SENSITIVITY OF CROP PLANTS TO WATER-STRESS AT SPECIFIC DEVELOPMENTAL STAGES - REEVALUATION OF EXPERIMENTAL FINDINGS

It has been suggested that in many crops differences in sensitivity to water stress occur at different growth stages. Since identical amount s of water may be applied, irrespective of whether a crop is exposed t o relatively severe and short periods of stress or to extended periods of mild stress, the responses to such differing conditions should be compared. Unfortunately, such a comparison has not been conducted in m ost studies on sensitivity to water stress at different growth stages. In the present study, based on three field experiments conducted for different purposes, such a comparison was made for three crops: corn, sunflower, and tomato. In corn, distinct responses of ear and kernel y ields to the timing of water stress were found. Withdrawal of irrigati on water during flowering and cob formation resulted in greater yield losses than during other stages, indicating that this is a critical gr owth stage. However, slight and uniform reduction of water during the entire growth period resulted in significantly less damage to kernel o r ear production, although the total amount of water applied was simil ar to that under staged withdrawal. In sunflowers, the withdrawal of i rrigation water even at noncritical growth stages caused a more marked reduction in grain yield than did a uniform reduction throughout the entire season. In tomatoes, on the other hand, the withdrawal of irrig ation water during specific growth stages caused minimal damage to fru it and total soluble solids yield as compared with fully irrigated con trol; reduction of irrigation water throughout the season brought abou t a significant decrease in yield. The difference between these crops is interpreted on the basis of the determinance of their floral merist ems.