ABSTRACT-The effect of solar radiation on photosynthesis, quantum yield of fluorescence and pigmentation under solar radiation in a laboratory system and in the natural habitat during a daily cycle was investigated in the red eulittoral alga Porphyra leucost~cta. Optimal (F,/F,,,) and effective (hF/Fm') quantum y~e l d of fluorescence, photosynthetic efficiency and maximal photosynthesis were drastically reduced by (!V-B (280-315 nm) radiation in algac transferred from the laboratory (grown in white light) to solar radiation for 3 h. The absorptance due to chlorophyll and biliproteins (phycoerythrin and phycocyanln) increased after 3 h of incubation In photosynthetically actlve radiation (PAR, 400-700 nm) and PAR+UV-A (315-400 nni) whereas no changes were observed under PAR+UV-A+UV-B In the field, the effcctive quanturn y e l d and oxygen production decreastd at noon durlng a diiily cycle, indicating photolnhibition. When solar UV-A and UV-B radiation was removed by means of selective filters, the average effcctive quantum yield increased by 28 -c 2 " . I SD. Chlorophyll and biliprotein concentration presented a daily pattern which was affected by UV radiation Both chlorophyll and biliprotein contents were reduced at noon, follo\~ved by recovery in the afternoon, but only when UV radiation was cut off. The difference between the maximal and minimal chl a concentration during the dally cycle was greater in the presence of UV radiation, but the difference for phycoerythrin and phycocyanin was greater under PAR alone. These results seem to indicate that the accuniulation of pigments is affected by solar radiation, with short-term changes induced by UV light. The ecological relevance of the daily variations of photosynthesis and pigmentation under solar radiat~on is discussed.

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Climatic dryness imposes limitations on vascular plant growth by reducing stomatal conductance, thereby decreasing CO uptake and transpiration. Given that transpiration-driven water flow is required for nutrient uptake, climatic stress-induced nutrient deficit could be a key mechanism for decreased plant performance under prolonged drought. We propose the existence of an "isohydric trap," a dryness-induced detrimental feedback leading to nutrient deficit and stoichiometry imbalance in strict isohydric species. We tested this framework in a common garden experiment with 840 individuals of four ecologically contrasting European pines (Pinus halepensis, P. nigra, P. sylvestris, and P. uncinata) at a site with high temperature and low soil water availability. We measured growth, survival, photochemical efficiency, stem water potentials, leaf isotopic composition (δ C, δ O), and nutrient concentrations (C, N, P, K, Zn, Cu). After 2 years, the Mediterranean species Pinus halepensis showed lower δ O and higher δ C values than the other species, indicating higher time-integrated transpiration and water-use efficiency (WUE), along with lower predawn and midday water potentials, higher photochemical efficiency, higher leaf P, and K concentrations, more balanced N:P and N:K ratios, and much greater dry-biomass (up to 63-fold) and survival (100%). Conversely, the more mesic mountain pine species showed higher leaf δ O and lower δ C, indicating lower transpiration and WUE, higher water potentials, severe P and K deficiencies and N:P and N:K imbalances, and poorer photochemical efficiency, growth, and survival. These results support our hypothesis that vascular plant species with tight stomatal regulation of transpiration can become trapped in a feedback cycle of nutrient deficit and imbalance that exacerbates the detrimental impacts of climatic dryness on performance. This overlooked feedback mechanism may hamper the ability of isohydric species to respond to ongoing global change, by aggravating the interactive impacts of stoichiometric imbalance and water stress caused by anthropogenic N deposition and hotter droughts, respectively.

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