Irreversible effects of water stress on growth and stomatal development in cotyledons of etiolated squash seedlings. Plant and Cell Physiology 27

Growth of etiolated squash cotyledons and hypocotyls was suppressed when the seedlings were subjected to 60 n u polyethylene glycol (PEG) in 1/5 strength of Hoagland solution. The fresh weight of the hypocotyl completely recovered when the water stress was relieved after one day of PEG treatment. The fresh weight of the cotyledons, however, did not completely recover even three days after the relief of water stress. The transpiration rate of the cotyledons was substantially reduced by the water stress, and it also did not completely recover after the water stress was relieved. Micro-scopic observation of stomata of the cotyledons by a replica method revealed that the water stress reduced the increase in both stomatal width and density, and this reduction did not completely recover after the water stress was relieved. After one day of water stress, the endogenous ABA content of the cotyledon was increased from 68 to 114 ng/g fr wt. After the water stress was relieved, the increased ABA content decreased linearly over two days to 20 ng/g fr wt, the same value as in unstressed cotyledons. These results indicate that the effect of water stress on the hypocotyl growth was reversible but that the effect on the cotyledons was irreversible. The irreversible effect of water stress on the growth of the cotyledons probably resulted from the inhibition of stomatal development, but endogenous ABA did not appear to inhibit the development. Key words: ABA — Cotyledon — Cucurbita maxima — Growth — Squash — Stomat