Annual plants, those that germinate, grow, flower and seed and die within one growing season, are particularly at risk when drought conditions occur. The lack of soil moisture causes wilting and stunted growth, a hastened flowering and a shortened life. The effects of drought are cumulative, as it disrupts the food-making in the plant and effectively modifies usual plant processes.
Fast-growing annuals take advantage of growing conditions but suffer quickly when these growing conditions are not favorable. Drought, the lack of water in the soil around plant roots, results in immediate stress and physical symptoms on the plant. Wilting is caused by plant cells that lack internal water pressure that sustains cell wall rigidness. Repeated wilting leads to degradation and death of plant tissues, first manifested as yellowing leaves and stems, eventually browning.
A lack of water in a plant disrupts the making of food, called photosynthesis, as well as other plant life processes. With fewer resources, the plant will not use its food for creation of more leaves, which in turn will require more water to sustain. Often a drought-stricken annual will look shorter than expected, stunted in leaf size and stem lengths.
Seeds of annuals will not sprout in drought conditions. This mechanism ensures that the embryo in the seed is not exposed to conditions that are inhospitable and likely not conducive for any growth of a plant to maturity. Thus, in soils where annual plants may typically pop up and grow each year, there may be no plants during a drought, a means to preserve the seeds to sprout next year when the drought is over.
Life Cycle Modifications
Annual plants are genetically programmed to germinate, flower and produce seed in one growing season. Seeds are the only means to ensure the plant species endures for another year. When drought conditions are encountered, the annual will expend its energy to create the flower and seed, regardless of time of year. Droughts can find annuals flowering earlier than normal, or in fewer numbers. The stress created by lack of water leads the plant to use all resources to flower and create a seed that will survive after the mother plant dies and can germinate and prosper in a future growing season that is more hospitable.