RESPONSES OF GAMETOPHYTES AND SPOROPHYTES OF PYRROSIA PILOSELLOIDES TO WATER STRESS

Abstract

The epiphytic habitat represents a highly dynamic environment and water deficit is one of the most common abiotic stress that affects growth and development of epiphytes. Although gametophytes of P. piloselloides lack elaborate protective devices, such as stomata for instance, our findings demonstrated that they are physiologically capable of enduring water stress, and in so doing, establish themselves in semi-arid niches. The mechanism by which these gametophytes compensate for water deficiency in their dry habitats is not by avoiding, but simply, by tolerating desiccation. In fact, they could survive in the air-dry state for as long as 21 days without incurring any permanent or lethal damage to photosynthesis. Based on chlorophyll a fluorescence measurements, the curtailment of photochemistry in 7-, 14- and 21-day desiccated gametophytes could be postulated to be the result of an inhibition of electron flow between the water-splitting side of PSII and PSI. The latter phenomenon was also well-correlated with the unstacking and degradation of thylakoids in swollen chloroplasts of desiccated gametophytes. In cases where gametophytes were desiccated for 28 days and beyond, they were able to regenerate new gametophytes from the surviving cells. Clearly, repair processes following cellular and metabolic disruptions are important in these gametophytes. Their rapid recovery upon rehydration after desiccation is an essential part of their adaptation which enables them to utilize intermittently available moisture in dry habitats. Unlike gametophytes, sporophytes of P. piloselloides survive in their semi-arid habitats by avoiding excessive dehydration. Given the water-conserving features of CAM, its presence in the sporophytes is not surprising. Sporophytes retain water essential for the maintenance of cellular function and viability by storing water in their succulent fronds and by opening their stomata at night, when VPD is low. With progressive dehydration, there were also continuous reductions in both stomatal conductance and capacity for nocturnal CO2