ABSTRACT
The production of small leaves is a common feature of water stressed cotton. Smaller leaves could result from either decreased cell expansion or a reduction in mitotic activity during leaf development. This study examined leaves that had developed under stress on T25 and T169, two exotic cotton strains that are nonwilting and wilting, respectively. Scanning electron micrographs of the upper and lower leaf surfaces were used to determine the epidermal cell and stomatal densities (number per mm) and sizes (um). These values were used to calculate the number of epidermal cells and stomata per leaf as well as the epidermal to stomata ratio. The epidermal cell density of T25 was increased less than 2% whereas in T169 the epidermal cell density was increased 22%. Thus, epidermal cell expansion was not inhibited in T25 by water stress, but it was inhibited in T169. Stomatal density was increased 16 and 5% in the upper and lower leaf surfaces, respectively, for T25 compared to 30 and 17% respectively, for T169. Water stress reduced the number of epideriqal cells per leaf 33% in T25 and 40% in T169 (both surfaces were virtually the same). The number of stomata per leaf was decreased by stress 27% in T25 and 40% in T169. The size of the epidermal cells and stomata were not significantly reduced by water stress in T25, but the size of both epidermal cells and stomata were reduced 20% in T169.The size of the epidermal cells and stomata were not significantly reduced by water stress in T25, but the size of both epidermal cells and stomata were reduced 20% in T169. The epidermal cell to stomata ratio was decreased more in T25 than in T169. In summary, the major effect of water stress that contributes to small leaves on T25 was an inhibition of mitotic activity and cell size was not affected. There was also a minor variation in cell differentiation ( as measured by the epidermal cell to stomata ratio). On the other hand, the stress sensitive strain, T169, exhibited a stress sensitivity to both cell expansion and to cell division. Stress did not alter the epidermal cell to stomata ratio in this strain. We conclude that the control of cell expansion, cell division and cell differentiation are all used by the cotton plant to cope with stress. Furthermore, our findings suggest that genotypic variations exist in these parameters.
|