Desiccation tolerance and sensitivity of vegetative plant tissue.

Abstract

There is a great deal of work currently being done in the field of desiccation tolerance. Generally workers studying desiccation-tolerant plant tissues have concentrated on the mechanisms of desiccation tolerance without concomitant studies on why most plants cannot survive desiccation. The present study considers both a desiccation-tolerant plant as well as a range of desiccation-sensitive plants. The work incorporates physiological, biophysical, biochemical and ultrastructural studies in an attempt to get a holistic picture of vegetative material as it dries and then rehydrates. The plant species used in this study are: Craterostigma nanum, a so-called resurrection plant; Garcinia livingstonei, a drought-tolerant small tree; Isoglossa woodii, an understorey shrub which shows a remarkable ability to recover from wilting; Pisum sativum seedlings, which have a very high water content at full turgor; and finally, Adiantum raddianum, the maiden hair fern, which wilts very quickly and does not recover from wilting. The desiccation-tolerant plant, C. nanum, had an unusual pressure-volume (PV) curve which indicated that while large volume changes were taking place there was little concomitant change in pressure or water potential. The unusual nature of this PV curve made it difficult to assess the relative water content (RWC) at which turgor was lost. The desiccation-sensitive plants exhibited standard curvi-linear PV curves. The amount of nonfreezable water in the five species was studied and found to show no correlation with the ability to withstand dehydration or with the lethal water content. There were no differences in the melting enthalpy of tissue water between the tolerant and most of the sensitive plants. Isoglossa woodii had a lower melting enthalpy than the tolerant and the other sensitive species. Survival studies showed that the desiccation-sensitive plants all had similar lethal RWCs. The tolerant plant survived dehydration to as low as 1% RWC, recovering on rehydration within 24 hours. Membrane leakage studies showed that the sensitive plants all exhibited membrane damage at different absolute water contents, but very similar RWCs and water potentials. The increase in leakage corresponded to the lethal RWC for all the sensitive species. The desiccation-tolerant plant recovered from dehydration to very low water contents and did not show an increase in membrane leakage if prior rehydration had taken place. Without prior rehydration this tolerant plant exhibited an increase in leakage at similar RWCs and water potentials to that of the sensitive species. There did not appear to be much difference in the RWC at which damage to membranes occurred whether the material was dried rapidly or slowly. Respiration and chlorophyll fluorescence were studied to determine what effect drying and rehydration have on the electron transport· processes of the leaf. The chlorophyll fluorescence studies gave an indication of damage to the photosynthetic apparatus. Both qualitative changes as well as quantitative changes in fluorescence parameters were assessed. Characteristics like quantum efficiency (Fv/Fm)remained fairly constant for a wide range of RWCs until a critical RWC was reached where there was a sharp decline in Fv/Fm. Upon rehydration, C. nanum recovered to pre-stress levels, I. woodii showed no recovery and no further damage on rehydration, whilst the other species exhibited even more damage on rehydration than they had on dehydration. Respiration remained fairly constant or increased slightly during drying until a critical RWC was reached at which it suddenly declined. The RWC at which this decline occurred ranged from 15% and 20% in P. sativum and C. nanum respectively, to 50% for G. livingstonei. On rehydration respiration exceeded the levels measured in dehydrated material for the sensitive species. Unsuccessful attempts were made to fix material anhydrously for ultrastructural studies so standard fIxation was used. The ultrastructural studies revealed that changes had occurred in the ultrastructure of leaves of the sensitive species dried to 30% RWC particularly in A. raddianum and P. sativum. Drying to 5% RWC revealed extensive ultrastructural degradation which was worsened on rehydration in the sensitive species. The tolerant species showed ultrastructural changes on drying but these were not as severe as occurred in the sensitive species. The cell walls of the tolerant species folded in on drying. This folding was possibly responsible for the unusual PV curves found in this species. At 5% RWC the cells were closely packed and very irregular in shape. The cell contents were clearly resolved and evenly spread throughout the cell. The large central vacuole appeared to have subdivided into a number of smaller vacuoles. On rehydration the cells regained their shape and the cell contents had moved towards the periphery as the large central vacuole was reformed. Beading of membranes, which was common in the sensitive species, was not found in the tolerant species suggesting that membrane damage was not as severe in the tolerant species. Western Blot analysis of the proteins present during drying was performed to determine whether a class of desiccation-induced proteins, called dehydrins, were present. These proteins have been suggested to play a protective role in desiccation-tolerant tissue. It was found that C. nanum did, in fact, possess dehydrins, but so did P. sativum. The other three sensitive species did not show any appreciable levels of dehydrin proteins. The presence of dehydrins alone is, therefore, not sufficient to confer desiccation tolerance. While physiologically the damage occurring in the sensitive plants was similar to that of the tolerant plant, at an ultrastructural level the damage appeared less in the tolerant plant. On rehydration from low RWCs damage appeared to become exacerbated in the sensitive plants. This was in contrast to the tolerant plant where damage was apparently repaired. There appears, therefore, to be a combination of protection and repair mechanisms responsible for the ability of C. nanum to tolerate desiccation. The lethal RWC of the sensitive species was higher than that at which protective mechanisms, such as water replacement, might come into play. So it is not just the possible ability to replace tightly bound water that set the tolerant plant aside. It must also have mechanisms to tolerate damage at the higher RWCs which were damaging and lethal to the sensitive plants. The lethal damage to sensitive species appeared to be related to a critical volume, thus it is concluded that the tolerant plant had the ability to tolerate or avoid this mechanical damage during drying as well as the ability to remain viable in the dry state. It is hypothesised that the ability of the walls to fold in and the unusual nature of the PV curve may provide some answers to the enigma of desiccation tolerance.