Modified Photochemical Reflectance Indices as New Tool for Revealing Influence of Drought and Heat on Pea and Wheat Plants

Abstract

In environmental conditions, plants can be affected by the action of numerous abiotic stressors. These stressors can induce both damage of physiological processes and adaptive changes including signaling-based changes. Development of optical methods of revealing influence of stressors on plants is an important task for plant investigations. The photochemical reflectance index (PRI) based on plant reflectance at 531 nm (measuring wavelength) and 570 nm (reference wavelength) can be effective tool of revealing plant stress changes (mainly, photosynthetic changes); however, its efficiency is strongly varied at different conditions. Earlier, we proposed series of modified PRIs with moderate shifts of the measuring wavelength and showed that these indices can be effective for revealing photosynthetic changes under fluctuations in light intensity. The current work was devoted to the analysis of sensitivity of these modified PRIs to action of drought and short-term heat stress. Investigation of spatially-fixed leaves of pea plants showed that the modified PRI with the shorter measuring wavelength (515 nm) was increased under response of drought and heat; by contrast, the modified PRI with the longer wavelength (555 nm) was decreased under response to these stressors. Changes of investigated indices could be related to parameters of photosynthetic light reactions; however, these relations were stronger for the modified PRI with the 555 nm measuring wavelength. Investigation of canopy of pea (vegetation room) and wheat (vegetation room and open-ground) supported these results. Thus, moderate changes in the measuring wavelengths of PRI can strongly modify the efficiency of their use for the estimation of plant physiological changes (mainly photosynthetic changes) under action of stressors. It is probable that the modified PRI with the 555 nm measuring wavelength (or similar indices) can be an effective tool for revealing photosynthetic changes induced by stressors.

Keywords: PRI; modified photochemical reflectance indices; pea; photosynthetic changes; short-term heat; soil drought; water shortage; wheat.

Figure 1

Dependence of difference between modified photochemical reflectance indices in leaves of pea plants under water shortage (PRI(λ,570)_E) and these indices in leaves of control plants (PRI(λ,570)_C) on wavelengths of the measuring reflected light (λ) (n = 5–6). PRI(λ,570)_E and PRI(λ,570)_C were measured after five days of water shortage. *, difference was significant (p < 0.05).

Figure 2

Dependence of the maximal quantum yield of photosystem II (Fv/Fm) (a) and non-photochemical quenching of chlorophyll fluorescence (NPQ) (b) in spatially fixed leaves of pea plants on duration of water shortage (n = 5–6). *, difference between experimental and control parameters was significant (p < 0.05).

Figure 3

Dependence of the modified photochemical reflectance index calculated on the basis of 515 and 570 nm wavelengths (PRI(515,570)) in spatially-fixed leaves of pea plants on duration of water shortage (n = 5–6) (a) and dependences of average values of PRI(515,570) on Fv/Fm (b) and NPQ (c) (n = 10). Average values of Fv/Fm and NPQ from Figure 2 were used. R² is the determination coefficient. *, difference between experimental and control indices was significant (p < 0.05).

Figure 4

Dependence of the modified photochemical reflectance index calculated on the basis of 555 and 570 nm wavelengths (PRI(555,570)) in spatially-fixed leaves of pea plants on duration of water shortage (n = 5–6) (a) and dependences of average values of PRI(555,570) on Fv/Fm (b) and NPQ (c) (n = 10). Average values of Fv/Fm and NPQ from Figure 2 were used. R² is the determination coefficient. *, difference between experimental and control indices was significant (p < 0.05).

Figure 5

Dependence of difference between modified photochemical reflectance indices in spatially-fixed leaves of pea plants 1 h after termination of the short-term heat (46.5 °C, 30 min) (PRI(λ,570)_E) and leaves of control plants (PRI(λ,570)_C) on wavelengths of the measuring reflected light (λ) (a), and influence of the heat stress on PRI(515,570) (b), PRI(555,570) (c), Fv/Fm (d), and NPQ (e) (n = 5). *, difference was significant (p < 0.05).

Figure 6

Dependences of difference between modified photochemical reflectance indices in canopy of plants under soil drought conditions (PRI(λ,570)_E) and the canopy of control plants (PRI(λ,570)_C) on wavelengths of the measuring reflected light (λ) in pea (a), wheat cultivated under conditions of a vegetation room (b), and wheat cultivated under open-ground conditions (c) (n = 45). PRI(λ,570)_E and PRI(λ,570)_C were measured after 13 (for pea plants cultivated under conditions of a vegetation room), 17 (for wheat plants cultivated under conditions of a vegetation room), and 11 days (for wheat plants cultivated under open-ground conditions) of this drought. *, difference was significant (p < 0.05).

Figure 7

Dependences of the PRI(515,570) (a) and PRI(555,570) indices (b) in the canopy of pea plants cultivated under conditions of a vegetation room on the duration of the soil drought (n = 45). *, difference between experimental and control indices was significant (p < 0.05).

Figure 8

Dependences of the PRI(515,570) (a) and PRI(555,570) indices (b) in the canopy of wheat plants cultivated under conditions of a vegetation room on the duration of the soil drought (n = 45). *, difference between experimental and control indices was significant (p < 0.05).

Figure 9

Dependences of the PRI(515,570) (a) and PRI(555,570) indices (b) in the canopy of wheat plants cultivated under open-ground conditions on the duration of the soil drought (n = 45). *, difference between experimental and control indices was significant (p < 0.05).

Figure 10

Dependence of Fv/Fm in leaves of pea plants cultivated under conditions of a vegetation room on the duration of the soil drought (n = 45) (a) and dependences of average values of PRI(515,570) (b) and PRI(555,570) (c) in the canopy on Fv/Fm (n = 14). Average values of PRI(515,570) and PRI(555,570) from Figure 7 were used. R² is the determination coefficient. *, difference between experimental and control indices was significant (p < 0.05).

Figure 11

Dependence of Fv/Fm in leaves of wheat plants cultivated under open-ground conditions on duration of soil drought (n = 45) (a) and dependences of average values of PRI(515,570) (b) and PRI(555,570) (c) in the canopy on Fv/Fm (n = 12). Average values of PRI(515,570) and PRI(555,570) from Figure 9 were used. R² is the determination coefficient. *, difference between experimental and control indices was significant (p < 0.05).

Figure 12

Hypothetical scheme of ways in which stressors influence the typical and modified photochemical reflectance indices. Red and green arrows show negative and positive influence of stressors on these indices, respectively.