NATURAL-ABUNDANCE H-2/H-1 RATIOS OF WATER STORAGE IN LEAVES OF PEPEROMIA-CONGESTA HBK DURING WATER-STRESS

The mixing patterns of water from the water-storage multiple-epidermis (ME) and the spongy parenchyma (SP) of leaves of Peperomia congesta HBK were studied by following variations in the natural abundance of deuterium. The H-2/H-1 ratios of water from the two tissues were different throughout the diel cycle and were always lower in the ME. Withholding water from the potted plants resulted in complete isotopic homogeneity of leaf water within 3 days. Isotopic homogeneity and the extent of H-2 enrichment of leaf water were constant for the duration of the S-week stress period while the weight of the ME differentially and continuously decreased during the same period. Reinstating irrigation resulted in the rapid restoration of pre-stress ME weight, and a simultaneous sharp drop in the H-2/H-1 ratio of water in both the ME and SP. The proportion of recharge by soil water was similar whether calculated based on changes in tissue weight or on H-2/H-1 ratios. In contrast to the rapid recovery in water content, isotopic enrichment and restoration of isotopic heterogeneity were gradual over several days. The results indicate that under favorable conditions water, and not only solutes, stored in the ME does not mix well with water in other tissues, but becomes a more integral part of total leaf water under water stress conditions. In recovery from water stress, recharge with soil water is rapid (hours) while communication with the external environment, as reflected by isotopic enrichment and restoration of isotopic heterogeneity in leaf water, is gradual (days). The results demonstrate the potential of using stable isotopes at the natural abundance level to study the dynamics of water movement within plants.