Water Management in Horticultural Crops

Introduction

• Water molecules are integral part of living systems being the solvent for metabolites and structural component of proteins and nucleic acids. Transpiration lowers water potential at evaporative sites within leaves, and this effect is immediately translated to root system through creating tension in plants vascular system. This demand of water must be satisfied continuously to maintain leaf water potential. The internal water balance of plants depends on relative rates of water absorption and water loss. Despite stomatal resistance, the steep gradient in vapour pressure from leaf to air favours transpiration rate well in excess of CO₂ fixation. Transpiration rate normally ranges from 500 to 2500 mg H₂O/dm²-h, whereas CO₂ fixation rates range generally 5 to 25 mg CO₂/dm²-h. This physiological phenomenon amply justifies high water requirement to maintain high dry matter productivity. For this reason, actively growing plants as the case of all the vegetable crops need to maintain liquid phase continuity from soil water through its vascular system and all the way to evaporative sites in leaves. Vegetables contain large amount of water and the product qualities like tenderness, succulence, crispness and flavor are very much related to water supply at proper stages. In fact, texture of vegetables is determined by combination of tissue structure, cell wall properties and turgor pressure.
• The phenology of the growing plant can be characterized by vegetative, flowering, fruiting and other distinctive characteristics in vegetative stage like, curding in cauliflower, heading in cabbage and lettuce, bulbing in onion and garlic and tuberization in potato and sweet potato. During vegetative stage, consumptive use continues to increase till the end of this stage of growth and flowering occurs near and during the peak of consumptive use. The fruiting stage is accompanied by a decrease in consumptive use until the transpiration ceases during the later part of development of seeds inside the fruit.
• Stomata are the compulsory passage way for CO₂ and H₂O gas exchange between plant and atmosphere. Therefore, to fix carbon, the plant losses water to the atmosphere, the ratio being variable depending on species and growing conditions. This ratio is called water use efficiency i.e., quantity water transpired/ unit of carbon fixed in drymatter. Water use efficiency increases as amount of water required /unit of dry matter production decreases. Transpiration ratio is used express water use efficiency. Transpiration ratio refers to the quantity of water transpired by plant to accumulate 1 g of dry matter. This ratio ranges from 200 to 1000 (200:1 to 1000 :1) depending on crop species, cultivation conditions and vegetation period. Vegetable crops can be categorized in to four groups according to such index of water exchange. Leaf vegetables require more water to produce 1 g dry matter than pumpkin. Vegetable crops showing high transpiration ratio can not endure water stress, and any shortage of water during the period of growth severely affects yield. On the other hand, vegetable crops characterized by very low transpiration ratio can somewhat endure water stress condition and can give satisfactory yield even in moisture deficit condition.