Costs and benefits of photosynthetic stems in desert species from southern California

Abstract

Woody plants with green photosynthetic stems are common in dry woodlands with the possible advantages of extra carbon gain, re-assimilation of CO2, and high water-use efficiency. However, their green stem tissue may also incur greater costs of water loss when stomata are closed. Our study focussed on evaluating the costs and benefits of having green stems in desert plants, addressing the water-use efficiency hypothesis. We measured water status, carbon and water exchange, and carbon, nitrogen and oxygen isotopic composition of 15 species in a desert wash scrub in Joshua Tree National Park, California, USA. We found that all woody species that have green stems relied on their green stems as the sole organ for carbon assimilation for most of the study period. Green stems had similar photosynthetic rate (Amax), stomatal conductance (gs) and intrinsic water-use efficiency (WUEi) to leaves of the same species. However, Amax, gs and cuticular conductance (gmin) were higher in green stems than in leaves of non-green stemmed species. Carbon isotopic composition (δ13C) was similar in both leaves and green stems, indicating no difference in integrated long-term WUE. Our results raise questions about the possible trade-off between carbon gain and water loss through the cuticle in green stems and how this may affect plant responses to current and future droughts.

Functional traits of leaves and photosynthetic stems of species from a sarcocaulescent scrub in the southern Baja California Peninsula

“…However, we did not measure gas exchange, so the similarity in fluorescence variables may not hold for actual photosynthetic rates. In relatively few cases, leaves and stems have had the same photosynthetic rate (Tinoco‐Ojanguren, 2008; Ávila‐Lovera et al, 2019a). Generally, the greater A max of leaves compared to stems is due to lower N concentration and chlorophyll content generally found in stems than in leaves (Osmond et al, 1987; Nilsen and Bao, 1990; Tinoco‐Ojanguren, 2008; Ávila et al, 2014a; Ávila‐Lovera and Tezara, 2018; Ávila‐Lovera et al, 2019a).…”

“…In relatively few cases, leaves and stems have had the same photosynthetic rate (Tinoco‐Ojanguren, 2008; Ávila‐Lovera et al, 2019a). Generally, the greater A max of leaves compared to stems is due to lower N concentration and chlorophyll content generally found in stems than in leaves (Osmond et al, 1987; Nilsen and Bao, 1990; Tinoco‐Ojanguren, 2008; Ávila et al, 2014a; Ávila‐Lovera and Tezara, 2018; Ávila‐Lovera et al, 2019a). In our study, N concentration was lower in stems than in leaves, which may indicate different allocation patterns of nitrogen in leaves and stems (Field and Mooney, 1986; Harrison et al, 2009; Ávila‐Lovera et al, 2019b).…”

“…The reduction of PPFD interception resulted in slight increase in ETR in stems but not in leaves. Similarly, species from desert sites are able to maintain photosynthetic activity in their stems through the hot and dry season (Ávila‐Lovera et al, 2017, 2019a). Second, K S did not change from dry to wet seasons.…”

“…Green photosynthetic stems are common in dry lands worldwide, including subtropical deserts and tropical woodlands, and these species rely on their photosynthetic stem during the hot and dry period of the year as the sole source of carbon (Nilsen, 1995; Gibson, 1996; Ávila‐Lovera et al, 2019a). Furthermore, stem recycling photosynthesis, also known as corticular photosynthesis (Strain and Johnson, 1963) or woody tissue photosynthesis (Pfanz and Aschan, 2001), has been found to positively affect stem hydraulic functioning in mangroves (Schmitz et al, 2012), Populus nigra (De Baerdemaeker et al, 2017), and Salix matsudana (Liu et al, 2019).…”

“…Given the importance of photosynthetic stems in species in arid and semi‐arid environments (Nilsen, 1995; Gibson, 1998), which rely on their photosynthetic stems during the hot, dry period of the year as the sole source of carbon (Ávila‐Lovera et al, 2019a), we sought to test the following hypotheses: (1) There is proportional allocation to photosynthetic capacity in leaves and photosynthetic stems, such that plants with high leaf photosynthetic capacity also have high stem photosynthetic capacity. (2) There is a positive relationship between stem water transport efficiency and leaf and stem photosynthetic capacity, as shown in plants from desert ecosystems.…”