Water deficit-dependent changes in non-structural carbohydrate profiles, growth and mortality of pine and spruce seedlings in hydroculture

Abstract

Sugars, sugar alcohols and starch are the main components of the mobile carbon pool in plants. Water stress substantially affects the carbon balance in plants and the reserves of mobile carbon and can lead to their exhaustion and the development of carbon starvation. In addition to their function as reserve compounds, many sugars and sugar alcohols perform specific protective functions under water stress, acting as osmotically active compounds, chemical chaperones, and ROS scavengers. Pinus sylvestris and Picea abies are isohydric species that are prone to substantial changes in carbon balance under water stress. Therefore, we analysed the dynamics of sugars, sugar alcohols and starch in seedlings of these two species under polyethylene glycol-induced water stress ranging from relatively weak (culture medium water potential of −0.15 MPa) to very strong (−1.0 MPa) intensities. The water stress throughout the intensity range significantly increased the mortality of the seedlings of both species, and the extent of mortality was not correlated with the degree of seedling dehydration. The greatest mortality of pine seedlings was observed under −0.5 MPa and coincided with a sharp (greater than 60%) decrease in the total pool of carbohydrates and sugar alcohols in the roots. We assume that the main cause of death of pine seedlings under such conditions was carbon starvation of the roots. The most likely causes of carbon reserve depletion in the pine roots were the rapid root growth during the initial period of water stress in combination with the disruption of the assimilate transport from the shoots, as mobile carbon reserves increased substantially in the pine needles under water stress. Unlike those of pine, changes in the mobile carbon pools in both the roots and needles of spruce did not correlate with seedling mortality. Therefore, carbon starvation was likely not the reason for spruce seedling death under water stress. Although severe water stress often depletes starch pools in plants, we observed an increase in starch accumulation in both species under water stress. In pine, this increase could be due to the inhibition of starch hydrolysis arising from a significant increase in the content of maltose under water stress. Based on an analysis of the dynamics of individual sugars and sugar alcohols under water stress, we can assume that raffinose and non-cyclic sugar alcohols play a role in the response of both species to water stress.