Dynamics of photosynthetic induction and relaxation within the canopy of rice and two wild relatives

Abstract

Wild rice species are a source of genetic material for improving cultivated rice (Oryza sativa) and a means to understand its evolutionary history. Renewed interest in non-steady-state photosynthesis in crops has taken place due its potential in improving sustainable productivity. Variation was characterized for photosynthetic induction and relaxation at two leaf canopy levels in three rice species. The wild rice accessions had 16%-40% higher rates of leaf CO₂ uptake (A) during photosynthetic induction relative to the O. sativa accession. However, O. sativa had an overall higher photosynthetic capacity when compared to accessions of its wild progenitors. Additionally, O. sativa had a faster stomatal closing response, resulting in higher intrinsic water-use efficiency during high-to-low light transitions. Leaf position in the canopy had a significant effect on non-steady-state photosynthesis, but not steady-state photosynthesis. The results show potential to utilize wild material to refine plant models and improve non-steady-state photosynthesis in cultivated rice for increased productivity.

Keywords: crop canopy dynamics; non‐photochemical quenching; non‐steady‐state photosynthesis; photosynthetic induction; rice.

FIGURE 1

CO₂ uptake (A) and stomatal conductance (g_s ) over time showing an increase during photosynthetic induction by high light (1,500 µmol m⁻² s⁻¹) followed by a decrease in response to low light (50 µmol m⁻² s⁻¹). Measurements were taken in three Oryza species and at two canopy levels in each plant (upper and lower). The measurement was taken at an ambient [CO₂] of 400 µmol mol⁻¹. Each point is the mean ($\pm$SE) of six plants (n = 6).

FIGURE 2

Mean performance and standard error for average CO₂ uptake ($\overline{A}$), average stomatal conductance (${\overline{g}}_s$), average intercellular CO₂ concentration (${\overline{C}}_i$), and average intrinsic water‐use efficiency ($\overline{\mathit{iWUE}}=\overline{A}/{\overline{g}}_s$) during photosynthetic induction. These traits were measured in three Oryza species (O. sativa, O. rufipogon, and O. nivara) at two canopy levels (upper and lower). Each bar is the mean of six plants (n = 6) $\pm$ SE.

FIGURE 3

CO₂ uptake once it has reached a steady‐state rate after photosynthetic induction (A_s) in three Oryza species (O. sativa, O. rufipogon, and O. nivara) and two canopy levels (upper and lower). Letters are indicative of significant differences between treatments. Six plants were measured per boxplot (n = 6).

FIGURE 4

Intrinsic water‐use efficiency (iWUE = A/g_s ) over time during photosynthetic induction (the transition from low light (50 µmol m⁻² s⁻¹) to high light (1,500 µmol m⁻² s⁻¹) and photosynthetic relaxation (the transition from high light to low light). Periods of low light are shown by the gray areas in the figure, and highlight is shown in white. This measurement was taken at an ambient [CO₂] of 400 µmol mol⁻¹ on three Oryza species and at two canopy levels. Each point is the mean ($\pm$ SE) of six plants (n = 6)

FIGURE 5

Mean performance and standard error for time to 50% induction of CO₂ uptake (IT_{50 A}), time to 90% induction of CO₂ uptake (IT_{90 A}), time to 50% induction of stomatal conductance (g_{s50 i}), and time to 50% induction of stomatal conductance (g_{s90 i}) during photosynthetic induction. These traits were measured in three Oryza species (O. sativa, O. rufipogon, and O. nivara) at two canopy levels (upper and lower). Each bar is the mean of six plants (n = 6) $\pm$ SE.

FIGURE 6

Response of CO₂ uptake (A) to intercellular [CO₂] (C_i ) in three Oryza species (O. sativa, O. rufipogon, and O. nivara) measured at two canopy levels (upper and lower). The CO₂ response curves were measured in saturating light conditions (1,500 µmol m⁻² s⁻¹). Each point is the mean ($\pm$SE) of six plants (n = 6).

FIGURE 7

Variation for CO₂ uptake in saturating light and [CO₂] (A _max), carboxylation efficiency (CE), the maximum rate of carboxylation efficiency (V_{c , max}), and the maximum rate of electron transport (J _max) in three Oryza species and two canopy levels. Letters are indicative of significant differences between species. Each boxplot represents six plants (n = 6)

FIGURE 8

The response of uncorrected leaf CO₂ uptake (A; ●) and the response of leaf CO₂ uptake corrected for stomatal limitation (A^* ; ) over time in different rice species and canopy levels. The vertical line indicates the mean time for the activation of Rubisco (τ) per accession. Each point represents the mean of 6 plants $\pm$ SE (n = 6)

FIGURE 9

A. The time to 50% stomatal closure during photosynthetic relaxation (g_{s50 r}) in three Oryza species and at two canopy levels. B. The time to 90% stomatal closure during photosynthetic relaxation (g_{s90 r}) in three rice species and at two canopy levels. Letters are indicative of a significant difference between species. Six plants were measured per boxplot (n = 6)

FIGURE 10

Non‐photochemical quenching (NPQ) during photosynthetic induction and relaxation measured in three Oryza species at two canopy levels. Each point is the mean of 6 plants (n = 6); p < 0.05.