Dimitra A. Loka

Potassium (K) is a primary macronutrient for overall plant growth, yield potential, product quality and stress resistance of crops. Potato (Solanum tuberosum L.) crops require a high amount of potassium to achieve the ideal yield and quality. Therefore, the determination of optimum K rate and efficient source for potato is necessary because K affects crop physiological processes, dry matter production, cooking, and processing requirements. Through modeling on the pooled data extracted from 62 studies, the highest tuber yields might be obtained at an exchangeable soil K level of 200 mg kg⁻¹ approximately, dependent on soil pH, texture, and organic matter. Through modeling on the data of 48 studies, it also revealed that application of potassium sulfate (K₂SO₄) and potassium chloride (KCl) at rates of 200 kg ha⁻¹ and potassium nitrate (KNO₃) at a rate of 100 kg ha⁻¹ might achieve the ideal yield, implying the importance of K sources in potato production. However, these values (either soil exchangeable K content, or fertilizer rates) might not be applicable in a specific growing environment for a specific potato variety. It seems that there is no discrimination among split, pre-plant or in-season application of K, although pre-plant fertilization might be a trustworthy strategy for economic tuber yield. Owing to the luxury consumption of K by potato crop, a combination of factors, including soil exchangeable K level, petiole K concentration, crop removal amount, soil conditions, management practices, climatic conditions, and potato variety, should be considered in order to make rational K fertilizer recommendations.

Heat stress constitutes a major threat to crop production, and according to climatic projections, night temperatures are expected to increase faster and to a greater extent compared to day temperatures. While extensive research has been dedicated to the effects of higher than optimum day temperatures on cotton physiology, metabolism, and yield, and while heat-tolerant cotton cultivars have been introduced, the responses of such heat-tolerant cultivars to high night temperatures have not been evaluated. The objective of this study was to assess the efficiency of heat-tolerant cultivars to high night temperatures stress by monitoring the physiological and biochemical responses of two cotton cultivars, differing in thermotolerance, subjected to higher than optimum night temperatures, during anthesis. To that end, growth chamber experiments were conducted using two cotton cultivars differing in thermotolerance, namely ST5288B2RF (thermosensitive) and VH260 (thermotolerant). Treatments consisted of normal day/night temperatures (32/24 °C) and high night temperatures (32/30 °C) for 2 weeks at flowering (approximately 8 eight weeks after planting). The results indicated that VH260 was more thermotolerant than ST5288 even under conditions of high night temperature stress, as it managed to maintain its net photosynthetic rates, cell membrane integrity, as well as pistil carbohydrate contents and ultimately achieved higher total reproductive weight. It was concluded that heat tolerance of thermotolerant cultivars selected under conditions of high day temperatures is also conserved under high night temperatures, while net photosynthetic rates and cell membrane integrity can be utilized as selection traits for heat tolerance under either high day or night temperatures.

Higher than optimum temperatures during cotton's (Gossypium hirsutum L.) growing season is a common occurrence in cotton‐growing areas around the world with negative consequences on productivity. According to climate projections, night temperatures are expected to increase more than day temperatures. The objective of this study was to monitor and record the effects of higher than optimum night temperatures during cotton's early reproductive stage on physiological (photosynthesis, respiration and membrane damage) and biochemical (leaf adenosine triphosphate (ATP) levels, as well as glutathione reductase (GR) content and soluble carbohydrate concentrations of the leaf subtending the flower buds. Number, dry weight, carbohydrate concentrations and GR levels of flower buds were also measured at the end of the experiment. Growth chamber experiments were conducted using cotton cultivar ST 5288 B2RF and treatments consisted of normal day/night temperatures (32/24 °C) and high night temperatures (32/30 °C) for 7 days at squaring (approximately 4 weeks after planting). The results indicated that high night temperatures had an immediate effect on leaf respiration rates and membrane damage by significantly increasing them compared to the control and a similar pattern was observed on leaf photosynthesis and ATP levels that were markedly decreased. Leaf GR levels were also substantially increased under conditions of high night temperatures, in contrast to flower bud GR content which remained unaffected. High night temperatures had a significant effect on leaf carbohydrate concentrations resulting in significant decreases in hexose, sucrose and starch levels. Nevertheless, flower bud sucrose content was substantially increased under conditions of high night temperatures, while hexose content was decreased and starch concentrations remained unaffected. A detrimental effect of elevated night temperatures on the number of flower buds per plant and on the dry weight of flower buds was observed, and it was concluded that high night temperatures had a negative effect on cotton flower bud production due to disruptions on flower bud carbohydrate metabolism as a result of the insufficient GR response.