This book consists of six parts i.e., Photosynthesis, Respiration, Water relations in plants, Plant nutrition, Stress physiology and Hormone. Photosynthesis deals with the role of light in photosynthesis, the structure of the photosynthetic apparatus, and the processes that begin with the excitation of chlorophyll by light and culminate in the synthesis of ATP and NADPH as well as we will examine the cyclic reactions that accomplish fixation and reduction of CO2, then consider how the phenomenon of photorespiration catalyzed by the carboxylating enzyme alters the efficiency of photosynthesis. Where, Photosynthesis provides the organic building blocks that plants (and nearly all other life) depend on. Respiration, with its associated carbon metabolism, releases the energy stored in carbon compounds in a controlled manner for cellular use. At the same time it generates many carbon precursors for biosynthesis. In the first part of this chapter we will review respiration in its metabolic context, emphasizing the inter connections and the special features that are peculiar to plants. We will also relate respiration to recent developments in our understanding of the biochemistry and molecular biology of plant mitochondria. In water relation in plants chapter we will examine the mechanisms and driving forces operating on water transport within the plant and between the plant and its environment. Transpirational water loss from the leaf is driven by a gradient in water vapor concentration. Long-distance transport in the xylem is driven by pressure gradients, as is water movement in the soil. Water transport through cell layers such as the root cortex is complex, but it responds to water potential gradients across the tissue. Plant nutrition outlines the primary reactions through which the major nutrients (nitrogen, sulfur, phosphate, cations, and oxygen) are assimilated. We emphasize the physiological implications of the required energy expenditures and introduce the topic of symbiotic nitrogen fixation. In hormone chapter, we will discuss the synthesis, physiological role of different natural hormones with chemical structure and uses in sustainable agriculture in present days. In stress physiology chapter we will examine principles, and the ways in which plants adapt and acclimate to water deficit, salinity, chilling and freezing, heat, and oxygen deficiency in the root biosphere. Air pollution an important source of plant stress. Although it is convenient to examine each of these stress factors separately, most are interrelated and a common set of cellular, biochemical, and molecular responses accompanies many of the individual acclimation and adaptation processes. The principles spread overall chapters are clearly elucidated in lucid manner with pictures, figures and tables for easy and comprehensive understanding.