High temperature (HT) stress is a major environmental stress that limits plant growth, metabolism, and productivity worldwide Plant growth and development involve numerous biochemical reactions that are sensitive to temperature Plant responses to HT vary with the degree and duration of HT and the plant type HT is now a major concern for crop production and approaches for sustaining high yields of crop plants under HT stress are important agricultural goals Plants possess a number of adaptive, avoidance, or acclimation mechanisms to cope with HT situations In addition, major tolerance mechanisms that employ ion transporters, proteins, osmoprotectants, antioxidants, and other factors involved in signaling cascades and transcriptional control are activated to offset stress-induced biochemical and physiological alterations Plant survival under HT stress depends on the ability to perceive the HT stimulus, generate and transmit the signal, and initiate appropriate physiological and biochemical changes HT-induced gene expression and metabolite synthesis also substantially improve tolerance The physiological and biochemical responses to heat stress are active research areas, and the molecular approaches are being adopted for developing HT tolerance in plants This article reviews the recent findings on responses, adaptation, and tolerance to HT at the cellular, organellar, and whole plant levels and describes various approaches being taken to enhance thermotolerance in plants

Physiological, biochemical, and molecular mechanisms of heat stress tolerance in plants

Among the plant nutrients, potassium (K) is one of the vital elements required for plant growth and physiology. Potassium is not only a constituent of the plant structure but it also has a regulatory function in several biochemical processes related to protein synthesis, carbohydrate metabolism, and enzyme activation. Several physiological processes depend on K, such as stomatal regulation and photosynthesis. In recent decades, K was found to provide abiotic stress tolerance. Under salt stress, K helps to maintain ion homeostasis and to regulate the osmotic balance. Under drought stress conditions, K regulates stomatal opening and helps plants adapt to water deficits. Many reports support the notion that K enhances antioxidant defense in plants and therefore protects them from oxidative stress under various environmental adversities. In addition, this element provides some cellular signaling alone or in association with other signaling molecules and phytohormones. Although considerable progress has been made in understanding K-induced abiotic stress tolerance in plants, the exact molecular mechanisms of these protections are still under investigation. In this review, we summarized the recent literature on the biological functions of K, its uptake, its translocation, and its role in plant abiotic stress tolerance.

Potassium: A Vital Regulator of Plant Responses and Tolerance to Abiotic Stresses

At the end of the class, students will be able to answer the following questions: • What is organic agriculture? • Where do I go to get certification in organic agriculture? • How can I get more information about organic agriculture? What is organic? • The term " organic " is not synonymous to the terms " natural " or " eco-friendly. " • The label " natural " on foodstuff does not guarantee complete adherence to organic practices as defined by a law. • Produce food of high quality in sufficient quantity • Maintain biological diversity within the farming system • Maintain long-term soil fertility • Rely on renewable resources in locally organized agricultural systems • Minimize pollution and protect the environment What is allowed in organic crop production? • New varieties of crops and agricultural technologies • Crop rotations, cover crops and natural-based products that maintain soil fertility • Biological, cultural and physical methods to limit pest expansion and increase population of beneficial insects

英国有机农业（Organic farming）的新举措

Abiotic stresses strongly affect plant growth, development, and quality of production; final crop yield can be really compromised if stress occurs in plants’ most sensitive phenological phases. Additionally, the increase of crop stress tolerance through genetic improvements requires long breeding programmes and different cultivation environments for crop performance validation. Biostimulants have been proposed as agronomic tools to counteract abiotic stress. Indeed, these products containing bioactive molecules have a beneficial effect on plants and improve their capability to face adverse environmental conditions, acting on primary or secondary metabolism. Many companies are investing in new biostimulant products development and in the identification of the most effective bioactive molecules contained in different kinds of extracts, able to elicit specific plant responses against abiotic stresses. Most of these compounds are unknown and their characterization in term of composition is almost impossible; therefore, they could be classified on the basis of their role in plants. Biostimulants have been generally applied to high-value crops like fruits and vegetables; thus, in this review, we examine and summarise literature on their use on vegetable crops, focusing on their application to counteract the most common environmental stresses.

Biostimulants Application in Horticultural Crops under Abiotic Stress Conditions

Plants are exposed to a wide range of environmental conditions and one of the major forces that shape the structure and function of plants are temperature stresses, which include low and high temperature stresses and considered as major abiotic stresses for crop plants. Due to global climate change, temperature stress is becoming the major area of concern for the researchers worldwide. The reactions of plants to these stresses are complex and have devastating effects on plant metabolism, disrupting cellular homeostasis and uncoupling major physiological and biochemical processes. Temperature stresses disrupt photosynthesis and increase photorespiration altering the normal homeostasis of plant cells. The constancy of temperature, among different metabolic equilibria present in plant cells, depends to a certain extent on a homeostatically regulated ratio of redox components, which are present virtually in all plant cells. Several pathways, which are present in plant cells, enable correct equilibrium of the plant cellular redox state and balance fluctuations in plant cells caused by changes in environment due to stressful conditions. In temperature stresses, high temperature stress is considered to be one of the major abiotic stresses for restricting crop production. The responses of plants to heat stress vary with extent of temperature increase, its duration and the type of plant. On other hand, low temperature as major environmental factor often affects plant growth and crop productivity and leads to substantial crop loses. The present review discusses how oxidative damage as a result of temperature stress is detrimental for various crops. Various strategies adapted by the plants to main redox homeostasis are described along with use of exogenous application of some stress protectants.

