Osmotic Adjustment in Wheat (Triticum aestivum L.) During Pre- and Post-anthesis Drought

Abstract

Pre-anthesis drought is expected to greatly increase yield losses in wheat (Triticum aestivum L.), one of the most important crops worldwide. Most studies investigate the effects of pre-anthesis drought only at maturity. The physiology of the plant before anthesis and how it is affected during drought is less studied. Our study focused on physiological patterns in wheat plants during pre- and post-anthesis drought. To this end, we measured leaf xylem water potential, osmotic potential and water content in different plant parts at a high temporal frequency: every 3 days, three times a day. The experiment started just before booting until 2 weeks after flowering. Drought stress was induced by withholding irrigation with rewatering upon turgor loss, which occurred once before and once after anthesis. The goal was to investigate the patterns of osmotic adjustment, when it is used for protection against drought, and if the strategy changes during the phenological development of the plant. Our data gave no indication of daily osmotic adjustment, but did show a delicate control of the osmotic potential during drought in both leaves and stem. Under high drought stress, osmotic potential decreased to avoid further water loss. Before anthesis, rewatering restored leaf water potential and osmotic potential quickly. After anthesis, rewatering restored water potential in the flag leaves, but the osmotic potential in the stem and flag leaf remained low longer. Osmotic adjustment was thus maintained longer after anthesis, showing that the plants invest more energy in the osmotic adjustment after anthesis than before anthesis. We hypothesize that this is because the plants consider the developing ear after anthesis a more important carbohydrate sink than the stem, which is a carbohydrate sink before anthesis, to be used later as a reserve. Low osmotic potential in the stem allowed turgor maintenance, while the low osmotic potential in the flag leaf led to an increase in leaf turgor beyond the level of the control plants. This allowed leaf functioning under drought and assured that water was redirected to the flag leaf and not used to refill the stem storage.

Keywords: Van't Hoff equation; carbohydrate mobilization; leaf water potential; osmotic adjustment; osmotic potential; plant stress; source-sink; turgor.

Figure 1

Growing environment of the wheat plants. (A) Vapor pressure deficit (VPD, kPa, n = 2) present in the growth chamber. (B) Soil water potential (kPa, n = 4) measured with tensiometers. Darker lines represent the mean while the lighter band the standard error. Rewatering events are marked with a blue dotted line.

Figure 2

Development of different physiological variables in time, expressed as “days after sowing” (DAS): xylem water potential in the flag leaf (A) and bottom leaf (B), osmotic potential in the flag leaf (C) and in the stem (D) and water content in the stem (E). Above the graphs, different phenological periods or events are visualized on the same timeline as the data. Rewatering events are marked with a blue dotted line. The control and drought treatment are depicted in blue and orange, respectively. Every 3 days, these variables were measured at predawn, midday and afternoon. Mean (n = 5) and standard error are depicted and the means are connected with lines to improve the visibility of the trends. Significant differences between the control and drought treatment are indicated below the measurements. Significance levels: *p < 0.05, **p < 0.01, and ***p < 0.001.

Figure 3

Calculated turgor (MPa) present in the flag leaves of the wheat plants. Rewatering events are marked with a blue dotted line. The control and drought treatment are depicted in blue and orange, respectively. Mean (n = 5) and standard error are depicted and the means are connected with lines to improve the visibility of the trends. Significant differences between the control and drought treatment are indicated above the measurements. Significance levels: *p < 0.05.

Figure 4

Relation between the osmotic potential and relative water content in the stems of wheat plants. The control treatment is represented by open circles while the drought treatment by full circles. Different colors represent different timeslots during the day.

Figure 5

Relative Osmolyte Content (ROC) over time in wheat stems, visualizing the osmotic adjustment. Rewatering events are marked with a blue dotted line. The control and drought treatment are depicted in blue and orange, respectively. Mean (n = 5) and standard error are depicted and the means are connected with lines to improve the visibility of the trends. Significant differences between the control and drought treatment are indicated below the measurements. Significance levels: *p < 0.05, **p < 0.01, ***p < 0.001.