect on stress tolerant efficiency of whole seedling. Under the drought stress, chloroplast ultra-structures are the first targets to be damaged in the cellular levels since it is the major site of ROS production (Munné-Bosch and Peñuelas, 2003).