Stomatal functioning in relation to leaf age in in vitro grown plum shoots

The ability of stomata to close in response to abiotic factors was studied on different-aged leaves of invitro- grown clone Mr. S. 2/5 plum (Prunus cerasifera) shoots. Epidermal peels removed from the first, third and fifth leaf in basipetal sequence from the shoot tip were exposed to 50 mM mannitol, 10 mM coumarin or dark treatment. The control solution consisted of 1 mM KC1 in 0.5 mM 2-(N-morpholino)ethanesulphonic acid. The percentage of stomata that closed following such treatments became progressively lower with increasing leaf age. The effect of mannitol was greater than that of coumarin. With dark treatment, pronounced closure was observed in the apical leaf, while on the third and fifth leaf, values were lower but not statistically different. Finally, the kinetics of stomatal closure assessed during mannitol incubation indicated a progressively slower response from the first to the fifth leaf. The more effective stomatal functioning of the youngest leaf was confirmed by a more pronounced stomatal re-opening observed in epidermal peels first treated with mannitol and then incubated again in the control solution.