Abstract:

In this study， we investigated the changes in leaf chlorophyll content and photosynthesis parameters of Elymus sibiricus cv. Qinghai No.1 in response to fertilization and aging. E. sibiricus is distributed and used widely in the grassland of Qinghai province on the Qinghai-Tibetan plateau. Three field experiments were set up： 1） a time series of plots from one to six years old without any treatments， 2） fertilizer addition to six-year-old plots， 3） annual fertilizer addition to field plots for six years since 2015. From 2015，sowings were carried out in May of each year. The seeding rate was 2.25 g?m^-2 in row with 30 cm in spacing between rows and 15 m² （3 m×5 m） per plot. The chlorophyll content of leaves was measured at the flowering stage for 10 randomly chosen plants from the flag leaf down to the third leaf， using a SPAD analyzer from Konica Minolta， Inc. The photosynthesis parameters of the flag leaf were recorded from 10：30 to 11：30 am in a sunny day at the flowering stage （July-August） with a LI-COR LI-6400XT portable photosynthesis meter. It was found that chlorophyll content was decreased from the three years of age， but increased again at six years of age. In the age-series of E. sibiricus cv. Qinghai No.1， the net photosynthesis rate decreased， intercellular CO₂ concentration increased， stomatal conductance and transpiration rate decreased significantly at three years of age， and remained unchanged thereafter from three to six years of age. When nitrogen and phosphorous fertilizer was applied to previously unfertilized six-year-old field plots of E. sibiricus cv. Qinghai No.1， the relative chlorophyll content （which varied in different parts of leaves from 33.30% to 48.23%）， and the net photosynthesis rate increased， and signs of ageing were reversed. The highest net photosynthesis rate observed （10.77 μmol·m^-2·s^-1） occurred at the nitrogen level of 60 kg?ha^-1. In the long-term fertilizer addition field， low level nitrogen addition （45 kg·ha^-1） enhanced chlorophyll content， while phosphorous addition had no significant effect. An intermediate nitrogen addition level （60 kg·ha^-1） with phosphorous resulted in six-year-old plants retaining a high net photosynthesis rate.

Key words: Elymus sibiricus, leaf ageing, chlorophyll, photosynthesis, fertilization