Floodwater Depth Causes Different Physiological Responses During Post-flooding in Willows

Abstract

Willows are widely planted in areas under risk of flooding. The physiological responses of willows to flooding have been characterized, but little is known about their responses during the post-flooding period. After the end of the stress episode, plants may modify some traits to compensate for the biomass loss during flooding. The aim of this work was to analyze the post-flooding physiological responses of willow under two different depths of stagnant floodwater. Cuttings of Salix matsudana NZ692 clone were planted in pots in a greenhouse. The experiment started when the plants were 2 months old with the following treatments: Control plants (watered to field capacity); plants partially flooded 10 cm above soil level (F10) and plants partially flooded 40 cm above soil level (F40). The flooding episode lasted 35 days and was followed by a recovery period of 28 days (post-flooding period). After the flooding period, height, diameter and total biomass were higher in F10, while F40 plants showed an increase in plant adventitious root production and leaf nitrogen content. During the post-flooding period, the photosynthetic rate, nitrogen, chlorophyll and soluble sugar contents were significantly higher in leaves of F40 than in Control and F10 treatments. Stomatal conductance and specific leaf area were higher in the previously flooded plants compared to Control treatment. Plants from F10 treatments showed a higher growth in height, root-to-shoot ratio, and carbon isotope discrimination than F40, while the opposite occurred for growth in diameter, vessel size and leaf area. We conclude that depth of floodwater not only causes different responses during flooding, but that its effects are also present in the post-flooding recovery period, affecting the growth and physiology of willows once the stress episode has ended. Even when flooding impacted growth negatively in F40, in the post-flooding period these plants compensated by increasing the photosynthetic rate, plant leaf area and xylem vessel size. Willows endurance to flooding is the result of both responses during flooding, and plastic responses during post-flooding.

Keywords: Salix matsudana Koidz; chlorophyll; photosynthetic rate; stomatal conductance; vessels.

FIGURE 1

View of the stem of a control plant, without development of hypertrophied lenticels or adventitious roots (A); hypertrophied lenticels (HL) and aquatic roots (AR) in plants of F40 treatment (B); aquatic roots and lenticels outside water in a plant from F10 treatment (C); a transversal view of F40 aquatic roots (D, 10×); and a transversal view of an aquatic root in F10 (E, 10×). (A) aerenchyma lacunae.

FIGURE 2

Aquatic roots of a F40 plant immediately after taken out of water (A) and 24 h later (B). (C) Dry matter partitioning and root-to-shoot ratio (in bold, above the bars) at the beginning of the experiment (day 0), at the end of the flooding period (35 days), and at the end of the post-flooding recovery period (63 days). Means followed by the same letter did not differ according to Tukey’s test (p < 0.05, N = 5). The letters indicate significant differences for each compartment, total biomass (above the bar) and root-to-shoot ratio (next to this value, in bold).

FIGURE 3

Height (A), growth rate in height (B), diameter (C) and growth rate in diameter (D, multiplied by 10³) for Control, F10 and F40 treatments at the beginning of the experiment (Day 0), at the end of the flooding period (Day 35), and at the end of the post-flooding recovery period (Day 63). Growth rates were calculated for the whole flooding and post-flooding periods. Means followed by the same letter did not differ according to Tukey’s test (p < 0.05, N = 12–22). Vertical bar: standard deviation.

FIGURE 4

Total leaf area (A) and specific leaf area (B) for C (Control), F10, and F40 treatments at the beginning of the experiment (Day 0), at the end of the flooding period (Day 35), and at the end of the post-flooding recovery period (Day 63). Means followed by the same letter did not differ according to Tukey’s test (p < 0.05, N = 4–8). Vertical bar: standard deviation.

FIGURE 5

Soluble sugars content (A, N = 5) and nitrogen content (N, B, N = 3) on fully expanded leaves after 35 days of flooding, and at the end of the post-flooding recovery period (Day 63). Means followed by the same letter did not differ according to Tukey’s test (p < 0.05). Vertical bar: standard deviation.

FIGURE 6

Vessel area (A), vessel number (B), percentage of vessel lumen fraction of the stem (LF, C), transversal cut of the stem with light blue arrows indicating the vessels developed during flooding, and green arrows indicating the vessels formed during post-flooding (D). Total refers to the average values of all vessels in the stem (formed during pre-flooding, flooding and post-flooding). Means followed by the same letter did not differ according to Tukey’s test (p < 0.05, N = 5). Vertical bar: standard deviation.

FIGURE 7

Comparison between the flooding and post-flooding responses among the treatments F10 (plants flooded 10 cm above soil level) and F40 (plants flooded 40 cm above soil level). The change of size of the bars indicates whether a trait increased or diminished compared to the other flooding treatment. Bars of similar size indicates no significant differences between the flooding treatments. The symbol “*” represents an average of L1 and L2 leaves.