The Physiological Basis of Alfalfa Plant Height Establishment

Abstract

Plant height plays an important role in crop yield, product quality, and cultivation management. However, the physiological mechanisms that regulate the establishment of plant height in alfalfa plants remain unclear. Herein, we measured plant height traits, leaf characteristics, photosynthetic physiology, cell wall composition, and endogenous hormone contents of tall- and short-stalked alfalfa materials at different reproductive periods. We analyzed the physiology responsible for differences in plant height. The results demonstrated that the number of internodes in tall- and short-stalked alfalfa materials tended to converge with the advancement of the fertility period. Meanwhile, the average internode length (IL) of tall-stalked materials was significantly higher than that of short-stalked materials at different fertility periods, with internode length identified as the main trait determining the differences in alfalfa plant height. Leaf characteristics, which are closely related to photosynthetic capacity, are crucial energy sources supporting the expression of plant height traits, and we found that an increase in the number of leaves contributed to a proportional increase in plant height. Additionally, a significant positive correlation was observed between plant height and leaf dry weight per plant during the branching and early flowering stages of alfalfa. The leaves of alfalfa affect plant height through photosynthesis, with the budding stage identified as the key period for efficient light energy utilization. Plant height at the budding stage showed a significant positive correlation with soluble sugar (SS) content and a significant negative correlation with intercellular CO₂ concentration. Moreover, we found that alfalfa plant height was significantly correlated with the contents of indole-3-acetic acid in stem tips (SIAA), gibberellin A₃ in leaves (LGA₃), zeatin in stem tips (SZT), and abscisic acid in leaves (LABA). Further investigation revealed that SS, SIAA, and LGA₃ contents were important physiological indicators affecting alfalfa plant height. This study provides a theoretical basis for understanding the formation of alfalfa plant height traits and for genetic improvement studies.

Keywords: endogenous hormones; leaf characteristics; photosynthetic physiology; plant height traits; reproductive period.

Figure 1

Alfalfa at different growth stages. (A) Branching period. (B) Budding period. (C) Early flowering period.

Figure 2

Indicators of plant height-related traits in tall- and short-stalked alfalfa materials. (A) Plant height. (B) Growth rate. (C) Number of internodes. (D) Average internode length. Different uppercase letters (A,B,C) indicate significant differences at different growth stages within the same variety (p < 0.05), whereas different lowercase letters (a,b,c,d) indicate significant differences among different varieties at the same growth stage (p < 0.05). Two short-stalked materials are WL354HQ and WL343HQ, and two tall-stalked materials are WL525HQ and Gannong No. 3. Orange, blue, green, and yellow represent the WL354HQ, WL343HQ, WL525HQ, and Gannong No. 3 varieties, respectively. T₁, T₂, and T₃ represent the branching stage, the budding stage, and the first flowering stage, respectively.

Figure 3

Leaf characterization indices of tall- and short-stalked alfalfa materials. (A) Number of leaves per plant. (B) Leaf area. (C) Leaf shape index. (D) Leaf dry weight per plant. (E) Leaf–stem ratio. Different uppercase letters (A,B,C) indicate significant differences at different growth stages within the same variety (p < 0.05), whereas different lowercase letters (a,b,c) indicate significant differences among different varieties at the same growth stage (p < 0.05). Two short-stalked materials are WL354HQ and WL343HQ, and two tall-stalked materials are WL525HQ and Gannong No. 3. Orange, blue, green, and yellow represent the WL354HQ, WL343HQ, WL525HQ, and Gannong No. 3 varieties, respectively. T₁, T₂, and T₃ represent the branching stage, the budding stage, and the first flowering stage, respectively.

