Assessing the physiological responses and shoot growth of potted 'Jen-Ju-Ba' guava (Psidium guajava L.) plants to drought

Guava is an important fruit crop in Taiwan, but is increasingly affected by drought because of global warming. Therefore, we investigated the effects of two water-withholding/recovery cycles on the physiology and shoot growth of 2-year-old potted guava (Psidium guajava 'Jen-Ju-Ba') plants. During the first cycle (days 0-9), leaf water potential (Ψ leaf) and soil moisture decreased to -1.9 MPa (89% relative water content (RWC)) and -55 kPa (40% RWC), respectively, and the temporary wilting point was reached. Furthermore, the plants exhibited a 28-86% reduction in leaf CO₂ exchange rate (Pn), chlorophyll fluorescence under light adaptation (ФPSII), stomatal conductance (Gs), transpiration (E), leaf and soil RWC, Ψ leaf, and soil moisture compared with well-watered control plants. However, no difference in shoot growth was recorded. During the second cycle (days 13-19), necrotic leaves appeared when Ψ leaf and soil moisture decreased to -2.9 MPa (81% RWC) and -71 kPa (24% RWC), respectively, and the permanent wilting point was reached. At this stage, the experimental plants exhibited a 16-89% reduction in the abovementioned parameters compared with well-watered control plants and a 20% reduction in growth and a 210% reduction in the relative growth rate of the shoots, suggesting that reductions in Pn and shoot growth were significantly associated with pronounced drying. These findings indicate that an irrigation regime should be developed to optimize the canopy development of guava trees and that Pn, ФPSII, E, and Gs might serve as useful physiological indices of drought.