Effects of nitrogen and vapour pressure deficit on phytomer growth and development in a C4 grass

Abstract

Phytomers are basic morphological units of plants. Knowledge of phytomer development is essential for understanding morphological plasticity, functional-structural modelling of plant growth and the usage of leaf characteristics to indicate growth conditions at the time of production (e.g. stable isotope signals). Yet, systematic analysis on the process of phytomer development is unavailable for wild or perennial C₄ grasses. Also, effects of environmental factors, such as nitrogen nutrition or vapour pressure deficit (VPD), on coordination events of developmental processes of C₄ grasses have not been studied. This study investigates phytomer growth and development in Cleistogenes squarrosa, a predominant C₄ grass in the Eurasian steppe, grown at low (0.63 kPa) or high (1.58 kPa) VPD with low or high nitrogen supply in controlled environments. Elongation of phytomers on marked tillers was measured daily for 13 days. Then lengths of immature and mature phytomer components (blade, sheath and internode) of all phytomers were measured following dissection. Nitrogen nutrition and VPD had no effects on coordination of growth within and between phytomers: phytomer tips emerged when phytomers reached 26 % of their final length, coincident with the acceleration phase of its elongation; blade elongation stopped when phytomers reached ∼75 % of their final length and elongation of the preceding phytomer was confined to the internode. The relationship between fraction of final phytomer length and days after tip emergence for all treatments was well described by a sigmoidal function: y = 1/{1 + exp[(1.82 - x)/1.81]}. C. squarrosa exhibited little morphological plasticity at phytomer-level in response to nitrogen supply and VPD, but a clear increase in tillering under high N supply. Also, the invariant coordination of elongation within and between phytomers was a stable developmental feature, thus the quantitative coordination rules are applicable for predicting morphological development of C. squarrosa under contrasting levels of nitrogen nutrition or VPD.

Keywords: Coordination; developmental dynamics; elongation rate; growth duration; internode; leaf appearance; leaf blade; phyllochron; plant growth model; sheath.

Figure 1

Schematic of a mature phytomer and its components, and arrangement of phytomers along a tiller of C. squarrosa. The ligule (or collar; data not shown) marks the blade-sheath junction, and the node forms the site of insertion of the leaf on the stem. Immature, growing phytomers are situated at the tip of the tillers (note: for simplicity, the scheme depicts only two growing phytomers, but up to five concurrently expanding phytomers in different developmental stages are found along the distal part of tiller axis upon dissection).

Figure 2

Final (mature) length of phytomers along the axis of the main tiller of C. squarrosa under contrasting nitrogen fertilizer and VPD treatments. Phytomer rank was counted acropetally from the base of the tiller. Error bars indicate SE (n = 16).

Figure 3

Fractional contributions of phytomer components to the final length of phytomers at successive stages of phytomer elongation of C. squarrosa under contrasting N fertilizer and VPD treatments. Panel A, blade; panel B, sheath; Panel C, internode. The solid lines were fitted to the data of all treatments. Stage of elongation is given by the fraction of final phytomer length, estimated as the ratio of the actual phytomer length to the predicted final phytomer length (see ‘Material and Methods’ section). All data were obtained from destructive measurements. Each point corresponds to a single measurement.

Figure 4

Relationships between the lengths of two successive immature (non-fully elongated) phytomers of C. squarrosa under contrasting N fertilizer and VPD treatments. The solid line was fitted to the data of all treatments. P_n refers to any given immature phytomer and P_n + 1 to the corresponding next younger phytomer. All data were obtained from destructive measurements. Each point corresponds to a single measurement.

Figure 5

Time course of the fraction of final phytomer length in C. squarrosa under contrasting N fertilizer and VPD treatments. Coloured circles represent the visible phase of the time course of phytomer development (see Materials and Methods): green circles, N1 V1; purple circles, N1 V2; red circles, N2 V1; blue circles, N2 V2. Black and white symbols represent the initial phase of phytomer development based on predictions of age and the final length: closed circles, N1 V1; open circles, N1 V2; closed squares, N2 V1; open squares, N2 V2. The curve denotes the two-parameter sigmoidal function for all data: y = 1/{1 + exp[(1.82− x)/1.81]}, (R²=0.96, residual standard error = 0.04).