**Monitoring of the Drought Tolerance of Various Cotton Genotypes Using Chlorophyll Fluorescence**

Erkin Zakhidov, Sherzod Nematov and Vakhobjon Kuvondikov

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/62232

#### **Abstract**

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In this chapter, chlorophyll fluorescence in plant leaves of three genotypes of cotton cultivated in Uzbekistan and characterized at different degrees of drought tolerance is studied. The light and CO2 responses of the chlorophyll fluorescence and the photosyn‐ thesis and possible mechanisms of adaptation of plants to moderate long-term drought are described. The chlorophyll fluorescence and various morpho-physiological indicators of well-watered and moderately drought-stressed cotton plants have been measured simultaneously over a long period of plant ontogenesis to establish direct correlations between them to estimate the magnitude of drought effect using fluores‐ cence parameters. It is shown that determination of such correlations and their calibration by photoacoustic signals generated in plant leaves at application of lowfrequency-modulated light may be used for monitoring of the drought tolerance of crops in the field.

**Keywords:** photosynthesis, chlorophyll fluorescence, cotton genotypes, drought effect

## **1. Introduction**

Drought is an important environmental stress exerting a critical effect on plants that can reduce their productivity, on average, up to 50% [1]. Approximately one third of Earth's arable land all over the world suffers from chronic water deficiency for agriculture and by various estima‐ tions; in 2050s, this area can be doubled [2]. Particularly, in Central Asia, located mostly in desert zones, the first-limiting factor of crop yield is water deficit and the agriculture can be prac‐ ticed only with additional irrigation. However, the irrational use of water resources of the region

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

for cotton production in the past has lead to an excessive soil salinization and to the exhaus‐ tion of its largest water resource—the Aral Sea. Therefore, revealing the adaptation potential of local agricultural crops to water deficit and creating their drought-tolerant genotypes are an important task: this would allow, in particular, to obtain higher cotton yield and quality in conditions of limited water resources and to improve local environment by stopping desertifi‐ cation of the region.

Creating drought-tolerant genotypes of agricultural crops is complicated because the lack of systematic knowledge on physiological parameters reflecting the genetic potential for improved productivity under conditions of water deficiency. The effect of drought stress on the photosynthetic performance and drought-induced morpho-physiological, biochemical and biophysical changes in various plant species have been extensively studied; stomatal and non-stomatal limitations to photosynthesis, their role and possible mechanisms have been suggested [3]. These studies have shown that photosynthetic performance is very informative and sensitive indicator of stress effects of drought in plants.

Nowadays, the methods of chlorophyll fluorescence control along with the classical measure‐ ments of photosynthesis based on gas-exchange analysis are widely used by agronomists in monitoring of crops and their response to environmental stresses [4]. Revealing physical characteristics of chlorophyll fluorescence in plant leaves and employing achievements in laser physics, optoelectronics and computer technologies allowed developing a variety of efficient experimental methods and easy to use devices for measuring such key fluorescence parame‐ ters, as a maximal (saturated) and a minimal (dark) fluorescence, a prompt and a delayed fluorescence, a kinetics of induction of chlorophyll fluorescence and their relationship with quantitative indicators of photosynthesis in plants [5, 6]. These methods are fast, noninvasive and estimate the photosynthetic performance of plants even under mid-day solar radiation, and portable devices commercially manufactured on their basis determine the parameters of plant photosynthetic performance with multiple replication of measurements and recording the results in a memory for subsequent statistical processing using relevant computer pro‐ grams [7, 8].

Here, the results of long-term effect's study of drought on the chlorophyll fluorescence and morpho-physiological indicators of cotton plants grown under field conditions are described. Literature on researches concerning to mechanisms of stress effect of drought on photosyn‐ thesis in plants are analyzed. The long-term effect of drought on cotton plants has been studied during the key period of their ontogenesis — in flowering and maturing stages from last July to last September by simultaneously measuring indicated parameters in well-watered and moderately drought-stressed plants. Correlations between the chlorophyll fluorescence and morpho-physiological indicators (leaf blade area and thickness, relative water content and transpiration) have been defined in three genotypes of cotton with different degrees of drought tolerance.

Comparative measurements of the operating quantum efficiency of photochemistry in Photosystem II, ФPSII, and its changes during the day time in well-watered and moderately drought-stressed plants have shown that in contrary to the widely accepted idea on tight links between ФPSII and the quantum efficiency of CO2 uptake [9], and decline of photosynthesis in plants under drought stress [10, 11], the sustainable higher values of ФPSII in drought-stressed plants have been registered [12, 13]. It was also defined considerable changes in morphophysiological parameters under drought stress.

For better understanding of mechanisms of such an unexpected increase in the quantum efficiency of primary photochemistry, the chlorophyll fluorescence was measured simultane‐ ously with the gas-exchange analysis at different light intensities and CO2 concentrations [14]. Drought-stressed plants displayed elevated rates of photorespiration playing a protective role in conditions of water deficit, when plants can gradually adapt to such a stress, regulating various phases of photosynthetic reactions.

The measurement of photoacoustic waves generated in plant leaves on application of a modulated light simultaneously with the chlorophyll fluorescence allowed us to determine quantitatively the magnitude of photosynthetic oxygen evolution. This has an especial importance in the case of elevated photorespiration, when tight links between ФPSII and the quantum efficiency of the CO2 uptake is broken. Photobaric component of the photoacoustic waves at low-modulation frequencies (~10 Hz) originated in the photosynthetic oxygen evolution process [15, 16], as quantitative indicator of the photosynthetic performance of plants, may be used for the calibration of the values ФPSII determined in chlorophyll fluores‐ cence measurements.

In this way, the chlorophyll fluorescence parameters measured simultaneously with morphophysiological indicators of plants proposed for monitoring of the drought tolerance of various cotton genotypes in the field that can be applied in the practice of a plant breeding.