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Temperature stress and redox homeostasis in agricultural crops

Two fieldexperiments wereconducted at El-Kassasein, IsmailiaGovernorate, Egypt.during the two successive seasons of 2003/2004 and 2004/2005 to study the response of vegetative growth and yield of some broccoli varieties(Southern star, Perennium crop, Prominence, Atlantic F1 and Monotop) to apply organic manures ( Cattle and poultry manures) compared with mineral fertilization.Results indicated that the highest vegetative growth of broccoli plants was recorded by Atlantic F1 cvs in the two seasons. Moreover, the highest total yield of broccoli was obtained by Perennium crop and Southern star cvs varieties. in the frist and second seasons, respectively. The highest vegetative growth of broccoli plants was recorded by plants which was supplied with 100% cattle manure. However, the highest total yield and quality of broccoli were recorded by adding poultry manure in the two seasons.Using poultry manure with Southern star cv. gave the highest total yield and quality of broccoli .

Effect of Organic Manure with or Without Chemical Fertilizers on Growth, Yield and Quality of Some Varieties of Broccoli Plants

The present study was carried out in two successive seasons of 2010 and 2011 to study the effect of foliar application of chitosan rates (1, 2, 3 and 4 mlL -1 ) and yeast rates (1, 2, 3 and 4 gL -1 ) on growth, yield and quality as well as chemical constituents of cucumber plants. Results of this work showed that foliar application with yeast and chitosan increased significantly the vegetative growth, yield and its quality of cucumber. Meanwhile, foliar spraying with active dry yeast at rates of 4 g L -1 recorded highest values of T.S.S., N (%), Fe, Zn, Cu and Mn (mg kg -1 ) in cucumber fruits. However, foliar application of chitosan at rates of 4 mlL -1 gave highest contents of P and K % in the two seasons of study. It can be concluded that foliar application with chitosan at rates of 4 mlL -1 recorded the best treatment to obtain the highest vegetative growth, yield and quality of cucumber plants.

Response of Cucumber Plants to Foliar Application of Chitosan and Yeast under Greenhouse Conditions

Two field experiments were carried out in the experimental station of the National Research Centre at EL-Nubaria, EL-Behira Governorate, Northern Egypt, to study the effect of 12 treatments, which were the combinations of three levels of mineral fertilizers application (100%, 65% and 35% of the recommended dose) with four levels of humic acid spraying (0, 1, 2 and 3 g humic acid/l.) on snap bean plant growth, pod yield and quality. Results showed that the vegetative growth of snap bean plants ( plant height, number of leaves and branches as well as fresh and dry weight of whole plant), green pod yields and its quality (pod length, weight, pod content of chlorophyll, fiber, total protein as well as N, P, K content) were improved by increasing the levels of mineral fertilizers application (100%). Results also clear that the vegetative growth of snap bean plants, green pod yields and its quality were improved by increasing the levels of humic acid spraying.

Response of snap bean plants to mineral fertilizers and humic acid application.

Two field experiments were conducted during the two successive summer seasons of 2009 and 2010 at the Experimental Farm of the National Research Centre in El-Nobaria region, Behira Governorate, to investigate the response of sweet pepper plants cv. California wonder to different rates of potassium fertilization (50, 100 and 200 kg/fed.) as potassium sulfate in addition to foliar application by potassium oxide (2 and 4 cm/L) and potassium humate (4 gm/L) as a stimulative dose. Potassium foliar applications were made 3 times in a 15 days interval with the same doses during the growing period (30, 45 and 60 days after transplanting). The highest potassium fertilization rate (200 kg/fed.) gave the tallest sweet pepper plants, the highest number of leaves and branches per plants and the highest fresh and dry weights of leaves as well as the highest total yield. Also, the obtained results reported that the fruit measurements expressed as fruit length, average fruit weight and vitamin C content, as well as leaves chemical composition (N, P, K and total chlorophyll) were increased with increasing potassium fertilization rate. On the other hand, spraying sweet pepper plants with potassium humate at rate of 4 gm/L markedly increased vegetative growth, yield, fruit quality and chemical composition. The favorable effects of the potassium on the growth, total yield and fruit parameters were obtained when sweet pepper plants fertilized with 200 kg/fed. potassium sulfate plus foliar application of potassium humate 4 gm/L followed statistically by 200 Kg/fed. potassium sulfate with foliar application of either 2 or 4 gm/L potassium oxide with no significant differenced between them but both of them were significantly higher than control.

http://jofamericanscience.org/journals/am-sci/am0612/81_4165am0612_722_729.pdf

Growth, Yield and Fruit Quality of Sweet Pepper Plants ( Capsicum annuum L.) as Affected by Potassium Fertilization

Two field experiments were carried out in two successive seasons of 2009/2010 and 2010/2011 at private farm in Berkash, Giza Governorate, Egypt, to study the effect of foliar application of some bio-stimulants, Amino acids, Seaweed extracts and Chitosan on growth, total yield, quality and some chemical constituents of Chinese garlic plants. Results showed that foliar spraying of Chinese garlic plants with bio-stimulants especially amino acids compound had a significant effect on vegetative growth, yield and quality. Moreover it could be concluded that foliar spraying of amino acids at rate of 3 cm/L let to obtain the highest values of vegetative growth, yield and its quality of Chinese garlic plants.

Z. F. Fawzy

Papers

Effect of Organic Manure with or Without Chemical Fertilizers on Growth, Yield and Quality of Some Varieties of Broccoli Plants

Response of Cucumber Plants to Foliar Application of Chitosan and Yeast under Greenhouse Conditions

Response of snap bean plants to mineral fertilizers and humic acid application.

Growth, Yield and Fruit Quality of Sweet Pepper Plants ( Capsicum annuum L.) as Affected by Potassium Fertilization

Response of garlic (Allium sativum, L.) plants to foliar spraying of some bio-stimulants under sandy soil condition.