Fraction Of Transpirable Soil Water Thresholds Research Articles

Broader leaves result in better performance of indica rice under drought stress

Leaf growth is one of the first physiological processes affected by changes in plant water status under drought. A decrease in leaf expansion rate usually precedes any reduction in stomatal conductance or photosynthesis. Changes in leaf size and stomatal opening are potential adaptive mechanisms, which may help avoid drought by reducing transpiration rate, and can be used to improve rice genotypes in water-saving cultivation. The indica rice cultivar IR64 and four of its near-isogenic lines (NILs; BC 3-derived lines) unique for leaf size traits, YTK 124 (long leaves), YTK 127 (broad leaves), YTK 205 (short leaves) and YTK 214 (narrow leaves), were compared in this study for changes in leaf growth and its water status. The plants were subjected to two soil water regimes, well-watered and progressive soil drying measured by the fraction of transpirable soil water (FTSW). Applied drought reduced leaf number, total leaf area, specific leaf area, plant biomass, tiller number, plant height, stomatal conductance, amount of water transpired, leaf relative water content, and leaf water potential more in IR64 and the NILs than in the respective controls; nonetheless, transpiration efficiency (TE) was slightly higher under drought than in the well-watered controls. NILs with broader leaves had higher biomass (and its individual components), less stomatal conductance, and higher TE under drought than NILs with narrow and shorter leaves. Under drought, leaf number was positively correlated with tiller number and plant height; nonetheless, root weight and total biomass, water transpired and TE, and plant height and TE were positively correlated with each other. However, a negative correlation was observed between stomatal conductance and the FTSW threshold at which normalized transpiration started to decline during soil drying. Overall, the IR64-derived lines with broader leaves performed better than NILs with narrow and short leaves under drought.

Plant Growth, Water Relations, and Transpiration of Spiderplant [Gynandropsis gynandra (L.) Briq.] under Water-limited Conditions

Plant growth and osmotic adjustment of spiderplant were investigated in a glasshouse and under field conditions. Two fast-growing genotypes (P-landrace and P-commercial) and a slow-growing landrace (G-landrace) were grown under soil water deficit and watered conditions. The fraction of transpirable soil water (FTSW) was used as an indicator of water availability in pots. In the greenhouse, transpiration was determined by changes in daily pot weights and the ratio of transpiration of plants in soil water deficit to watered treatments expressed as normalized transpiration ratio (NTR). Water use in the field experiment was determined by gravimetric methods. The fast-growing genotypes had a higher rate of soil drying due to a higher rate of leaf area development. They were also more sensitive to soil water deficit with NTR beginning to decline at FTSW of 0.55-0.77 as compared to 0.29 for the slow-growing landrace. Also, the fast growing genotypes had FTSW thresholds for the stem elongation rate of 0.35-0.55 as compared to 0.20 for the slow growing landrace. The rate of leaf development declined when 40% to 60% of available water in the soil was removed, regardless of genotype. Leaf area of plants under field conditions decreased when the soil moisture was <60% field capacity. Under severe soil water deficit stress in pots, plants partitioned more biomass to roots than above ground; however, biomass partitioning between leaves and stems was not influenced by soil water deficit. Spiderplant showed limited osmotic adjustment (OA) in the range of 0.10-0.33 MPa at the highest soil water deficit (FTSW = 0). Thus, spiderplant is mainly a drought avoiding species. To achieve maximum growth, it is necessary to keep FTSW above 0.6.

Comparative effects of partial root drying (PRD) and regulated deficit irrigation (RDI) on water relations and water use efficiency in common bean (Phaseolus vulgaris L.)

Abstract In order to investigate the hypothesis that plant responses to altered water relations was related to hormonal signals in water-restricted plants, the comparative effects of partial root drying (PRD) and regulated deficit irrigation (RDI) were investigated by measuring plant growth, water relations, leaf abscissic acid (ABA) concentrations and xylem sap pH in common bean ( Phaseolus vulgaris L.). In a first experiment, plants were grown in a split-root system, with roots divided between two compartments and subjected to one of three irrigation treatments: well-watered (WW) receiving 100% of plant transpiration (T), PRD with half of the root system exposed to soil drying and other half kept well-watered with 50% T, and RDI with 50% T supplied, half to each side of the root system. The results showed a significant decrease in leaf water potential for both PRD and RDI treatments compared to the WW, but without any significant differences between PRD and RDI. Transpiration did not show any significant effect of irrigation treatments during the first 10 days of drought stress application, and thereafter decreased in PRD and RDI to a level around 50% of WW. Both RDI and PRD treatments resulted in a significant reduction of shoot and pod biomass compared to the control. However, since the total water used by transpiration was reduced by half in both water deficits treatments, this has resulted in a substantial increase in plant water use efficiency. Leaf ABA concentrations did not change under PRD and RDI throughout the experiment, until day 25 of water deficit application, where plants under RDI treatment accumulated higher amounts of ABA compared to WW and PRD plants. It was therefore concluded that ABA might not be directly involved in the initial stages of stomatal control under PRD or RDI. A second experiment showed that the relationship between relative transpiration rate and the fraction of transpirable soil water (FTSW) was well described by a linear plateau function that allowed the calculation of the FTSW thresholds at which transpiration started to decrease for RDI and PRD compared to WW. These thresholds were similar for RDI and PRD treatments. These results confirmed the conclusion of the first experiment, and did not support the hypothesis of a specific root-to-shoot signaling mechanism under PRD triggering stomatal closure, since no earlier decline in transpiration rate was observed in PRD compared to RDI.