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Development and Growth Regulation

Rapid Wall Relaxation in Elongating Tissues ¹

Department of Soil and Crop Sciences, Texas A & M University, College Station, Texas 77843, Department of Biochemistry and Biophysics, Texas A & M University, College Station, Texas 77843

Reported differences in the relaxation of cell walls in enlarging stem tissues of soybean (Glycine max [L.] Merr.) and pea (Pisum sativum L.) cause measurements of the yield threshold turgor, an important growth parameter, to be in doubt. Using the pressure probe and guillotine psychrometer, we investigated wall relaxation in these species by excising the elongating tissue in air to remove the water supply. We found that the rapid kinetics usually exhibited by soybean could be delayed and made similar to the slow kinetics previously reported for pea if slowly growing or mature tissue was left attached to the rapidly growing tissue when relaxation was initiated. The greater the amount of attached tissue, the slower the relaxation, suggesting that slowly growing tissue acted as a water source. Consistent with this concept was a lower water potential in the rapidly elongating tissue than in the slowly growing tissue. Previous reports of wall relaxation in pea included slowly growing tissue. If this tissue was removed from pea, relaxation became as rapid as usually exhibited by soybean. It is concluded that the true relaxation of cell walls to the yield threshold requires only a few minutes and that the yield threshold should be constant during so short a time, thus reflecting the yield threshold in the intact plant before excision. Under these conditions, the yield threshold was close to the turgor in the intact plant regardless of the species. The presence of slowly growing or mature tissue delays wall relaxation and should be avoided during such measurements. However, this delay can be used to advantage when turgor of intact growing tissues is being measured using excised tissues because turgor does not change for a considerable time after excision.

³ Present address: Environmental Stress Physiology Laboratory, Department of Applied Physiology, National Institute of Agrobiological Resources, Tsukuba Science City, Yatabe, Ibaraki 305, Japan.

⁴ Present address: College of Marine Studies, University of Delaware, Lewes, DE 19958.

¹ Contribution of the Texas Agricultural Experiment Station, paper No. 23103 in the technical article series. Partial support for this work was also provided by grants from the Fulbright Commission (R. M.), Government of Japan (S. M.), and the Department of Energy grant DE-FG05-84ER13273 (J. S. B.)

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