Effects of desiccation rate and inhibition of protein synthesis and the violaxanthin cycle on the rewetting recovery of Porella platyphylla

Abstract

Following 5 different rates of 1-week desiccation to which Porella platyphylla (a desiccation-tolerant leafy liverwort) was subjected, physiological recovery upon rehydration (after 1 h, 24 h, 48 h) in light was monitored by cytological, photosynthetic (e-transport rate, photoprotective mechanisms), and other metabolic parameters using nuclear- and chloroplast-encoded protein synthesis inhibitors and a violaxanthin cycle inhibitor (DTT: dithiothreitol). Desiccation tolerance is mainly constitutive in P. platyphylla, allowing survival of rapid drying, and employs an active rehydration-induced repair and recovery mechanism. Following 24 hours of rehydration after the one-week dehydration in laboratory air (∼35% relative humidity /RH/), the chloroplasts had an irregular round shape. The recovery within 1 h of the thylakoid-function-related photosynthetic processes was extremely fast and independent of protein synthesis, while the overall recovery (except plants dried up at 76% RH, which depended on protein synthesis only to a limited extent) required de novo protein synthesis during rewetting. During the course of rehydration protein synthesis affects rehydrin expression, osmolite and general carbohydrate metabolism, the main repair and antioxidant mechanisms. Recovery was the best in samples dried up at 76% RH. Membrane permeability increased upon rehydration following 1-week dehydration. Normal membrane function recovered after 24 h (max. 48 h), except in plants that suffered drastic dehydration. Total protein content of plants dried under different conditions and of samples taken during recovery was generally lower than in the control plants at full turgor. The xanthophyll cycle has a great importance during the recovery in light; we confirmed the existence of a greater zeaxanthin-dependent and a smaller zeaxanthin-insensitive NPQ (nonphotochemical quenching). Plants dried under more favorable conditions had better light protection. The highest values of the osmotic potential belonged to the plants subjected to intense dehydration. Total soluble carbohydrate content was well-balanced and seemed to be unchanged following the different treatments. There was a significant increase in proline, while total fructan showed lower values at different degrees of 1-week desiccation. The cytoplasmic protein synthesis is likely to be involved in the change of the fructan content during rehydration.