Ultrastructure, adaptability, and alleviation mechanisms of photosynthetic apparatus in plants under waterlogging: A review

Shifts in resource allocation and aggravation of foliage development restrict the growth rate of Picea abies under increasing atmospheric humidity at high latitudes

Global warming is accompanied by rising precipitation, atmospheric water vapour content, and specific humidity at high latitudes. The rising amount and frequency of rainfall increase the air relative humidity (RH) on a local scale, especially within forest canopies. We studied the effects of artificially elevated environmental humidity (RH and soil moisture) on leaf gas exchange, stomatal responses and growth of young Picea abies trees at the Free Air Humidity Manipulation site in eastern Estonia. Manipulation did not affect the net assimilation rate (An) but affected the stomatal responses, net photosynthetic efficiency (An/ci), and photosynthetic water-use efficiency (WUE). At an elevated air humidity (H), trees exhibited the highest stomatal conductance (gS) and lowest WUE, An/ci, and stomatal sensitivity to air vapour pressure deficit compared to trees growing under ambient conditions (C) and elevated soil moisture (I). Compared to C trees, H trees demonstrated reduced height growth, foliage biomass, and enhanced investments in fine roots referring to worsening soil nutrient availability. Tree growth decline can be explained by (1) foliage development retardation, (2) resource allocation changes, causing a shift in the photosynthetic to non-photosynthetic tissue ratio in favour of the latter, and (3) impaired nutrient uptake from the soil. Changes in stomatal responses make trees grown in a higher RH more vulnerable to weather extremes, also limiting tree growth and forest productivity. Increasing precipitation with concomitant increase in atmospheric humidity at high latitudes counteracts the expected enhancement of tree growth due to climate warming in mesic northern forests.

Psammophytes Alyssum desertorum Stapf and Secale sylvestre Host Are Sensitive to Soil Flooding

While morphological and functional traits enable hydrophytes to survive under waterlogging and partial or complete submergence, the data on responses of psammophytes-sand plants-to flooding are very limited. We analyzed the effect of 5- and 10-day soil flooding on the photosynthetic apparatus and the synthesis of alcohol dehydrogenase (ADH), heat shock proteins 70 (HSP70), and ethylene in seedlings of psammophytes Alyssum desertorum and Secale sylvestre using electron microscopy, chlorophyll a fluorescence induction, and biochemical methods. It was found that seedlings growing under soil flooding differed from those growing in stationary conditions with such traits as chloroplast ultrastructure, pigment content, chlorophyll fluorescence induction, and the dynamics of ADH, HSP, and ethylene synthesis. Although flooding caused no apparent damage to the photosynthetic apparatus in all the variants, a significant decrease in total photosynthesis efficiency was observed in both studied plants, as indicated by decreased values of φR0 and PIABS,total. More noticeable upregulation of ADH in S. sylvestre, as well as increasing HSP70 level and more intensive ethylene emission in A. desertorum, indicate species-specific differences in these traits in response to short-term soil flooding. Meanwhile, the absence of systemic anaerobic metabolic adaptation to prolonged hypoxia causes plant death.