Abstract
Abiotic stress can alter key physiological constituents and functions in green plants. Improving the capacity to monitor this response in a non-destructive manner is of considerable interest, as it would offer a direct means of initiating timely corrective action. Given the vital role that plant pigments play in the photosynthetic process and general plant physiological condition, their accurate estimation would provide a means to monitor plant health and indirectly determine stress response. The aim of this work is to evaluate the response of leaf chlorophyll and carotenoid (Ct) content in wheat (Triticum aestivum L.) to changes in varying application levels of soil salinity and fertilizer applied over a complete growth cycle. The study also seeks to establish and analyze relationships between measurements from a SPAD-502 instrument and the leaf pigments, as extracted at the anthesis stage. A greenhouse pot experiment was conducted in triplicate by employing distinct treatments of both soil salinity and fertilizer dose at three levels. Results showed that higher doses of fertilizer increased the content of leaf pigments across all levels of soil salinity. Likewise, increasing the level of soil salinity significantly increased the chlorophyll and Ct content per leaf area at all levels of applied fertilizer. However, as an adaptation process and defense mechanism under salinity stress, leaves were found to be thicker and narrower. Thus, on a per-plant basis, increasing salinity significantly reduced the chlorophyll (Chlt) and Ct produced under each fertilizer treatment. In addition, interaction effects of soil salinity and fertilizer application on the photosynthetic pigment content were found to be significant, as the higher amounts of fertilizer augmented the detrimental effects of salinity. A strong positive (R2 = 0.93) and statistically significant (p < 0.001) relationship between SPAD-502 values and Chlt and between SPAD-502 values and Ct content (R2 = 0.85) was determined based on a large (n = 277) dataset. We demonstrate that the SPAD-502 readings and plant photosynthetic pigment content per-leaf area are profoundly affected by salinity and nutrient stress, but that the general form of their relationship remains largely unaffected by the stress. As such, a generalized regression model can be used for Chlt and Ct estimation, even across a range of salinity and fertilizer gradients.
Highlights
The accurate estimation of leaf photosynthetic pigments is an important element in monitoring plant stress and fertilizer application and managing the overall vegetation health— in agricultural systems, where productivity levels are directly related to plant condition
The leaf pigment content as influenced by salinity and fertilizer application is presented in two ways: (i) pigment content per unit leaf area, and (ii) the total content of pigments produced per plant, as analyzed in the following two sub-sections
Wheat plants under salinity stress showed a significant increase in the chlorophyll and Ct content per leaf area, whereas salinity stress significantly reduced leaf dry matter and total content of the produced pigments when accounting for pragmatic changes in leaf area
Summary
The accurate estimation of leaf photosynthetic pigments is an important element in monitoring plant stress and fertilizer application and managing the overall vegetation health— in agricultural systems, where productivity levels are directly related to plant condition. Agronomy 2017, 7, 61 photosynthetic pigments are key variables in characterizing photosynthetic response and gross primary production in the biosphere [1,2,3,4], with the pigments playing a central role in light harvesting, photosystem protection, and other growth functions [5,6,7]. Chlorophylls control the photosynthetic potential of plants by capturing light energy from the sun [8], and represent one of the most important photosynthetic pigments. In addition to the direct contribution in the photosynthetic process, Ct are involved in the defense mechanism against oxidative stress [16,17,18], and play an essential role in the dissipation of excess light energy and provide protection to reaction centers [19,20,21]
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