Native American tribes have been cultivating peaches [Prunus persica (L.) Batsch] since their introduction to North America in the 1600s. In the American Southwest, peach orchards derived from centuries of seed selections have been maintained in relative isolation from commercial cultivars. These Native American peach selections may be better adapted to the arid climate of the Intermountain West. We compared physiological robustness during water stress of seedling peaches from a 60-year-old orchard maintained by Navajo farmers in southwestern Utah to the commercial peach rootstock Lovell. Six replicate trees of each rootstock were subjected to eight cycles of controlled drought on an automated lysimeter system, which monitored transpiration rate continuously. Trees were selected for uniform size and transpiration rate at the start of the study. During the drought cycles, individual trees were watered when their transpiration rate decreased to less than 250 g of water per day, ≈20% of their well-watered daily transpiration rate. After the first cycle of drought, the transpiration rate of the Navajo trees was greater than the Lovell trees, so they depleted their root-zone water more rapidly and experienced greater water stress. Despite greater stress, the Navajo selection had greater leaf area and dry weight at harvest. Because the root system was confined, these results indicate that the Navajo selection may have greater resilience when experiencing drought, independent of the depth and distribution of the root system. However, this study was not able to determine whether physiological resilience during drought was a result of canopy or root characteristics. Field studies are needed to determine whether root distribution or depth also contribute to drought tolerance in the Navajo selection.

Summary. Sudden changes in photoactive radiation (PAR) (wavelength, 400-700 nm) induces rapid surface area changes in chloroplast thylakoid membranes. Although this response may have important photo-acclimative functions for the plant, little is known about the mechanisms by which changes in irradiance are detected or how thylakoid membranes actually increase or decrease surface area. Knowledge of the time required for significant changes in thylakoid area would help eliminate or support several possible mechanisms that may be involved in this aspect of photo-acclimation in plants. Leaf tissues were acclimated to a PAR of 500 ~tmol quanta per m 2 per s then exposed to low irradiance (PAR, 50 gmol quanta per m 2 per s) and sampled at 5, 15, 30, and 60 min post exposure. Tissue and cell structure were quantified and results showed a significant increase in the surface-to-volume ratio and surface area per unit of standard leaf volume for both appressed and nonappressed thylakoids within 5 min of exposure to low irradiance. On the basis of the ratios of appressed to nonappressed thylakoids, the surface area of the nonappressed thylakoids was found to increase faster than that of the appressed thylakoids throughout the sample period. The portion of the appressed thylakoids in contact with the stroma was defined as margin thylakoids. Margin thylakoid surface-tovolume ratio did not change relative to the high-irradiance control during the sample period but did remain significantly lower than the low-irradiance control during the sample period. The ratio of appressed to margin thylakoids indicated a broadening and shortening of the appressed thylakoid stack within the first 5 min of lowirradiance exposure. The rapidity of the shade response indicates that the early events in this response probably do not directly involve gene activation pathways.

Cherry orchards are transitioning to high-density plantings and dwarfing rootstocks to maximize production, but the response of these rootstocks to drought stress is poorly characterized. We used a 16-container, automated lysimeter system to apply repeated water stress to ungrafted Krymsk® 5 and 6 rootstocks during two growing cycles. Drought stress was imposed by withholding irrigation until the daily transpiration rate of each tree was 25% and 30% of the unstressed rate during the first trial and second trial, respectively. After this point was reached, the root-zone water status was restored to field capacity. Whole-tree transpiration measurements were supplemented with leaf-level gas-exchange measurements. Krymsk® 6 had a higher rate of photosynthesis, more vigorous vegetative growth and less conservative stomatal regulation during incipient drought than Krymsk® 5. At harvest, carbon partitioning to roots was greater in Krymsk® 6 than Krymsk® 5. The conservative rate of water use in Krymsk® 5 could be a function of greater stomatal control or reduced carbon partitioning to roots, which thereby limited transpiration rates. Further studies are needed to confirm that these results are applicable to trees grown using a common grafted scion under field conditions.