レタスの潜在的な干ばつ耐性のためのセミハイスループットスクリーニング（<em>レタス</em>）遺伝資源のコレクション

Water availability for irrigation has been a concern across much of the United States and globally for decades, but research into the response to drought-stress, along with other abiotic stresses, has lagged behind work in the areas of disease and insect resistance, at the industrial, academic, and governmental levels largely due to a lack of funding. Water availability for agriculture has historically been only an afterthought at the level of policy makers. Recently, due to several severe droughts in important agricultural production regions in both the United States and Australia^1,2 fresh water availability has been thrust into the spotlight at both the national and international levels leading to many more resources being directed towards research into developing drought-tolerant cultivars of the major grain crops. While this shifting focus toward the development of drought-tolerant major crops is beneficial, like many areas of plant research specialty crops have largely been left behind.

A severe drought is currently limiting vegetable production in California, the largest production region for Lettuce (Lactuca sativa L.) in the United States³. These short-term weather patterns coupled with legislative and judicial action along with long-term climate change^4,5 have combined to reduce water available for agriculture in many of the most productive regions of California. Lettuce production in California represents a 1.5 billion dollar industry accounting for nearly 80% of lettuce production in the United States³. Leafy vegetables have high leaf water content and lettuce, in particular, has a shallow root system^6,7 which leaves the crop vulnerable to water-stress. In lettuce, as in all crops the development of drought-tolerant varieties will become increasingly important as fresh water supplies for irrigation become more constrained⁸.

This protocol lays out a method by which an initial screen was performed on a large collection of lettuce germplasm in order to identify a pool of potentially drought-tolerant candidate lines for use in physiological, molecular, and genetic studies to identify specific drought-tolerance traits. These candidates can also be used directly as sources of drought-tolerance in breeding programs to improve water use efficiency in commercial cultivars. This protocol was developed specifically to address the challenges that arise during the course of any screen of an extensive germplasm collection, especially issues of space availability and labor. Also, the protocol as presented was developed for use in lettuce, but has been successfully adapted for use in the screening of a germplasm collection of spinach (Spinacia oleracea) for potential drought-tolerance and could be modified simply to screen any leafy vegetable crop.

An important consideration before initiating a drought-tolerance screen is to understand what this method is and is not. This protocol is meant to represent a rapid method by which a large germplasm pool can be quickly and efficiently narrowed to a manageable number of candidate germplasm for use in more focused and thorough studies to identify individual tolerance traits. This protocol subjects the plants to a rapidly induced severe water-stress in contrast to a more natural slowly-induced moderate continuous drought-stress that would be observed under field conditions. The type of response induced within the plant during these two types of water-stress events (rapid dehydration versus natural drought) is not identical⁹ which could lead to the exclusion of some materials from future trials. These two stress events are not without overlap though¹⁰ and this protocol should serve as an effective way to identify potentially drought-tolerant germplasm contained within a large collection. This protocol alone should not be considered sufficient to identify with a high degree of certainty germplasm that contains durable drought-tolerance under field water-stress conditions, but it represents a significant step forward in the rapid screening of leafy green vegetables for potential drought-tolerance traits.