Figure 4

Photosynthetic parameters of tall- and short-stalked alfalfa materials. (A) Transpiration rate. (B) Net photosynthetic rate. (C) Intercellular CO₂ concentration. (D) Stomatal conductance. Different uppercase letters (A,B,C) indicate significant differences at different growth stages within the same variety (p < 0.05), whereas different lowercase letters (a,b,c) indicate significant differences among different varieties at the same growth stage (p < 0.05). Two short-stalked materials are WL354HQ and WL343HQ, and two tall-stalked materials are WL525HQ and Gannong No. 3. Orange, blue, green, and yellow represent the WL354HQ, WL343HQ, WL525HQ, and Gannong No. 3 varieties, respectively. T₁, T₂, and T₃ represent the branching stage, the budding stage, and the first flowering stage, respectively.

Figure 5

Photosynthetic product content of tall- and short-stalked alfalfa materials. (A) Soluble sugar content. (B) Sucrose content. (C) Starch content. Different uppercase letters (A,B,C) indicate significant differences at different growth stages within the same variety (p < 0.05), whereas different lowercase letters (a,b,c) indicate significant differences among different varieties at the same growth stage (p < 0.05). Two short-stalked materials are WL354HQ and WL343HQ, and two tall-stalked materials are WL525HQ and Gannong No. 3. Orange, blue, green, and yellow represent the WL354HQ, WL343HQ, WL525HQ, and Gannong No. 3 varieties, respectively. T₁, T₂, and T₃ represent the branching stage, the budding stage, and the first flowering stage, respectively.

Figure 6

Cell wall composition of tall- and short-stalked alfalfa materials. (A) Lignin content. (B) Cellulose content. (C) Hemicellulose content. Different uppercase letters (A,B,C) indicate significant differences at different growth stages within the same variety (p < 0.05), whereas different lowercase letters (a,b,c) indicate significant differences among different varieties at the same growth stage (p < 0.05). Two short-stalked materials are WL354HQ and WL343HQ, and two tall-stalked materials are WL525HQ and Gannong No. 3. Orange, blue, green, and yellow represent the WL354HQ, WL343HQ, WL525HQ, and Gannong No. 3 varieties, respectively. T₁, T₂, and T₃ represent the branching stage, the budding stage, and the first flowering stage, respectively.

Figure 7

Endogenous hormone content in tall- and short-stalked alfalfa materials. Zeatin content in leaves (A) and stem tips (B). Gibberellin A₃ content in leaves (C) and stem tips (D). Indole-3-acetic acid content in leaves (E) and stem tips (F). Abscisic acid content in leaves (G) and stem tips (H). Salicylic acid content in leaves (I) and stem tips (J). Different uppercase letters (A,B,C) indicate significant differences at different growth stages within the same variety (p < 0.05), whereas different lowercase letters (a,b,c,d) indicate significant differences among different varieties at the same growth stage (p < 0.05). Two short-stalked materials are WL354HQ and WL343HQ, and two tall-stalked materials are WL525HQ and Gannong No. 3. Orange, blue, green, and yellow represent the WL354HQ, WL343HQ, WL525HQ, and Gannong No. 3 varieties, respectively. T₁, T₂, and T₃ represent the branching stage, the budding stage, and the first flowering stage, respectively.

Figure 8

Principal component analysis of alfalfa plant height-related traits, leaf characteristics, and physiological indices. (a) The same shapes in black, red, and green represent different growth stages; (b) the same shapes in black and red represent short- and tall-stalked materials, respectively. Plant height (PH); average internode length (IL); number of leaves per plant (NLP); leaf area (LA); leaf dry weight per plant (LDWP); leaf–stem ratio (LSR); intercellular CO₂ concentration (Ci); soluble sugar (SS); sucrose (Suc); indole-3-acetic acid in stem tips (SIAA); gibberellin A in leaves (LGA); gibberellin A in stem tips (SGA); abscisic acid in leaves (LABA); abscisic acid in stem tips (SABA); zeatin in leaves (LZT); zeatin in stem tips (SZT).

Figure 9

Physiological basis of alfalfa plant height establishment, with black arrows indicating interactions. Correlations (+ or −) between blue, orange, and green represent branching, budding, and early flowering stages, respectively. (**) Indicates a highly significant correlation at the 0.01 level, whereas (*) indicates a significant correlation at the 0.05 level.