Diurnal and seasonal patterns of net photosynthesis by irrigated Chrysothamnus nauseosus under field conditions.

Chrysothamnus nauseosus ssp. albicaulis (white rubber rabbitbrush) is a common deciduous C3 desert shrub which is a potential source of natural rubber and which exhibits high rates of net photosynthesis (Pn) for a woody species. When maintained under irrigated conditions in the field, rates of Pn on cloudless days were generally found to be highest during the cool morning hours (peak rates observed between about 0900 and 1000 hrs). Pn declined during mid-day hours and was approximately 75% of that observed in the early morning for the remainder of the light period. This decline in Pn was accompanied by about 26 μmol CO2.m(-2).s(-1) in mid-May (approximately 30 days after bud break) to about 48 μmol CO2.m(-2).s(-1) in early August. Thereafter, Pn declined gradually until leaf fall in November. These data indicate that rabbitbrush is capable of relatively high rates of Pn throughout a substantial portion of the growing season when maintained under irrigated conditions. This may, at least in part, account for the relatively high rates of growth observed under conditions of ample soil moisture.